Improvement of Processability Characteristics of Porcelain-Based Formulations Toward the Utilization of 3D Printing Technology

Abstract

A study of the feasibility of porcelain-based formulations for 3D printing was performed. Based on commercial materials characterization, the binder jetting process properties requirements were defined. Porcelain powder-based formulations were prepared and evaluated with different binder solutions. The powder-binder formulations were characterized (e.g., particle size distribution and wettability of powder, viscosity and surface tension of liquid binder) and showed some different and similar characteristics when compared with commercial materials. The addition of solid (sodium alginate, sucrose) and liquid (glycerol, ethanol) additives in the powder-binder composition improved the experimental printed tests. The effect of binder composition and operating process parameters (binder saturation level, bleed compensation, and printed layer thickness) was analyzed and optimized to obtain a printed saucer with different designs. Results revealed some limitations related to the materials and the technology, thus justifying the introduction of technological improvements. This study showed the possibility to process industrial porcelain powders by additive manufacturing, paving the way for a new development challenge in the productive process of ceramic products.

Introduction

The next generation of digital manufacturing can help the adaptability of companies to meet the divergent needs of society. Technological innovation establishes new paradigms of creating, making, and selling products, whereas transformations of scaling additive manufacturing from prototype to production are happening in some industrial sectors. Emerging technologies, including 3D printing (3DP), provide to the ceramic industry the production of the innovative free form without dedicated tooling, with reduced costs and lead time. New porcelain products can open up new markets with a high potential for “mass customization.”¹

The 3DP is a powder-based additive manufacturing technique based on the binder jetting process.^2,3 A suitable choice of binder and powder is very important for the final quality of 3DP parts, with the printing resolution being mainly affected by the relationship of layer thickness and printhead resolution. There are specific powder and binder characteristics that can be relevant to this issue, such as flowability, particle size and morphology, binder droplet size, viscosity, and surface tension. Several studies are reported in the literature on the use of solid and liquid additive agents to adapt different materials to the commercial 3DP equipment, including for ceramic powder formulations.^4–17

Miyanaji et al.^18–21 used different additives to improve the flowability and mechanical performance of porcelain 3DP parts for dental applications. High powder flowability allows the formation of homogenous layers and facilitates the depowdering. Moghadasi et al.²² and Miao et al.²³ evaluated the effects of particle size on bed powder and final properties of ceramic 3DP parts. The binder jetting process requires not only good flowability, good spreading of powders but also stable layers and good compaction of the manufacturing powder bed.

The particle morphology affects flowability and powder packing density, consequently interfering in the final properties (density and mechanical properties) of ceramic parts.^24–26 Beyond powder characteristics, the binder characteristics, operating parameters, and postprocessing are the main factors that have been understudied to achieve the required final properties.^{3,19,20,22,23,25,27–32}

Porcelain is a traditional ceramic material that is distinguished by its density and high mechanical strength. Porcelain compositions are based on kaolin, clay, feldspar, and quartz.^33–35 Conventional techniques are available to produce porcelain products through three different physical states of raw materials: (1) suspension, (2) pastes, and (3) powders. These spray-dried powders are commonly used in the tableware ceramic industry to produce planar and dished simple shapes (e.g., plates) through the powder pressing technique.^36–38 The 3DP could bring an alternative way to transform porcelain powders to obtain complex three-dimensional shapes without form limitation.

One of the challenges in porcelain 3DP is to develop an adequate powder linkage of green parts to support all the following operating steps (depowdering and heat treatment/binder infiltration steps). To achieve that requirement, different binders may be incorporated into the liquid composition, as a binder applied by the print head, or into the solid composition of the powder, as a constituent of the powder. Basic and acid aqueous solutions can promote a physical aggregation of powder particles through a mechanical interaction.

Chemical approaches, thermoset resin binders, are able to promote a stronger linkage but are not very feasible with common used commercial jetting devices. After 3DP, green parts can be submitted to a debinding process to remove the binders and to provide the porcelain final properties with thermal treatments. Another postprocessing strategy uses infiltration techniques of binders to increase the mechanical strength of the green parts. Some organic components, as thermosets resins, are applied to the green parts and, after resin cure-time, the parts obtain the final properties.⁹

The present study aims at exploring the possibility of using industrial porcelain in the 3DP technique, thus contributing to the development of knowledge in the advanced manufacturing of 3D porcelain parts. Experimental design focuses on: (1) studying the feasibility and applicability of industrial porcelain powders for 3DP of porcelain parts with intent commercialized products with a new technology; (2) developing porcelain powder and binder formulations, with adequate solid and liquid additives, without changing the inorganic composition of porcelain; (3) studying powder-binder properties (e.g., particle size distribution, viscosity) that are relevant for 3DP processing; and (4) studying the operating parameters (e.g., binder saturation) that lead to an adequate fabrication of porcelain parts by 3DP.

This work opens up the possibility to create the required tools to integrate this technology into the industrial ceramic fabrication process targeted to free forms of small production and customized products.

Materials and Methods

Commercial powders and binders characterization

Commercial materials were analyzed to identify reference requirements to prepare new materials. The commercial powders used in 3DP marketed equipment were characterized by different techniques. The crystalline phases were identified by X-ray diffraction (XRD) (Rigaku Geigerflex Dmax-C, Japan, equipment with a CuKα radiation source), and the thermal properties were assessed by differential thermal analyses (DTA) and thermogravimetric analyses (Seteram Labsys 1600 DTA/TG-DSC, Switzerland, heating rate of 5°C/min from 75°C to 1400°C in nitrogen).

The average particle size and distribution were performed by Coulter technique, and the morphology and chemical composition were assessed by scanning electron microscopy (SEM) (Hitachi, SU-70, Japan) with coupled energy dispersive spectrometry (Bruker, Quantax 400, USA). The powder flowability was determined with a glass funnel (stem diameter of 30 mm and funnel opening diameter of 130 mm), by determining the time spent to flow 100 g of powder.

The specific surface area was determined by physical adsorption of nitrogen gas by the Brunauer–Emmett–Teller (BET) method (Micromerites, Gemini 2370V5, USA). Wettability was evaluated by a droplet method to account for the non-spherical shape and non-mono-size of the particles.^39–41 Powder sample disks with 20 mm diameter were pressed with 500 kg. These were used as an optical contact angle meter and interfacial tensiometer (KINO, SL200HT, USA) equipped with CAST3.0 software. The powder samples disks were tested at room temperature with drops of 2 μL of distilled water.

Liquid commercial binders were also analyzed in terms of composition by Fourier transform infrared spectroscopy (FTIR) (Shimadzu, IRPrestige-21, Japan). The viscosity and surface tension of the binders were determined by using, respectively, a viscometer (Brookfield Viscometer, DVE-LV, Canada) and a tensiometer (Attension, Sigma 700). These analyses aim at identifying reference values for development binder formulations, for porcelain powders, compatible with printhead requirements of 3DP equipment.

Powder and binder formulations

Spray-dried industrial porcelain powder from a ceramic company was used in the present study. The porcelain powder was calibrated and characterized by several techniques. To achieve an efficient binder jetting with an adequate distribution, the porcelain powders were sieved to obtain an average particle size smaller than 150 μm and near to the commercial powder. Sodium alginate (Sigma-Aldrich, USA) was also sieved under 63 μm and was added to the porcelain powder formulation in a percentage of 10 wt% as a solid additive.

This powder formulation was designated by P10Al. The average particle size was determined by using a Coulter Counter equipment (Coulter LS Particle Size Analyser 230, USA). The P10Al was also characterized by XRD and SEM. The flowability measurements were also carried out to predict the powder behavior deposition of homogeneous layers. The P10Al powder wettability ability for the 3DP process was measured by contact angle. The results of P10Al characterization were compared with commercial powder to evaluate their 3DP processing adequacy.

An aqueous binder solution with sucrose was prepared and used in the commercial 3DP equipment. The effect of additional liquid agents such as ethanol and glycerol (Table 1) was also tested. Different binders were formulated and optimized to improve the green linkage of printed powder. Some binder properties were evaluated to ensure the compatibility with the print head. The viscosity, surface tension, and pH were analyzed and compared with the commercial binder.

Table 1.

Composition of Binder Formulations

Material	binder 1 (wt%)	binder 2 (wt%)	binder 3 (wt%)	binder 4 (wt%)	binder 5 (wt%)
Sucrose	15	15	15	15	15
Glycerol	2	4	4	8	8
Ethanol	2	2	4	4	8

Additive manufacturing

The 3D digital models were designed by using computer-aided design (CAD) software (SolidWorks^®) and converted to STL files for 3DP. Operating parameters were controlled in the 3DPrint software. The powder and binder formulations prepared were tested in commercial binder jetting equipment (ProJet^®360—3D System, USA). To optimize and select the most adequate binder formulation for P10Al, different aqueous solutions (Table 1) were tested with standard operating parameters for the commercial powder. The binder that promoted the best esthetic and mechanical results of green saucer printed was selected. The effects of some operating parameters (binder saturation, layer thickness, bleed compensation) were also evaluated with additive manufacturing tests of saucer models with different designs.

Two CAD models of prismatic samples (80 × 10 × 4 mm and 10 × 10 × 5 mm) were prepared for printing standard samples (with commercial materials) and porcelain samples (with formulated—P10Al and previously selected binder) for different physical and mechanical characterization. The manufacturing of the samples always followed the same build orientation (samples were 0° relatively to the machine x-axis) in 3DP machine coordinates: length in the x-axis, width in the y-axis, and thickness in the z-axis.

Postprocessing and printed parts characterization

The printed standard samples were infiltrated, with commercial cyanoacrylate (Z-Bond, 3D System, USA) used for infiltration of printed plaster models. In the case of P10Al printed samples, it was used as conventional heat treatment but without glaze. A thermal treatment reproducing the industrial schedule (first firing: 1030°C at 5°C/min and 2 h of dwelling time and second firing with glaze: 1350°C at 5°C/min and 2 h of dwelling time) was applied to the porcelain printed models.

Morphological surface characteristics of the sintered and glazed saucer model were analyzed by SEM and compared with those obtained by the conventional powder pressing technique. The dimensions deviations were evaluated by measuring the length (x-axis), width (y-axis), and thickness (z-axis) of the printed samples in relation to the project CAD dimensions. The shrinkage was calculated through dimensions measurement.

The mechanical performance was also assessed by flexural tests, according to ASTM C1161—13,⁴² in a universal mechanical testing machine (Shimadzu, AG-IS model, 10 kN maximum load cell, Japan). The standard and P10Al samples were loaded in a three-point bending setup with a span of 30 mm and a test speed of 1 mm/min. The maximum flexural stress (σ_fM) was determined on plots of flexural stress (σ_f) versus flexural strain (ɛ_f) from the following equations:

$${\sigma }_f=\frac{3FL}{2b{d}^2},$$ (1)

$${\delta }_f=\frac{6Dd}{L^2},$$ (2)

where F is the applied force, L is the span length between supports, b is the width, d is the thickness, and D is the deflection of the sample.

The density (ρ) was determined by the mass-to-volume ratio. A digital balance (±0.001 g precision) and a micrometer (±0.005 mm precision) were used. The percentage of water absorption (%WA) was calculated through the following equation:

$$WA=\frac{ms-md}{md},$$ (3)

where ms is the saturated mass, and md is the dry mass. The apparent porosity (%AP) was evaluated by Archimedes method and calculated by the equation:

$$AP=\frac{ms-md}{ms-ma},$$ (4)

where the apparent mass (ma) was measured with the sample submerged in water. The determination of ρ, %WA, and %AP was performed according to ISO 10545-3:2018.⁴³

Results and Discussion

Properties requirements

Commercial powers characterization

Powders usually used in 3DP technologies are composed of hydrated calcium sulfate and other solids additives (Fig. 1a, b). This sulfate develops a stable reaction with a binder, promoting a fast and strong linkage between the powder particles. The commercial powder shows a bimodal particle size distribution with a large volume percentage around 20–100 μm and an average size of ∼47.8 μm (Fig. 1c, d), similar to the results reported with commercial powder.⁴⁴ These physical characteristics affect some technical parameters of the process (e.g., layer thickness, flowability, uniformity of powder bed) and the part resolution.⁴⁴

FIG. 1.

Reference properties of the commercial powder: XRD and chemical elements identified by EDS (a), thermal properties—DTA/TGA (b), average particle size and distribution—Coulter technique (c) and morphology— SEM (d). X-ray diffraction, (e, f) wettability-contact angle analyses. DTA, differential thermal analyses; EDS, energy dispersive spectrometry; SEM, scanning electron microscopy; TGA, thermogravimetric analyses; XRD, X-ray diffraction.

The shape, size, and distribution of powder influence the part porosity. The commercial powder exhibits a flowability of 3.22 s and a specific surface area of 0.9851 m²/g. An adequate powder flowability allows the preparation of homogeneous layers at the start of the process and simplifies the depowdering steps at the end of the process. Thin particles increase the part resolution and decrease the part porosity but are more prone to develop particles agglomeration that inhibit the flowability.^8,12 The measurements of commercial powder wettability (Fig. 1e, f) show a hydrophilic material because of its hydrated composition. The results show a complete wetting (spreading) of the powder corresponding to a contact angle of 0°.

This powder demonstrated a fast interaction with the binder with a very low absorption time of 0.27 s. The binder jetting process involves the production of layers through a reaction of the binder droplets with the powder particles. This prompt reaction helps to follow the layering process.

Commercial binder characterization

The commercial binders are composed of 75–95 wt% of H₂O (Table 2) and a remaining content consisting of a humectant, surfactant, and other compounds.^29,45

Table 2.

Typical Composition of Commercial Binder^4,33,42

Composition	% wt
Glycerol	1–10 or <5.27
preservative (sorbic acid salt)	1–5
Surfactant	<1
Salt	<1
Pigment	1–10
H2O	75–95

The FTIR spectrum (Fig. 2) evidences the presence of glycerol and H₂O in the composition of the binder. The individual FTIR analyses of constituents (glycerol and water) help to identify the spectrum peaks of the binder. Compared with the FTIR spectrum of the original commercial binder, there is an evident presence of water in its composition. To identify the glycerol, heat treatment was performed to eliminate the water. After this treatment, the FTIR spectrum of the heated commercial binder is similar to the spectrum of glycerol.

FIG. 2.

Individual FTIR analyses of constituents of commercial binder (a) and FTIR analyses of commercial binder (b). FTIR, Fourier transform infrared spectroscopy.

The main binder requirements are based on the interaction of the binder versus powder and the compatibility with operating equipment devices. The binder composition should promote a stable and strong linkage with powder to ensure adequate green mechanical properties. The piezoelectric print head has some hundreds of micro nozzles, each one around 24 μm diameter, where an aqueous binder is deposited on powder to selectively join the particles.⁴⁶ The binder composition can include aqueous solutions, solvents, and oils monomers but cannot expel large particles (>1 μm) to not block the micro nozzles.⁶

The binder should have adequate properties (e.g., viscosity, pH, and surface tension) to ensure the proper functioning of the print head and the droplet deposition in the powder layer.⁴⁴

The surface tension of the commercial binder was 33.15 ± 0.01 mN/m (at 20°C). It is reported that for 3DP the surface tension of the commercial binder should be in the range 33.5–40 mN/m, values below that for water (50 mN/m).^{6,12,16,45,46} This lower surface tension value aims at promoting a better flow and a stable droplet formation.^16,46 The measured viscosity was 1.2–20 Pa.s at 0.84–0.06 s⁻¹ shear rate, similar to the results reported by Shirazi et al.,¹² which is important in the drop formation mechanism. Note that shear thinning performance of the binder can lead to satellite drop formation, affecting the geometrical accuracy of the printed parts.^12,47 The pH 9.2 obtained indicated that the solution is alkaline basic.

Powder and binder formulations

Powder formulations

The industrial porcelain powders of this study show a typical spherical shape resulting from the spray drying process that they were subjected to (Fig. 3). In industry, spray-dried powders are mostly spherical with a few doughnut shapes and some dispersed fine irregular particles.³⁷ Usually, in industrial processes such as powder pressing, these powders are directly used as obtained and their sizes can vary in the range 20–500 μm.^36–38 In the present study, powders were obtained by atomization in a bi-fluid nozzle (pneumatic nozzle) spray-dryer. These powders exhibited particle sizes below 200 μm in a bimodal distribution with a greater volume around 10 to 80 μm (Fig. 3).

FIG. 3.

Particle size distribution of spray-dried porcelain powders (a) and SEM micrographs of sodium alginate (b) and P10Al formulation (c).

After sieving powders (below to 150 μm), the porcelain powders were mixed with a solid organic binder. Industrial porcelain powder is an inert material. It was necessary for the use of additives to provide a medium to interact with the binder in the binder jetting process. As the piezoelectric mechanism system of the print head in the 3DP equipment is very sensitive and tends to fail with some liquid additives, we tested solid additives to complement the liquid binder (Table 1).⁶ Within this context, sodium alginate was tested as the solid binder in the powder bed.

Sodium alginate particles appeared with an irregular sharp-edged morphology (Fig. 3b) and were added to the porcelain powders in a low volume percentage to avoid high porosity after heat treatment of the final porcelain parts.¹² Grant et al.⁷ also applied this solid additive strategy to 3DP ceramic parts to improve the green mechanical properties. The solid additive is an alternative of binder options more frequently used, such as a liquid-based binder. Ceramic powders with maltodextrin and sugar were also tested in 3DP in powder form with a particle size below 150 μm.^7,8

The porcelain-sodium alginate powder formulations P10Al are composed of irregular and spherical particles (Fig. 3c) with a bimodal distribution (Fig. 3a) close to that of the commercial powder (Fig. 1c). The P10Al powder formulation presents a higher flowability (1.38 s) compared with the commercial powder (3.22 s). The morphology of commercial powder shows more resistance to flow by the irregular and planar contact of particles. In the case of P10Al formulations, the presence of spherical shapes promotes an apparent decrease of flow times even with the presence of fine irregular particles.

Spherical shapes usually promote good flowability for spreading the powder layers, and planar and irregular shapes support better stability of the layers. Some authors^24–26 refer to the same behavior. Figure 3c of P10Al formulation shows these required morphologic characteristics, presenting a particle distribution with fine and large sizes. The combination of different morphologies and different sizes allowed stable and compact powder layers that are suitable for binder jetting.

Non-spherical shapes and small sizes affect the flowability due to the mechanical interlocking and some possible agglomeration (particles size <10 μm). Spherical shapes and larger sizes promote the increase of flowability and are easier to displace from the powder bed.²⁴ Powder mixtures and bimodal distributions showed to be more suitable to obtain an adequate flowability and powder packing that results in better green and sintered parts properties.²⁵

The XRD results (Fig. 4) confirmed the presence of the crystalline phases kaolinite, albite, quartz, calcite, and dolomite, which are in agreement with the porcelain crystalline phases found in other studies.^48–50 The XRD quantitative analyses showed that P10Al formulation mainly consisted of 25 wt% of quartz—Silicon Oxide phase (SiO₂), 11 wt% of kaolinite—Aluminum Silicate Hydroxide phase (Al₂Si₂O₅(OH)₄), and 64 wt% of muscovite—Potassium Aluminum Silicate Hydroxide phase (KAl₃Si₃O₁₀(OH)₂).

FIG. 4.

XRD pattern of P10Al powder formulation. XRD, X-ray diffraction.

For sodium alginate, three crystalline peaks at 2θ values of 13.7°, 23°, and 40° are associated with an interaction between the alginate chains through intermolecular hydrogen bonding.^51,52 However, in the P10Al formulation, these peaks are also identified but not so obvious because of low intensity compared with porcelain peaks.

For 3DP, a very important property of the powders is wettability. The powder wettability can affect the fluid displacement and saturation in each powder layer. The binder droplets are deposited into the powder layer through mechanisms of capillary attraction and gravity. The analyses showed that P10Al powder formulation is a hydrophilic material with a contact angle of 0°. As shown in Figure 5, the binder droplets totally penetrated the P10Al powder with a higher absorption time (2.01 s) than for the commercial powder (Fig. 1e, f). Absorption times with similar values were also found in other studies with powder clay mixtures.⁴¹

FIG. 5.

Wettability of P10Al powder (a–c) observed by contact angle analyses.

The connection of the liquid with the porous surface layer is dependent on the powder size and morphology that influence the absorption mechanism and the fluid displacement. The P10Al powders exhibit a bimodal distribution such as the commercial powder but with large irregular and spherical particles. In addition, a wide number of fine porcelain and additive particles is present in P10Al, which, when agglomerated, offer more resistance to the binder propagation, promoting the increase of the absorption time. This is a typical behavior with formulations containing fine particles.⁵³

Binder formulations

Aqueous binder solutions with sucrose, glycerol, and ethanol were prepared and used in commercial 3DP equipment. Soluble solid additives (sucrose) were used to prevent clogging of the print head. Glycerol acts as a lubricate and an emollient agent. It was added to decrease the surface tension of the formulated aqueous solution.^14,54,55 Glycerol improves the wettability and bond of binder versus powder, but it cannot be applied in high quantities as it may clog the fluid system.

An active dehydrating compound (ethanol) was used to prevent excess wettability and ensure better control of dropping in the powder bed.^12,16,56 The surface tension values (Table 3) of the formulated binders (Table 1) showed that the presence of ethanol allows the decrease of surface tension. The surface tension value obtained for binder 5 (with 8% ethanol) 39.39 ± 0.55 mN/m (at 20°C) is near the commercial binder and is within the advisable range of values for this type of 3DP inks.^12,16,45,46

Table 3.

Properties of the Formulated Binders

Material	binder 1	binder 2	binder 3	binder 4	binder 5
Surface tension (mN/m)	40.79 ± 1.00	42.45 ± 0.63	40.65 ± 0.65	41.41 ± 0.64	39.39 ± 0.55
Viscosity at 0.84–0.06 s−1 shear rate (Pa.s)	2.1–27	2.4–30	2.0–28	2.1–30	1.8–30
pH	7.9	7.6	7.6	7.8	7.8

The viscosities of the formulated binders (Table 3) are slightly higher than those of the commercial binder, with a value of 1.2–20 Pa.s for the same shear rate. The average pH 7.7 of the formulated binders (Table 3) indicates more neutrality of the aqueous solutions compared with the pH 9.2 for the commercial binder. Sucrose does not have the chemical ability to change the concentration of protons (H+) or hydroxide ions (OH–) in the solution and the addition of glycerol and ethanol was not expected to produce significant pH changes, as well.

Additive manufacturing

A saucer digital model was printed with different binders (Fig. 6) aiming at selecting the binder formulation that most adequately ensured a proper droplet deposition and an effective linkage of P10Al particles. The presence of a solid additive in the powder formulation improved the printed parts, but the binder formulation revealed to be a determining factor in the process. In the case of ceramics, the binder is commonly a nonpermanent solution method for particle bonding.

FIG. 6.

Printed saucers with P10Al powder and different formulated binders: binder 1 (a), binder 2 (b), binder 3 (c), binder 4 (d), and binder 5 (e, f).

After the printing process, the parts need to be sintered and the liquid and solid binders are removed during the heat treatment. A high binder concentration promotes higher porosity and the mechanical behavior can be impaired.¹² Within this context, a balance between standard values and the performance of the developed materials was established in this study.

The printed tests show better geometry accuracy with binder 5 (Fig. 6e, f). The presence of ethanol in the same proportion of glycerol allowed a higher control of binder droplets and promoted better geometrical accuracy but did not significantly improve the green mechanical resistance. Green printed saucers were obtained, but the adhesion of powder layers inside of parts was generally weak and unable to resist the depowdering postprocessing step.

To increase the mechanical resistance of green printed parts, some process parameters were calibrated. Based on standard operating parameters, new values were established for binder saturation in the shell and core of the printed area, powder layer thickness, and bleed compensation function. The process parameter calibration is dependent not only on the physical and chemical powder characteristics but also on the geometrical model characteristics. Some experimental tests with a variation of operating parameters were performed with P10Al and binder 5 in different models to analyze the cause–effect relationship.

The binder deposition affects the geometric accuracy of printed parts. The 3DPrint software automatically defines for each layer the contour droplet deposition (shell) and the inside droplet deposition (core) (Fig. 7a). In the core, the head printed does not deposit binder droplets in all project areas of the part so it is necessary to optimize the saturation that connects a maximum of ceramic powders. High binder saturation levels can promote a binder spreading out of the required projected printed area, affecting the printed part accuracy.

FIG. 7.

Printed saucers [(c) saucer with fine thickness and (d) saucer with large thickness] with P10Al powder and binder 5 with different saturation levels on core and shell (a) to avoid deviations of printed layers (b).

When the wettability of the powder is too high, the powder layer spreading is not homogeneous and deformation of printed layers occurs (Fig. 7b). In contrast, insufficient binder saturation leads to a lack of connection between printed layers, resulting in a decrease of mechanical resistance of green printed parts (Fig. 6). Two different saucers models were printed with P10Al and aqueous solution with binder 5. Different binder saturation levels were applied to obtain different saucers without a break.

The saucer with the finest thickness was printed with a saturation level of shell = 100% and core = 200%. In the saucer with a larger thickness, it was necessary to increase the saturation levels (shell = 200% and core = 300%) (Fig. 7c, d). The increase of the saturation levels was decisive to increase the binder dispersion on the print area and control the powder wetting.

Model geometries involving larger thickness require higher binder saturation values to ensure wettability and bonding between layers, improve the particles bond, and enhance the mechanical resistance of green printed parts. The minimum thickness available (0.101 mm) was also parameterized to promote binder diffusion between powder layers. The bleed compensation parameter was activated in all tests to ensure better accuracy in the deposition of the binder droplets in each layer of the projected area, avoiding spreading outside the model contour.

The printed saucers showed a better “green” mechanical resistance compared with previous tests (Fig. 6) but they are porous. The porosity is a typical characteristic of binder jetting processes that involve inert powdered materials.

Postprocessing and printed parts characterization

To increase the density of the printed parts, two conventional heating treatments were used. The first heating treatment was performed at 1030°C (first firing), and the second heating treatment was performed at 1350°C (second firing). In each heating treatment, a 120 min step was used on heating and cooling rates controlled. After the first firing, the biscuit saucers were glazed and submitted to second firing. The surface of the sintered printed saucers was observed in SEM and compared with saucers produced by the conventional porcelain powder pressing technique.

The intrinsic porosity and consequent mechanical resistance of the printed saucers was improved with postprocessing steps. Figure 8 showed a higher porosity of saucers obtained by 3DP compared with a conventional technique. The porosity of the biscuit printed saucer (Fig. 8a) promotes a higher depth penetration of glaze, resulting in a surface that is less permeable (Fig. 8b) than the saucer obtained by the powder pressing technique (Fig. 8d).

FIG. 8.

SEM micrograph of surface of saucers produced by 3DP technique (a, b) and powder pressing technique (c, d) with first heating treatment (1030°C) (a, c) and second heating treatment (1350°C) (b, d). 3DP, 3D printing.

The dimension deviations of prismatic samples of commercial materials and porcelain formulated materials were compared with the projected CAD dimensions (Table 4). For both types of samples, Table 4 shows a higher dimensional deviation in the z-axis compared with the x-axis and y-axis. This less precision on the z-direction is also reported in the literature.⁵⁷

Table 4.

Physical and Mechanical Characterization of Standard Samples and P10Al Samples After Postprocessing Steps

Samples	Dimensional deviations (%) (digital part—green body)			Dimensional deviations (%) (green body—postprocessing)			σfM (MPa)	ρ (g/cm)	WA (%)	AP (%)
	x-axis	y-axis	z-axis	x-axis	y-axis	z-axis
Standard samples	0.21 (± 0.06)	−1.23 (± 0.40)	5.21 (± 2.99)	0.11 (± 0.06)	0.91 (± 0.38)	0.44 (± 0.36)	31.76 (± 3.18)	1.48 (± 0.03)	—	—
P10Al samples	0.07 (± 0.30)	0.87 (± 1.93)	−5.33 (± 7.98)	−27.14 (± 0.68)	−29.47 (± 1.19)	−22.50 (± 8.22)	14.23 (± 3.7)	1.60 (± 0.13)	12.99 (± 1.93)	23.89 (± 2.80)

Dimensional deviations between printed model and CAD model, shrinkage after postprocessing (infiltration or heat treatment).

AP, apparent porosity percentage; CAD, computer-aided design; σ_fM, maximum flexural strength, ρ, density; WA, water absorption percentage.

The weak bonding strength between particles is one disadvantage of the 3DP process. When compared with other additive manufacturing processes, the mechanical performance is typically lower.^44,58 After printing, the samples are dried and depowdering steps are made. As the green mechanical resistance of samples is low, postprocessing steps are required to improve their strength. P10Al samples were sintered (without glaze), and standard samples were infiltrated with commercial Z-Bond.

These postprocessing steps help to eliminate the voids of the printed parts, increase the density and, consequently, mechanical properties.⁵⁸ In the case of the sintering method, the P10Al samples had an expected shrinkage (linear isotropic shrinkage of −26.37% ± 3.5%) that had not occurred in the case of commercial infiltrated samples (0.49% ± 0.4%) (Table 4).

The mechanical performance of the postprocessing samples was evaluated through bending tests. The flexural strength of standard samples was higher than that of P10Al (Table 4) and even higher than those reported in the literature for the same commercial materials or gypsum powder formulations.^55,59 It was not possible to determine the water absorption percentage and apparent porosity percentage of standard samples due to its degradation behavior in hot water.

Regarding the porcelain, the values for flexural strength and density were lower and the values of water absorption percentage and apparent porosity percentage were higher than the values for porcelain manufactured by slip casting and 3DP.^21,50,60 The intrinsic porosity of 3DP parts was improved by heat treatment but was not enough to get the standard values of industrial processes. The investigation of lighter tableware ceramic by controlling the porosity was reported.

The results demonstrated a reduction of weight parts and lower mechanical properties.⁶¹ The results achieved through the new process strategy—3DP explored in the present study—could also contribute to the production of these lightweight products.

Conclusion

The present study showed the possibility of manufacturing porcelain products by the 3DP technique. Based on the examination of commercial materials, new powder and liquid formulations were developed to improve the mechanical behavior of porcelain parts, particularly before sintering. The inert characteristic of the porcelain formulations represented a relevant limitation for the utilization of 3DP technology. In order to improve the processability and bond connection of powder and binder, new formulations were developed by using some solid and liquid additives (sodium alginate and sucrose).

Based on the production of different saucers models, the importance of adapting the operating process parameters toward the achievement of enhanced green mechanical resistance and better accuracy of printed parts was shown. The postprocessing steps improved the mechanical behavior of the printed samples. The P10Al samples presented a flexural resistance of 14.23 ± 3.7 MPa and a linear isotropic shrinkage after heating treatment of −26.37% ± 3.5%. Water absorption was 11.48% ± 1.51%, and apparent porosity was 23.89% ± 2.80%. The P10Al printed models exhibited higher porosity and weaker bending strength than the models obtained by conventional processes. This study opens horizons in the field of industrial design on ceramic artefacts.

Author Disclosure Statement

No competing financial interests exist.

Funding Information

This work was developed within the scope of the PrintCer3D project—Rapid manufacturing of porcelain products by three-dimensional printing, with reference number 33988, financed by Fundo Europeu de Desenvolvimento Regional (FEDER), through COMPETE—Programa Operacional Factores de Competitividade (POFC). This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020 and UIDP/50011/2020, financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement.