Recent advances in ceramic injection molding (CIM) make it technically feasible and economical to produce medium to large volumes of complex, ultra-high precision ceramic components. Engineers can now combine the intricate geometry, which has long been the domain of plastic and metal parts, with the superior performance characteristics of ceramics.
The well-known benefits of ceramic material include: high hardness and mechanical strength; wear and corrosion resistance; dimensional stability over a wide range of temperatures; ability to withstand high working temperatures; good electrical insulation and excellent dielectric properties. However, until recent advances in CIM technology, production engineers and product designers did not view ceramics as a viable option for complex parts requiring tight dimensional tolerances.
Limitations of Earlier Technology
Manufacturing small, intricate shapes in volume before the advent of CIM had significant limitations. The obvious challenge for ceramics is the inherent fragility of parts prior to sintering and the hardness of the material, which makes machining processes difficult and expensive post sintering.
Commonly used manufacturing processes, such as dry pressing or extrusion, are well-suited for high volume production, but they can produce only relatively simple shapes. For example, blind holes and undercuts are not possible. For more complex shapes, tight tolerances and improved surface finish, secondary machining is generally required.
Ceramic injection molding allows for features such as re-entrant angles, multi-shaped blind holes, screw threads, surface profiles, perpendicular holes, undercuts and intricate cavities. Unfortunately, until recently CIM did not provide the tight tolerances and high repeatability that is required for many applications. Achieving precise dimensional control has been difficult for CIM because the manufacturing process involves significant shrinkage of the component.
The Ceramic Injection Molding Process
The CIM process begins with very fine ceramic powders. The powders are compounded with polymer binders to produce a pelletized feedstock. During molding, binders melt to form a liquid medium that carries the ceramic powders into the mold during the injection stage.
Using an injection molding machine similar to that used in conventional plastic molding, the feedstock is forced into a mold cavity forming a net shape part. Molds can be single-cavity or multi-cavity configurations.
After forming, the part goes through a two-stage process. First is pyrolysis or “debinding” to remove the binder, followed by sintering in a high temperature kiln to form a fully dense ceramic component. Sintering is the process of heating the material to a temperature below the melting point but high enough to allow fusion of individual particles and densification of the material.
During sintering, the component shrinks by as much as 20% while retaining the original geometric shape. With good process control, it is possible to achieve a uniform and repeatable shrinkage leading to tight tolerances, obviating any need for machining of the part afterwards.
Advances in CIM
Morgan Technical Ceramics, a leading manufacturer of innovative ceramic, glass, metal and engineered coating solutions, has introduced several refinements to the CIM process to control shrinkage. These techniques achieve tolerances of ±0.3% of nominal (e.g., 1.000" = ±0.003") with excellent batch-to-batch repeatability and Cpk’s in excess of 1.66.
The high degree of dimensional control comes from process improvements implemented by Morgan Technical Ceramics at all stages of production.
First, during the design phases, Morgan Technical Ceramics conducts mold flow simulation and analysis in order to optimize the part and mold design. On the computer, adjustments are made to gate positions, wall thickness and cooling parameters to help achieve success. By performing this analysis early in the design process, prior to commissioning the injection mold, many problems can be addressed and improvements can be implemented quickly at low cost and without causing expensive delays in the production schedule.
A second key factor in achieving tight tolerances and high repeatability is quality control during the mixing process to create a homogeneous pelletized feedstock. The ceramic particles must have a consistent size and they must be distributed evenly in the polymer binder. Morgan Technical Ceramics’ engineers have implemented sophisticated processes to achieve uniform mixing and eliminate minute air pockets that could cause distortion or cracking in the final product. The use of sub-micron powders allows for smaller features that would otherwise not be possible with larger granulate-based forming methods such as dry pressing.
The third critical refinement that Morgan Technical Ceramics has implemented is cavity pressure containment and control. Cavity pressure is the process variable that correlates most directly with part quality. Morgan Technical Ceramics uses pressure transducers inside the mold tool cavity to provide process control as the feedstock flows into the cavity. The transducers gives the system an “eye” inside the cavity, allowing Morgan Technical Ceramics’ engineers to closely control part weight and dimensions, and eliminate flash, sinks, shorts and warp. In today’s world of Six Sigma, the standards are rising: “just fine” and “good enough” are not acceptable any more. Jobs with high volume and tight tolerances demand a level of capability that can only be achieved with cavity pressure containment and control.
Trend towards net-shape fabrication
Engineers bring parts to Morgan Technical Ceramics and are amazed that the ceramic injection molding process can now produce similar geometries to those available in plastic and metal. Morgan Technical Ceramics’ tolerances are typically within 25 microns on anywhere from 10 to 200 different dimensions. This new process is attractive because it is repeatable. Customers receive highly consistent quality with little part-to- part variation that enables Cpk’s in excess of 1.66. With fewer rejects and proven statistical process control, incoming inspection by the customer is no longer required.
The advancedCIM process gives engineers more versatility in the use of ceramics when designing new products and replacing plastic and metal components that fail to perform adequately.
In fact, the wider use of CIM is part of an overall philosophic trend in component manufacturing. There is a discernible move away from the energy-inefficient and wasteful practice of machining off material, towards more efficient net-shape fabrication, which takes advantage of computer-driven technology. This trend, in turn, has allowed production engineers and product designers to improve productivity, lower manufacturing costs and improve product performance.
By: Paul Manison, Project Manager, Morgan Technical Ceramics