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Rapid Prototyping with CeramicsUsing Injection Molding and Stereolithography
Rapid Prototype and Manufacturing Institute
Materials Science and Engineering
IntroductionAs design and development engineers continue to compress the product and production life cycle, there is a need to extend rapid prototyping from the polymer materials that aided its development, to other materials involved in complex systems. Extending rapid prototyping to ceramic products is difficult due to their inherently high melting temperatures and resulting processing difficulties associated with sintering and shrinkage. Technologies such as Fused Deposition Modeling (FDM) and Laminated Object Manufacturing (LOM) have shown initial promise, but have not met the mechanical property requirements or surface finishes demanded of advanced ceramic products. By combining rapid tooling techniques and ceramic injection molding, it is possible to produce functional ceramic prototypes in one week.
Technical Ceramics and Their ApplicationsIndustrial uses of ceramics represent an $11 billion market (1) . Applications include automotive spark plugs, insulators, connectors, cutting tools, textile thread guides, wear parts, nozzles, heat engine parts, cores for investment casting, catalytic and electronic substrates, filters, sensors, and biomedical implants as shown in Figure 1. Many materials systems are commercially available, some of the more common being aluminum oxide, zirconium oxide, silicon oxides, carbides and nitrides, titanium oxide, as well as combinations of these materials.
Ceramic Injection MoldingCeramic components are formed by consolidating high purity powders into the desired shape and densifying it at high temperatures. Because of their high hardness, it is almost always cost prohibitive to form a component by removing material from a solid ceramic block or shape. Some of the most common shape forming processes used in ceramic manufacturing include hot or cold pressing, casting, extruding, and injection molding. Binders and other additives are required to impart flowability to the powders during processing and to give the part strength for handling in subsequent processing steps.
Ceramic injection molding is a ceramic forming technology that has been derived from plastic injection molding, promising to produce complex parts to near net shape in high volume. However, technical ceramics formed by injection molding can shrink from 10 to 20% due to the amount of binder required to obtain a flowable mixture.
This shrinkage is not always isotropic, but is related to the flow direction and part geometry. There are some important distinctions between plastic and ceramic injection molding, and understanding of this is critical to achieving successful ceramic products (2) . In ceramic injection molding, a two phase mixture of ceramic powder and wax or polymeric binder system is necessary to achieve an injection that completely fills the mold. In order to minimize the shrinkage of the part during sintering, a minimal amount of binder is desired, however, this causes the viscosity to increase. In plastic injection molding, increasing injection pressure reduces the viscosity of the polymer. In ceramic injection molding, the mixture is dilatent, and an increase in pressure increases the viscosity of the ceramic mixture. Therefore, relatively low pressures are utilized in ceramic injection molding. The size of the part is also a limitation in ceramic injection molding. Parts with large and/or varying crossections are difficult to mold and debind without introducing defects such as cracking and blistering.
The challenges facing ceramic injection molding are the high cost and the long lead time for tooling, and the ability to predict the shrinkage of new geometries. When these are coupled, several iterations or modifications to the mold can take several months to a year to deliver a useable part to a prospective customer. Herein lies the beauty of combining rapid prototyping with ceramic injection molding. The quick turn around and flexibility of the tooling significantly reduce the time and cost of getting the first part to the customer for their initial trials, FDA or other certifications, and design feedback.
Initial SuccessesThe feasibility of ceramic injection molding was demonstrated using an epoxy tool produced by stereolithography.
Several significant advantages were realized by using the SLA tool to injection mold ceramic parts:
Research Objectives and ApproachThe objective of the research work involved with this project, which is just underway, is to develop a shrinkage model that will predict the dimensions of the fired ceramic part with dimensional accuracy within 0.1% of design dimensions. Both experimental and modeling efforts will be utilized. A test shape will be designed to accentuate the issues. Existing sintering models will be adapted to provide process simulation suitable for geometric design. Geometric design methodologies will be integrated with all of the process steps from the CAD model to the functional component through rapid inspection processes (also being developed at Georgia Tech’s RPMI) to measure parts, compare them with design and make corrections to develop the model. Application of this methodology will allow rapid, first-time fabrication of precision functional prototypes of sintered ceramics that could be extended to metallic and composite systems.
The Ceramic Injection Molding Process used at Georgia Tech
MoldingA Peltsman Low Pressure Injection Molding Machine was used to mold an aluminum oxide and wax slurry into an epoxy mold made by stereolithography (Figure 2). The part was a demonstration keychain with the RPMI logo, approximately 2.5" x 1.25" x 0.125" thick, with two through holes. The mold was originally made for plastic injection molding. The gate was opened by drilling the epoxy mold with a 1/4" drill bit. The slurry containing 72 volume % aluminum oxide powder was injected at 85° C and 8 psi pressure. The part took about 3 minutes to cool in the mold before it could be removed. The mold had no cooling channels or vents. A mold release spray was used before each shot. Over 150 parts have been molded from a single epoxy mold without any signs of wear.
Thermal ProcessingThe binders are removed from the green part by a slow heating process that ramps to 200° C over two days. The parts are packed in fine clay to aid in wicking out the binder. After the parts have cooled and the clay cleaned off, the parts are sintered to 1650° C in an overnight cycle. Residual clay on the surface can cause reactions during sintering that affect the surface texture. The final ceramic part is fully dense after shrinking 5 to 12% depending on the geometry and processing conditions.
The shrinkage is anisotropic, or nonuniform for the orthogonal dimensions, but is consistent from part to part when good process control practices are utilized. The amount of anisotropy may seem to be insignificant. For example, the linear shrinkage in the direction of flow of a cylinder is approximately 9.5% in the axial direction, and 10.5% diametrically.
However, when the objective is to produce a net shape part, such as a nozzle, a difference of this magnitude can easily throw a feature of small dimensions out of tolerance and cause a mold to be reworked.
The Practical Benefits and JustificationsThe solid epoxy mold made by stereolithography was built in 33 hours. The elapsed time from CAD file to final part can be less than one week. Cost of the mold would range from about several hundred dollars if the SLA machine was in-house, to several thousand dollars through a service bureau. Continuing improvements in rapid tooling by AIM are reducing the time and cost to about half by shelling out the mold and backfilling with an aluminum or copper filled epoxy. However, this would eliminate the ability to observe the mold as it is filling.
To create an aluminum mold by CNC machining using a traditional tool shop would take about 2 weeks and cost about $4500.
These estimates increase exponentially for increasingly complex geometries.
Conclusions and Future WorkThe feasibility of ceramic injection molding using rapid tooling has been demonstrated. There are several advantages to using the tooling for prototypes, including lower cost and less time that significantly compress the product and production development cycle. Future work will explore the molding of different geometries of parts, tooling enhancement such as cores and part ejection systems, different materials systems such as zirconia and whisker filled ceramic composite materials, and developing a shrinkage model to predict the dimensions of the fired ceramic part with dimensional accuracy within 0.1% of design dimensions.
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