**5. Investment casting with positive molds using PBJ printed material**

#### **5.1. Theory, chemical, and physical basics**

#### *5.1.1. Conventional investment casting*

The investment casting process is used to produce metal parts that have finer details, thinner wall thicknesses, and superior surface quality compared with parts generated by the sand casting process. Since the foundation of the process is a positive model, the process differs in its substantial process steps from the sand casting process. See **Figure 17** for details. In this section, various methods are described to create cavities which can then be applied in the metal casting process (compare [18]).

**Figure 17.** Steps of investment casting: (a) positive model, (b) fixed gating system, (c) mounting of the (wax) cluster, (d) embedding in plaster or ceramics slurry, (e) setting of the shell, (f) burn-out of the model and the gating system with hardening of the shell, (g) casting, and (h) production result.

The basis of the investment casting process is a positive model, which are always lost models in contrast to the models used in sand casting. These models can be produced by artisans manually using the traditional way or by applying industrial mass-production technology.

The models are often produced by injection molding out of wax. As in the plastics processing, the wax is injected under high pressure into a metal mold and removed from the mold after cooling. In order to increase the dimensional stability and to change other properties, the wax is often filled with microgranules. The granules themselves are mostly plastics such as polystyrene or polyethylene.

The individual wax models are attached to further wax model in the production cycle. This wax model serves as a carrier and defines the gating in the subsequent casting of the liquid metal. The individual models are connected using thermal wax to the gating. The upper part of the gating widens conically and later represents the pouring funnel.

The shape thus obtained is further processed in the conventional method in two ways. The first method is to immerse the mold in liquid plaster. After the plaster has hardened, the mold is placed in an oven for heat treatment. Here, the wax evaporates or burns out and exposes the actual casting cavity. The second option is to generate a ceramic shell by repeated applying a coat of ceramic slurry. It is then fired in a kiln. During this process, the model evaporates or gets burnt out.

Both methods differ in the cooling rates which are attained later in the casting process. Especially, the simpler method using gypsum often produces an inferior metal structure and thus a casting of poor strength.

#### *5.1.2. A PBJ process for the generation positive burn-out models*

**5. Investment casting with positive molds using PBJ printed material**

The investment casting process is used to produce metal parts that have finer details, thinner wall thicknesses, and superior surface quality compared with parts generated by the sand casting process. Since the foundation of the process is a positive model, the process differs in its substantial process steps from the sand casting process. See **Figure 17** for details. In this section, various methods are described to create cavities which can then be applied in the metal

**Figure 17.** Steps of investment casting: (a) positive model, (b) fixed gating system, (c) mounting of the (wax) cluster, (d) embedding in plaster or ceramics slurry, (e) setting of the shell, (f) burn-out of the model and the gating system with

The basis of the investment casting process is a positive model, which are always lost models in contrast to the models used in sand casting. These models can be produced by artisans manually using the traditional way or by applying industrial mass-production technology. The models are often produced by injection molding out of wax. As in the plastics processing, the wax is injected under high pressure into a metal mold and removed from the mold after cooling. In order to increase the dimensional stability and to change other properties, the wax is often filled with microgranules. The granules themselves are mostly plastics such as

The individual wax models are attached to further wax model in the production cycle. This wax model serves as a carrier and defines the gating in the subsequent casting of the liquid

**5.1. Theory, chemical, and physical basics**

hardening of the shell, (g) casting, and (h) production result.

polystyrene or polyethylene.

*5.1.1. Conventional investment casting*

76 New Trends in 3D Printing

casting process (compare [18]).

In principle, several methods are suitable for the realization of the above-described investment casting process using additives manufacturing methods. These methods are wax or acrylate direct print processes, laser sintering of polystyrene, or stereolithography. Each method demonstrates specific characteristics which in turn make them the preferred method to apply depending on the production environment and related variables (see [25]). Different levels of components can be built by these technologies: from the single part model up to a complete cluster (see **Figure 18**).

An essential feature underlying all methods is the melt-out or burn-out property of the additive material that is being applied. Here, residue-free burn-out during the process (as far as possible) is important from an economic point of view as purification steps can thus be avoided. Individual melting is not sufficient, as the melt cannot run out from complex shapes and the material is able to leave the channels only in a gaseous state.

**Figure 18.** Production levels of components using AM processes for precision casting tools: (a) positive-model, (b) model with sprues, and (c) complete cluster with gating.

The PBJ method uses an acrylate-based printing process for investment casting. PMMA powder serves usually as base material.

The process is described in detail in the subchapter "sand casting." After lowering the build platform, a layer of PMMA powder is spread over the built platform with the help of the recoater. The achievable minimum layer thickness is well below that of the sand casting process and reaches 80 μm. Through a binder system technology, the powder particles stick to each other whereby binder is applied on to the powder bed by an inkjet print head by using preprogrammed bitmap data. For this purpose, solvent-based adhesives can be used as well as polymerized adhesive. For the latter process, one component of a multicomponent adhesive system is added to the powder.

The basic chemical reaction of the polymerizing system is a free-radical polymerization. An initiator, for example, dibenzoylperoxide, is present in the powder itself. The concentration can be adjusted as required during powder production. At higher temperatures, the initiator decomposes into two radicals depending on the type of initiator used.

The actual ink contains monomers. These monomers have radically polymerizable chemical double bonds, which are the key reactants for the chain reaction in curing of such materials (**Figure 19**). In addition, the binder includes an activator which brings about the decomposition of the initiator and at the same time generates initiating radicals. In this case, the activator reduces the temperature of the disintegration under the room temperature. When ink pene‐ trates into the powder, initiator from the powder is released and is activated by the activator. The radicals are bound to the monomers (chain initiation), whereby a new radical is formed, which reacts with another monomer (chain continued) in turn form further radicals. Thus, a chain reaction that leads to macromolecules is formed which makes up the strength of the bond (see micrographs of the bonds in **Figure 20**). The reaction stops (chain termination) if, for example, radicals react with each other (recombination) or the reaction mixture of reactive monomers increasingly reduces leading to a failure in reaction. The achievable strength depends on many factors, which in turn affect the chain reaction (see [26]).

**Figure 19.** Chemistry involved in the vx-acrylate process.

recoater. The achievable minimum layer thickness is well below that of the sand casting process and reaches 80 μm. Through a binder system technology, the powder particles stick to each other whereby binder is applied on to the powder bed by an inkjet print head by using preprogrammed bitmap data. For this purpose, solvent-based adhesives can be used as well as polymerized adhesive. For the latter process, one component of a multicomponent adhesive

The basic chemical reaction of the polymerizing system is a free-radical polymerization. An initiator, for example, dibenzoylperoxide, is present in the powder itself. The concentration can be adjusted as required during powder production. At higher temperatures, the initiator

The actual ink contains monomers. These monomers have radically polymerizable chemical double bonds, which are the key reactants for the chain reaction in curing of such materials (**Figure 19**). In addition, the binder includes an activator which brings about the decomposition of the initiator and at the same time generates initiating radicals. In this case, the activator reduces the temperature of the disintegration under the room temperature. When ink pene‐ trates into the powder, initiator from the powder is released and is activated by the activator. The radicals are bound to the monomers (chain initiation), whereby a new radical is formed, which reacts with another monomer (chain continued) in turn form further radicals. Thus, a chain reaction that leads to macromolecules is formed which makes up the strength of the bond (see micrographs of the bonds in **Figure 20**). The reaction stops (chain termination) if, for example, radicals react with each other (recombination) or the reaction mixture of reactive monomers increasingly reduces leading to a failure in reaction. The achievable strength

decomposes into two radicals depending on the type of initiator used.

depends on many factors, which in turn affect the chain reaction (see [26]).

**Figure 19.** Chemistry involved in the vx-acrylate process.

system is added to the powder.

78 New Trends in 3D Printing

**Figure 20.** SEM micrographs of particles bound by the PolyPor method (a) magnified 600 times and (b) magnified 1200 times.

#### **5.2. Methods for the characterization of PBJ generated parts for investment casting**

In investment casting processes, just as in the case of sand casting, special component prop‐ erties need to be achieved in order to support further processing steps.

Material strength values are important in order to allow safe handling of the component. But too high material strength values and a high modulus of elasticity are in fact damaging and thus harmful for follow-up processes. Here as well, a low thermal expansion coefficient is important. The mechanical characteristics are determined with a tensile testing machine of the type ZMART.PRO from the company Zwick. Here, the tensile testing program is carried out at room temperature at a speed of 1 mm/s. The measurement is carried out without displace‐ ment transducer. As a sample, a tensile testing rod is used with the cross-sectional dimensions 10 × 4 mm2 (see **Figure 21**). The load cell has a measuring range of 2 kN.

**Figure 21.** Test specimen dimensions for PBJ-parts used in investment casting.

For the embedding process, a closed surface is required. Without this closed surface, water can penetrate into the model. This could negatively affect the mold during burn-out in terms of cracks or disintegration of the shell. The surfaces of the PBJ-printed models are coated as porous surface adversely affect the follow-up processes of the above-mentioned processes. Thus, it is advisable to treat the model with liquid wax. The wax solidifies on the surface or penetrates by capillary action deep into the model, depending on the immersion conditions and temperatures of the wax or the component. A quick immersion of a cool component leads to a wax layer forming on the surface of the actual component. On the other hand, if a component is immersed over a longer period it gets almost completely filled with wax and has a significantly rougher surface.

The surface must be sealed for the follow-up process. If only a thin surface layer is obtained by the dipping method, there is a risk of microporosities causing surface damage during solidification and shrinkage of the wax (see **Figure 22**). This may lead to water seeping into the model. Thus, the model may be damaged during burn-out. The method of immersing components for a longer time poses the risk of component distortion due to the temperature reaction as well as the creation of rough surfaces.

**Figure 22.** SEM microscopy of a wax treated surface with pinhole defect.

This microporosity can be detected with the dye-penetration technique in accordance with DIN/ISO EN 571-1. Here, the dye accumulates in the microporosity and reveals the critical areas. Such critical areas must be reworked before the furnace process (**Figure 23**).

**Figure 23.** (a) Diagram showing after-treatment of porous components and (b) test: (i) waxed component, (ii) test color, (iii) the component after cleaning, (iv) the component after application of the developer.

After burning out, the residual ash is also an important factor for safe processing. In particular, undertaking cleaning steps after the casting cavity has been formed through burn-out is not advisable economically.

The residual ash is determined using the conventional ash measuring equipment. Since the quantities are very small, the procedure has to be performed with great care (see the steps in **Figure 24**). For the measurement at least 100 g bound material is required. These are added to a previously extensively heated bowl. After weighing, the plate is placed in an oven and heated up to 700°C. This temperature is maintained for at least 4 h. After cooling down for several hours, the dish is weighed again. The difference to the tare value will be read from the amount weighed. The quotient is thus residual ash. A residual ash content of less than 0.1% is consid‐ ered as free of residual ash.

**Figure 24.** Methodology for determination of residual ash.

In this method, the properties are determined with a test job, just as in the case of the sand casting process. As material system, the PolyPorB system from voxeljet is used. The test job is built on a VX1000 machine from voxeljet. The surface quality is determined based on test blocks. Waxed as well as nonwaxed samples are used.

#### **5.3. Experimental results**

and temperatures of the wax or the component. A quick immersion of a cool component leads to a wax layer forming on the surface of the actual component. On the other hand, if a component is immersed over a longer period it gets almost completely filled with wax and has

The surface must be sealed for the follow-up process. If only a thin surface layer is obtained by the dipping method, there is a risk of microporosities causing surface damage during solidification and shrinkage of the wax (see **Figure 22**). This may lead to water seeping into the model. Thus, the model may be damaged during burn-out. The method of immersing components for a longer time poses the risk of component distortion due to the temperature

This microporosity can be detected with the dye-penetration technique in accordance with DIN/ISO EN 571-1. Here, the dye accumulates in the microporosity and reveals the critical

**Figure 23.** (a) Diagram showing after-treatment of porous components and (b) test: (i) waxed component, (ii) test color,

(iii) the component after cleaning, (iv) the component after application of the developer.

areas. Such critical areas must be reworked before the furnace process (**Figure 23**).

a significantly rougher surface.

80 New Trends in 3D Printing

reaction as well as the creation of rough surfaces.

**Figure 22.** SEM microscopy of a wax treated surface with pinhole defect.

The density of the plastic components differs from the density of these parts because of the massively different density in relation to sand components. A mean of 0.63 g/cm3 is meas‐ ured .The scattering is relatively small with a range within a build direction of 0.02 g/cm3 . The anisotropy follows the above-mentioned measures, but is less.


**Table 10.** Mechanical parameters of components from the material PolyPorB for use in the investment casting process.

There is a close correlation between the modulus of elasticity and tensile strength for this material system. The tensile strength reaches a maximum of 3.3 MPa and a mean of 2.7 MPa. Here, the relative anisotropy of the strongest to weakest direction is 60%. The elongation at break is consistently less than 1.5%. Therefore, in the fracture behavior, it behaves similar to a glass-like material (refer to **Table 10**).


**Table 11.** Surface characteristics of PolyPorB parts for investment casting.

The length deviation of the material depends in part on the storage and aging of the compo‐ nents. In compliance with the prescribed standard processes, the measurement shows an average percentage deviation from 0.7%. The deviation is always negative, and thus produced shrinkage. The shrinkage is anisotropic and reaches its minimum in the *Z*-direction. This shows a correlation to the direction of the weakest point.

On average, q roughness Ra of 19.9 μm is achieved. The highest roughness is an Ra of 25.8 μm. If parts are waxed, the roughness varies according to the waxing procedure. If the component is dipped briefly, an Ra significantly below 10 μm is achieved. The components that are dipped longer the roughness corresponds to that of a nonwaxed part (**Table 11**).

The thermal expansion of nonwaxed components largely corresponds to the base material PMMA. The measurement shows a thermal expansion coefficient of 100 × 10−6 K−1. In the region of low temperatures, the wax on the surface of the waxed components will melt. Thus, the thermal expansion seen in the investment casting process can be compensated during the burnout of the printed component.

The test pieces which were immersed for a short time show a homogeneous image with no color inclusions along the surfaces during the penetration test. The edges of the component exhibit a slightly porous surface, which can be identified due to its pink color. Long immersed parts turn totally pink and penetrant testing confirms the (adverse) reading of the surface condition. In bodies having complex geometries, particularly the outer and inner edges are placed where errors occur. Cracks and tears in the wax surface can be clearly observed by their distinct red color (**Figure 25**).

**Figure 25.** Results of the color penetration test: (a) short immersion, (b) long immersion, and (c) problem areas.

For the material system used, the measurement of the residual ash of samples indicates an ash content of less than 0.3 per thousand based on the initial weight.

#### **5.4. Discussion**

**Orientation X Y Z**

**Orientation X Y Z**

longer the roughness corresponds to that of a nonwaxed part (**Table 11**).

**Table 11.** Surface characteristics of PolyPorB parts for investment casting.

a correlation to the direction of the weakest point.

out of the printed component.

Density (g/cm3

82 New Trends in 3D Printing

glass-like material (refer to **Table 10**).

**Min. Mean Max. Min. Mean Max. Min. Mean Max.**

**Min. Mean Max. Min. Mean Max. Min. Mean Max.**

) 0.62 0.63 0.64 0.63 0.64 0.65 0.62 0.63 0.64

Modulus of elasticity (GPa) 0.34 0.35 0.37 0.28 0.30 0.37 0.26 0.28 0.29 Ultimate tensile strength (MPa) 3.3 3.4 3.5 2.2 2.6 3.5 1.7 2.0 2.1 Elongation at break (%) 1.4 1.5 1.6 1 1.2 1.5 1.7 0.9 2.1

**Table 10.** Mechanical parameters of components from the material PolyPorB for use in the investment casting process.

There is a close correlation between the modulus of elasticity and tensile strength for this material system. The tensile strength reaches a maximum of 3.3 MPa and a mean of 2.7 MPa. Here, the relative anisotropy of the strongest to weakest direction is 60%. The elongation at break is consistently less than 1.5%. Therefore, in the fracture behavior, it behaves similar to a

Surface quality Ra (μm) 15.9 18.8 22.4 18.1 21.4 25.8 17.2 19.5 22.2

The length deviation of the material depends in part on the storage and aging of the compo‐ nents. In compliance with the prescribed standard processes, the measurement shows an average percentage deviation from 0.7%. The deviation is always negative, and thus produced shrinkage. The shrinkage is anisotropic and reaches its minimum in the *Z*-direction. This shows

On average, q roughness Ra of 19.9 μm is achieved. The highest roughness is an Ra of 25.8 μm. If parts are waxed, the roughness varies according to the waxing procedure. If the component is dipped briefly, an Ra significantly below 10 μm is achieved. The components that are dipped

The thermal expansion of nonwaxed components largely corresponds to the base material PMMA. The measurement shows a thermal expansion coefficient of 100 × 10−6 K−1. In the region of low temperatures, the wax on the surface of the waxed components will melt. Thus, the thermal expansion seen in the investment casting process can be compensated during the burn-

The test pieces which were immersed for a short time show a homogeneous image with no color inclusions along the surfaces during the penetration test. The edges of the component exhibit a slightly porous surface, which can be identified due to its pink color. Long immersed parts turn totally pink and penetrant testing confirms the (adverse) reading of the surface condition. In bodies having complex geometries, particularly the outer and inner edges are In the case of investment casting processes, as opposed to the conventional sand casting methods, the finished models are not directly comparable with PBJ-printed models. In the first process, the part to be compared is built up by injecting a liquid, and in the second process the part is made using particulate material. The strength of PBJ-printed components is sufficient for safe handling during subsequent process steps. The high brittleness needs to be considered during handling. However, the lower density, which is present due to the porosity of the component, makes the parts easy to handle.

The geometry is strongly influenced by shrinkage. However, this shrinkage can be easily simulated and nearly completely compensated by an increase in the geometry of the raw data. The surface roughness of the models in the untreated state does not meet the requirements of a "precise casting procedure". Through a postprocess procedure which is suitably adapted to the task (in this case wax treatment), the desired measurements can be achieved here as well.

The surface of the model has to be sealed, as during the post-treatment process it is immersed in a liquid. The surfaced can be sealed by immersing them in wax. The color infiltration method shows that special care is needed and further steps to ensure that the part is completely coated (sealed) may be necessary.

Thermal expansion of the basic structure of the particulate material can be compensated by applying a coating of wax. The type of wax selected and its melting point play an important role.

During burn-out which exposes the casting cavity, very little residual ash is left behind, this can be attributed to the combination of the raw materials, namely, PMMA and wax used in the process. By applying a suitable process, cleaning of the form can be avoided after burn-out and metal can be poured directly into the hot mold.
