Study of 3D printing process: Optimization, quality analysis, and comparison of 3D printed and cast ceramic properties

Contents lists available at ScienceDirect Open Ceramics journal homepage: www.sciencedirect.com/journal/open-ceramics Study of 3D printing process: Optimization, quality analysis, and comparison of 3D printed and cast ceramic properties Adam Boleslavskýa,∗, Hana Ovčačíková b, Milan Mihola a, Aki Mikkola c, Michaela Topinková b, Zdenko Bobovskýa aVSB – Technical University of Ostrava, Faculty of Mechanical, Engineering, Department of Robotics, 17. listopadu 2172/15, 708 00 Ostrava - Poruba, Czech Republic bTechnical University of Ostrava, Faculty of Materials Science and Technology, Department of Thermal Engineering, 17. listopadu 2172/15, 708 00 Ostrava - Poruba, Czech Republic cMechanical Engineering, LUT School of Energy Systems, Yliopistonkatu 34, Lappeenranta, Finland A R T I C L E I N F O Dataset link:zenodo.org/records/14161699 Keywords: Additive manufacturing(AM) Direct ink writing (DIW) Ceramic 3D printing methodology Clay A B S T R A C T The goal of this study was to optimize and validate the procedures and methods used to form ceramic objects using 3D-print molding instead of cast molding. Chamotte refractory clay was shaped into 5 x 5 x 5 cm cubes using both the 3D-print-molding and cast-molding methods. These cubes were then evaluated, fired, and evaluated again. The 3D-print molding method used was an adaptation of ’direct ink writing’. Since refractory clay dries during manufacturing, a Photoneo 3D scanner was used to monitor cube shrinkage before firing. Other basic properties such as mineralogical composition, evaluated via X-ray diffraction, were also measured. X-ray fluorescence spectroscopy determined chemical composition. After firing, compressive strength, bulk density, porosity, and water absorption were measured and structural aspects such as cracking and porosity were evaluated. The 3D-print molding of the chamotte clay was largely successful. The measured compressive strength of the fired 3D-print-molded and cast-molded ceramic cubes was 31.4 MPa and 30.4 MPa, respectively. The 3D-print-molded ceramic parts were slightly more porous (14.5%) and absorptive (7.1%). Total volumetric shrinkage was 36 %. Detailed cross-sectional analysis of the samples identified defects related to specific shortcomings of both molding methods. This information suggests areas that could be targeted for refinement. Addressing them could lead to significant advancements, allowing 3D-print-molded ceramics and similar materials to achieve superior properties compared to conventional manufacturing methods. 1. Introduction Three-Dimensional (3D) printing, also referred to as additive manufacturing, has brought about a small revolution in technology while significantly impacting materials science. The main advantages it brings include rapid prototyping, design flexibility, high precision, time savings, cost-effectiveness, and variability of the printing materials [1]. For example in these studies [1,2], the authors reported their expectation that the world’s additive manufacturing market would reach approximately $8.6 billion by 2020. In fact, revenues in 2018 were $14.5 billion, almost double their expectation. 3D printing has been under development since 1981, when Hideo Kodama pioneered a solution for rapid prototyping [3]. Kodama’s innovation laid the foundation for modern 3D printing by enabling the manufacture of physical objects directly from digital designs. From a technological perspective, the process of constructing a 3D model or ∗Corresponding author. E-mail address: [email protected] (A. Boleslavský). 3D haptic physical model [4] is consistent with currently available technologies [5]. The process can be divided into the following three parts. (1) A virtual 3D model is built using CAD software. (2) A specialized program called a slicer converts the 3D model into code that controls a 3D printer. The slicer optimizes parameters such as height and width of the line, print speed, etc. (3) Finally, code data are uploaded to the printer for printing. The 3D-printing method not only efficiently produces plastic and metal objects. It can also be used with other materials that can be stored, extruded with precision, and fused with previous layers before solidifying under normal room conditions [6]. Example materials include food and other biomaterials [7–9]. Photopolymer materials can be 3D printed based on the principle of UV light hardening. This method is called stereolithography [10]. Concrete structures [11–13] are another example of recent advancements in 3D printing. Ceramic raw materials such as silicates, cements, https://doi.org/10.1016/j.oceram.2025.100797 Received 31 January 2025; Received in revised form 27 April 2025; Accepted 12 May 2025 Open Ceramics 23 (2025) 100797 Available online 27 May 2025 2666-5395/© 2025 The Authors. Published by Elsevier Ltd on behalf of European Ceramic Society. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ). A. Boleslavský et al. or refractories can also be 3D printed [14]. Direct Ink Writing (DIW), a technology for printing pastes that harden over time, is commonly used to 3D print these materials [15]. DIW is also referred to as fused deposition modeling [16] or fused filament fabrication [17]. The manufacture of ceramics is undoubtedly challenging, both in terms of energy and raw material handling [18]. 3D-printing technology can help to mitigate some of this challenge. Generally, ceramic raw materials can be classified according to chemical composition into oxide ceramics such as Al2O3, ZrO2, TiO2, ZnO, and SiO2 and nonoxide ceramics such as carbides (SiC, B4C, TiC, ZrC), nitrides (Si3N4, AlN), titanates, phosphates, aluminates, and mixtures of ceramic materials (Al2O3-ZrO2, Al2O3-GdAlO3, Al2O3-GdAlO3-ZrO2, Al2O3-YAG, Al2O3-SiO2, Si3N4-SiO2) [18]. Traditional manufacturing technologies for ceramic parts include pressing, casting, extrusion, injection molding, gel casting, and tape castings. The raw materials may come with or without additives and binders [19]. Many of these technologies are limited by mold design parameters. The conventional molding processes that are currently available cannot be used to form shapes with complex geometries or interconnected holes [19]. Relatively new to the scene, ceramic additive manufacturing via 3D printing has developed to include a wide range of ceramic materials using different molding or forming technologies. Compared to standard manufacturing methods, ceramic additive manufacturing presents new ceramic material possibilities. 3D printing can shape more complex geometries. Moreover, 3D printing can also print powders and slurries. Both compact and porous parts and macroporous lattice structures can be prepared [20–22]. 3D printers can also be used to extrude different ceramic and waste materials mixtures. Aluminosilicate and zirconium dioxide clays [23–25], bioceramics [26,27], composite or nanocomposite materials [28,29], function ceramics [30], geopolymers [31], and concrete [32,33] can all be 3D printed as a general rule. Thanks to modern technologies in materials science and computer control a relatively large number of new technologies have been developed to 3D print raw ceramic materials whether in suspension, as a powder, or as a bulk material. 3D printing from suspensions involves liquid or semi-liquid materials in which fine ceramic particles are dispersed. For this type of additive technology, methods such as photopolymerization, inkjet printing, and extrusion are used. A good overview of the distribution of these technologies is also presented in these studies [21,34]. Currently, ceramic components are printed using many modern technologies such as selective laser sintering [35–37], stereolithography [38,39], binder jetting [40], material extrusion [41], and material jetting [42,43]. Particle size plays an important role. For binder jetting, particles should be smaller than 10 μm to limit surface roughness, high porosity, and poor densification during sintering [19]. Size particles <20 μm are preferable for the robocasting technique to achieve better flow during the extrusion and sintering processes. Direct ink writing was developed by Cesarani et al. at Sandia National Laboratories in 1997 to print ceramic pastes. This technique is easy, adaptable, and inexpensive and appropriate for a variety of raw materials such as monolithic or composite ceramic materials, polymers, and alloys [19,44,45]. As stated by the authors of [37], most scientific publications have focused on the DIW of advanced ceramic materials based on ZrO2 and Al2O3 as well as on non-oxides. Few authors have researched the printing of traditional clays. The principle of the DIW method is extrusion. The raw material, a non-Newtonian viscous suspension with rheological properties containing both a liquid and a solid phase, is printed at room temperature. The advantage of ceramic materials with viscoelastic behavior is that they retain their original shape even as additional layers are applied [46,47]. The final properties of the fired ceramic component are determined by viscosity, density, granulometry, shape and diameter of the nozzle, drying, and sintering. Compared to other relevant manufacturing Table 1 Chemical composition of ceramic materials tested by XRF analysis. Oxides (wt. %) Na2O MgO Al2O3SiO2P2O5Fe2O3K2O CaO 0,37 0,4 28,9 54,3 0,09 2,6 3,4 0,4 SO3TiO2V2O5MnO SrO ZrO2Cl LOI 0,27 1,4 0,05 0,01 0,01 0,05 <0,001 1,47 methods, DIW is economical and timesaving, and it results in minimum defects [48]. In study [49], the authors listed several companies active in the field of ceramic 3D printing. This shows the connection between academic research and industrial innovation. In addition, the 3D printed ceramics market shows an annual growth rate of more than 4%. The market is expected to reach $3.6 billion by 2030 [50]. In this work, ceramic clay was 3D-print molded using the DIW method. Comparison samples were also prepared using cast molding. As part of the experiment, a detailed print optimization study was performed, including a description of the entire printing system, calibration, and optimization of printing parameters. The parameters of interest were compressive strength, bulk density, porosity, and the structure of the tested samples. Compared to other literature, this experiment describes several problems and reveals the deficiencies in 3D printing of the ceramic raw materials via results analysis. Observed deficiencies are described, and the paper offers solution ideas and suggests ways to improve the 3D-print molding process and the final 3D printed part. 2. Experimental part The following paragraphs describe the materials tested and the equipment used to carry out this research work. The 3D-printing methodology is also described. Calibrations needed to achieve the results are also outlined. This section concludes by describing the fabrication of the test samples. 2.1. Description of the material This raw ceramic material used to mold the test samples is a commercial product called chamotte clay. It is produced by Pavek s.r.o. (Czech Republic). From the manufacturer, the water content of the material may be different for each batch. For this material, the most common value is 22% water content. This should be increased by adding and mixing water up to 31%. This mix meets the requirements for printing on the equipment used. Specifically, the requirements for smooth movement of the material through the printing system, printing to the desired heights and consistency of the print line shape. The recommended firing temperature is between 980 and 1250 ◦C. Firing temperature influences the color of the ceramic, which becomes a light cream. X-ray fluorescence was used to determine the chemical composition of the molded clay. See Table 1. The iron oxide (5.44-9.89 wt%) results in dark red coloration after firing at high temperature [51]. The significant amount of Na2O and K2O content in the sample reflects the presence of feldspars. These are responsible for the formation of the amorphous phase, which improves the sintering process and lowers the required firing temperature. The high measured percentage of K2O (3.37 wt%) comes from the mineral illite present in the clay [52]. The powdered clay had a relatively low Loss On Ignition (LOI) of 1.47 wt%. The LOI calculation reveals the amount of organic matter that was in the sample. Figs. 1and 2 show the mineralogical composition of ceramic clay detected by X-ray Diffraction (XRD) phase analysis using the powder method (XRPD). Quartz is shown as the main crystalline phase (SiO2), and mullite (3Al2O3⋅2SiO2) as the second phase. The main quartz peak Open Ceramics 23 (2025) 100797 2 A. Boleslavský et al. Fig. 1. XRPD diffractogram of ceramic material before firing. Fig. 2. XRPD diffractograms of ceramic material after firing up 1060 ◦C. is in the range of 30–32 ◦ 2𝜃. The mullite phase frequently appears in the record. Crystalline mullite is the most common phase in silicate materials. It is a high-temperature ceramic phase that forms in the Al2O3-SiO2 system. Crystalline mullite is the most common phase in ceramic materials. It forms when kaolin is heated to a high temperature. Additionally, silica oxide and a silicon-rich liquid phase are also formed. Mullite has low thermal expansion, chemical resistance, and good resistance to Open Ceramics 23 (2025) 100797 3 A. Boleslavský et al. Table 2 Rheological parameters of the mixture (MA). Mixture CP of G’ = G’’ (kPa) CP of 𝜏 (Pa) LVR tan (𝛿) LVR 𝜏 (Pa) MA 42.62 2581 0.37 489.7 thermal shock [53,54]. Potassium mica as dehydroxylated muscovite is the next most prominent phase. Water is released at temperatures above 850 ◦C and the feldspars phase as albite (NaAlSi3O8). The presence of quartz and muscovite proves that the mixture will have good plastic properties and good binding capacity for hydroxyl or water molecules. The mixture is also suitable for 3D printing [55–57]. Shear-thinning behavior is crucial for efficient Direct Ink Writing (DIW) 3D printing of ceramics, which allows easy extrusion by reducing viscosity as shear stress increases [58]. The G’ (storage modulus) and G’’ (loss modulus) play an important role — their equality indicates the initiation of flow. Inappropriate flow stress can lead to either difficult extrusion (high) or spontaneous flow (low) [59]. Rheological measurements were performed using a rotational rheometer Thermo Haake MARS iQ Air at a temperature of 23 ◦C with parallel plates (Ø 25 mm, gap = 2 mm). Shear-thinning behavior was investigated using the Rotation Ramp test in Control Rate mode with a shear rate range from 0.0026 to 120 s−1. The oscillatory Amplitude Sweep test (f = 1 Hz, 𝜏 = 1–5000 Pa) was used to determine the elastic modulus 𝐺′ and the loss modulus 𝐺′′, enabling the observation of changes in the viscoelastic properties of the material. This test also allowed the identification of the linear viscoelastic region (LVR) boundary and the critical point 𝐺′=𝐺′′, which marks the transition from elastic to viscous behavior. Furthermore, the value of the loss tangent tan 𝛿 (𝐺′′∕𝐺′) was determined, which defines the ratio of the viscous and elastic components. This ratio, 𝐺′′∕𝐺′, allows comparison of whether the material behaves more viscously or elastically. If tan 𝛿 < 1, elastic behavior dominates; if tan 𝛿 > 1, viscous behavior dominates; and if tan 𝛿= 1, it represents the transition between elastic and viscous behavior. The correct ratio of these components is key to controlling flow during 3D printing — if the material is too fluid, layers are prone to flowing, while if the material is too solid, irregular extrusion may occur. The results of these values allow print parameters to be optimized, including extrusion speed and layer setting time. The results from the Rotation Ramp and Amplitude Sweep measurements are presented in Fig. 3. The specific values of Crossover Points (CP) for 𝐺′=𝐺′′, expressed in kPa and 𝜏, Pa, are listed in Table 2. Additionally, the values for the End of LVR for 𝜏, Pa, and the Damping Factor tan(𝛿) are also included. Rheological measurements from the Rotation Ramp test in Control Rate mode confirmed the shear-thinning behavior of the ceramic paste used for 3D printing. The results from the Amplitude Sweep test validate the significance of the gel point (𝐺′=𝐺′′ ≥103 Pa) for achieving printability of 3D ceramic structures, as stated in the study [60]. The attainment of this crossover point is crucial for optimizing the composition of ceramic pastes, aligning with previously published criteria. The measurements confirm that the paste exhibits rheofluidity, as shear-thinning behavior is observed — when higher shear stress is applied, the paste structure breaks down and begins to flow. This property is desirable for DIW 3D printing, as the paste must remain sufficiently solid at rest (𝐺′> 𝐺′′) while flowing easily when force is applied during extrusion. For the estimation of the shear rate range at the nozzle, Eq. (1) [61] was used: 𝛾 =𝑄 𝜋𝑟3 ⋅3𝑛+ 1 𝑛(s−1)(1) where 𝛾 is the shear rate (s-1); 𝑄 is the volumetric flow rate (m3s-1); 𝑟 is the nozzle radius (m); and 𝑛 is the flow behavior index (–). Applying the Power-Law model to the measured viscosity data Fig. 3a, the flow behavior index was determined to be 𝑛= 0.2483. The volumetric flow rate 𝑄 was calculated as the product of the crosssectional area of a single printed layer and the movement speed of the printhead. The nozzle radius was 𝑟= 2 mm. For a layer width of 4.5 mm, the shear rate was calculated as 75.5 s−1, and for a layer width of 5.5 mm, as 92.5 s−1. 2.2. 3D printing A Make-R type printer modification for fused deposition modeling was adapted by the authors of [62] to print clay using the DIW method. Repetier Host software was used to configure the printing process. The following printers were used: (a) a 3D-Bioplotter by EnvisionTEC with a 0.26 to 1.27 mm nozzle diameter, pneumatic pressure of 0.01–0.5 MPa, and speeds from 6 to 25 mm/s;. (b) a Techcon (Cypress CA, USA) adhesive dispensing system repurposed for clay extrusion featuring a 6-axis robotic arm (IRB 120, ABB, Switzerland), Ø0.51–3 mm nozzles with pneumatic pressures ranging from 0.14– 0.69 MPa, and speeds from 1 to 80 mm/s [37]; and (c) a Delta Wasp 2040 3D with liquid deposition modeling, three nozzle sizes (1, 2, and 3 mm), and an air compression pump (0.4 and 0.6 MPa optimum pressure) and tank with a mechanical screw extruder [57]. Clay samples were also printed using a 3D Potter 7 pottery printer with a circular nozzle of Ø5mm [63]. The mentioned printers are examples of printers with settings and equipment for DIW printing. Fig. 4 illustrates the in-house-developed printer system used to DIW print the chamotte clay. The printer shown in Fig. 4(a) is based on the Tronxy X5SA model, which was originally designed for printing various plastic materials. A CoreXY printer was modified to print ceramic materials, utilizing equipment already available in existing inventory. The modification was designed to withstand the substantial loads generated during ceramic printing. Additionally, safeguards were implemented to prevent contamination from dirt and water. The decision to modify existing equipment for the purpose of printing ceramic materials proved to be more cost-effective. The results demonstrate that this choice was also successful from a technological standpoint. The printing process begins by filling the almost 4 l steel cylinder cartridge. It is necessary to ensure the elimination of air in the system. For this reason, a screw feeder is used for loading the cartridge. In this process, the material is manually loaded to the feeder and it mixes, compresses, and moves material into the cylindrical cartridge. This method is significantly more effective than manual filling, which can trap air within the material. To remove any residual air from the system, the extruder is equipped with venting holes in the section above the screw. This is a standard feature in commercial extruders. Once filled the ends of the cartridge are sealed by a piston and threaded lids. The compressor pressurizes the piston, which then pushes the raw material through a hole on the opposite end and through a tube connected to the extruder. The cartridge is suspended above the printer on a pulley system, with the compressor positioned nearby. The new extruder designed for this experiment (Fig. 4(b)) was made specifically for the ceramic raw material. Development of the extruder was inspired by several studies on printing ceramic raw materials [64–66]. It accepts easily changeable plastic nozzles of 2, 4, 5, 8 and 10 mm diameter. Ceramic material is fed into the extruder from the side, and the extrusion itself is carried out utilizing a screw. The described procedures and guides for 3D printing have been based on existing methodologies developed by other authors, particularly those presented by Jonathan Keep, who has been involved in 3D printing of ceramic materials for a long time [67]. Other authors and Open Ceramics 23 (2025) 100797 4 A. Boleslavský et al. Fig. 3. (a) Shear-thinning behavior, (b) Storage and Loss modulus measurement. Fig. 4. (a) current state of the printing equipment, (b) design of new extruder. manufacturers also have their own procedures, which differ slightly. The procedure described here is suitable for the equipment and material used. The first step, after preparation of the material, is to calibrate the printer according to the recommended procedures and guides that deal with the correct adjustment and calibration of a CoreXY printer. In this experiment, the print bed is the tile on which the sample is printed. Since the tile is replaceable, it can be moved with the printed product that can be left to dry outside of the printer. A new tile is then provided for the next print. The correct Z-axis offset must be set for each new tile. Dimensional error can be on the order of millimeters in the case of incorrect calibration, which is noticeable when printing with smaller nozzle diameters. A simple test is performed to check that the printer is set up correctly and that the tile used is flat enough for printing. Fig. 5 shows a bed-leveling print, a set of nested squares centered on a tile bed. The widths of the extruded lines correspond to the precise distance of the nozzle from the surface of the bed. If they are equal, then the printer gantry is level and the tile if flat. If, however, the print happens to be wider somewhere, this indicates that the printer gantry must be leveled or the tile bed is not sufficiently flat. For this experiment, the substrate was first set up using a large straight tile bed. The prints themselves were then printed on the smaller tile pads. The bed was leveled for each new tile. The next step is to set the material flow to the desired print line profile. This is done by printing out a square shape and adjusting the flow. Three different results can occur when measuring dimensions, as shown in Fig. 6. In case (a),insufficient material was extruded. Material flow must be increased. Flow rate must be monitored and adjusted until the result looks like (b). If material flow exceeds the correct value, it accumulates between the lines and packs onto the nozzle as shown in (c). If the flow rate is even higher, a situation can occur at low speeds where the layers are pushed sideways by the material, and the material is smoothed from above by the nozzle. See (Fig. 7). Although there is accumulation on the nozzle, it is not as bad as the case in Fig. 6 (c). Fig. 8 demonstrates the overall workflow of the experimental print process. It begins by preparing the 3D CAD model, which must be designed or adjusted to be printable. Overlaps, details, and manufacturing methods in respect of the material must be considered [68–70]. The model is then uploaded to a slicer where print setup takes place and Open Ceramics 23 (2025) 100797 5 A. Boleslavský et al. Fig. 5. Image of calibration pad print. G-code is generated. Before printing, the printer is calibrated to ensure accuracy. The material is prepared and loaded. Then, the printing line is calibrated. Afterwards, printing can begin. Printing must be monitored and flow adjusted as needed to compensate for variations in raw material consistency. This issue can be mitigated by careful material mixing and handling. Printing multiple copies of the same item reveals defects, limitations, and relationships specific to the variability of the ceramic clay. Print settings affect appearance. Printer size determines the maximum size that can be printed. And small system changes lead to different print results. A relationship map was prepared to determine how a given system change would affect other areas within the 3D printing process. Fig. 9 illustrates these relationships. Printer choice affects the accuracy of the print, the size of the layers, and therefore the quality of the print. Printer calibration improves print accuracy. Poor calibration will affect the print from the beginning, and for this reason, careful calibration is critical to the printing process. The equipment is also affected by the material being printed. The equipment must be able to store and precisely extrude the material as directed by the G-code. Equipment and printer dimensions affect slicer settings such as print dimensions and layer sizes. Another example of a relationship can be the requirement for serial printing. This is directly influenced by the equipment and print quality requirements of the product. Here, it is mainly about the degree of automation. For example, if the bed is changed, refills and calibration are done automatically. This would be the ideal situation for serial printing. Otherwise, when a human is involved in the process (changing the print pad, refilling the material for each print, etc.), serialization is still possible but becomes more challenging. The print setup will also affect the seriality of the print, indirectly through the desired quality of the desired printout. The relationship map shows in Fig. 9 is a simplification that can be developed in greater detail. The map makes it possible to better examine the methodology and calibration. It can also be used to improve the print device, as even these small improvements can have a big impact on the end result. 2.3. Sample preparation To produce the test samples for this research, chamotte clay was shaped into 5 𝑥 5 𝑥 5 cm cubes using traditional cast molding and by 3D-print molding. In its original raw form, the clay had 22% water content. This percentage was increased to 31% by adding 1 l of water to 10 kg of clay to achieve optimal consistency and facilitate the molding methods. The cast-molded cubes were made by pouring the chamotte clay into a rubber mold. After pouring, the clay and mold were vibrated for four minutes. After 24 h, solidified clay cubes were demolded. Fig. 10 shows the cast-molded cubes. The 3D-print-molded cubes were printed onto unglazed ceramic tiles. Fig. 12 shows the 3D-print-molded cubes. The demolded cast cubes and 3D-print-molded cubes were then dried at 105 ◦C for 24 h. They were then fired at 1060 ◦C for 5 h with 1 h of maintaining the maximum temperature in an electric resistance furnace without sheet and without cooling. To avoid clogging during the 3D-print molding process, the clay must be extruded at pressures greater than 0.6 MPa. Lower pressures are not sufficient to overcome the pressure loss in the system. At 0.8 MPa, clogging does not occur, and extrudability is ensured. The volume of clay that will be required is fed into the feed cartridge. With the 3D-print molding process, shortages or excesses of material are not significant issues. At 0.8 MPa and with a 4 mm diameter nozzle, a print speed of 15 mm/s was suitable. The distance between the nozzle and the printing bed was set to 4 mm. The same height was maintained for the other layers. Clay water content remained relatively constant. The flow of extruded clay was steady. It did not accumulate nor did it drop out. Assuming equal pressure, the ideal print speed would be lower for larger diameter nozzles and higher for smaller diameter nozzles. Water content also affects extrudability, so the system must be carefully calibrated to accommodate the specific print medium. Fig. 11 illustrates the print result for the 0.8 MPa, 15 mm/s, 4 mm nozzle setup for the 31% water chamotte clay. The photo was captured within 5 min after the print was completed. The print bead was 4 mm thick and 4.5 to 5.5 mm wide. This corresponds to a setting of 5 mm in the slicer. When the layers are laid side by side, they overlap. This deforms the clay material, which serves to flatten and align the bead and connect it to adjacent lines. Refer back to Fig. 6 (b). The chamotte clay material can slump under some conditions [71]. The diameter of the nozzle and the height of the print plays a role as does whether or not the print is single-walled, multi-walled, or has supports. Print speed also affects slumping. If the material is fed too fast, previous layers are flattened by the material flow. The cubes printed for this experiment were singlewalled, the beads were of average height, and the nozzle diameter was 4 mm. Because, each bead was supported by adjacent lines and layers, slumping was minimal and not visible during printing. After printing, however, during the initial drying process, some slumping occurred. Twelve of the 3D-print-molded cubes are shown in Fig. 12. Fig. 12(a) and b show the top and bottom of the cubes after initial drying. Figs. 12(c) and 12(d) show the tops and bottoms after firing. The tops (last printed layer) are relatively smooth (12(a) and 12(c)). On the other hand, each bead remained distinct on the bottom, which was in contact with the ceramic tile during extrusion (12(b) and 12(d)). The gaps between layers are relatively large. These gaps may be due to the choice of print bed surface or poor setup of the first layer. Figs. 12(b) and 12(d) show that firing did not affect these gaps. Despite the effort to choose a print pad with low water absorption, because the tile used was unglazed, excess absorption probably resulted in the first layer bead gaps. Since firing did not affect these gaps, and because they only occur on the first layer, it is likely that they are the result of excess absorption of water into the print pad. A lower quality glazed tile was chosen as the printing bed. The tile was chosen to avoid rapid water drainage, which when it occurs, creates cracks. Glass was not chosen as the experience with printing this material on glass is that the material does not hold in place and slides across the glass, ruining the print at the very beginning. Fig. 13 shows a side view of the 3D printed cubes. Here you can see the deflection of the sample, which is completely negligible due to Open Ceramics 23 (2025) 100797 6 A. Boleslavský et al. Fig. 6. Print line profile settings. Fig. 7. The case of high material flow and low printing speed. Fig. 8. Printing phases. the setup. Picture A is after drying and picture B is after firing. Even during firing, the profile did not change during shrinkage. A more detailed analysis was performed by the authors [72]. A condition is presented where the elastic behavior of the clay is characterized, which tends to have a relatively linear deformation slope. Another state is characterized by plastic deformation, which is manifested by an exponential increase in deformation until complete collapse. The test results presented here agree with the authors and suggest that these states lead to a reliable estimate of the mechanical dynamics that occur during printing. This approach can therefore form the basis for the development of a comprehensive design method for the entire printing operation. Kruger et al. [73], define buildability as the ability of a material to retain its shape after several layers have been printed. Which is measured by counting the number of stable printed layers before collapse [74]. Nozzle diameter and print speed significantly affect the buildability of the 3D-print-molded mixture [75]. To avoid print failures caused by the weight loading of each layer, certain stress constraints must be satisfied. These authors used the model to avoid the rheological parameters of materials such as concrete, which is a thixotropic material. Therefore, a 3D-print molded material is subject to plastic stress if the self-weight of the layer exceeds its static yield strength. As concrete gains strength over time, its static yield strength changes. To Open Ceramics 23 (2025) 100797 7 A. Boleslavský et al. Fig. 9. Relationships in the printing process. Fig. 10. Test cubes prepared by casting. Fig. 11. Extruded line and its application to 3D-print-molded parts. support the buildability of the optimized design, the geometric overlap of adjacent layers must be appropriately determined. This is especially important for structures with large curvatures or large overhangs. Lack of overlap may be one reason why the lowest layer has shifted outward in some experiments [76]. In article [77], print speed for the first layer was set to 50% for better adhesion to the print bed. In this case, cylindrical shapes with a diameter of 8 mm and a height of 4 mm were extruded. 3. Results and discussion The following paragraphs describe the test results and how the test cubes were evaluated. Key parameters of interest include shrinkage and compressive strength. Test samples were also cut in half and analyzed, which helped to better reveal some defects and led to more discussion. Bulk density 𝐵𝐷 is mass per unit volume. For the sample cube evaluated, this is influenced by the presence of cavities and pores. Bulk density was determined by weighing the samples and dividing by measured volume. Bulk density was determined as part of absorption measurement. Saturated cubes were weighed on hydrostatically balanced scales in water at a temperature of 23 ±2 ◦C. Each newly formed watersaturated cube was suspended from an auxiliary structure, completely submerged, and positioned away from the walls of the container. The submerged weight 𝑚2 was measured after removing air bubbles. Weight in air 𝑚3 was measured after removing each cube from the water and wiping off the surface moisture with a damp cloth. 𝜌liq is the density of the liquid in which the samples was immersed during the measurement. In our case it is water. Therefore, 𝜌liq is equal to 998,2 kg/m3. After oven drying, another measurement was made to determine the weight of the dry cube 𝑚1. Eq. (2) were used to calculate bulk density of the cubes. (EN 771-1, EN 72 2603) 𝐵𝐷 =𝑚1 𝑚3−𝑚2 ⋅𝜌liq (kg.m−3)(2) Open Ceramics 23 (2025) 100797 8 A. Boleslavský et al. Fig. 12. Cubes prepared by 3D-print molding (a) top of cubes after drying (b) bottom of cubes after drying (c) top of cubes after firing (d) bottom of cubes after firing. where 𝜌liq is the density of the liquid in which the samples were immersed during the measurement (𝑘𝑔.𝑚−3); 𝑚1 is dry sample weight (kg); 𝑚2 is mass of the sample saturated with liquid in water (kg) and 𝑚3 is mass of the liquid-saturated sample in air (kg). Absorption (EN 771-1) is the ability of the fired ceramic material to absorb liquid. It was determined in percentage as the ratio of the weight of water absorbed by the test cube to the weight of the dried cube (absolute mass absorption). The weighed and dried cubes were placed in the container so that they did not touch the sides or each other. Water was then poured into the container until they were completely submerged. The water was then heated to the boiling point and held there for 2 h. As it evaporated away, more water was continually added to ensure the cubes remained submerged. After boiling, the cubes were allowed to air cool to ambient temperature. They were then surface dried and weighed. The absorbance 𝑊 𝐴,(3) in % is expressed as follows. 𝑊 𝐴 =𝑚4−𝑚5 𝑚5 ⋅100 (%) (3) where 𝑚4 is weight of the sample after the absorption test (kg) and 𝑚5 is weight of the dried sample (kg) Apparent porosity 𝐴𝑃 is the ratio of the volume of open pores and voids of a cube to its total volume including all pores and voids. The true porosity is the ratio of the volume of open and closed pores and cavities of the sample to its total volume including all pores and cavities. The calculation is performed according to Eq. (4). 𝐴𝑃 =𝑚3−𝑚1 𝑚3−𝑚2 ⋅100 (%) (4) The compressive strength 𝐶𝑆 of each sample was measured using a hydraulic press (EN 772-1, EN 72 2605). 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