Optimization of Kaolin Clay Composition for Enhanced Mechanical Properties in 3D-Printed Structures

Abstract

Clay 3D printing is an emerging field within additive manufacturing that presents significant opportunities for both structural and artistic applications. Driven by the increasing interest in this technology, there is a growing demand for optimized printing protocols tailored to clay, a readily available and versatile material. This study investigates the optimal processing parameters for kaolin clay composites and assesses the influence of clay-to-water ratios on the physical and mechanical properties of printed specimens. Experimental results demonstrate that higher clay content enhances the dimensional stability and structural integrity of printed components. The optimal formulation was determined to be 60% clay and 40% water, which produced the highest mechanical performance: the flexural strength of sintered specimens reached 1.3125 MPa and the compressive strength attained a maximum of 6.14 MPa. Shrinkage analysis indicated that specimens with greater water content experienced increased volumetric shrinkage, with reductions of up to 10% in linear dimensions and 14% in mass during drying and sintering. These findings highlight the critical relationship between material composition and final part performance in clay 3D printing and provide guidance for optimizing material formulations to enhance the mechanical robustness of printed clay composite structures for diverse applications.

1. Introduction

The advancement of three-dimensional (3D) printing technology has experienced rapid growth, exerting a profound influence on manufacturing practices. Originally employed primarily for prototyping, it has now evolved into a pivotal technique across multiple industries, including outdoor equipment, construction, healthcare, art, and the automotive sector [1,2,3]. This technology enables the precise fabrication of intricate components [4,5], patient-specific implants [6,7], and advanced parts [8,9], thereby accelerating product development cycles while fostering innovation and customization in production [10,11,12]. Within the construction industry, 3D printing has redefined project execution by reducing labor requirements, shortening timelines, and enhancing resource efficiency, while simultaneously allowing for flexible and highly accurate design solutions [13,14]. Ongoing advancements have further expanded its scope to include clay-based 3D printing [15,16,17,18], which supports the creation of complex and sustainable structures applicable in architecture, art, and product design [9,13]. Unlike conventional materials such as plastics and metals, however, clay presents unique challenges due to its rheological characteristics, necessitating precise regulation of parameters such as extrusion rate, applied pressure, printing speed, and moisture content [19,20].

Optimizing printing parameters is crucial to achieving high precision, structural stability, and production efficiency in clay-based 3D printing [21,22,23]. Among these parameters, extrusion speed plays a particularly critical role, as it directly influences the material flow rate and thereby affects the consistency and overall quality of the printed object. Proper calibration of extrusion speed ensures a continuous and uniform deposition of material, which is essential for maintaining accuracy and the structural integrity of complex designs [2,17,24,25]. Beyond extrusion speed, other factors such as extrusion pressure and printing speed also significantly impact print quality [10,16,26]. Inadequate extrusion pressure often leads to irregular material flow, while excessive pressure can result in over-deposition that diminishes fine details [27,28,29]. Similarly, printing speed must be carefully controlled: excessive speeds can compromise structural stability, whereas overly slow speeds may cause deformation due to gravitational effects or pressure buildup [29,30,31,32,33].

Kaolin is a clay-based material predominantly composed of the mineral kaolinite, valued for its fine particle size and chemical stability. [34,35,36,37]. Its versatility has made it an essential material across various industries, including ceramics, paper, and cosmetics, owing to its excellent plasticity and ability to withstand high temperatures. These properties render kaolin indispensable in the production of intricate pottery and high-quality porcelain. In the firing process, the heating rate significantly affects the thermal behavior and mullite formation of kaolin clay [10,26], making it a critical factor in manufacturing. Beyond clay type, the mechanical strength of 3D-printed products is also strongly influenced by the water content in the clay mixture [10,38]. An optimal water ratio ensures suitable viscosity for smooth extrusion and prevents nozzle clogging, while also promoting strong interlayer adhesion during printing [28,39]. Excessive water content, however, can result in overly soft or fluid clay, leading to deformation and difficulty in maintaining the intended geometry [17,38]. Conversely, insufficient water content produces a mixture that is too dry or viscous, restricting extrusion and increasing the likelihood of cracking and delamination between layers [40]. Thus, careful control of both clay type and water content is essential to ensure the production of stable, durable, and precise structures in clay-based 3D printing.

Although additive manufacturing has seen growing use in ceramics, most prior research has concentrated on demonstrating feasibility rather than optimizing material composition to improve structural integrity and printability. Much of the existing literature highlights the printability of clay-based materials. However, it provides a limited analysis of how clay–water ratio variations influence mechanical strength, shrinkage, and dimensional accuracy. This study addresses that gap by researching the relationship between material composition and performance, ensuring that printed structures retain strength and precision after drying and sintering. The originality of this work lies in refining the clay formulation to improve mechanical performance while reducing common defects such as shrinkage and deformation. This research advances clay-based 3D printing through parameter optimization, expanding its potential applications in architectural fabrication, ceramic prototyping, and sustainable construction. The capability to produce durable, customizable, and environmentally friendly clay structures represents a valuable opportunity for industries aiming to merge modern manufacturing technologies with traditional materials.

2. Materials and Methods

2.1. Materials Preparation

The specimens in this study were prepared using kaolin clay powder and distilled water. The kaolin powder employed was of cosmetic-grade quality and accompanied by a Certificate of Analysis (CoA) from the manufacturer. Classified at 325 mesh, the powder achieved a 99.5% pass rate, with an inherent moisture content of approximately 2–3%. Distilled water was selected to eliminate the presence of impurities, ensuring a contaminant-free mixture that enhances both the accuracy and reproducibility of the experiments [10]. This emphasis on material purity contributes to the reliability of the study and provides a consistent basis for achieving high-quality results. The chemical composition of the kaolin powder is presented in

Table 1. Chemical composition of Kaolin clay powder.

Al2O3	CaO	MgO	MnO2	Cr2O3	Na2O	K2O	SiO2	TiO2
33–39	0.01–0.07	0.01–0.10	<0.01–0.05	<0.01–0.05	0.01–0.17	0.25–0.80	45–47	0.25–0.37

, while a detailed schematic of the experimental framework is illustrated in Figure 1a–c.

2.2. Mixing of Clay Mixture

The kaolin clay powder was blended with distilled water using a Thinky ARE-310 centrifugal mixer (Thinky Corp., Tokyo, Japan) (Figure 1a). This high-speed mixing process generates a vortex effect, enabling homogeneous dispersion and producing a smooth, consistent slurry. Mixing was conducted for 3–5 min at 2000 rpm, followed by a 1 min defoaming cycle at 2200 rpm to eliminate entrapped air.

2.3. Moisture Content Analysis

Moisture content was determined using an Ohaus MB95 moisture analyzer (Ohaus Corp., Parsippany, NJ, USA) (Figure 1a) [41]. A standardized sample weight of 9 g was taken from the clay–water mixture to ensure consistency and reproducibility for each measurement. The analyzer was operated in standard drying mode at 110 °C, with the A60 auto shut-off function enabled. This setting automatically terminates the measurement once mass loss is less than 1 mg within 60 s [23]. Two measurements were conducted for each batch, while five specimens were tested for each mixture variation. In addition, the moisture content of the dry, unmixed kaolin powder was measured separately as a reference, with three replicate tests performed to improve accuracy.

2.4. Three-Dimensional Modeling and Slicing of Specimen

Three-dimensional modeling was performed using Autodesk Inventor Pro 2024 on a Windows-based system. Two test models were designed per relevant ASTM standards, as illustrated in Figure 2a,b. The first model is a solid block prepared to meet flexural testing specifications, with dimensions of 6 mm in width, 8 mm in thickness, and a nominal length of 90 mm. To compensate for shrinkage during drying and sintering, the model was extended to 102 mm, ensuring accurate representation of its final behavior under stress. The second model is a solid cylinder designed in compliance with ASTM C773-88 [42,43], with a diameter of 25 mm and a height of 50 mm. Both models were developed with an emphasis on precision and reliability to secure valid experimental outcomes. Following the design stage, the 3D models were imported into Ultimaker Cura 5.11 for slicing, preparing them for printing.

2.5. Printing Process

Two batches of clay mixture were prepared and combined for each printing session. The blended clay was transferred into a larger container and manually kneaded to achieve a uniform texture, eliminating entrapped air bubbles and ensuring the mixture fit appropriately into the feeding barrel. The barrel, measuring 50 mm in diameter and approximately 25 cm in length, was then squeezed with the kneaded clay from the rear side. Once filled, the barrel was securely connected to the piston and feed housing. The piston advanced gradually to expel any remaining air before a tube linked the barrel to the screw extruder [17,38,44]. At this stage, they extruded a small amount of clay to verify consistency, preparing for printing. The 3D printer (Tronxy Moore 1, Shenzhen, China) and clay barrel are depicted in Figure 1c.

Following printing, the specimens were air-dried for 48 h prior to furnace sintering. They avoided handling the specimens during the first 24 h to minimize the risk of deformation or breakage. After this initial period, the specimens could be removed safely from the build plate and left to finish drying [45,46].

2.6. Sintering

Sintering of the specimens was performed using a Thermolyne F48010-33 (Thermo Fisher Scientific, Waltham, MA, USA) benchtop muffle furnace, a closed-air system equipped with precise electronic temperature controls (Figure 1c). The programmable settings enabled accurate regulation of both the heating ramp and the target temperature [47,48,49]. Specimens were sintered at 1100 °C for one hour, with a controlled heating rate of 5 °C per minute to minimize the risk of cracking during the initial heating stage. Following sintering, the specimens were allowed to cool naturally inside the furnace, remaining undisturbed until the internal temperature stabilized at ambient conditions.

2.7. SEM and FTIR

After the material was sintered, structural observations were continued using Scanning Electron Microscopy (SEM) with a Thermo Scientific Phenom Pro X (Thermo Fisher Scientific, Eindhoven, The Netherlands) device to compare the morphology of the material before and after the sintering process. This analysis aims to identify changes in particle size, pore distribution, and the level of densification that occur due to the heat treatment. Samples before sintering were taken from the dried material from the molding process, while samples after sintering were taken from specimens that had undergone a heating process at the optimum temperature. Comparison of the two conditions provides an overview of the effect of sintering on the microstructure arrangement and interparticle bonding, which contributes to increasing the mechanical strength of kaolin-based materials. Further analysis was carried out using FTIR with a SHIMADZU IRX Ross (Shimadzu Corp., Kyoto, Japan) instrument to obtain data on the difference in spectra between kaolin before and after the sintering process.

2.8. Flexural Testing

The mechanical testing was conducted using a Universal Testing Machine (Carson CRN-50, Carson corp., Taipei, Taiwan). Flexural strength was evaluated using a fully articulating three-point bending fixture to ensure accurate measurement. The fixture was configured with an 80 mm support span and a 40 mm loading span to achieve optimal force distribution. Each 3D-printed specimen conformed to the standard dimensions of 8 mm × 6 mm in cross-section and a minimum length of 90 mm. Testing was performed on specimens in their as-sintered condition without additional surface treatments. Careful alignment of the samples on the fixture was maintained to avoid premature.

2.9. Compression Testing

Compression testing was conducted in accordance with ASTM C773-88 [42,43]. Cylindrical specimens were prepared with dimensions of 25 mm in diameter and 50 mm in height. The tests were conducted at a controlled loading rate of 0.5 kN/s to ensure precision and consistency. To improve load distribution and minimize localized stress concentrations, a 0.8 mm aluminum plate was placed on each specimen’s upper and lower contact surfaces. This cushioning layer protected the samples during testing while facilitating uniform force application, as illustrated in Figure 1b.

3. Results and Discussion

3.1. FTIR Spectroscopy

As illustrated in Figure 3, the FTIR spectrum of pure kaolin reveals distinct absorption peaks characteristic of its mineral composition. Broad peaks observed at 3618.46 cm⁻¹ and 3367.71 cm⁻¹ correspond to O–H stretching vibrations, confirming the presence of hydroxyl groups in kaolinite. The absorption band at 1641.42 cm⁻¹ is associated with H–O–H bending vibrations, indicating bound water within the structure. A sharp peak at 908.47 cm⁻¹, attributed to Al–OH bending, represents a defining feature of kaolin [1,50]. Furthermore, absorption bands at 527 cm⁻¹ and 485 cm⁻¹ correspond to Si–O and Al–O–Si bending vibrations, respectively, validating the silicate framework of the mineral. Collectively, these spectral characteristics confirm that the sample consists of pure kaolin with no significant impurities compromising its structural integrity [51].

FTIR analysis of pure and sintered kaolin demonstrates notable structural transformations induced by sintering. In pure kaolin, distinct hydroxyl (–OH) stretching vibrations are observed at 3618.46 cm⁻¹ and 3367.71 cm⁻¹, characteristic of kaolinite, while the band at 1641.42 cm⁻¹ corresponds to adsorbed water. Following sintering, these hydroxyl peaks disappear, indicating dihydroxylation and the conversion of kaolinite into metakaolin. Concurrently, the emergence of new peaks at 2970.38 cm⁻¹ and 2358.94 cm⁻¹ may reflect interactions with atmospheric gases or carbonization. Shifts in Si–O and Al–O vibrations at 1065.99 cm⁻¹ and 794.67 cm⁻¹ further suggest structural rearrangements, potentially associated with forming amorphous aluminosilicates or mullite at elevated temperatures [52].

These spectral changes confirm that sintering significantly alters kaolin’s chemical composition and phase structure. The loss of hydroxyl groups through dihydroxylation and the subsequent formation of new structural phases highlight kaolin’s transformation into thermally modified materials, suitable for applications requiring enhanced stability and performance [15,47].

3.2. SEM Analysis

The Scanning Electron Microscope (SEM) analysis of the kaolin sample reveals a heterogeneous particle size distribution with pronounced non-uniformity, suggesting potential aggregation or inconsistencies during processing. The surface morphology displays significant roughness, characterized by pores, cracks, and other microstructural features contributing to the material’s textural complexity. Particles appear irregular in shape—often elongated or angular rather than spherical—features that can strongly influence material properties such as packing density, sintering response, and mechanical strength [51,53].

Comparative analysis of pure and sintered kaolin (Figure 4) highlights notable structural transformations induced by the sintering process. In its raw state, kaolin particles appear highly irregular, loosely packed, and marked by broad size distribution, surface roughness, and microporosity. Following sintering, the microstructure becomes more consolidated, with particle fusion producing a denser matrix while retaining some surface roughness [18]. Grain growth contributes to a more uniform particle distribution; residual layered structures and cracks indicate incomplete sintering [54]. These morphological changes reflect enhanced particle bonding and improved mechanical strength, underscoring the role of sintering in optimizing kaolin’s structural performance.

3.3. Clay Mixture Uniformity

Six different clay–water mixtures were prepared and evaluated to determine the optimal mixing configuration. For each trial, 116 g of cosmetic-grade kaolin clay was combined with 84 g of distilled water, and the mixture was placed in a container with a 90 mm diameter. The variations in mixing conditions and their corresponding results are presented in Figure 5, offering a comparative overview of mixture performance.

When following Thinky’s default mixing configuration, moisture distribution within the container proved inconsistent: the upper portion of the mixture appeared soft and wet, whereas the lower portion remained dry and compact. Increased water content produced higher stickiness, while extending the mixing duration improved uniformity [5]. At 4 min of mixing, the clay achieved a more consistent blend, though occasional dry pockets were still detected during manual inspection [10,13,55]. Extending the mixing time to 5 min eliminated all visible signs of non-uniformity, producing a fully homogeneous mixture. However, the additional mixing also introduced minor heat due to frictional effects.

The defoaming function successfully removed air bubbles formed during mixing, but its use significantly reduced mixture uniformity within the container. Furthermore, prolonging mixing beyond 5 min did not yield further improvements; instead, it caused the clay to overheat, potentially compromising material properties. Based on these observations, the most effective procedure for preparing 200 g of clay–water mixture is mixing for exactly 5 min without employing the defoam mode [38,56,57]. This approach achieves optimal uniformity while preserving the material’s temperature stability and structural integrity.

3.4. 3D Printing of Clay

One of the most critical parameters influencing successful 3D printing with clay is the water concentration within the mixture. Moisture content must be precisely controlled to achieve the ideal paste-like consistency, which enables smooth extrusion while preserving structural integrity. Additional factors, including clay type, extrusion pressure, and printing speed, also contribute to the accuracy of layer deposition and the overall stability of the printed structure. Post-printing drying is equally important, as inadequate drying can result in cracking or warping prior to sintering. When these parameters are carefully managed, high-quality and mechanically robust clay prints can be consistently produced. The results of the 3D-printed specimen evaluations are presented in Figure 6.

3.5. Effects of Drying and Sintering on Printed Clay

During the drying process, a significant portion of the moisture contained in the printed products is gradually removed. This reduction in water content induces dimensional changes in the components, leading to shrinkage that may alter their overall size and fit. Since water also contributes to the total mass, its loss results in a measurable decrease in specimen weight. Both shrinkage and weight reduction primarily occur during the air-drying and sintering stages. Direct measurement of freshly printed specimens is challenging; therefore, the degree of shrinkage during air-drying is assessed by comparing the dimensions of the sintered products with those of the original 3D CAD model used for printing. In addition, weight loss during sintering is carefully recorded, offering valuable insights into the material’s transformation throughout the drying and sintering processes [58,59].

It is essential to make precise design adjustments to ensure that the final printed specimens or products meet the required dimensions after processing. For example, in the case of flexural test specimens, the 3D CAD model must be designed 10 mm longer than the intended final length. This adjustment compensates for dimensional shrinkage during drying and sintering, ensuring the final product conforms to the specified size requirements [28,60]. The detailed shrinkage behavior of all specimens is presented in Figure 7a,b.

As shown in Figure 7d, most moisture is eliminated during the air-drying stage, as indicated by the significant shrinkage observed in this phase. Specimens with higher initial water content exhibit greater volumetric shrinkage, as illustrated in Figure 7a,c. Similarly, Figure 8 demonstrates that these moisture-rich specimens also undergo the largest weight reduction. This behavior occurs because specimens with elevated water content inherently contain more bound and free water, which is progressively expelled during drying and sintering, leading to a more pronounced reduction in volume and weight.

As shrinkage increases during the printing process, the dimensional accuracy of the final product tends to decrease significantly [35,61]. For applications where modifications to the 3D model are not feasible, such as slice-and-print methods, employing a higher clay content in the printing mixture offers advantages. A higher clay ratio enhances the structural integrity of the printed object and helps reduce dimensional inaccuracies. However, to achieve precise final dimensions, it remains essential to adjust the 3D model size according to the expected shrinkage associated with the chosen mixture [62]. This calibration ensures closer alignment between the printed product and the intended specifications, improving overall quality outcomes [61].

3.6. Flexural Strength Testing

The appearance of specimens before and after flexural testing is shown in Figure 9, while the flexural strength results are presented in Figure 10. The findings demonstrate a positive correlation between clay content and flexural strength. Among the tested specimens, the one containing 60% clay exhibited the highest strength, reaching 1.3125 MPa. In contrast, the specimen with 56% clay recorded the lowest strength, showing an average difference of 0.125 MPa. This indicates that the 60% clay specimen is approximately 10.5% stronger than its 56% clay counterpart. Similarly, the specimen with 58% clay displayed a 5.2% increase in strength compared to the 56% clay mixture, further confirming the beneficial effect of higher clay content on the structural integrity of the printed specimens.

In 3D printing clay, a higher clay concentration (with less water) results in greater flexural strength due to several factors [44,63]. Increased clay content enhances particle cohesion and bonding, creating a denser, more interconnected structure that resists bending forces [23]. Less water reduces deformation during printing and shrinkage during drying [37,39]. The lamellar structure of clay particles interlocks more effectively, further improving structural integrity. Additionally, if the printed object undergoes sintering, a higher clay concentration facilitates the formation of a denser and stronger ceramic phase, significantly increasing flexural strength [64]. However, while a higher clay ratio improves strength, adequate water content is still necessary to ensure smooth extrusion and prevent nozzle clogging during printing.

3.7. Compressive Strength Testing

The failure modes and fractured specimens are presented in Figure 11a,b, while Figure 12 illustrates the compressive strength of the sintered samples. The results indicate that specimens with higher clay content exhibit greater compressive strength. Among the tested samples, the 60% clay specimen achieved the highest value at 6.14 MPa, whereas the 56% clay specimen recorded the lowest strength at 4.546 MPa. This difference demonstrates that the 60% clay specimen is approximately 35% stronger than the 56% clay specimen, despite the relatively small variation of only 4% in clay content. The enhanced compressive strength at higher clay concentrations can be attributed to several factors. A greater clay fraction produces a denser microstructure with fewer pores, thereby minimizing potential weak points. Reduced water content further decreases shrinkage and the formation of microcracks [65], resulting in a more compact and uniform material. Moreover, the strong cohesive and binding characteristics of clay particles enhance structural integrity [17,36,66], improving resistance to compressive forces. The lamellar morphology of clay particles also promotes interlocking, which reinforces the overall structure against compression.

3.8. Moisture Analysis

Moisture is crucial in determining the strength of 3D-printed clay throughout its printing, drying, and sintering stages [26,50,54]. Proper humidity is vital for successful printing. Too much water makes the material too soft and causes deformation. Too little can lead to nozzle clogs and uneven prints. During drying, moisture levels affect shrinkage [35], and cracking excess water leads to more significant shrinkage and a higher risk of cracks [47], weakening the material. Before sintering, optimal moisture content ensures a denser, more compact final structure, as residual moisture can create internal pores that reduce compressive and flexural strength [67]. Proper moisture control at each stage is key to achieving a stable, durable, high-strength 3D-printed clay structure. The detailed moisture analysis of both the sintered and mixed clay samples is depicted in Figure 13a,b, providing valuable insight into their respective moisture characteristics.

3.9. Visual and Physical Examination

To prepare the mixed clay for printing, hand-form it to fit into the feed barrel. This process also helps eliminate any air bubbles in the mixture [38]. The manufacturer recommends performing a hand-forming step before loading the clay into the barrel. Each of the three mixtures behaves differently during the forming process. The 60%C mixture feels the most solid and requires more force to shape, but it holds its form well and is easy to load into the barrel.

In contrast, the 56%C mixture is very wet and sticky during mixing. It does not hold its shape and is difficult to load into the barrel. Figure 14a–c presents a detailed representation of the hand-formed shape of the mixtures before their insertion into the barrel. The outcomes of the visual examination in different magnifications are illustrated in detail in Figure 15a,b, providing a comprehensive overview of the observed results.

3.10. Imperfection During the Manufacturing Process

The optimal results were achieved by maintaining a pressure ratio between 0.5 and 0.8. This range ensures stable extrusion performance and consistent print quality. A higher pressure ratio indicates that a larger volume of clay is being forced through the barrel and into the extruder tube compared to the amount dispensed through the screw extruder. In practice, a pressure ratio of around 0.65 is typically used for most printing tasks, with adjustments made only when specific conditions require them. Careful regulation of this parameter is essential for balancing efficiency and print accuracy.

If the pressure ratio is too low, the piston does not supply enough material during extrusion, causing gaps in the feeding tube [43,68]. Conversely, an excessively high ratio forces too much clay into the system, particularly problematic for mixtures with higher water content. This can lead to excess material and moisture exiting the nozzle [39], producing overly wet and inconsistent prints [26]. Additionally, clay with elevated water content exhibits higher viscosity, which can obstruct smooth movement through the screw extruder. Under such conditions, the piston may push excessive material into the system, resulting in jamming. This overload increases the risk of motor gear slippage, potentially damaging the extruder motor and barrel, leading to costly repairs and downtime. The effects of incorrect pressure ratios during printing are shown in Figure 16.

Noticeable slumping occurs when printing compression specimens using the 56% clay mixture. This issue originates from the movement of the printing bed along the y-axis, which displaces the clay and results in a skewed alignment in the upper portion of the specimen body, as illustrated in Figure 16c. To minimize this problem, it is advisable to reconsider the slicing strategy and reduce bed movement along the y-axis as much as possible. Interestingly, this phenomenon is less pronounced in specimens with higher clay content [6]. Mixtures with a greater proportion of clay contain a higher volume of solid particles, which enhances both shear and storage moduli. An increased storage modulus substantially improves the specimen’s resistance to external forces such as gravity and shear, producing a more stable and structurally sound product [23,69].

3.11. Discussion

The results of this study demonstrate a clear and consistent trend showing that increasing clay content significantly enhances the mechanical performance, dimensional stability, and overall print quality of 3D-printed kaolin specimens. Both flexural and compressive strength tests reveal a direct and positive correlation between higher clay proportions and improved structural integrity, with the 60% clay mixture yielding the highest recorded mechanical strengths. This outcome highlights the influence of clay-to-water ratio on the densification behavior and microstructural consolidation of printed and sintered clay bodies. A higher solid fraction contributes to a more compact particle network, which strengthens interparticle bonding and reduces void formation, thereby producing specimens with superior load-bearing capacity and reduced deformation.

The enhanced performance at 60% clay content is primarily attributed to three interconnected mechanisms. First, the denser particle packing facilitated by higher clay concentration minimizes internal porosity and enhances stress transfer between adjacent particles. Second, the reduction in water content limits volumetric shrinkage during drying and sintering, mitigating crack propagation and geometric distortion. Third, improved particle cohesion and stronger van der Waals interactions among clay platelets contribute to the increased resistance to both bending and compression forces. These effects collectively produce a more rigid and dimensionally stable material, making the 60% clay formulation the most balanced mixture for achieving both printability and mechanical robustness under the studied conditions.

However, from an optimization perspective, it is important to emphasize that the identified “optimum” clay content of 60% represents an optimum within the specific experimental range (56–60%) investigated in this study. The observed improvement trend between these increments suggests that higher clay proportions—beyond 60%—might yield further increases in mechanical strength. Nevertheless, such an assumption must be approached with caution, as practical considerations related to rheology and extrusion behavior play a crucial role in additive manufacturing. As the clay fraction increases, the reduction in water content correspondingly increases the slurry’s viscosity, potentially making the material too stiff or dry to be effectively extruded through the nozzle. Under such conditions, extrusion pressure must be elevated to maintain flow, which may result in inconsistent deposition, nozzle clogging, or discontinuous layers. These processing challenges directly affect the buildability and surface finish of the printed components.

Furthermore, overly viscous clay mixtures may also exhibit poor interlayer adhesion, as insufficient moisture limits the diffusion of particles across layer interfaces. This can lead to delamination and anisotropic strength distribution in the printed structures. Conversely, if additional water is added to offset high viscosity, the resultant mixture may again suffer from excessive shrinkage, surface cracking, or deformation during drying. Hence, a balance must be maintained between rheological stability, printability, and mechanical performance—a delicate equilibrium that defines the optimal processing window for clay-based 3D printing. While the present work identifies 60% as the optimal clay composition for the tested conditions, it also delineates a transitional threshold between desirable printability and mechanical enhancement. Future research should extend the exploration of clay content beyond 60%, for example at 62% or 64%, to determine whether mechanical performance continues to improve or if extrusion limitations become predominant. Such studies should be complemented by detailed rheological characterization—examining yield stress, viscosity profiles, and thixotropic recovery—to establish quantitative correlations between material flow behavior and printing outcomes. Additionally, systematic investigations on extrusion pressure control, nozzle geometry optimization, and the incorporation of plasticizers or dispersing agents could help mitigate flow resistance and expand the printable range for clay-rich formulations.

From a broader perspective, these findings contribute to the growing understanding of process–structure–property relationships in clay-based additive manufacturing. By revealing how subtle variations in the clay–water ratio govern microstructural evolution and mechanical performance, this work provides essential insights for optimizing feedstock formulations in sustainable and scalable ceramic printing applications. Extending this research to include different clay types (e.g., ball clay, bentonite, or halloysite), as well as hybrid systems combining natural fibers or inorganic additives, could further enhance printability and strength while maintaining eco-friendly material characteristics. Ultimately, such studies will aid in developing predictive models that integrate rheological parameters, extrusion dynamics, and microstructural evolution—thereby advancing clay-based 3D printing toward more reliable and industrially relevant applications in architecture, art, and green construction.

4. Conclusions

This study successfully identified the optimal parameters for 3D printing with kaolin clay and analyzed its physical and mechanical properties through a comprehensive experimental approach. The key conclusions drawn from this research listed as follow:

1.

The ideal clay mixture consists of 60% clay and 40% water, striking the best balance between structural strength, printability, and ease of handling. This composition ensures a smooth extrusion process while maintaining the shape stability of the printed objects.

2.

The highest flexural strength recorded was 1.3125 MPa, achieved with the 60% clay mixture, highlighting the positive correlation between higher clay content and mechanical strength. In contrast, the lowest flexural strength was observed in the 56% clay mixture, showing a decrease of approximately 10.5% compared to the optimal composition.

3.

The highest compressive strength was 6.14 MPa, also obtained with the 60% clay mixture, making it 35% stronger than the 56% clay composition. This improvement is attributed to the enhanced particle bonding and reduced porosity in the sintered specimens with a higher clay ratio.

4.

Shrinkage analysis revealed that specimens with higher water content experienced up to 10% volumetric shrinkage and 14% weight reduction during drying and sintering. This underscores the importance of precisely controlling the clay-water ratio to minimize deformation and dimensional inaccuracies in the final product.

5.

Proper moisture control is essential throughout the printing, drying, and sintering processes. Excess water increases shrinkage, deformation, and the risk of cracks, while insufficient water can hinder extrusion, resulting in poor layer adhesion and nozzle clogging.

6.

The manufacturing process must account for the inherent shrinkage of kaolin-based materials by adjusting the initial design dimensions accordingly. This is crucial to achieving accurate final product dimensions after sintering.

7.

From an optimization perspective, the 60% clay mixture represents an optimum within the tested range (56–60%). Although a higher clay fraction might further enhance mechanical performance, it could also make the slurry too viscous or dry for smooth extrusion. Increasing clay content beyond 60% may lead to processing challenges such as nozzle clogging, irregular deposition, or poor interlayer bonding. Therefore, the present results define a practical balance between strength and printability within the studied parameters.

8.

Future research is recommended to explore higher clay contents (e.g., 62–64%) and to investigate their effects on rheology, extrusion pressure, and nozzle design optimization. Such studies could provide insights into the trade-offs between improved mechanical strength and printability limitations, ultimately expanding the printable range for clay-rich formulations.

These findings provide valuable guidance for optimizing clay formulations in additive manufacturing, particularly for applications requiring high precision, dimensional stability, and mechanical robustness. The study highlights the critical interplay between material composition, rheology, and processing parameters in determining the final performance of 3D-printed clay-based materials. Furthermore, it establishes a foundation for future studies aiming to enhance the processability, sustainability, and structural performance of clay-based additive manufacturing systems.