**3. Production of casts by powder-binder-jetting**

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

3

3 6 . *<sup>V</sup>*

(7)

*p*

*D*

ç ÷ è ø (5)

(6)

(8)

(9)

*p*

*D*

with the number of particles per voxel *N*p, the dimension *l*p, a cubic voxel, and the grain

For the volume of a single bond *V*B and the total binder volume per voxel *V*v, the following

As with the approximation of the print volume, in this case only one-half, it is again assumed

For calculating the bond area, relevant for the maximum mechanical stress at break, the

2 *V*

æ ö = ××ç ÷ ç ÷ è ø

2 2 3

*V l <sup>D</sup> l V AV D h D h l D lh* æ ö æ ö æ ö <sup>×</sup> = × = ×× × = × ç ÷ ç ÷ ç ÷ ç ÷ ç ÷ <sup>×</sup> × × × × è ø è ø è ø

s

<sup>2</sup> 2 6 <sup>4</sup> . *B V <sup>p</sup> V V*

*p Vp V*

The relationship between *σ*Max and the particle diameter *D*p is therefore, at *V*V, *l*V, and *h* = const,

The particle diameter is directly proportional to the breaking stress value in the same way as the amount of liquid. This implies that the use of finer particles leads to a reduction of strength. Similarly, when particle size is reduced (improving the surface properties), the amount of

*<sup>l</sup> A a*

and by applying the above relationship for *V*B , we obtain

3 3

2

*V P*

*Max p D* .~ (10)

*p*

*D* p

ç ÷ è ø

6 6 *<sup>V</sup> P*

æ ö =× =×ç ÷

*<sup>l</sup> N N*

diameter *D*p of the considered perfect monomodal particle size distribution.

*VB B*

*<sup>l</sup> V VNV*

æ ö = × = ××ç ÷

<sup>2</sup> <sup>2</sup> . <sup>3</sup> *V ah <sup>B</sup>* = × ×× p

equation applies:

60 New Trends in 3D Printing

to form a binding as shown below:

following equation applies:

almost linear:

The introduction describes the conventional process—to produce metal parts by casting. Sand casting or the combined use of molds and sand cores is explained as follows.

Casting provides important components for industrial production because they provide high functional integration densities with cost-effective implementation. The production of castings is carried out in two steps: First, to create a multipart mold having a cavity which corresponds substantially to the later casting. Second, the casting where the mold is filled with molten metal and after its solidification forms the actual part.

Depending on the complexity, quantity, or cost requirements, the mold can be designed as a disposable or reusable form. Simple structures can be produced using reusable forms. Complex shaped channels in the casting are only possible using disposable molds to realize such complex cores. Often the techniques are hybridized with each other to take advantage of the required assembly (see [13]).

Thus, the cavity of a casting for metallic molds, and molds and cores, may be surrounded by sand. These different parts are mounted before they are cast, some with a large degree of automation (for process steps, see **Figure 4**).

Disposable molds and cores are made from a base molding material that is a particle material using a binder. The binder is often a fluid, which is mixed with the particle material. This mixture is then added to the model and solidifies. The model is shaped in such a way that the model or the core can be removed from the model without being destroyed. Thus, high complexities cannot be achieved using this method. More complex mold sets have to be assembled from individual parts by mounting (see [14]).

**Figure 4.** Diagram of the conventional sand casting process: (a) model, (b) preparation of the molding material mix‐ ture, (c) molding of the model and solidification of the molding material mixture, (d) part of mold (drag), (e) mounting of molds and cores, (f) pouring liquid metal, (g) solidification and weakening of molding material, (h) extraction, and (i) casting with gating.

For the production of series, the molding material is often mixed with the binder and activated in an automated process (**Figure 4**). This reactive mixture is then injected via a pneumatic conveyer into a model form and gets coated alongside the walls. The component thus produced is cured in the model form. After curing, the part is often automatically removed by a robot and is deburred by an automated process as well. Thereafter, the mold for the actual casting process is again mounted with the help of the robot. The casting process too can thus be carried out fully automated.

The method described requires several basic materials properties for the individual process steps. The particle material must be free flowing to fill the model form. The binder must have very low viscosity for metering and for the resulting molding material flow characteristics (**Figure 5** ).

such complex cores. Often the techniques are hybridized with each other to take advantage of

Thus, the cavity of a casting for metallic molds, and molds and cores, may be surrounded by sand. These different parts are mounted before they are cast, some with a large degree of

Disposable molds and cores are made from a base molding material that is a particle material using a binder. The binder is often a fluid, which is mixed with the particle material. This mixture is then added to the model and solidifies. The model is shaped in such a way that the model or the core can be removed from the model without being destroyed. Thus, high complexities cannot be achieved using this method. More complex mold sets have to be

**Figure 4.** Diagram of the conventional sand casting process: (a) model, (b) preparation of the molding material mix‐ ture, (c) molding of the model and solidification of the molding material mixture, (d) part of mold (drag), (e) mounting of molds and cores, (f) pouring liquid metal, (g) solidification and weakening of molding material, (h) extraction, and

For the production of series, the molding material is often mixed with the binder and activated in an automated process (**Figure 4**). This reactive mixture is then injected via a pneumatic conveyer into a model form and gets coated alongside the walls. The component thus produced is cured in the model form. After curing, the part is often automatically removed by a robot and is deburred by an automated process as well. Thereafter, the mold for the actual casting process is again mounted with the help of the robot. The casting process too can thus be carried

The method described requires several basic materials properties for the individual process steps. The particle material must be free flowing to fill the model form. The binder must have

the required assembly (see [13]).

62 New Trends in 3D Printing

(i) casting with gating.

out fully automated.

automation (for process steps, see **Figure 4**).

assembled from individual parts by mounting (see [14]).

Both properties are also a basis for processing of materials in machines within the PBJ process. Therefore, the materials can be used in 3D printers without major modifications in their physical or chemical properties.

Both methods can also be combined. Forms created with conventional tools or molds are able to support PBJ-printed cores (compare [15] or [16]).

The heat occurring during casting is used to weaken the molds and cores to such an extent that after solidification of the casting the molding material can easily be removed from the casting. This is called de-coring and is significantly defined by the binder.

Among the additive manufacturing methods, the laser-sintering process is the only other AM method by which it is possible to create cores and molds for the foundry industry. In this method, resin-coated sand is melted with a laser and thus selectively bound. The technique is similar to the Croning® method (see [17]).

**Figure 5.** Brief overview of highly automated core production: (a) half section of a core model, (b) preparation of the molding material mixture, (c) core shooting process, (d) inserting the core in the mold, (e) casting process, (f) removal of the casting with the core (gating is not shown), (g) clamping the casting, (h) shake-out of the core, and (i) casting.

Metals that can be used in the sand casting process, in certain cases, exhibit very different properties (see **Table 3**). As far as weight goes, cast iron is the most widely used material for casting. It is characterized by a low melting point of about 1250°C and excellent mold filling properties. Less common casting is the use of steel whose melting point is about 1700°C. Here, form filling is much more difficult. Due to the ready availability of a variety of cast iron types having high ductility, the ductility of the material is only a weak criterion for choosing the appropriate casting material. Therefore, cast steel has become less important. Aluminum is superior due to the lightweight requirements of modern products and has thus become increasingly popular. Aluminum–silicon alloys are excellent for mold filling and melt at around 700°C. In sand casting processes, there are plenty of other metals that can be used. These include the nonferrous metals such as copper, brass, and bronze. Similarly, other light metals or alloys can also be used, for example magnesium (see [18]).


**Table 3.** The most important metals in the foundry industry.

#### *3.1.1. Sand and particle material*

Particle material is the basis of the powder-binder-jetting process. Therefore, the properties of the particles and the particle clusters are of particular importance and are described below (see **Figure 6**).

**Figure 6.** Methods for the characterization of particulate matter: (a) angle of repose and density determination, (b) mi‐ croscopy, (c) pH value determination, and (d) particle size analysis.

Different groups of sand types are available for sand casting processes. The natural sands are characterized by low cost. The main group forms the quartz sand whose purity permits predictable behavior in the casting. The contents of SiO2 often exceed 95% in weight. The other components are usually also oxidic in nature such as Al2O3 or MgO. Since the sand prior to use is strongly heated in order to dry, organic admixtures are usually excluded.

increasingly popular. Aluminum–silicon alloys are excellent for mold filling and melt at around 700°C. In sand casting processes, there are plenty of other metals that can be used. These include the nonferrous metals such as copper, brass, and bronze. Similarly, other light

Particle material is the basis of the powder-binder-jetting process. Therefore, the properties of the particles and the particle clusters are of particular importance and are described below (see

**Figure 6.** Methods for the characterization of particulate matter: (a) angle of repose and density determination, (b) mi‐

Different groups of sand types are available for sand casting processes. The natural sands are characterized by low cost. The main group forms the quartz sand whose purity permits predictable behavior in the casting. The contents of SiO2 often exceed 95% in weight. The other

Strength 0 + ++ Elongation at break 0 0 ++ Damping 0 + − Corrosion behavior + − − Machining + + 0 Density ~3 ~6.5 ~7 Market importance + 0 −

**Aluminum Cast iron Steel**

metals or alloys can also be used, for example magnesium (see [18]).

**Table 3.** The most important metals in the foundry industry.

croscopy, (c) pH value determination, and (d) particle size analysis.

*3.1.1. Sand and particle material*

**Figure 6**).

64 New Trends in 3D Printing

For special applications, there are also natural sands which are a mixture of Al2O3 and SiO2. Here, the costs are not as low as with pure quartz sands. Other minerals, for example chrome ore or zircon sands, are common when a high density of the molds or cores is required (see [19]).

Artificial sands, although substantially more expensive, are also common. Here, mostly Al2O3 or SiO2 are used as the base. These sands are chemically highly pure and the particle spectrum is precisely adapted to the requirement (see [20]).

In the case of natural sands, the grain structure depends on the origin of the sands. Cracked products are made up of sharp-edged grains that are generally more square shaped than spherical in shape. Sand qualities can be obtained from natural deposits such as rivers, whose particle shape is nearly spherical. These sands also often exhibit particularly favorable microtopology for binding reactions (see [20] and **Figure 7**).

Depending on the production process, the form of artificial sand is usually spherical. This property allows these products to flow very well and there are smooth surfaces.

**Figure 7.** Various mold raw materials: (a) SEM picture of natural sand and (b) optical microscopy artificial molding material.

The sand particles react with various chemicals. Many oxides react alkaline in an aqueous environment. Therefore, the pH value of the applied media changes, which is especially significant for PBJ processes when the sand is premixed with a liquid.

A measurement of the chemical properties can be carried out by measuring the pH of the wash solution for the sand. For example, for an amount of water of 100 g, 10 g sand is given and after a waiting period of 10 min the pH value is determined with a pH meter with a glass electrode or is determined titrimetrically against an alkaline solution.

Natural sands have a significantly broad particle size distribution. Here, often a noticeable amount of fine particle distribution of up to <10 μm particle size can be noticed. This affects various properties, among other things, the flow property, but also the bond because the micrograins attract the binder in a capillary manner and thus withdraws from the binding region of the mold-forming grains (find different qualities in **Table 4**).

Undesirably large grains are often separated by sieving. This leads to a sharp divide in the grain size range. The bulk density of the sieved as well as unsifted materials falls way behind theoretical possibilities. With natural sand, about 50% volume space filling can be achieved. Targeted blends can positively affect the bulk density (see [21]).

Artificial sands are nearly monodisperse, i.e., they have essentially only one grain size. This property and the round grain often lead to an increased bulk density. Again, this effect can be enhanced by adding further monodisperse powders having different grain size.

Natural sands are often preferred because of their low cost. However, especially with quartz sands, a change in the crystal structure of the elementary cell (quartz inversion) takes place at a temperature of 573°C. These stretches abruptly produce a sudden increase in volume of the core or mold. This often leads to flaking of layers close to the mold cavity, thus causing casting defects (see [14]).

For artificial sands, this effect is usually only minimal. Here, additionally a high density is often implemented in order to reduce the buoyancy of cores in the melt.

In the PBJ process, much finer sands are used compared with sands used in the standard foundry. This is necessary in order to achieve surface qualities that are comparable to the conventional process because the grains are not aligned toward the surface of the component as is the case while using a model.


\* the shown figures are simplified and do not take account of the small amount of additional oxides.

**Table 4.** Examples of data for sand systems which are suitable for the PBJ process.

#### *3.1.2. Binder systems*

The bonding strength between particles is caused by the binder. In the case of PBJ, the binder is selectively applied with the help of an inkjet print head.

micrograins attract the binder in a capillary manner and thus withdraws from the binding

Undesirably large grains are often separated by sieving. This leads to a sharp divide in the grain size range. The bulk density of the sieved as well as unsifted materials falls way behind theoretical possibilities. With natural sand, about 50% volume space filling can be achieved.

Artificial sands are nearly monodisperse, i.e., they have essentially only one grain size. This property and the round grain often lead to an increased bulk density. Again, this effect can be

Natural sands are often preferred because of their low cost. However, especially with quartz sands, a change in the crystal structure of the elementary cell (quartz inversion) takes place at a temperature of 573°C. These stretches abruptly produce a sudden increase in volume of the core or mold. This often leads to flaking of layers close to the mold cavity, thus causing casting

For artificial sands, this effect is usually only minimal. Here, additionally a high density is

In the PBJ process, much finer sands are used compared with sands used in the standard foundry. This is necessary in order to achieve surface qualities that are comparable to the conventional process because the grains are not aligned toward the surface of the component

/g) 176 172 105

The bonding strength between particles is caused by the binder. In the case of PBJ, the binder

Chemical base SiO2 Al2O3/SiO2 Al2O3/SiO2 Purity/contents (%) >98 62/38 60/40 Medium grain size (μm) 140 150 200

Sinter temperature (°C) >1550 >1660 >1660 Residual water content (%) 0.2 0.7 0.7 pH of washing solution 6.3 6.7 6.4

the shown figures are simplified and do not take account of the small amount of additional oxides.

**Table 4.** Examples of data for sand systems which are suitable for the PBJ process.

is selectively applied with the help of an inkjet print head.

**Quartz sand GS14RP Cera beads AFS100 Kerphalite HA**

enhanced by adding further monodisperse powders having different grain size.

often implemented in order to reduce the buoyancy of cores in the melt.

region of the mold-forming grains (find different qualities in **Table 4**).

Targeted blends can positively affect the bulk density (see [21]).

defects (see [14]).

66 New Trends in 3D Printing

Specific surface (cm2

*3.1.2. Binder systems*

\*

as is the case while using a model.

**Figure 8.** Basic binding mechanisms: (a) solidification by evaporation of solvent and (b) curing by polymerization.

Basically, two binding mechanisms can be used for the above-mentioned PBJ process (as depicted in **Figure 8**).

The material may be dissolved in liquid binder. The adhesive effect of the material is obtained by drying. Often plastics or inorganic binders are dissolved in water or suitable solvents. The drying must take place as long as the layer is exposed and the parts lie covered in powder during the construction process. A variation on this procedure is to dissolve the particle material with the help of a solvent which is part of the printing liquid and then to let it dry again. Thus, a binder-free medium can be printed. This is favorable for the life of the print head. But the problem is often the long setting time.

Polymerizing binders can also be used. Polymerization is a chemical reaction where mostly low molecular weight substances react in longer chains of molecules. Thus, a transition between low viscosities, i.e., good printability and extremely high viscosities (solid), is made possible.

Usually, it is not only necessary but also useful to use both methods as a hybrid. This allows moderate reaction rates that safeguard all the machine components and support a drying process deep inside the large powder cake.

The properties of the binder which are suitable for the PBJ methods are very similar to those of the conventional molding production. In the following, the most common systems are described according to their chemical and physical characteristics (**Table 5**).

Furan resin is a polymer binder which is derived from corn cobs. The resin system is available in the form of a resin and an activator component. The resin itself is made up of monomers and oligomers, for example, furfuryl alcohol, bisphenol A, and resorcinol. In addition, adhesion promoters are included. The activator is an acid, usually a mixture of several acids. It has a catalytic effect on the reaction, but is itself also consumed in the process. The polymer with is produced through polycondensation is a cross-linked, high-strength thermoset which is highly temperature resistant due to furan groups in the molecule (see [14]).

In contrast to furan resins, phenol resins consist of phenol units, which are preferably linked to each other in *para* and/or *ortho*-positions via methylene groups. No continuous conjugated system is formed. The curing process is carried out using formaldehyde depending on the prepolymer. For example this can be made available for the process by the thermal decompo‐ sition of the substance Urotropin. A thermoset plastic is again formed. The temperature resistance is even higher than that of furan resin (see [14]).

Inorganic binders are gaining importance not only because of their historical importance, but also because of new requirements for environmental-friendly production. They are character‐ ized by very low and harmless cast emissions. For this process, liquid water glass is of particular importance. It can achieve high strengths. However, the de-coring process after casting requires more effort with this binder system than with the organically bound varieties (see [14]).


**Table 5.** Relevant binder systems and properties, \*typical figures, depending on the amount of binder.

#### **3.2. Methods for the characterization of PBJ generated parts for foundry purposes**

Using PBJ printed components as tools for casting processes requires a special range of characteristics for the parts. The porosity of the parts is essential for the process. Similarly, the strength values are important to ensure safe handling, as well as safe and easy de-coring after casting. In this process, some values are linked and, under certain circumstances, represent contradictory optimization criteria (**Figure 9**).

**Figure 9.** Schema of interdependent properties.

The resulting amount of gas emitted from the burning binder during casting must be diverted through the porous structure of the molds and cores. The volume of the gas impact will depend on the proportion of the binder. This in turn significantly affects the strength. The strength is dependent upon the particle size, the related resolution, and the achievable surface quality of the process in relation to the geometry. Similarly, high strength often means the tendency for shrinkage and thus distortion. The geometry works once again due to the effect of the grain size on gas permeability, as the finer particles cause the gas channels to become narrow.

prepolymer. For example this can be made available for the process by the thermal decompo‐ sition of the substance Urotropin. A thermoset plastic is again formed. The temperature

Inorganic binders are gaining importance not only because of their historical importance, but also because of new requirements for environmental-friendly production. They are character‐ ized by very low and harmless cast emissions. For this process, liquid water glass is of particular importance. It can achieve high strengths. However, the de-coring process after casting requires more effort with this binder system than with the organically bound varieties

) 250\* 350\* 300\*

Temp. stability Sufficient Very stable Extremely stable

Shelf life (months) 12 6 Unlimited

**Table 5.** Relevant binder systems and properties, \*typical figures, depending on the amount of binder.

**3.2. Methods for the characterization of PBJ generated parts for foundry purposes**

Using PBJ printed components as tools for casting processes requires a special range of characteristics for the parts. The porosity of the parts is essential for the process. Similarly, the strength values are important to ensure safe handling, as well as safe and easy de-coring after casting. In this process, some values are linked and, under certain circumstances, represent

De-coring ++ + −

Emissions High High No

contradictory optimization criteria (**Figure 9**).

**Figure 9.** Schema of interdependent properties.

**Furanic resin Phenolic resin Inorganics**

resistance is even higher than that of furan resin (see [14]).

(see [14]).

68 New Trends in 3D Printing

Bending strength (N/cm2

**Figure 10.** (a) Diagram showing the arrangement to determine gas permeability and (b) process cycle during the deter‐ mination of ignition loss: (i) taring process, (ii) (initial) weighing material, (iii) glowing, and (iv) weighing material.

The range of printed applications as well as conventionally manufactured cores and molds is affected by gas permeability. To determine the behavior of the gas, the flow rate can be measured with the help of a test specimen at a given pressure. In this case, a tester of the type SPDU of + GF + is used. A cylinder having dimensions 50 mm × 50 mm is used as test specimen (for schematic, see **Figure 10a**).

The second important parameter characterizing the moldings with respect to the gas problem is the loss on ignition. This is determined by taking a sample from a bound form or a core. A furnace of type KLS 05/11 from Thermconcept and scales of the type of PCB2500-2 core were used in the above-described experiment.

According to the standard method for determining the loss on ignition, about 30 g pulverized molding, usually from test bars, is weighed and put into a ceramic bowl. Before the bowl is filled, it is constantly heated to constant weight in an oven to extract moisture and organic residues. Before and after filling, the bowl is weighed. The binder or moisture in the compo‐ nents is burned out or expelled at about 900°C for approximately 30 minutes. After cooling, the weight is determined again. The difference between the weight of sand sample before and after heating is the absolute loss on ignition which is usually expressed in percent by weight on the initial weight (see **Figure 10b**).

**Figure 11.** (a) Detecting the main dimensions with a vernier height gauge and (b) basic determination of triangulation.

The accuracy of the construction method is determined based on test specimens. These determine the scope of the machine. Rods measuring up to 500 mm can be measured in this manner. The simplest and most common method is the direct measuring with a caliper or vernier height gauge. More complex shapes can be determined using optical methods. Some of them are based on the principle of triangulation. Here, a device of the type MICRO-OPTRONIC optoNCDT is used (ref. overview [22]) (see **Figure 11**).

The surface parameters of the molds and cores are determined by a tactile measuring method. Here, the extreme microstructure and the weak binding of the grains are taken into account so that an imprint of the surface is made before measurements are carried out (**Figure 12a**).

For the purpose of forming an imprint, plasticine may be used as a quick means yielding good accuracy. Plasticine is soft and produces a clean and accurate imprint. The impression thus obtained is cured at 100°C in the oven. The imprint is then scanned with a surface measuring device, i.e., type Mahr MarSurf SD 26. The evaluation of the values Ra and Rz is calculated automatically (**Figure 12b**).

**Figure 12.** (a) Creating an imprint of the surface and (b) determination of the surface finish using a tactile device.

The strength of 3D printed parts is determined using destructive testing just as in conventional production methods. Conventional test equipment is used. A universal type ZMART.PRO of the company Zwick is used for the determination of the *E* modulus, the elongation at break, and the tensile strength. For a three-point bend test which is commonly used in the foundry industry, a test device of the type +GF+ PFO is used.

A test job is specifically built having the appropriate material system for recording the measured values. This includes all relevant tests specimens in different variations for the described tests. The material system furan is processed on a Prometal S15 system. The phenolic resin and inorganic binding material are processed on a voxeljet VX1000. **Figure 13** represents a test job on a VX1000.

**Figure 13.** Arrangement of different test samples within a test job.

#### **3.3. Results for PBJ printed parts**

**Figure 11.** (a) Detecting the main dimensions with a vernier height gauge and (b) basic determination of triangulation.

The accuracy of the construction method is determined based on test specimens. These determine the scope of the machine. Rods measuring up to 500 mm can be measured in this manner. The simplest and most common method is the direct measuring with a caliper or vernier height gauge. More complex shapes can be determined using optical methods. Some of them are based on the principle of triangulation. Here, a device of the type MICRO-

The surface parameters of the molds and cores are determined by a tactile measuring method. Here, the extreme microstructure and the weak binding of the grains are taken into account so that an imprint of the surface is made before measurements are carried out (**Figure 12a**).

For the purpose of forming an imprint, plasticine may be used as a quick means yielding good accuracy. Plasticine is soft and produces a clean and accurate imprint. The impression thus obtained is cured at 100°C in the oven. The imprint is then scanned with a surface measuring device, i.e., type Mahr MarSurf SD 26. The evaluation of the values Ra and Rz is calculated

**Figure 12.** (a) Creating an imprint of the surface and (b) determination of the surface finish using a tactile device.

OPTRONIC optoNCDT is used (ref. overview [22]) (see **Figure 11**).

automatically (**Figure 12b**).

70 New Trends in 3D Printing

Within PBJ, the furan resin system is the most common and long-term proven system. The measured values refer to a system with the sand type GS 14 RP Strobel quartz sand. As sand binder, a binder from ASK type ASKURAN with the corresponding hardener is used.

The test samples show a strength with an average of 270 N/cm2 at a density of 1.35 g/cm2 . The scattering of 20% within an orientation can be measured. The maximum anisotropy of the strength values is shown in a deviation of nearly 40% in strength for bending-test samples built in the *Z*-direction (see **Table 6**).


**Table 6.** Mechanical properties of PBJ-printed parts.

The measurements of the length of the bars exhibit low scatterings in the range of about 0.2%. The surface quality of differently arranged surfaces widely varies from each other. In this case, the surface roughness Ra of the top surface with 19.8 μm is the lowest. Subsequently, parallel to the *XY* plane, surfaces having a Ra of 22.4 μm follow. The area with the roughest surface is oriented in the *Z*-direction (Ra = 25.2) (**Table 7**).


**Table 7.** Geometric characteristics of PBJ-printed parts.

In the settings used for the test job, a loss on ignition of 1.8% can be measured. The gas permeability of the material system is 68 l/h. **Table 8** provides the gas permeability of more sand systems. It is assumed that each system has a for the strength reasonable compaction, and thus also a reasonable density.


**Table 8.** Gas permeability of PBJ-printed parts using different grain sizes.

A comparative measurement of different chemical binder systems is performed on the basis of a variety of systems, and in each case an optimized configuration for the intended use is applicable. **Table 9** provides a brief comparison of the results.


**Table 9.** Properties of PBJ-printed parts using different binder systems.

#### **3.4. Discussion**

strength values is shown in a deviation of nearly 40% in strength for bending-test samples built

Modulus of elasticity (GPa) 1.4 1.3 1.1 1.1 1.3 1.5 1.0 1.1 1.2 Ultimate tensile strength (MPa) 0.6 1.1 1.3 0.8 0.95 1.2 0.2 0.4 0.6 Elongation at break (%) 0.05 0.06 0.08 0.04 0.07 0.09 0.02 0.06 0.16

The measurements of the length of the bars exhibit low scatterings in the range of about 0.2%. The surface quality of differently arranged surfaces widely varies from each other. In this case, the surface roughness Ra of the top surface with 19.8 μm is the lowest. Subsequently, parallel to the *XY* plane, surfaces having a Ra of 22.4 μm follow. The area with the roughest surface is

Surface quality Ra (μm) 19.8 22.0 25.7 21.4 24.3 25.6 22.4 25.2 29.1

In the settings used for the test job, a loss on ignition of 1.8% can be measured. The gas permeability of the material system is 68 l/h. **Table 8** provides the gas permeability of more sand systems. It is assumed that each system has a for the strength reasonable compaction,

A comparative measurement of different chemical binder systems is performed on the basis of a variety of systems, and in each case an optimized configuration for the intended use is

Grain size (μm) 140 190 250 Gas permeability (l/h) 65/75 140 250

**Table 8.** Gas permeability of PBJ-printed parts using different grain sizes.

applicable. **Table 9** provides a brief comparison of the results.

) 1.33 1.37 1.41 1.33 1.36 1.41 1.32 1.33 1.36

) 222 276 326 236 278 331 219 245 268

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

**GS14 GS19 GS25**

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

**Orientation X Y Z**

**Orientation X Y Z**

in the *Z*-direction (see **Table 6**).

Ultimate bending strength (N/cm2

**Table 6.** Mechanical properties of PBJ-printed parts.

oriented in the *Z*-direction (Ra = 25.2) (**Table 7**).

**Table 7.** Geometric characteristics of PBJ-printed parts.

and thus also a reasonable density.

Density (g/cm3

72 New Trends in 3D Printing

The special characteristics of the printed components relative to the conventionally manufac‐ tured components are the smaller grain size, the alignment of the grains, and built-layer characteristics unique to each method. Thus, the 3D printing itself exhibits different values. At a comparable grain size, the surface finish of 3D printed parts is substantially lower compared with that of the conventional parts. Similarly, the loss on ignition is higher at the same strength, as the conventional methods have a greater densing effect on the material.

The methodology for measuring the properties can be well transferred to the PBJ process. But there are special considerations to be observed. Most dimensions are anisotropic. Additionally, the artifacts "stair steps" have to be considered for an objective surface quality comparison and each surface orientation has a different surface finish.

The use of 3D printed cores is by now widely spread. Even complex cores can be produced by this method with a suitable choice of the binder system, although some characteristics of the conventional production may not be achievable. To produce challenging cores, it is possible to find good compromises for test series. The material systems described above provide such processes, material and binder combinations.
