3D Printing of Biomimetic Functional Nanocomposites *via* Vat Photopolymerization

*Tengteng Tang, Dylan Joralmon and Xiangjia Li*

## **Abstract**

The complex structures and functional material systems of natural organisms effectively cope with crisis-ridden living environments such as high temperature, drought, toxicity, and predator. Behind these excellent survival strategies evolved over hundreds of millions of years is a series of effective mechanical, optical, hydraulic, and electromagnetic properties. Bionic design and manufacturing have always attracted extensive attention, but the progress has been limited by the inability of traditional manufacturing techniques to reproduce microscopically complex structures and the lack of functional materials. Therefore, there is an urgent need for a fabrication technique with a high degree of fabrication freedom and using composites derived from biological materials. Vat photopolymerization, an emerging additive manufacturing (aka 3D printing) technology, exhibits high manufacturing flexibility in the integrated manufacturing of multi-material systems and multi-scale structures. Here, biomaterial-inspired heterogeneous material systems based on polymer matrices and nanofillers, and the introduction of magnetic and electric fields on the basis of conventional 3D printing systems to spatially and programmably distribute nanofillers are summarized, which provides a new strategy for fabricating anisotropic structures. The application of this versatile 3D printing system in fabricating mechanically reinforced structures, polymer/metal structures, self-actuating, and superhydrophobic structures is also elaborated.

**Keywords:** 3D printing, biomimicry, nanocomposite, magnetic field, electric field

## **1. Introduction**

Organisms in nature exhibit unique survival strategies due to their special multiscale and multi-material structures. These biological structures are not only highly hierarchical but also highly flexible and have a clear division of labor in function. The multi-scale or multi-layer structure allows it to exhibit excellent heat conduction efficiency and energy dissipation when it is subjected to external stimuli, such as high temperature and impact force. Biomaterials make it have excellent mechanical, optical, and hydraulic performance. For example, the eggbeater-like structure on the surface of *Salvinia molesta* has excellent superhydrophobic properties, keeping the leaf surface clean all the time [1]. Nacre's bricks-and-mortar multilayer structure allows it to effectively disperse the energy shock when it is subjected to external force, showing excellent energy dissipation capability [2]. Due to the mechanical reinforcement of the arrayed nanofiller in the polymer, the limpet teeth, as the hardest structure in nature, make it firmly attached to the rock wall in the turbulence of the ocean [3, 4]. The microneedles on the surface of the cactus can efficiently absorb moisture in the air even in arid environments [5]. These structures with different functions all contribute to improving the survival rate of the natural organisms. Hierarchical structure [1, 6, 7], biomaterial matrix [8, 9], and organic/inorganic fillers [8, 10, 11] are all essential to realize highly functional structures, which also provide broad opportunities for fabricating next-generation highly integrated functional structures and materials.

Bionic design and manufacturing have always been hot spots in scientific research, and a lot of research work has been carried out in this field. Some reported preliminary results also demonstrate the feasibility of artificial biomimetic structures. For example, an anisotropic structure whose deformation can be programmably controlled is produced based on the hygroscopic properties of cellulose [9], and the mechanical reinforcement caused by the electric field arrangement of multiwalled carbon nanotubes (MWCNTs) inspired by shrimp claws [12], and the superhydrophobic structure based on *S. molesta* leaves showed excellent performance in oil separation [1]. Among the manufacturing technologies corresponding to these artificial structures, vat photopolymerization (VPP) [13], a layer-by-layer projection and selective solidification printing technology, can not only manufacture complex structures but also far outperform 3D printing methods such as fused deposition modeling (FDM) [14] and direct ink writing (DIW) [15] in terms of printing accuracy and efficiency. However, the degree of biomimeticity of current artificial structures and the development of bioinspired materials are still limited.

In order to solve the aforementioned problems, the researchers made the following new attempts based on the previous bionic manufacturing. By learning the material composition of natural structures, a series of heterogeneous hybrid systems with mixtures of polymer matrix and nanofillers have been developed. The introduction of magnetic and electric fields makes it possible to align nanofillers during printing, which further endows the structure with anisotropic behavior. Specifically, vat photopolymerization of mechanically enhanced structures inspired by Limpet teeth and magnetic field-assisted alignment of nanofillers will be described in Section 2. The heterogeneous material system inspired by the multilayer metal-containing shell of scaly-foot snail, polymer/metal structure, and electric field-assisted 3D printing system will be described in Section 3. The anisotropic gradient distribution and deformationprogrammable porous structure inspired by the mechanism of Delosperma nakurense's seed compartment moisture absorption and release of seeds, and liquid crystal templating assisted vat photopolymerization will be described in Section 4. The immersed surface accumulation 3D printing of superhydrophobic structure inspired by *S. molesta* leaves will be described in Section 5. Finally, the summary and prospect of materials and methods for biomimetic manufacturing will be detailed in the conclusion section.

## **2. Limpet teeth inspired nanocomposite for mechanical reinforcement**

Limpets are a type of aquatic gastropod mollusk with flat, cone-shaped shells that use a specialized organ called the radula to acquire food from hard ocean rocks [16, 17]. As shown in **Figure 1**, the radula is composed of rows of mineralized teeth that exhibit

*3D Printing of Biomimetic Functional Nanocomposites* via *Vat Photopolymerization DOI: http://dx.doi.org/10.5772/intechopen.110413*

#### **Figure 1.**

*Images of limpet tooth. (a) Top view of limpet shells; (b) microscale image of radula organ with exposed teeth; (c) microscale image of a single limpet tooth showing the base and cusp; (d) scanning electron microscope (SEM) image of limpet teeth microstructure with aligned mineral fibers. (a) Copyright from ref. [18]; (b) copyright from ref. [16]; (c, d) copyright from ref. [17].*

the highest known mechanical strength, surpassing that of any other naturally occurring materials [16]. This excellent mechanical strength results from biomineralization, a cyclical process, that reinforces the chitin protein matrix through the distribution of aligned iron oxide minerals to create distinctive hierarchical structures [17]. These mineral-based microstructures have evolved to provide limpets with the necessary strength to graze on rough ocean surfaces, enabling them to extract nutrients from their environment without incurring damage [16]. However, the structural rigidity of mature limpet teeth eventually declines due to repeated feedings, which prompts the formation of new rows of biomineralized teeth [17]. The naturally occurring nanocomposite, a combination of soft protein and aligned mineral phase, is primarily dictated by the presence of mineral nanorods, which enhance the mechanical performance of otherwise weak biomaterials [17]. Consequently, limpet teeth provide an ideal design inspiration for the fabrication of a biomimetic nanocomposites with hierarchical microstructures and superior mechanical strength.

### **2.1 Functional architecture of limpet teeth**

Biomaterials found in living organisms have adapted through natural selection processes to develop unique microstructures with enhanced physical properties [2, 12, 16, 17]. For example, the complex microstructures found in mantis shrimp have adapted a twisted plywood structure, known as a Bouligand structure, to enhance its mechanical properties and flexibility in order to withstand high-impact punches using its fist-like clubs [2, 12, 19]. Recent studies have determined that limpet teeth exhibit a linear elastic modulus of 120 GPa under tensile testing conditions [16]. Furthermore, the mineral phase of tooth samples was demonstrated to have a linear elastic of 180 GPa, which, compared to the overall structure, indicates that the observed mechanical strength is primarily attributed to the mineral phase [16]. This superior mechanical strength,

through the integration of biomineralized reinforcement of a polymer matrix, relaxes challenges associated with manufacturing technologies to fabricate microstructures with highly complex geometries and excellent mechanical performance. Current fabrication methodologies have certain limitations that must be addressed given the high degree of difficulty in achieving microstructures with aligned mineral biomaterials. Through recent advancements in biomimetic AM technologies, these limitations can be mitigated by rigorously controlling the microstructures using 3D printing methods that were previously unachievable through traditional fabrication methods [3].

Considerable challenges arise from the capability to create uniform mineral nanofillers with adjustable dimensions, such as length and cross-sectional area. The interaction of the mineral nanofillers within the chitin-polymer matrix plays a critical role in the observed tensile strength in the natural composite material based on varying dimensions and orientation. Moreover, the ability to develop mineral nanofillers with desired dimensions, with high repeatability, is of key importance to maximizing the mechanical strength in 3D printed nanocomposite parts. Therefore, these challenges have led to strategies that incorporate physical fields to aid in the alignment of magnetic microbundles during photopolymerization [2, 3, 12].

Integrating a physical field during the 3D printing process allows for high control of the alignment direction of mineral nanofillers in order to reinforce the printed part such that the compressive, tensile, and bending are deflected by the mineral constituents. The controlled alignment of mineral nanofillers within the 3D printed part is imperative to effectively create an AM process capable of fabricating hierarchical microstructures into a fully functional 3D object using a bioinspired nanocomposite. Using methods such as physical field-assisted 3D printing, a bioinspired nanocomposite material can be created and manipulated into a fully functional 3D shape that exhibits a significantly high mechanical strength in comparison to other microscale manufacturing processes as well as highly accurate and controllable micro features.

## **2.2 Magnetic field-assisted vat photopolymerization**

In order to reproduce the microstructures observed in limpets' teeth, an AM process, known as magnetic field assisted vat photopolymerization (MF-VPP), can be employed because of its flexibility to control the alignment direction of ferromagnetic nanofillers in polymeric resins, depicted in **Figure 2a**. For example, when a dynamic magnetic field is applied to the printing region, randomly distributed magnetic nanoparticles align along magnetic flux lines and subsequently agglomerate to form magnetic microbundles. Furthermore, the magnetic-field-assisted 3D printing process is highly advantageous because of its capability to rapidly align magnetic nanofillers in any spatial direction without direct contact between the magnet and nanocomposite material. A digital light projector (DLP) selectively cross-links the polymer resin on the printing platform, using a microscale mask image, which as a result constrains the magnetic nanofillers in the structure, as seen in **Figure 2b**. This process of alignment and photopolymerization is consecutively repeated, in a layerby-layer fashion, to fabricate the desired 3D printed object.

Furthermore, optimization of printing parameters is highly dependent on the strength of the magnetic field and nanofiller concentrations because of light scattering effects that inhibit photopolymerization. As shown in **Figure 2c**, as the concentration of magnetic nanofiller increases, microbundle length also increases while the gap between adjacent bundles is reduced. Moreover, increasing the strength of the magnetic field significantly increases microbundle length, when compared to

*3D Printing of Biomimetic Functional Nanocomposites* via *Vat Photopolymerization DOI: http://dx.doi.org/10.5772/intechopen.110413*

#### **Figure 2.**

*Magnetic field assisted vat photopolymerization. (a) Schematic of MF-VPP printer with COMSOL Multiphysics simulation of magnetic flux lines in the printing region; (b) images showing the alignment of different concentrations of iron hydroxide nanoparticles within photopolymer matrix; (c) compression strength of reinforced iron hydroxide with different iron oxide concentrations. All figures' copyright from ref. [4].*

microbundle length under weaker magnetic fields, while simultaneously decreasing the bundle diameter and gap. Consequently, nanocomposite material with high concentrations of magnetic nanofiller requires longer exposure times since it is difficult for the projected 2D light beam to penetrate the material to initialize photopolymerization. Taking these printing parameters into consideration, the process of coupling a dynamic magnetic field combined with 2D light projections during cross-linking allows for the creation of bioinspired microstructures with attractive anisotropic mechanical performance greater than that seen in limpet teeth. The 3D-printed microstructures can be fabricated at a low cost with unique features that are modifiable resulting in differing morphologies for various applications.

#### **2.3 Biomimetic material and structures**

Recent studies have revealed that the excellent mechanical performance exhibited by limpets' teeth can be attributed primarily to the reinforcement of a soft protein matrix by the controlled alignment of embedded iron-based minerals, specifically a mineral known as goethite [16]. Thus, the anisotropic mechanical strength of biomimetic hierarchical microstructures should be controlled by managing the spatial orientation of the magnetic nanofiller within the polymer resin. With the purpose of achieving high mechanical performance, a nanocomposite is prepared through the homogenous distribution of goethite nanoparticles in photocurable polymer resin. When initially distributed in the photocurable resin, the goethite nanoparticles have a random spatial orientation and must be coupled with the MF-VPP process in order to reinforce the normal weak polymer material *via* controlled nanofiller alignment.

Based on the morphologies seen in limpets' teeth, mechanical reinforcement is strongest when the mineral nanofillers are aligned parallel to the direction of the applied force. For example, this can be clearly observed when comparing the compressive strength of random and aligned magnetic nanofillers with just the pure polymer. As depicted in **Figure 2c**, the compressive strength of aligned nanofillers outperforms test specimens with random alignment and pure polymer. The maximum compressive load of aligned iron oxide particles-based composite is 80 times greater than that of the pure polymer. Anisotropic mechanical reinforcement of the microstructures is heavily influenced by the magnetic field intensity, dimension of magnetic nanofiller, and the magnetic nanofiller concentration in the 3D printable nanocomposite. However, only compression strength was enhanced due to the microbundle alignments, and the alignment of magnetic particles have no significant effect on the improvement of the tensile strength and bending strength. This is because there are no constraints between two adjacent magnetic particles, and the dissipations may lead to an early failure between each particle and polymer under the bending and tensile load.

Furthermore, bundles of magnetic nanofillers can be annealed, using a hightemperature furnace, to form long fibers that better mimic the microstructures seen in limpets' teeth. Similarly, these aligned magnetic nanofibers provide anisotropic mechanical reinforcement to the nanocomposite when a compressive load is applied. For example, the reinforced printed microstructures show superior mechanical performance when compared to the pure polymer and randomly oriented magnetic microbundles. However, high annealing temperatures, above the critical temperature for the photocurable resin, can cause cracks in the polymer material, which can lead to a reduction in the overall compressive strength. This can be easily mitigated with different polymers, such as polymer-derived ceramics, to fabricate high-strength composites with enhanced thermal performance. Thus, the reinforcement mechanism coupled with customizable alignment of magnetic nanofillers opens intriguing perspectives for designing high-strength 3D printed material based on bioinspired features with modifiable configurations.

## **2.4 Applications of anisotropically enhanced structure**

Limpets' teeth provide a promising design inspiration for the creation of a nanocomposite material with anisotropic functions with enhanced mechanical thermal and electrical properties. The MF-VPP method has been demonstrated to have a wide range of capabilities, including the ability to fabricate unique geometries and distinctive microstructures that are difficult to manufacture using conventional manufacturing techniques. These advancements have the potential to open new possibilities for creating intricate structures at the microscale, with exceptional mechanical strength. This technology is expected to have a significant impact in various fields such as aerospace, biomedical, and electronics in the coming years [4, 20–23]. For instance, rapid prototyping of highly accurate and low-cost scale models can aid in the design and optimization of lightweight and fuel-efficient airplane components [24]. Furthermore, 3D printing of nanocomposites can be used to generate complex components for jet engines that have enhanced damage tolerance and corrosion resistance [24].

This method of 3D printing nanocomposites is also advantageous for electrical components where heat dissipation is crucial for the functionality of key components that are suspectable to overheating. Effective heat transfer in electrical components can be achieved by adjusting the orientation of aligned nanofiller materials with anisotropic thermal properties [23]. Moreover, polymers with enhanced energy capacity and anisotropic electrical conductivity can be realized through the alignment

## *3D Printing of Biomimetic Functional Nanocomposites* via *Vat Photopolymerization DOI: http://dx.doi.org/10.5772/intechopen.110413*

of functional nanofillers for a wide variety of electronics [25]. Additionally, highresolution and porous biomaterials, such as scaffolds for bone regeneration, can be easily designed and fabricated using the MF-VPP method to create implants capable of withstanding shearing, compressive, and tensile loading [26]. In conclusion, creating complex structures with high mechanical strength on demand, which are both low cost and reliable, can be realized for numerous applications using the biomimetic approach described here.

## **3. Scaly-foot snail inspired multimaterial for property enhancement**

## **3.1 Functional architecture of scaly-foot snail**

Organisms in nature have evolved excellent survival strategies due to their special living environments. Scaly-foot snail (**Figure 3a**), a creature that lives in a deep-sea crater, is the only organism with metal in its skeleton (**Figure 3c**) [6, 29]. The multilayer structure of its shell can effectively deal with the threat of toxic substances and high temperature in the environment (**Figure 3b**) [6, 29]. The multi-layer multi-material structure also endows shell with extremely high hardness and excellent energy dispersion capability under impact. This outstanding property has widely attracted the attention of researchers, but it is extremely difficult to reproduce this structure in an integrated manner, both in terms of materials and manufacturing. Inspired by the excellent performance of the heterogeneous material system inside the scaly-foot snail's shell, the researcher Tang et al. developed a heterogeneous mixture that can be both photocured and electroplated. The matrix of the heterogeneous material system is PEGDA with good biocompatibility, PEDOT:PSS with excellent conductivity is used as filler to improve the conductivity of the matrix, photoinitiator is used to initiate the crosslinking of PEGDA, and CuSO4 solution is used as electrolyte to adjust rheological properties to meet the viscosity requirements of DLP printing.

#### **Figure 3.**

*Structural images of scaly-foot snail. (a) Image of the scaly-foot snail; (b) multilayered structure shown in crosssection of scaly-foot snail shell; (c) columnar channels and dispersed iron sulfide inside the shell. (a) Copyright from ref. [27]; (b, c) copyright from ref. [28].*

## **3.2 Electrical field-assisted vat photopolymerization**

The printing system in the reported work is a typical digital light processing (DLP) [30], which is mainly composed of digital micromirror device (DMD), linear stage, solution tank, printing platform, and control system (**Figure 4a**). The difference is that in order to introduce electroplating in the conventional layer-by-layer photocuring printing, two copper sheets are placed in the solution tank and connected to the negative pole of the power supply as an anode, and one corresponding copper sheet is placed on the bottom of the printing platform for both adhering the cured layer and as a cathode (**Figure 4b**). As shown in **Figure 4c**, when curing the polymer matrix, the projector projects a 2D light pattern of a specific shape to selectively cure the local area, and the conductive filler PEDOT:PSS and CuSO4 electrolyte are sealed in the PEGDA polymer chain. The projection of UV light is stopped during electrodeposition, the power supply is connected to the electric field between the cathode and anode, and the copper particles migrate to the bottom of the polymer layer to obtain electrons and then reduce to copper (**Figure 4d**). So far, this article has

### **Figure 4.**

*Manufacturing of bioinspired polymer/metal structures using electrical field-assisted vat photopolymerization and heterogeneous material systems. (a) Schematic diagram of the electrical field-assisted vat photopolymerization set-up; (b) illustration diagram of the electric field and projection system of the printing set-up; (c) demostration diagram of photocuring process in a single printed layer; and (d) schematic diagram of electrical field assisted metal deposition process. All figures' copyrights from ref. [29].*

## *3D Printing of Biomimetic Functional Nanocomposites* via *Vat Photopolymerization DOI: http://dx.doi.org/10.5772/intechopen.110413*

demonstrated the composition of the heterogeneous material system and the working logic of the printing system, and the electroplating results also show the effectiveness of the materials and manufacturing methods [29].

## **3.3 Fabrication of polymer/metal structures**

Polymer/metal structures are widely used in flexible circuits [31, 32], sensors [33, 34], and soft robots [35, 36] because of their light weight, extraordinary corrosion, and wear resistance. Most of the existing manufacturing methods are to first manufacture the polymer base by injection molding or 3D printing and then perform electroplating [37]. Since most polymers are not conductive, it is necessary to spray a conductive layer before electroplating [37]. Here, electric field-assisted 3D printing avoids the tedious and energy-wasting, time-consuming, and labor-intensive steps of traditional manufacturing [29]. It can print the polymer base and electroplate the metal surface in one step. The specific steps are as follows. The 3D model is first established by SolidWorks, and the output STL format file is imported into the self-built program for slicing. The system automatically assigns different material indexes to different layers during slicing. An index of zero corresponds to the photocuring step, and an index of one corresponds to the electroplating process. The obtained black and white mask images are then loaded into the printing program. Before printing starts, the motion system needs to be initialized, and the motion control program is used to move the printing platform to the zero point. At this time, the ground of the printing platform is in contact with the Teflon film at the bottom of the resin pool. After the printing process starts, the printing platform rises to leave a gap to solidify the base layer. Afterward, the system judges the material index corresponding to the picture. When the index is zero, the system projects UV light to cure polymer. The cured layer is then lifted by the platform to generate a new printing layer gap, and all following layers can be printed in this way. When the index is one, the system automatically progresses to the electroplating step. At this time, the polymer layer is lifted as an anode and only the bottom touches the mixture solution. After the electric field is turned on, the copper ions accept electrons on the surface of the anode and reduce to copper. As shown in **Figure 5**, the pitchfork polymer base was first printed, and then

#### **Figure 5.**

*Polymer/metal structure. (a) Schematic diagram of the polymer ASU pitchfork with copper surface. (b-e) Electroplating results and SEM images of the printed ASU pitchfork. All figures are copyrighted from ref. [29].*

the structure was placed in the same mixture for electroplating to obtain the metal surface. The copper and polymer layer can be clearly seen in **Figure 3c**. Due to the polymer base photocuring material and the material used for electroplating being the same material, the copper grows toward the polymer layer, which strengthens the polymer/metal interlayer bonding to avoid delamination.

## **3.4 Applications of polymer/metal structure**

Benefiting from the aforementioned photocurable heterogeneous materials that can be used as electrolytes at the same time, it is possible to manufacture polymer/ metal structures in an integrated manner, making up for the time-consuming and redundant multi-step process of traditional manufacturing methods. After testing the curing properties and plating properties of the material, the authors printed a series of different structures in **Figure 6** [29]. In order to demonstrate the corresponding multi-layer printing logic, the author printed a triangular base, electroplated a layer of copper and continued to print a layer of triangular polymer cylinders, and finally obtained a polymer-metal-polymer sandwich structure (**Figure 6a, b**). Metals are used in circuit manufacturing because of their excellent conductive properties, and the metal/polymer structure not only makes up for the low conductivity of polymers but also has the advantages of lightweight and flexibility of polymers. Based on this, circuits printed with pure polymers and electroplated circuits were manufactured (**Figure 6c**-**e**). The experimental results show that the circuits after metal plating have better electrical conductivity. The LED lamp beads light up when the circuit is connected, but the polymer circuit cannot be lighted because of its high resistivity. As a result, the multi-layered complex polymer-metal-polymer sandwich structure and the enhanced conductivity after electroplating demonstrate the high degree of manufacturing flexibility, structural complexity, and functionality of electric field-assisted printing heterogeneous materials in integrated packaged circuits [38], flexible sensors [38], and electromagnetic interference (EMI) shielding [39, 40].

### **Figure 6.**

*Demonstration of the sample made by electric field assisted vat photopolymerization. (a, b) schematic diagram and the manufacturing results of the polymer-metal-polymer sandwich structure. (c-e) Effect of electrodeposition on circuit conductivity. All figures are copyrighted from ref. [29].*

## **4. Delosperma nakurense inspired nanocomposite for shape changing**

## **4.1 Functional architecture of Delosperma nakurense**

Delosperma nakurense, a plant that lives in arid environments, chooses to open its seed chambers after rain or when there is high humidity to release its seeds to increase seed survival rate. This specific strategy of hygroscopic deformation is reflected in many plant species, such as pinecones [41], wheat awns [42], seedpods of orchid trees [43], and spikemoss stems [43]. These hygro-responsive structures choose to open the structure under wet or dry conditions to release seeds to complete reproduction. The spontaneous hygroscopic deformation of the seed chamber structure is of great significance to the design and manufacture of flexible devices, self-responsive sensors, and soft robots [44]. **Figure 7** describes the process of the seed chamber absorbing moisture and expanding to release the seeds. The researchers found that the three main points of this process are: porous structures are the basis of moisture absorption deformation, arranged cell wall realizes anisotropic deformation ratio, and swellable cellulose fiber is used to absorb water. It is essential to replicate these three elements in principle to bionically manufacture this structure. In order to achieve this goal, the researchers Tang et al. used liquid crystals and nano-fillers as the materials and used the phase separation of the two liquid crystals during curing to obtain a porous structure that can absorb moisture.

## **4.2 Liquid crystal templating assisted vat photopolymerization**

Liquid crystal, a substance that is both liquid and crystal and has a specific effect on light, has a strong polarization property that enables it to align under the action of electric field. This electro-alignment property is exploited to align nanofillers to impart anisotropic deformation of the structure. Correspondingly, using an electric field to align polymers and nanofillers is a proven approach to obtain anisotropic structures. Unlike extrusion molding, which uses shear force or ultrasonic vibrations to align polymers and nanofillers, ellipsoidal liquid crystal molecules are deflected under the action of an electric field force so that the long axis aligns with the direction of the electric field. At the same time, the deflected liquid crystal molecules drive the nanofillers to align in the direction of the electric field. This indirect arrangement makes up for the deficiency that the electric field can only arrange conductive substances (such as CNT [46], graphene [47]), and makes it possible to use the electric field to arrange non-conductive substances, such as the SiC nanofiller used in this study.

Based on the characteristics of liquid crystal materials, the authors Tang et al. propose an electric field-assisted printing strategy, which places electrode pairs facing

#### **Figure 7.**

*Seed capsule releases the seeds after absorbing moisture. (a, c) copyright from ref. [9]; (b) copyright from ref. [45].*

in different directions in the resin tank on the basis of traditional digital light processing. The printing system is mainly composed of a projector that projects a specific 2D light pattern, a stage that carries a cured layer and moves linearly, and DC power supply that is used to generate a high-voltage electric field. The electrode pair in a certain direction is first connected during printing, and after the LC and nanofillers are arranged for a period of time under the action of the electric field, the projector projects a beam of light to cure the selected area. By connecting electrode pairs in different directions and printing repeatedly layer by layer, LC and nanofillers with different alignment directions can be obtained by curing in a single layer or inside different layers.

## **4.3 Dynamic electro-alignment characterization**

Controlling the strength and timing of the electric field is critical for aligning liquid crystals and SiC nanofillers. Even though both liquid crystal and SiC are poor conductors, applying a high-intensity electric field for a long time generates a lot of heat, which causes the liquid crystal to undergo a nematic to isotropic phase transition [48]. Once the phase transition occurs, the liquid crystal becomes disordered [49], and the SiC that was previously aligned indirectly is simultaneously driven out of the ordered alignment state by the moving liquid crystal. When the electric field strength is weak, the polarization force applied to the liquid crystal molecules is not enough to drive the liquid crystal to move or it takes a long time to complete the arrangement. Therefore, choosing an appropriate electric field strength and application time is an indispensable condition for obtaining a homogeneous unidirectional alignment. In addition, the researchers also found that the alignment results using an alternating current (AC) electric field are significantly different from those using a direct current (DC) field. Even if the AC electric field is applied for a long time, there is no significant movement once the alignment is completed. In the case of the same voltage value, although the DC electric field increases the rate of arrangement, if the electric field is still applied after the arrangement reaches the highest degree of anisotropy, the order of the arrangement changes significantly, and the mixture undergoes approximately macroscopic disordered turbulence. This is detrimental to the desired result and should be avoided.

Specifically, when an AC voltage of 1 kV is applied, the long axis of the liquid crystal is slowly deflected to the direction of the electric field while driving the dispersed SiC nanofillers to gather and arrange in a line. The Fourier transform results in **Figure 8** show that the mixture is not directional when the electric field is not applied (0 s). After 195 s of alignment, the prominent peak curve in the probability distribution curve indicates that the mixture is anisotropic. Continuing to apply electric field, the degree of alignment does not change. In order to demonstrate the sustainability of the arrangement, within 2 mins after the electric field was turned off, the directionality of the arrangement did not decrease significantly, see the relative positions of the red and green curves in the **Figure 9**. When aligning mixture with 1 kV DC compared to AC, the results were significantly different in terms of time and sustainability of the alignment. The DC electric field makes the alignment more rapid, and an application time of 12 s is sufficient to obtain the most anisotropic alignment, compared to the 195 s required for the AC electric field (**Figure 10**). However, this directionality is destroyed with the prolongation of time. When the arrangement is 120 s, the mixture again becomes chaotic and enters a turbulent state. The potential reason for this phenomenon is that the direction of the DC electric field is always constant, and the liquid crystal molecules

*3D Printing of Biomimetic Functional Nanocomposites* via *Vat Photopolymerization DOI: http://dx.doi.org/10.5772/intechopen.110413*

#### **Figure 8.**

#### **Figure 9.**

*Liquid crystal templating assisted vat photopolymerization. (a) Schematic illustration of the liquid crystal templating assisted vat photopolymerization; (b) schematic diagram of electric field alignment of liquid crystal monomer and SiC nanofiller. All figures are copyrighted from ref. [45].*

are subjected to the electric field force of a single direction and then always migrate in the same direction. In the AC electric field, the liquid crystal molecules are subjected to alternating electric field forces, and there is no dominant force to move them in a specific direction, so the liquid crystals are only deflected *in situ*. The alignment state of the LC/SiC nanofiller does not change over time as long as the heat caused by the electrification does not cause the liquid crystal to undergo a nematic to isotropic phase transition. In summary, the AC electric field can make the arrangement more uniform and avoid entering a chaotic state, while the SiC nanofiller under the arrangement of the DC electric field has a more significant bundle, and the degree of anisotropy of the arrangement is higher but need to avoid applying an electric field for a long time to maintain the directionality of the arrangement.

**Figure 10.** *Probabilistic analysis of the directionality of the alignment under direct current electric field.*

### **4.4 Photopolymerization induced phase separation**

As mentioned in Section 4.1, the core of realizing hygroscopic deformation is an anisotropic porous structure, and liquid crystal as a functional material is well suited for this purpose. The phase separation of the two liquid crystals during solidification provides the possibility to fabricate a porous structure, and the arrangement of the applied electric field allows the pores to be aligned along a specific direction, and finally, an anisotropic and gradient porous structure can be obtained [45]. As shown in **Figure 11a**, under the action of an electric field in the RM257/5CB/SiC homogeneous mixture, the long axes of the liquid crystals RM257 and 5CB are deflected to be in line with the direction of the electric field, and this microscopic movement then indirectly drives the SiC nanofillers to converge and align in a straight line. Subsequent irradiation of UV light triggers the crosslinking of RM257, and 5CB, which does not participate in the reaction as a solvent, separates from the crosslinked RM257 and gathers together, and the area occupied by it is cleaned by acetone, leaving a large number of pores. The timing of phase separation is different due to the difference in light intensity at the bottom and top of the cured layer. The bottom is rapidly cross-linked under strong light irradiation, and it is difficult for 5CB to gather locally on a large scale, so the pore size at the bottom is smaller. On the contrary, due to the lower cross-linking rate at the top, the liquid crystals phase-separated to a higher degree, eventually leaving larger-sized pores. **Figure 11b**-**(i, ii)** show a porous structure with a gradient distribution, and iii and iv show elongated pores with long and short axes and a SiC nanofiller whose arrangement direction is consistent with the long axis of the pores. Therefore, the anisotropic porous structure mechanically enhanced by SiC can be obtained using liquid crystal phase separation and electric field indirect alignment of SiC nanofiller, which opens the possibility of programmable deformations and hygroscopic actuation described in the following section.

*3D Printing of Biomimetic Functional Nanocomposites* via *Vat Photopolymerization DOI: http://dx.doi.org/10.5772/intechopen.110413*

#### **Figure 11.**

*Fabrication process of anisotropic porous structure. (a) Schematic illustration of aligning composite, phase separation during the photopolymerization, and the anisotropic porous structure after extracting unreacted 5CB; (b) SEM images of the gradient anisotropic porous structure with aligned SiC nanofiller. All figures are copyrighted from ref. [45].*

### **4.5 Applications of gradient anisotropic porous structure**

As previously conceived, when the LC/SiC mixture is aligned, the cured structure undergoes deformation in a specific direction after cleaning the unreacted 5CB and drying it. Since the major axis of the pores is aligned with the alignment direction, the dry structure curls up along the major axis. As shown in **Figure 12a**, the curling direction corresponding to the blue area is perpendicular to the red area. The annular structure in **Figure 12b** is divided into four regions and has a bowl shape when dry. The smaller rectangular region in **Figure 12c** corresponds to the hinge during deformation, along which the larger rectangle deflects. When two adjacent areas in the same layer are at +45° and −45°, respectively, the deformation directions of the two areas are perpendicular to each other after drying, and the deformations are mutually restrained and folded inward (**Figure 12d**). In order to demonstrate the deformation constraints between different layers due to the different alignment directions, a double-layer ring structure as shown in **Figure 12e** was printed, and the deformation directions perpendicular to each other in the two layers lead to a saddle-shaped deformation. However, the structure in **Figure 12f** is diagonally curled between the respective layers, and the opposite curling directions of the two layers are perpendicular to

#### **Figure 12.**

*Spatially programmable alignment and deformation with the prediction of simulation. All figures are copyrighted from ref. [45].*

**Figure 13.** *Dynamic grasping process of hygroscopic gripper. All figures are copyrighted from ref. [45].*

each other. So far, by setting different alignment directions in different regions in the same layer or between different layers, a series of programmable deformations can be obtained. Furthermore, the final deformation result can be predicted in advance through simulation, and the experimental results also confirm that the simulation is highly consistent with the real deformation.

Soft grippers have attracted widespread attention because of their flexibility and unique driving methods. The common ones are pneumatic drive [50, 51], electromagnetic drive [52–54], and cable chain drive [55, 56]. However, the structures corresponding to these driving methods are very complex and require the input of external energy. Therefore, designing and manufacturing a self-actuating flexible gripper is the focus of current research. The spontaneous deformation property of the porous structure upon moisture absorption and drying is a favorable way to fabricate self-propelled flexible grippers. The author designed a four-beam gripper, and the liquid crystal alignment direction is perpendicular to the direction of the beams. As shown in **Figure 13**, the beam is fully bent, and the entire gripper is closed in the dry state. After placing it in the acetone, the gripper opens after the pores absorb moisture and expand, and then moves the gripper over the yellow target. As acetone volatilizes in the air, the pore previously filled with acetone collapses, and the gripper bends and closes to grab the target. Finally, the target is moved into the acetone, and the beam continues to expand hygroscopically to release the target. The entire grasping process is about 206 s, and the deformation of the gripper is reversible, as a hygro-responsive structure stretches in acetone and contracts in the air. In summary, the programmable control of the hygroscopic deformation of the anisotropic porous structure opens up new opportunities for the fabrication of flexible grippers [57], ultrafiltration membranes [58, 59], and flexible sensors [60]. The future research direction is mainly to control the shape and distribution of pores on the micron scale, improve the actuation response rate, and enhance the mechanical properties of composites.

## **5.** *Salvinia molesta* **inspired nanocomposite for controllable wettability**

## **5.1 Functional architecture of** *S. molesta*

Since Dettre and Johnson discovered in 1964 that the superhydrophobicity of the lotus leaf is related to the nano/microscale dual-scale structure of its surface [61], a large number of biomimetic structures have been designed and manufactured by researchers, such as the eggbeater-like structure of *S. molesta* that exhibits long-term *3D Printing of Biomimetic Functional Nanocomposites* via *Vat Photopolymerization DOI: http://dx.doi.org/10.5772/intechopen.110413*

**Figure 14.** *Superhydrophobic structures of Salvinia molesta in nature. All figures are copyrighted from ref. [62].*

air retention capability, which enables water droplets to remain on the structure's surface (**Figure 14a, b**). The core of the superhydrophobicity exhibited by this structure is that the hair on the head of the eggbeater is coated with nano-scale wax crystals and the hair ends have hydrophilic patches (**Figure 14c**). This nano/microscale dual-scale roughness effectively reduces the surface energy of the structure. In order to bionically manufacture this structure, researchers mostly use polymer resin to print structures below the millimeter level and the smallest size is only a few microns [63]. Although this method can reproduce the natural structure excellently and has good superhydrophobic performance, the dual-scale roughness characteristics of the surface of the natural structure cannot be reflected. Therefore, Yang et al. tried to add multiwalled carbon nanotubes (MWCNTs) to the resin to increase the roughness at the nanometer level. The results showed that MWCNTs improved the superhydrophobicity of the polymer structure.

## **5.2 Immersed surface accumulation vat photopolymerization**

Different from conventional top-down or bottom-up layer-by-layer printing, immersed surface accumulation innovatively uses optical fiber to guide the light beam, combined with computer numerical control (CNC) multi-axis printing platform to obtain greater degree of printing freedom (**Figure 15a**). The mask image obtained by slicing the three-dimensional model is projected onto the focusing objective lens by the digital micromirror device (DMD), and the shrunken optical pattern

### **Figure 15.**

*Immersed surface accumulation 3D printing. (a) Schematic diagram of the vat photopolymerization set-up; (b) schematic diagram of the optical system of the immersed surface accumulation strategy. All figures are copyrighted from ref. [1].*

#### **Figure 16.**

*Scanning electron microscope (SEM) images of the eggbeater-like structure. (a) Structures made from polymer; (b) structures made from polymer/MWCNTs mixture. All figures are copyrighted from ref. [1].*

is guided into the photocurable resin solution through the optical fiber. The polymer undergoes photoinitiated polymerization under light, and the mixture is selectively cured. The MWCNTs are sealed in the polymer network, some MWCNTs protrude from the surface of the structure to form nanoscale roughness. The polydimethylsiloxane covering the end of the optical fiber makes the cured polymer easily separated from the optical fiber. At the same time, the printing result can be observed through the beam splitter, so as to detect the printing quality in real time. At the level of printing freedom, immersed surface accumulation can not only realize the layer-by-layer printing of conventional vat photopolymerization on the plane but also can print on complex curved surfaces or sides. The scanning electron microscope image in **Figure 16** shows the effectiveness of the printing system. The shape of the structure is similar to the natural structure. The structure below the micron level has a minimum size of only tens of microns. The subsequent contact angle test also fully confirmed the superhydrophobicity of the structure.

## **5.3 Fabrication of eggbeater-like microstructure**

By using pure polymer and polymer/MWCNTs to print the eggbeater structure, the effect of MWCNTs on the surface roughness can be compared. **Figure 16a** is the top view and side view of the structure made of pure polymer. It is obvious that the entire *Salvinia molesta*-like structure has been completely reproduced, the surface is very smooth, and there is no overcuring between layers. This demonstrates that it is feasible to replicate micron-scale biomimetic structures using photocurable polymers and digital light processing. When MWCNTs were mixed into the polymer, the surface of the structure was distributed with microscale microgroove (**Figure 16b**), and the roughness increased. More importantly, part of the MWCNTs protrudes from the surface of the structure (**Figure 16b**), which constitutes the nanoscale roughness. Experiments prove that the nanoscale roughness caused by the microgrooves on the surface of the structure and the protruding MWCNTs significantly reduces the surface energy of the structure, and the two synergistically enhance the superhydrophobicity of the structure.

*3D Printing of Biomimetic Functional Nanocomposites* via *Vat Photopolymerization DOI: http://dx.doi.org/10.5772/intechopen.110413*

## **5.4 Applications of superhydrophobic structure**

In recent years, with the increasingly frequent exploitation and transportation of marine resources, emergencies such as offshore oil spills have occurred more and more frequently, which not only caused huge economic losses but also caused serious damage to the marine ecological environment. Traditionally, spilled crude oil has been dealt with by physical, chemical, and biological methods. For example, a porous lipophilic material absorbs oil through capillary action and stores it in the pores of the material, and finally recovers the crude oil adsorbed by the porous material through extrusion or centrifugation. In addition, the oil-water separation ability of the structure can be greatly improved by changing the lipophilic and hydrophobic properties of the structure. Therefore, through the design and surface treatment of the surface micro-nano structure, the oil absorption and hydrophobicity of the oil-absorbing material can be effectively improved. At the same time, due to the capillary force caused by the surface micro-nano structure, its adsorption capacity to the oil layer is greatly enhanced. As shown in **Figure 17**, under the action of surface tension and gravity of the droplet, the droplet of the oil-water mixture remains spherical on the surface of the eggbeater-like structure. Since the structure is superhydrophobic and has a strong capillary effect on oil droplets, the oil droplets penetrate into the gaps of the columns while the water droplets remain spherical. After 6 s, the oil droplets were completely absorbed by the biomimetic structure, achieving oil-water separation. When the eggbeater-like structure holds the oil-water droplets in different relative positions, even if the droplets are suspended below the surface of the structure, the result of oil-water separation does not change. At tiny scales, oil-liquid separation will not be affected by gravity but is mainly determined by the surface tension of the droplets and the surface energy of the structure. Therefore, regardless of the relative position of the droplet and the eggbeater-like superhydrophobic structure, the result and rate of the oil-water separation did not change significantly. In summary, the bionic superhydrophobic structure will have a wide range of applications in surface self-cleaning, oil-water separation, and anti-icing.

#### **Figure 17.**

*Oil/water separation of the printed eggbeater-like structure. Oil/water separation performance under the tilt condition of (a) 0°, (b) 180°, (c) 45°, and (d) 90°. All figures are copyrighted from ref. [1].*

## **6. Conclusion**

The implications for biomimetic fabrication of highly functional structures found in nature are both significant and challenging. This paper summarizes the difficulties and shortcomings of existing research in terms of materials, structures, and manufacturing methods required for bionic manufacturing, and proposes a series of preparation methods for bionic materials and highly flexible additive manufacturing methods. Firstly, since a single material is not enough to make the structure highly functional, the bioinspired polymer/nanofiller composite can effectively endow the structure with better mechanical performance, optical characteristics, thermal conductivity, hygroscopic deformation, and superhydrophobicity. Secondly, in view of the fact that there are still some slight differences between the bionic structure and the natural structure, and these differences are decisive for the performance, it is necessary to replicate the natural structure both macroscopically and microscopically, which will play an important role in improving performance. Thirdly, at the level of manufacturing methods, vat photopolymerization provides an effective means for manufacturing multi-scale complex structures, and the introduction of magnetic and electric fields has created the possibility to increase the complexity of the structure, and the nanofillers spatially arranged in a specific direction have brought the structure mechanical, thermal, optical, and deformational manifestations of anisotropy. Although the current research on biomimetic manufacturing is quite effective and the performance of the structure is excellent, there are still many potential research fields worth studying in the future: 1) The study and imitation of the structure, material, function, and interaction of natural organisms must be the result of comprehensive application and cross-learning of biological principles, material chemistry, advanced manufacturing technology, and numerical simulation; 2) Bionic structures need not be limited to natural structures despite their superior performance, the next-generation functional structures can be predicted through big data analysis and numerical simulation; 3) Continue to develop advanced manufacturing methods to cope with multi-scale, multi-material, and multi-physical-field printing needs; 4) The intelligent bionic structure should not only originate from nature but also return to nature, which realizes the purpose of green manufacturing and sustainable development. Overall, deconstructing at the material and structural level and using additive manufacturing to reproduce natural structures is effective in promoting bionic design and manufacturing, and opens new doors for the manufacture of next-generation high-performance mechanical, thermal, optical, and hydraulic structures.

## **Acknowledgements**

The authors acknowledge ASU startup funding, ASU FSE Strategic Interest Seed Funding, and National Science Foundation (NSF grant No. CMMI-2114119).

## **Conflict of interest**

The authors declare no conflict of interest.

*3D Printing of Biomimetic Functional Nanocomposites* via *Vat Photopolymerization DOI: http://dx.doi.org/10.5772/intechopen.110413*

## **Author details**

Tengteng Tang, Dylan Joralmon and Xiangjia Li\* Arizona State University, Tempe, USA

\*Address all correspondence to: xiangjia.li@asu.edu

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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## **Chapter 6**

## Advances in Large-Scale Robotic 3D Printing with Plastic Pellets

*Adolfo Nadal Serrano and José María Espejo Bares*

## **Abstract**

3D printing with robotics is reaching an unprecedented level of maturity both in the market and at a technological level. This paper discusses current applications of large-scale robotics applied to 3D printed real-scale final parts for the construction and product design industries, including state-of-the-art methodologies, technologies and applications. Furthermore, an in-depth view of technologies and applications developed by the author will be provided, including robot-end effector integration and the automated generation of machine code through an ad-hoc computer aided design to computer aided manufacturing (CAD-CAM) integration. This integration accounts for parametric capabilities and design-time feedback. Consequently, advances in the seamlessly integrated design-to-manufacturing workflow will be presented: (i) design (by means of employing parametric modeling software), (ii) geometry analysis (by means of ad-hoc machine manufacturing process simulations), (iii) CAD-CAM integration (by means of automated geometry processing and machine code generation), and (iv) manufacturing and testing (by combining 6-axis robots and large-scale plastic extruders).

**Keywords:** advanced additive manufacturing, large-scale, 3D printing, robotics, recycled plastic extrusion, pellets, automation

## **1. Introduction**

Additive manufacturing has exponentially increased its adoption and widened its use in the industry in the last 10 years after a short but significant decay in its implementation relative to former adoption expectations [1]. Its use focused on smallscale parts and rapid prototyping mainly, while allowing a decent amount of market space aside in terms of final parts for the automotive, aerospace, product design or construction industry, to name just some. One of the main issues behind this shortfall was high, unstable production costs, deriving simultaneously from both raw printing material and manufacturing costs—including operating and entry—level ones, such as high machinery prices. Besides, most industries have traditionally operated on metal materials—mostly aluminum or steel—and Computerized Numerical Control (CNC) based techniques, such as milling, drilling or similar.

Furthermore, the main drivers at a technological level for the adoption of additive manufacturing techniques, in general, and 3D printing, in particular, showed a bottom-up pattern [2]. These were generally pushed by technically-oriented

workforce but barely precepted by top-management implementations, often unable to carry out the necessary innovations due to a series of impediments, namely: (i) market constraints, (ii) misleading and conflicting market interests or (iii) lack of capacity to prove the feasibility of the investment in 3D printing technologies [3]. Finally, 3D printing technology has proven unable to scale appropriately in terms of size, production costs and time, despite the efforts made to bridge this gap [4].

On the material side, despite the fact that many metals have gained presence and microstructural stability in the 3D printing market, especially in small-scale applications related to jewelry, biomedicine or high-end parts [5] are still barely affordable for most end-use products. Besides, the precision and manufacturing tolerances of 3D printing techniques with titanium, for example, are far from those reachable by milling or CNC machining techniques. Therefore, mixed manufacturing technologies [6] have augmented their importance, thus allowing firms to gradually adhere to those in order to optimize their production processes while refining their product quality and expanding to otherwise unapproachable market opportunities [7].

As a consequence, there is plenty of room to implement other materials and largesize printing technologies with wide potential use in sectors such as the automotive industry or the construction and engineering fields, where large parts are needed to replace their more factory-like, traditionally-made counterparts at an affordable price while maintaining a well-balanced material performance and consumption [8, 9]. Moreover, automation processes are nowadays commoditized, pushed by the power of Supervisory Control and Data Acquisition and Artificial Intelligence (SCADA-AI) integrative applications [10] that allow to obtain large sets of end-effector data and produce behavioral patterns for a variety of practices and routines. For instance, data pattern analysis enables firms to control the overall performance of their production lines and tools in integrated user interfaces, favoring the use of "intelligent devices" able to provide data in real time. Robots, as fully customizable and programmable machines, gain further momentum over simpler CNC-driven architectures.

This research combines the two aforementioned opportunities into a single solution while providing affordable hardware and software solutions in an attempt to democratize technology and widen its use and implementation in predominantly the engineering and construction sectors. Thus, a solution for plastic pellet extrusion is presented, combined with a 6-axis robotic arm through seamless software and hardware integration.

## **2. Current research and technological development**

While most large-scale 3D printing technologies rely typically on 3-axis gantry-like machines, these have proven to have relevant limitations in terms of usability and scope: (i) these require large structures and initial investments, (ii) the crane-like structures must be used off-site and present constraints when located on-site, (iii) they are restricted to a horizontally layered logical structure, which (iv) restricts the geometrical capabilities of the systems, such as its use in cantilevers or inclined geometries in general. As a result, the use of robots and thereby adapted 3D printing technologies is growing steadily, activating research projects and technological advancements worldwide. Much research has been conducted by ETH Zurich, a pioneer in the development of advanced industrialized methods through an architectural lens leading the research in the field. Numerous initiatives have been carried out by this institution in the last 10 years since the creation of the Gramazio and Kohler

research group [11]. ETH's block research group presented in 2021 the first-ever pure unreinforced concrete offsite 3D printed bridge alongside Zaha Hadid Architects' computation and design group and other industrial partners, showing a unique dryassembled construction stabilized solely by its geometry through shape-based topological optimization [12], thus minimizing material usage [13] through a compressiononly, computationally pre-optimized design. This bridge was displayed at the Venice Biennale of Architecture in 2021, allowing visitors to see the lightweight, singlelayered shelled assembled panels generated by a fusion of FDM 3D printing with casting methods [14].

Further mentioned worthy efforts have been realized by market players and private companies alike. Aectual, a Netherlands-based company producing furniture and architectural products, focuses on the use of terrazzo, bioplastics and plastic pellets. The firm implements an ad-hoc setup employing 6-axis robotic arms and a regularsize pellet extruder controlled by Siemens PLCs, thus externalizing a meaningful part of their printing technology [15]. Furthermore, Aectual uses a 9-meter-long track to extend its maximum buildable volume capacity to 170cubic meters in order to create interior design elements.

Also located in the Netherlands, MX3D claims to introduce the advantages of 3D metal printing to new high-impact industries. MX3D uses a robot-mounted wire arc additive manufacturing (WAAM) system [16]. WAAM allows the use of conventional welding filler materials, which may reduce material costs drastically when compared with SLS manufacturing in a ratio of 10:1 [17], despite the high energy requirements of the system. The MX3D bridge, built with that technology at an early stage of development, took 6 months to print and required extensive testing in order to test the material's mechanical properties and calibrate the calculus of the structural section. Printed steel's properties and characteristics proved to differ significantly from regular steel in the elastic modulus, which affected the overall stability and stiffness of the bridge [18].

## **3. 3D printing design-to-manufacture. A comprehensive software and hardware solution for plastic pellet extrusion**

Optimizing the manufacturing capacity is one of the most substantial aims of every production industry. These intend to achieve mass customization without sacrificing efficiency or the benefits of economy of scale in terms of return on investment. As a result, the ability to respond to customer requirements in a quick and flexible manner while keeping high version numbers at low batch sizes must increase [19]. The implementation of a customizable, programmable and fully integrated large-scale production system is of use precisely to tackle this issue at (i) software, (ii) hardware and (iii) material levels. This concept is a great advancement in the design industry, due to the time-saving factor and the subsequent cost-effectiveness component. This design method also challenges sustainability goals. It enhances the material reduction in the manufacturing process and subsequent effect of generating less waste at the end of the product lifetime.

Therefore, a comprehensive 3D printing design-to-manufacture technology is proposed, which entails: (i) A parametric or-feature based modelling tool that grants the designer the possibility of modifying an established geometry in a matter of instances by easily generating algorithms grounded on the Rhino-based visual programming interface Grasshopper or others, which allow for single- or multi-solutionsbased algorithmic design [20] and receive intense optimization attention [21]; (ii) a quick-response, integrated slicing simulation algorithm capable of dealing with

complex and intricate geometries of various topologies including boundary representations (BReps) and meshes [22]; (iii) a CAD-CAM translator for a variety of robot models; (iv) a visualization interface whereby production can be simulated and robot signals set and programmed; and (v) a fully-functional end-effector comprising an extruder and low-cost, reliable control electronics for (vi) pellets obtained from thermoplastics including Polylactic Acid (PLA), polyethylene terephthalate (PET) and polyethylene terephthalate glycol PETG.

## **3.1 Parametric modeling and geometrical analysis**

Parametric modeling is a rather restrictedly employed term that defines the ability to design parts and products based on implicit geometrical relationships rather than via explicit dimensioning. This associative way of modeling and depicting solids and other topologies—such as BReps, meshes, and others—relies on the ability of 3D CAD platforms to define geometry through either (i) a history-based hierarchical objectdependency tree, (ii) customizable parameters and equations, (iii) programmable functions or (iv) a combination of the above. Each 3D modeling software offers a variety of interfaces, including but not limited to (i) equation editors, (ii) visual programming interfaces, (iii) application programming interface (API) accessibility through programming interfaces or a (iv) combination of any of those. McNeel Rhinoceros is used for testing and programming purposes, as well as a platform to program and extend its built-in modeling capabilities. Rhinoceros is a relatively lightweight NURBS-based software able to produce and handle all sorts of geometry, which constitutes an ideal system for advanced users that like to generate their own custom scripts, create generative systems [23], or create logic models.

For the purpose of this research, a set of different geometries comprising numerous variable conditions are tested, such as those displayed in **Figures 1** and **2**. These

**Figure 1.** *3D printing of a Striatus bridge block. Studio Naaro.*

**Figure 2.** *Software and hardware layers. Integration of electronics.*

display entirely parameterized sets of sizes, curvature settings, cantilevers and overhangs, among others, allowing the designer to modify and adapt the design according to manufacturing results and analyse optimal printing setup and material results in the printed part. Besides, three main 3D printing set-up-related parameters are implemented in the design of the parts: (i) nozzle diameter, (ii) shell overlap (which define the overall wall thickness for the part), and total part length, which relates simultaneously to printing speed and temperature cool down—the latter crucial to the successful layering and cohesion of the parts. Parts were designed hollow and single or two-cycled (this is, employing just one or two outlines per layer). **Figures 3** and **4** show the analysed printed configuration and the variable parameters used throughout the design process that defines the non-uniform rational Bezier spline (NURBS) model. Please note that the single-cycle chair design considers variable separations between contours in order to test the actual part's thickness as compared to the nominal size and adjust material flow.

**Table 1** shows the relationship between material flow, layer height setup, resulting part thickness (for a single wall pass) and qualitative results at a printing speed of

**Figure 3.** *Test parts (I). Single-cycled chair-like design showing dimensioning.*

**Figure 4.** *Test parts (II). Hopper showing dimensioning constraints.*

40 mm/s. These affected the design tolerances to a great extent, showing that speed could be adjusted to ranges of 60mm/s to 200mm/s if desired. Optimal material flow was found at 375.5 to 392 mm3/s at speed rates of 40 mm/s to 60mm/s, although further research must be conducted to reduce data dispersion. As seen in **Table 1**, layer heights could also be modified accordingly.


**Table 1.** *Material flow adjustments and results.*

## **4. Pre-processing for manufacturing. CAD-CAM integration**

Geometrical processing entails a three-step software-based workflow: (i) robot model and overall setup, (ii) model slicing and (iii) target generation. The software, an extension of Rhinoceros' Grasshopper visual interface, as indicated above, is capable of processing both mesh-like and surface-like topologies, including solids. From a mesh point of view, a wide range of extensions are allowed, including but not limited to \*.stl, \*.igs, \*.obj, \*.ply, \*.msh and similar mesh-compliant file formats. This functionality is provided by the platform itself. Surface-like and solids can also be processed as BReps, which support ASCII encoding; some formats allowed are \*.3 dm (native Rhinoceros format), \*.brep, \*rle, \*.step and \*.sat, widely used in engineering and product design. Solids or surface-like geometries may be translated into meshes to the user's intent, who will decide fabrication tolerances or further geometrical affections (**Figure 5**).

The workflow comprises the following stages:


**Figure 5.** *Programming workflow diagram.*

**Figure 6.** *Test part (II). Hopper in a simplified simulation. Printing bed width 700 mm.*

either by length or by curvature to obtain the points that will become the targets' origin by assigning them an orientation plane.


**Figure 7.** *Test part (II). Hopper in actual printing process. Printing bed width 700 mm.*

**Figure 6** above demonstrate the software's flexibility and accuracy in depicting the system's behavior via an ad-hoc geometry processing algorithm. **Figure 7** depicts the corresponding printing stages on the 700 mm-wide printing bed proving the workflow's feasibility.

## **5. Manufacturing and testing**

As introduced above, the manufacturing test setup conveys a series of components, entailing mechanical, logical and ad-hoc control units. The main components are shown in **Table 2**, including robot, end-effector, feeding system and printing bed.

The main component for the present setup is the robotic arm ABB IRB 1200 with the IRC 5 compact controller. Secondly, an adapted pellet extruder is used. Heating is delivered by three heating coils that work at 220 volts and are controlled by 3 PT100



#### **Table 2.**

*Main components of manufacturing unit.*

type thermistors. An Arduino code developed ad-hoc for the purpose runs on an Arduino Mega 2560 board with an added ramps 1.4 to control the three solid state relays (SSR) that regulate the three 220 V heating coils. Thirdly, with the objective of controlling the extruded material flow a Nema 23 stepper is used, which supplies a maximum torque of 3 Nm. Fourthly, an air-refrigerating system is mounted on the nozzle to cool down the printed material, comprising a 4 mm aluminum pipe curved around the tip of the nozzle. The extruder body is cooled by liquid refrigerant pumped from a tank placed aside the robotic arm. The printing bed consists of a 4 mm thick glass surface covered with a single layer of Scotch 3D printing tape, which enhances the adhesion of the part to the bed (**Table 3**).

Whereas the robot program includes the necessary DO-related commands, the wiring was set up to minimize the need for electronic components and coding. An Arduino MEGA board is used to control the overall behavior of the end-effector (this is, to turn the stepper motor on or off according to needs) while keeping track of the temperature regulation in each coil, as mentioned previously.

ABS was not tested due to its thermal instability while printing and in order to prevent warping. Additionally, ABS is not a suitable printing material in open spaces and in the absence of properly heated printing beds. Solid-state relays are employed as gates to power the coils, which cool down under regular atmospheric circumstances.

**Table 4** shows the test part specifications including operating time for both specimens. Note that the single-cycled design presents tighter layer heights and a lower printing speed at the same material flow rate, thus expelling more material per length



**Table 3.** *Printing parameters.*

#### **Figure 8.**

*Closeup of single-cycled design showing irregular geometry, material burns (left) and warping (right), scale bar shows a mark every 10 mm.*

unit. A close-up view of the part's layers is displayed in **Figure 8**, depicting geometry inaccuracies caused by intricate geometry and low precision zones (**Figure 8** left). Furthermore, material burns can be seen in intermediate layers (**Figure 8** left).

In addition to material burns, an excess of remaining latent heat in a layer can result in undesired material fluidity, causing the part to collapse partially or completely. More importantly, this may cause the nozzle to overheat, melting already printed parts or layers, thus affecting the overall stability and geometry in undesired ways. This effect may be seen in **Figure 8** right above.

In addition to adjusting printing speed per layer perimeter, a layer cooling system was developed and mounted (**Figure 9**). Also, geometry needs to be reviewed to minimize the amount of material at that height (**Figure 10**).

Tested materials include PLA and PETG presented in spherical pellet form and flake shape. Sizes are constrained to a maximum diameter of 6 mm for the former and up to 6.25 mm in any direction for the latter. The main characteristics of the used materials are displayed in **Table 5** for clarity purposes.

Two main material arrangements are employed: (i) the use of purely spherical pellets alone and (ii) a combination of spherical and flake-like pellets. These tested material combinations include a series of mixes that allow checking for the feasibility

### **Figure 9.**

*Closeup of single-cycled design showing test part (left) and regular layering (right). Scale bar shows a mark every 10 mm.*


#### **Table 4.**

*Test part specifications.*

of employing recycled and reused materials alongside new ones. Spherical pellets are provided as new, while flake ones are recycled from a variety of untraceable sources.

Therefore, their specifications and applications may only be deemed as approximate values. **Table 5** specifies the results and issues tested on different material mixes. As shown, most reliable results are obtained when combining new pellets and recycled ones in a proportion of 80–20% or higher due predominantly to two factors: (i) the substantial decrease in the reliability of the venturi feeding system while using flakes even at high air pressures and (ii) the affection on printing temperature requirements when combining different material sources, some of which might include unknown origins and previous physical or chemical treatments or pre-processing (**Figure 11** right).

*Advances in Large-Scale Robotic 3D Printing with Plastic Pellets DOI: http://dx.doi.org/10.5772/intechopen.109438*

**Figure 10.** *Collapse of hopper's tip due to heat accumulation and part's layer quality (detail and overall).*



#### **Table 5.**

*Printing material parameters.*

**Figure 11.** *Photography of ϕ2–ϕ4 mm PLA spherical and ϕ1.5–ϕ3 mm flake-shaped pellets used in the experiment.*

## **6. Results and conclusions**

In terms of size, the presented robot-based 3D-printing system is easily scalable to produce as-large-as-required parts simply by modifying or selecting robot models with a higher movement range, such as an IRB 1600, IRB 2600 or higher. Above mentioned parameters such as printing speed or layer air-cooling might need further

adjustment, despite the fact that larger parts allow more time for layers to cool down, which plays a crucial role in the stability of the parts while printing. In other words, larger sizes should be easier to print.

Special attention should be paid to warping, which highly depends on the geometry of the parts. The single-cycle chair-like design presents warping, in every instance of the process, exactly where the geometry is more acute and the part's printing length is higher, causing the part to be less condensed or present a lower level of adhesion to the printing bed. Warping, as opposed to layer cooling, will become a challenge with bigger parts.

Results show that a proper material mix combining new-to-used PLA and PETG is suitable for large-scale printing as shown in **Table 6**. Proportions of 80% new spherical pellets to 20% reused flake-shaped pellets and higher work properly. Whereas printing with pellets from equal thermoplastic types is achievable, mixing PLA and PETG has proven pointedly a more delicate task. Nonetheless, manufactured parts have proven to be stable and are well preserved through time, thus evidencing the feasibility and reliability of the entire system. The implications of this technology on the circular economy and environmental impact are left aside for future discussion.



## **Table 6.**

*Material combination results.*

Further research and funding will be required to challenge the thesis hereby presented and to be able to print bigger, more intricate geometries. Also, a significant part of future work will focus on the development of a more stable extruder, despite efforts made in that direction.

## **Acknowledgements**

Special thanks are given to Archiologics and its staff for their invaluable support and for providing funding, materials, hardware and valuable knowledge. This research would have been impossible without them.

## **Conflict of interest**

The authors declare no conflict of interest.

## **Author details**

Adolfo Nadal Serrano<sup>1</sup> \* and José María Espejo Bares<sup>2</sup>

1 Universidad Francisco de Vitoria, Archiologics, Madrid, Spain

2 Universidad Francisco de Vitoria, Madrid, Spain

\*Address all correspondence to: adolfo.nadal@ufv.es; adolfo.nadal@archiologics.com

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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## **Chapter 7**

## Can the DryLyte® Technology Polish 3D Printed Ceramic/Metal Samples and in Particular WC-Co?

*Guiomar Riu Perdrix and Joan Josep Roa Rovira*

## **Abstract**

DryLyte® Technology is an effective surface finish technique, which follows the same traditional electrolytic cell principle, but uses an electrolytic solid nonconductive medium rather than a liquid one. For the last 10 years, this technology has been attracting a lot of attention compared to conventional ones due to the selective smoothing of the surface technique, interacting only with the roughness peaks and not with the valleys, etc. In this book's chapter, for 3D-printed cemented carbides (WC-Co) polished with DryLyte® Technology, it is shown the correlation between the microstructure and the surface integrity, in terms of mechanical properties, at submicrometric length scale. Also, a particular case study is presented of 3D-printed WC-Co as a function of the testing temperature, ranging from room temperature up to service-like working conditions. Finally, the mechanical properties are correlated as function of the chemical nature and/or crystallographic phase.

**Keywords:** DryLyte® technology, roughness evolution, surface integrity, ceramic-metal materials, cemented carbides

## **1. Introduction**

The tribological and mechanical behavior of a tool under service-like working conditions depends not only on intrinsic properties of the constitutive phases but also on the material surface and subsurface properties—such as topography and residual stress state. During the last 10 years, extensive research has been dedicated to investigating the relation between chemistry, microstructure, and the resulting mechanical properties [1–3]. In addition to those bulk-related features, surface characteristics are also crucial in determining the functional response of a given material. Commonly, materials used for structural components are shaped into final dimensions and geometry changed as the material goes through different surface modification processes as mechanical working operations, material removal methods, heat treatment, and other finishing practices.

In this regard, surface alterations and/or modifications become critical in controlling the properties and performance of final products, particularly if service

conditions involve contact loading (e.g., wear, impact, fatigue, etc.) and/or environmental (e.g., corrosion, oxidation, etc.) interaction.

In the case of manufacturing stages involving material removal (e.g., grinding, lapping, etc.), a complex surface interaction exists between the tool and workpiece. In this sense, the temperature near the surface increases which could produce microstructural changes, including oxidation and also possible local melting at the surface level. Furthermore, plastic deformation, tearing, and fracture also occur. For all the aforementioned information, mechanical and thermal changes at the surface level take place along thermal stages which may induce relevant residual stresses [4].

The existence of a pronounced influence of manufacturing methods on mechanical properties and service performance, as a result of the type of surface produced, is well-establish. Within this framework, the concept of surface integrity in terms of roughness has been introduced as a key parameter. In this sense, the surface integrity, which contained not only the geometry consideration, including surface roughness and accuracy, but also other microstructural aspects at the surface and/or subsurface level.

Within this framework, this topic demands synergic interdisciplinary expertise in different fields: materials science, machining and shaping technology, as well as surface integrity in terms of mechanical properties [5, 6].

Ceramic/metallic materials and in particular the WC-Co cemented carbides (also known as hardmetals), are the key materials for tooling industry. These materials present an excellent combination of properties (e.g., hardness, strength, fracture toughness, wear, and abrasion resistance) [7–12]. Hardmetal tools are produced through a powder metallurgical (PM) route, where mixed WC and Co powders are sintered at high temperatures to consolidate the composite material [4, 13]. WC-Co grades are classified according to their Co content and WC grain size. The proportion of WC phase is generally between 70 and 97% of the total weight of the composite and its grain size averages between 0.4 and 10 μm. This range of cemented carbides can be subdivided into its major application area as described below [14, 15]:


However, during the last decade 10 years, the Additive Manufacturing (AM) routes are gaining more importance in this field. However, due to the complex shapes as well as closed cavities, conventional post-processing routes do not give out the desired quality in terms of surface finish.

## **2. Surface modification techniques on WC-Co grades**

In the production of conventional WC-Co components, it is occasionally necessary to carry out a number of shaping operations before final sintering. Green compacts can then be produced in simple shapes, such as rectangular and round blanks, by means of conventional methods, such as turning, drilling, and grinding.

After sintering, the corresponding blank has achieved its fully density and final mechanical properties, and it is ready to be dispatched. In this regard, most blanks need to be further post-processed in order to get the desired shape, size, flatness, and surface finish by using the traditional post-processing routes on WC-Co grades; being the most common techniques: diamond wheel grinding, diamond lapping, and grinding and electrical discharge machining (EDM) [16]. All these traditional postprocessing technologies will be briefly described in Section 2.1.

During the post-processing process, two aspects need to be taken into account: (a) the functionality of the machined workpiece and (b) the economic efficiency. According to different applications, the functionality of the workpiece after the postprocessing process can be divided into different groups as follows [17]:


Each of the aforementioned steps in a manufacturing chain influences the workpiece properties, which directly link to its functionality. Along this chapter of the book, we will focus attention to changes induced at the surface level changing the resulting roughness. In particular, the traditional post-processing techniques will be briefly explained (see Section 2.1) and in particular compared with a disruptive dryelectropolished technology (see Section 2.2) focusing the attention to these technologies on WC-Co AM specimens.

## **2.1 Traditional polishing techniques**

The main requirements which lead to use post-processing routes on WC-Co cemented carbides are:


• to reduce the polishing times.

The most widely employed post-processing techniques on WC-Co cemented carbides are:


Mainly, the most common post-processing technology to process the workpieces of WC-Co is the grinding (**Figure 1a**) and EDM process (**Figure 1b**), as shown in the SEM micrographs presented on **Figure 1**.

After that, it is necessary to reduce the surface roughness induced by these techniques by using chemo-mechanical process and/or chemical polishing process until the WC-Co cemented carbides workpieces present a precise dimension and shape that fulfill requirements in real applications [22–25]. However, the media used to reduce the surface roughness induced by the ground and chemo-mechanical polishing

*Can the DryLyte® Technology Polish 3D Printed Ceramic/Metal Samples… DOI: http://dx.doi.org/10.5772/intechopen.110299*

#### **Figure 1.**

*Scanning electron microscopy (SEM) micrographs of the microstructural quality for WC-Co post-processed by (a) grinding and (b) EDM technologies.*

**Figure 2.**

*Scanning electron microscopy (SEM) micrographs of the microstructural quality for the WC-Co chemomechanical polished. (a) Surface micrograph and (b) focused ion beam (FIB)—cross section showing the leaching induced (painted in red in the SEM micrograph) between both constitutive phases.*

process can induce localized leaching at the metallic Co binder as shown in **Figure 2a**. As reported in Refs. [26–28], when the pH media is below 7, the media induces a galvanic couple, where the ceramic reinforcement particles (WC) act as the cathode while the metallic Co binder as an anode. Thus, the potential difference between both parts generates a microgalvanic couples that induce a corrosion effect generating a difference in height between both constitutive phases of around 100 nm as shown in **Figure 2b**. This difference in height may produce a reduction of the workpiece under service-like working conditions.

From all the aforementioned information, the conventional post-processing techniques present some drawbacks for the WC-Co workpieces when these have been superficially treated by a ground and/or EDM process. Furthermore, these drawbacks are more evident when the geometry of the specimens' have complex shapes. In this sense, the AM specimens cannot be post-processed using conventional techniques as these technologies do not lead to the final workpiece:

(1) keep the geometry and preserve the tolerances and

(2) be able to polish complex shapes in a short polishing time.

For all the aforementioned information, recently the DryLyte® Technology increases their applicability in the market.

## **2.2 DryLyte® technology**

This technology was developed to overcome the limitations of classic polishing technologies (e.g., mechanical process, chemo-mechanical process, electropolishing, etc.). In this sense, DryLyte® (contraction of the words dry- and electrolyte) Technology is based on a dry-electrolyte and dry-suspension electrolyte made up of an ionic exchange resin.

In this sense, the structural characteristics, together with the functional group's nature and degree of the resin, has an important effect on its exchange-ion behavior. Morphologically, they can be classified as macroreticular or gel-type resins and macroreticular or macroporous resins:


The electrolyte presents a multi-modal particle size distribution (see **Figure 3a**) with an interconnected porous microstructure (see **Figure 3b**) and is chemically constituted of PolyStyrene DiVinylBenzene (PS-DVB). In this sense, the electrolyte particles are arranged in such a way that they make electrical contact with the negative pole of the power supply (cathode) and with the metal part to be polished (anode), generating a close circuit as schematically depicted in **Figure 3b**.

From all the aforementioned information, unlike traditional polishing systems, DryLyte® Technology achieves a uniform surface finish, avoiding surface marks patterns such as those generated by machining, and it is able to process complex geometries without generating micro-scratches on the surfaces, preserving the edges of complex geometries as the one produced by AM routes. Furthermore, this technology leads to polish close cavities by using internal cathodes. This is the main advantage to employ this technology to polish AM specimens in comparison with the traditional post-processing routes.

As it does not generate any polishing and/or surface modification texture (e.g., grinding patterns, etc.), this technology improves surface integrity in terms of mechanical properties (e.g., fatigue and wear) and chemical properties (e.g., corrosion resistance, aging, etc.). In this sense, along the entire chapter, this technology will be applied to ceramic/metal samples and in particular, to WC-Co cemented carbides produced by AM routes.

#### **Figure 3.**

*(a) SEM micrograph of the PS-DVB electrolyte showing a multi-modal particle size distribution; (b) SEM magnification micrograph showing the internal porosity; and (c) schematic representation of how the electricity passes through the PS-DVB particles during the dry-electropolishing process.*

## **3. AM of WC-Co hardmetals**

## **3.1 Theoretical background: bibliometric review**

AM technologies process has emerged as an alternative to traditional manufacturing process that can fabricate very complicated geometries with high efficiency and reduce post-processing [31–37].

AM technology is increasingly considered as a key manufacturing technology of tomorrow's society due to the fact, it makes possible the production of complex near-set shaped parts which are not feasible with conventional technologies (e.g., PM and/or subtractive). With this bottom-up approach, material is added layer-by-layer where necessary and the complex shape obtained. This technology offers several advantages, like:


From all these advantages, AM technology is being employed in many industries, like biomedical [38] and electronic [39] ones, as well as within scientific fields, such as mechanical devices [40, 41], periodic microstructures [42], and ceramics [43]. In this regard, the increasing interest in this technology is driven by its ability to produce custom devices, its simplicity and speed.

During the last 10 years, the AM of WC-Co has been the subject of several research efforts, and it is still in an early stage of development. AM technologies offer attractive advantages in terms of producing WC-Co hardmetal cutting tools with complex geometries, such as U-shaped or helical cooling channels inside. These internal, contour-adapted cooling channels allow higher cutting speeds and, consequently, a remarkable increase in the productivity of the machining process. The main AM technologies suitable for metal are selective laser melting (SLM), selective electron beam melting (SEBM), laser powder deposition, binder jet AM (BJAM), and wire arc AM (WAAM) [44–54]. So far, AM technologies have been successfully applied to stainless steels [55–59], Ni alloys [60, 61], Ti alloys [62, 63], refractory metals [64, 65], Al alloys [66, 67], etc.

For WC-Co, it remains very challenging to use AM due to its very high melting temperature. SLM and BJAM are the most attempted AM processes for manufacturing WC-Co hardmetals. Besides, a few studies on SEBM [46], 3D gel-printing (3DGP) [68], and fused filament fabrication (FFF) [69] of WC-Co hardmetal samples were deeply investigated.

The AM techniques used for fabrication of WC-Co hardmetals and the main research groups working on WC-Co AM were summarized in **Tables 1** and **2**.


*Can the DryLyte® Technology Polish 3D Printed Ceramic/Metal Samples… DOI: http://dx.doi.org/10.5772/intechopen.110299*


**Table 1.**

*Main research groups working on AM of WC-Co hardmetals [46, 68, 70–84].*



#### **Table 2.**

*State of the art of the AM techniques used for printing WC-Co hardmetals.*

## **3.2 Can the DryLyte® technology act as a post-processing technique for the AM specimens?**

However, laser-based AM technologies for manufacturing WC-Co hardmetals have encountered several issues as thermally induced cracks, heterogeneous microstructures, and embrittlement due to the formation of undesirable phases [71, 126]. On the other hand, sinter-based AM technologies offer the highest potential to process complex WC-Co parts with properties similar to commercial grades [127]. For these techniques, at this point in time limitations are found in regard to layer deposition quality, part size,

#### **Figure 4.**

*(a) Low SEM micrograph of the WC-Co sintered granules; (b) high-magnification SEM micrograph showing the internal part of the sintered WC-Co granules where it is evident the low amount of metallic Co content; (c) 3DP WC-Co drill bit head; (d) confocal laser scanning microscopy (CLSM) 3D reconstruction of the AM WC-Co drill bit investigated [40]; and (e) SEM micrograph after being polished by using the DryLyte® Technology [128].*

and batch size. Recently, solved on granules 3D-Printing (SG-3DP) has been successfully used to process fully dense WC-Co complex parts. In this chapter, the feasibility to polish SG-3DP by means of the DryLyte® Technology with lower Co content is explored. This technique consists in spreading powder-binder granules produced by a spry-drier technique (see **Figure 4a**) which were deposited layer-by-layer. **Figure 4b** shows the WC-Co granules microstructure where low Co content is clearly visible. As it is shown in **Figure 4c** by using the SG-3DP technology a drill bit head was printed with a thickness layer of around 74 μm as determined by using the Confocal Scanning Laser Microscopy, CSLM (see **Figure 4d**). As it is clearly evidenced in the SEM and CLSM micrographs, the sintered drill bit produced by the AM technology, presents a complex shape with high roughness values. To reduce the roughness of this complex specimen is not possible using conventional post-processing technology as the AM specimen will not preserve the desired geometry. In this sense, by using the DryLyte® Technology, it is possible to reduce considerably the superficial average roughness (*Sa*) until reaching a value of around 20 nm as shown in **Figure 4e**. This *Sa* reduction implies a roughness reduction of around 33% of the initial roughness.

After using the post-processing technologies, it is possible to superficially change the mechanical integrity in terms of hardness and elastic modulus. In this sense, preliminary results highlight that the surface mechanical integrity for the WC-Co specimens polished with DryLyte® Technology remains constant [128, 129] and equals as the once reported in the literature after being polished using conventional post-processing techniques [2]. Then, by using the DryLyte® technology leads to polish complex shapes processing from the AM routes keeping constant the mechanical surface integrity at the superficial level.

## **4. Conclusions**

The conventional post-processing techniques do not lead to homogeneously polish the AM specimens due to the fact these techniques do not keep constant the geometry, preserve tolerances, etc.

DryLyte® Technology presents several advantages compared with the conventional post-processing techniques as it allows to homogeneously polish complex specimens. Keeping constant the surface integrity in terms of mechanical properties under service-like working conditions.

## **Acknowledgements**

The authors are grateful to HILTI AG (Schaan, Liechtenstein) to provide us with the different samples investigated here. Furthermore, the authors are also grateful to Zeppelin 3D methodology company (especially to Javier Ledesma and María Gil) and the *Centro de Fabricacción Avanzada Aeronáutica* (especially to Guillermo González) to conduct the 3D measurements by using the InfiniteFocus G5 plus from Bruker Alicona. Finally, G.R. acknowledges the Ministerio de Ciencia e Innovación for the industrial PhD (Grant number: DIN2021-011846).

## **Conflict of interest**

The authors declare no conflict of interest.

*Can the DryLyte® Technology Polish 3D Printed Ceramic/Metal Samples… DOI: http://dx.doi.org/10.5772/intechopen.110299*

## **Appendices and nomenclature**


## **Author details**

Guiomar Riu Perdrix and Joan Josep Roa Rovira\* Steros GPA Innovative S.L., Barcelona, Spain

\*Address all correspondence to: jj.roa@gpainnova.com

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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