But not all injection molding systems are created equal. Ceramic manufacturers that have investigated high-pressure injection molding (HPIM), which requires hundreds of pounds per square inch of pressure (psi), have often discounted the feasibility of the technology due to its high equipment, tooling, energy and maintenance costs.
Over the past decade, a low-pressure alternative has been making inroads in the advanced ceramics industry. With a molding pressure of less than 100 psi, low-pressure injection molding (LPIM) is helping manufacturers achieve high-quality components with reduced energy consumption; less expensive, longer-lasting dies; and lower maintenance requirements, along with other benefits. As ceramic components have become increasingly smaller and more complex, new applications for the LPIM technology continue to be discovered. Perhaps some of the most notable advances are in the areas of microcomponents, silicon carbide (SiC) components, and thin and large-cross-section ceramic parts.
LPIM can work with simple and inexpensive molds and is economical for both small and large quantities of parts. The good flowability of the low viscous feedstock, the reduced wear of the tooling and the ability to use fine powders also points to LPIM as a good candidate for the injection molding of microparts. However, the mechanical strength of the wax or paraffin binder used for LPIM is even lower than the polymer binders used for HPIM. For this reason, LPIM is normally limited to microparts with lower complexity.
This problem can be overcome by using customized tooling, such as silicone rubber molds.1 Silicone rubber can reproduce the finest structures, including details at the sub-micrometer range, and can also demold fragile details.2 Due to the elasticity of the material, parts with parallel side walls or even undercuts-which are challenging or impossible to demold with other materials-can be easily withdrawn from the mold cavity.
First, a model of the final part is fabricated with an easily machinable material, such as brass, aluminum, plastic or even wax. The model is enlarged to compensate for the subsequent sintering shrinkage of the ceramic. Micromachining or rapid prototyping methods like micro-stereolithography are used to achieve a resolution in the micrometer range and reduce the production time on the model to a few days. The model is embedded in a two-component silicone rubber, and the finished mold is ready to be used in the LPIM process within a few minutes or hours, depending on the type of silicone used. The entire process chain, including debinding and sintering of the ceramic microparts, lasts just one to four weeks, depending mainly on the time required for the fabrication or supply of the model. This is only slightly longer than the typical processing time required for direct rapid prototyping methods.
Close collaboration with LPIM equipment and feedstock suppliers can help ensure the successful injection molding of ceramic microparts in production operations.
At Instituto de Cer mica de Galicia, Santiago de Compostela, Spain, researchers investigated the feasibility of producing silicon carbide (SiC) components with a high level of detail using the semi-automatic low-pressure molding machine shown in Figure 1, combined with carburization, a method that uses standard furnaces but relatively low temperatures compared to many other SiC fabrication procedures.7
The first step was to produce porous silicon preforms with the appropriate shape. The process of carburization (or eventually nitruration) involves both a weight gain and a volume increase (approximately 7%) that must be accommodated by the preform without deformation. As a result, some remnant porosity is needed on the silicon preforms. The institute had previously determined that relatively coarse grain sizes (200 mesh) present an adequate kinetics during the process of carburization. In this project, the average porosity of the silicon preforms used was about 20% after the binder removal process had been completed.
The institute chose LPIM as the forming method for the SiC components because it is a cost-effective near-net-shape forming process that offers automated production of complex-shaped, thin-walled ceramic parts. Among other advantages, pieces fabricated with LPIM show high levels of isotropy, and the molds are less expensive and last longer compared to HPIM.
To benefit from these advantages in the production of dense SiC ceramics, a suspension with a controlled solids loading and appropriate rheological properties is needed. At the Instituto de Cer mica de Galicia, a variety of processing aids were used to modify the rheological properties of a silicon-wax system. The effect of different fatty acids on the rheological properties of the LPIM feedstock was investigated at different concentrations and temperatures.
The researchers' goal at this stage was to develop slurries with high solids loading and low viscosity. A mixture of Si powder (Elkem Silgraim, 200 mesh, with an average particle size of 12 µm) with a 5% addition of 230 mesh Merck graphite (CAS number 7782-42-5) was prepared. A slurry with an 80% solids loading was obtained by mixing the ceramics powders with Siliplast(r) LP65 paraffin (Zschimmer-Schwarz), with a melting point of 50°C. To achieve this solids loading, it was necessary to deflocculate the slurry by adding different percentages of various fatty acids (oleic, stearic, etc.). Processing conditions for this stage were:
The binder system was removed by thermal debinding assisted by wicking, and the pieces were immersed in a carbon-based powder bed during heating. The thermal cycle consisted of an initial ramp with a rate of 5°C/minute up to 100°C, with a dwell time of 30 minutes; a second ramp at 1°C/minute up to 300°C, with a dwell of 30 minutes; a third ramp at 1°C/minute up to 600°C, with a dwell time of 30 minutes; and a final ramp at 2.5°C/minute up to 900°C, with a dwell of 60 minutes.
This cycle was followed by a "carburization cycle," consisting of a ramp of 5øC/minute up to 1200°C and a final ramp of 1.5°C/minute up to 1350-1400°C, with dwell times from 5 to 10 hours depending on the piece thickness. Finally, the system was cooled to room temperature at a rate of 10°C/minute. The debinding process occurred during this latter heat treatment, completing the carburization.
With the process described here, the institute has been able to produce a variety of different SiC pieces, including rectangular bars, conic crucibles and small nozzles (see Figure 4). The process was simple, fast and inexpensive. Chemical and mineralogical analyses of the components have shown that they are mainly composed of SiC, although small percentages of silicon nitride (Si3N4) and silicon oxynitride are also present.
Through this research, the institute concluded that combining LPIM with a carburization process (or eventually a nitruration process) could be considered as an affordable and efficient technique for producing detailed SiC (or Si3N4) components without shrinkage or deformation.
The ceramic powder used for this study was a submicrometer-sized alumina (Al2O3,/sub>) A-1000SG (Alcoa), with a specific surface area of 9 m2/g, 99.9% purity and a particle size of about 0.4 µm. The binder used in the formulation included only low-molecular-weight waxes.8 The mixture was prepared directly in a semi-automatic low-pressure molding machine (see Figure 1) and consisted of 86 wt.% alumina (dried for three hours at 150°C before use) and 14 wt.% binder (~45 vol.%). The major binder component was paraffin wax, which represented 75 wt.% of the binder. Small amounts of other components, including polyethylene wax, carnauba wax, stearic and oleic acids, were also added to the binder mixture (see Table 2).8 For the debinding of thin ceramic parts (up to 7 mm), a fast ramp with wicking was adequate.8 For the large-cross-section ceramic parts (greater than 10 mm), a fast ramp to 250°C under oxidative conditions caused a hard skin to form on the surface of the parts,9 which interfered with the debinding process. Researchers overcame this challenge by lowering the temperature to 170°C8,9 and extending the dwell time. Very large pieces, with cross-sections greater than 20 mm, needed an even longer dwell time at 170¡C to obtain ceramic bodies free of defects.10 The density of the ceramic parts sintered at 1600°C for two hours, as measured by the Archimedes method, varied from 96 to 98.5% of the theoretical density of alumina.8 The process used in this study allowed the production of high-quality ceramic parts with good reproducibility. The parts exhibited good performance in tests conducted under working conditions. Through this research, the university concluded that the overall process is robust, reliable and adequate for the production of complex-shaped ceramic parts in quantities ranging from 100 to 10,000 units. As ceramic manufacturers search for affordable ways to form smaller, more complex parts with improved capabilities, LPIM offers a solution.
An Advanced SolutionIn addition to the examples cited in this article, other research has confirmed the ability of LPIM to produce high-quality alumina and zirconia fasteners and numerous other ceramic components with good reproducibility and little or no post-processing requirements. These capabilities have been demonstrated both on a small test scale and in large production operations.
For the debinding of thin ceramic parts (up to 7 mm), a fast ramp with wicking was adequate.8 For the large-cross-section ceramic parts (greater than 10 mm), a fast ramp to 250°C under oxidative conditions caused a hard skin to form on the surface of the parts,9 which interfered with the debinding process. Researchers overcame this challenge by lowering the temperature to 170°C8,9 and extending the dwell time. Very large pieces, with cross-sections greater than 20 mm, needed an even longer dwell time at 170¡C to obtain ceramic bodies free of defects.10 The density of the ceramic parts sintered at 1600°C for two hours, as measured by the Archimedes method, varied from 96 to 98.5% of the theoretical density of alumina.8
The process used in this study allowed the production of high-quality ceramic parts with good reproducibility. The parts exhibited good performance in tests conducted under working conditions. Through this research, the university concluded that the overall process is robust, reliable and adequate for the production of complex-shaped ceramic parts in quantities ranging from 100 to 10,000 units.
As ceramic manufacturers search for affordable ways to form smaller, more complex parts with improved capabilities, LPIM offers a solution.