Advancing Components with Low-Pressure Injection Molding

Low-pressure injection molding is leading to advances in ceramic microcomponents and other ceramic parts.

Numerous ceramic manufacturers have faced the challenges of manufacturing advanced components-including the difficulty of forming increasingly small, complex shapes; the high costs of machining; and the problem of quickly and cost-effectively scaling from test to production quantities. Injection molding has long been touted as the solution. Capable of forming net-shaped or near-net-shaped parts in vast quantities with minimal or no additional processing requirements, injection molding has helped manufacturers in a variety of industries gain a competitive edge.

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.

Figure 1. A semi-automatic low-pressure molding machine (Peltsman LPIM Machine MIGL-33).

Microcomponents

Ceramic microcomponents (or microparts) are defined as parts with structural dimensions of a few millimeters and below. One of the most challenging steps in micro-injection molding, as in other micro-replication techniques, is withdrawing the green body from the injection tool. The binders used for HPIM are often not strong enough to allow successful demolding-strong interlocking between the tool and the small wall thicknesses of the parts easily results in the breaking of patterns during the demolding step. Ejector concepts are often too complex or completely unsuitable for the fragile structures. Additionally, due to the high precision required, the tools used for high-pressure micro-injection molding are typically elaborate and expensive to fabricate, making this forming method profitable only for high production levels.

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.

Figure 2. The rapid prototyping process chain (RPPC) used at Institut fuer Materialforschung III to produce ceramic microcomponents.

Institut fuer Materialforschung III, Eggenstein-Leopoldshafen, Germany, is demonstrating how the use of silicone rubber molds combined with LPIM technology can lead to faster prototyping of ceramic components. Using a semi-automatic low-pressure molding machine (see Figure 1), the institute has developed a rapid prototyping process chain (RPPC) that is capable of rapidly and cost-effectively producing ceramic microparts with full functionality, from single prototypes to preliminary or small-lot series (see Figure 2).^3,4

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.

Figure 3a. Alumina test columns produced at Institut fuer Materialforschung III using LPIM.

The process has a reproducibility of 0.3 to 0.5% for many ceramic materials, and details can be reproduced with an accuracy of few micrometers (see Figure 3). Due to the high solids content of LPIM feedstocks, this accuracy is comparable to or even better than conventional forming processes. However, the softness of the mold demands some adaptations of the injection step to ensure a solidification that is free of stresses.

Figure 3b. An alumina charging electrode for an inkjet printing system produced at Institut fuer Materialforschung III using LPIM.

For microparts, the debinding step is less critical than with larger parts due to the small cross-sections, which facilitate the removal of the volatile decomposition products. However, the low viscosity of the LPIM feedstocks makes the debinding procedure more challenging, as the molten binder phase reduces the strength of the green part. Deformation of the part can occur if a sufficiently high yield point does not exist. Gravitational forces and surface tension lead to plastic deformation, which alters the shape of fine detail-from rounding initially sharp edges to completely leveling surface patterns.

Figure 3c. Alumina nozzles produced at Institut fuer Materialforschung III using LPIM.

For large parts, a wicking powder is often used to prevent collapsing and create capillary forces that help remove the excess binder. However, the wicking powder leads to a higher surface roughness by producing indents and adhering particles.⁵ While the surface of large parts can be improved by finishing, this option is not available for microparts that have to be applied as-sintered.

For that reason, an adapted feedstock rheology and a highly absorptive support are essential for the debinding of microparts (see Table 1). It is important to note, however, that while deformation has a negative effect on the shape of the component, it can be used to improve mechanical properties or eliminate subsequent processing steps in some cases. For example, the smoothing and edge rounding that can occur during debinding eliminates surface defects comparable to the fire polishing of glasses. On smoothed zirconia microbars with a square cross section (approximately 200 x 200 µm) and rounded edges, a tensile strength of more than 3000 MPa was measured in three-point bending experiments.⁶

Close collaboration with LPIM equipment and feedstock suppliers can help ensure the successful injection molding of ceramic microparts in production operations.

Figure 4. SiC components produced at Instituto de Cer mica de Galicia using LPIM and a carburization process.

Silicone Carbide Components

Silicon (Si) is undoubtedly one of the most important materials used in semiconductor devices for electronic and photoelectric applications. However, the use of this material in structural applications has been restrained, at least in part, by a lack of favorable mechanical properties and geometrical freedom due to process-related material characteristics.

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:

Mixing tank temperature: 75°C
Mixing time: 2.5 hours
Deairing time: 20 minutes
Pipe temperature: 70°C
Mold support temperature: 70°C
Injection pressure: between 15-70 psi, depending on the specific piece dimensions
Injection time: between 2 to 10 seconds, depending on the specific piece dimensions

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.

Thin and Large-Cross-Section Parts

Successfully producing thin and large-cross-section ceramic parts using LPIM requires a powder and binder mixture that is optimized for high fluidity, along with an advanced debinding process. Researchers at the Universidade de Caxias do Sul in Caxias do Sul RS and the Universidade Federal do Rio Grande do Sul in Porto Alegre, Brazil, have developed a promising solution.

The ceramic powder used for this study was a submicrometer-sized alumina (Al₂O_{3,/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}

Figure 5. Thread guides for the textile industry produced at Universidade Federal do Rio Grande do Sul using LPIM with a specialized binder and an advanced debinding process.

These components were mixed for 20 hours at 90°C, and the resulting mixture was then injected into brass molds at 90°C and 400 kPa of pressure for 12 seconds. Most of the ceramic parts molded with this mixture were thread guides for the textile industry (see Figure 5). After injection molding, the ceramic parts were immersed in alumina powder for debinding (wicking). The powder bed consisted of the same alumina used to make the parts, thereby providing a physical support for the ceramic bodies and preventing major distortions during debinding.^9,10

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°C^8,9 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.

An Advanced Solution

In 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.

As ceramic manufacturers search for affordable ways to form smaller, more complex parts with improved capabilities, LPIM offers a solution.

For more information about LPIM, contact Peltsman Corp., P.O. Box 27484, Minneapolis, MN 55427; (763) 544-5915; fax (612) 871-5733; e-mail mp@pelcor.com ; or visit http://www.pelcor.com .