Micromini ceramics sport supersized properties

Compared to heavy-duty processes used to make bulky injection-molded ceramic spark plugs in the 1930s, today's sophisticated ceramic injection-molding (CIM) technology can produce microminiature components with features as small as 18 µm (0.0007 in.). MicroCIM parts weighing as little as 0.0004 gm serve in a wide variety of applications where high temperatures and caustic environments preclude most metals and engineered thermoplastics.

CIM basics

CIM is a combination of particulate materials, injection molding, and sintering science. Submicron ceramic powder is one key to microminiature molding. Powder selection is based on several factors including particle size, shape, and distribution. Under carefully monitored conditions, powders are blended with proprietary binders and additives, giving the mixture the fluidity to consistently fill finely detailed molds similar to those in traditional plastic-injection molding. Production CIM tooling, however, must be made from hardened materials to better resist the highly abrasive ceramic powders and requires tighter tolerances.

Depending on the application, tooling can be single or multicavity. Slides, cores, and other common tooling techniques for plastic injection molding are also used. To minimize up-front costs, molders often use different prototype building techniques to finalize part configurations before building production tooling. Common prototype methods range from machining components to using rapid-prototype tooling to produce injection-molded test pieces.

Injection-molding machines modified for CIM heat the feedstock and inject it into the mold. Parts solidify and are ejected from the mold as "green." Green parts are oversized and somewhat fragile. Technicians can inspect internal features with an in-process monitoring system, even in opaque materials, before parts are fired. This gives designers real-time feedback, reduces production costs, and ultimately improves quality.

After inspection, green parts are debound through evaporation and exothermic reactions that remove all but a small fraction of the binder. Parts then undergo prolonged exposure to temperatures up to 1,800° C in sintering furnaces. Sintering consolidates the powder just below the melting point of the ceramic constituents. During solid-state diffusionary processes, atoms and molecules migrate through the ceramic via a mass-transport mechanism of either lattice diffusion, surface diffusion, or evaporation-condensation. Sintering furnaces must be tailored with either oxidizing or reducing atmospheres that match the diffusionary mechanism of each ceramic. As ceramic particles fuse together, part dimensions predictably shrink around 20% and part density increases. Upon sintering, surface finish of 8 rms can readily be achieved.

For certain applications, CIM parts undergo a second hot-isostatic pressing (HIP) process that boosts the ceramic density and strength. CIM parts are small, so the secondary HIPing process doesn't appreciably increase part costs.

CIM produces net-shape parts. But for applications demanding exacting surface finishes or precision beyond as-sintered capabilities, finishing operations similar to those used for ultra-hard materials are required. These may include diamond grinding, lapping, or honing.

Material selection

There is a wide array of technical ceramics available. Here are a few of those most commonly used for microminiature components.

Aluminum oxide or alumina (Al₂O₃) is the most widely used and cost-effective engineered ceramic. It comes in several crystalline structures. Hexagonal alpha phase is the most stable at high temperatures. Alpha-phase alumina is the strongest and stiffest oxide ceramic. It also has good dielectric properties, resists most reagents except hydrofluoric and phosphoric acid, has good thermal properties, and high hardness. Alumina lacks fracture toughness, which can limit its use in structural applications under tension. However, it is important to note that alumina, for example, can perform well in tension. It has three to five times the tensile strength of many plastics, and half that of 316 stainless steel.

Zirconia (ZrO₂) has a high melting point (2,700° C), low thermal conductivity, and is not as hard as alumina. It exists in three crystalline forms: cubic, tetragonal, and monoclinic. At high temperatures zirconia undergoes phase transformations which reduce its mechanical properties and limit its use as an engineered ceramic. However, doping or alloying zirconia with oxides, such as CaO, MgO, and Y₂O₃, fully or partially stabilizes it. The resulting ceramics no longer undergo phase transformation during heating and cooling. Stabilized zirconia can also be heat treated (transformation toughened) to produce an intragranular tetragonal-zirconia precipitate which compressively stresses the entire structure and increases fracture toughness and bending strength.

Fully stabilized zirconia is harder and better resists thermal shock. It also has higher oxygen-ion conductivity, so it is used in oxygen sensors and solid-oxide fuel cells. Partially stabilized zirconia (PSZ), on the other hand, is a mixture of both cubic and tetragonal ZrO₂ crystals. Its bending strength rivals that of hardened tool steel. It also holds a sharp edge so it finds use in industrial and domestic knives and surgical scalpels.

Zirconia toughened alumina (ZTA) combines alumina's high hardness and wear properties with zirconia's toughness and bending strength. ZTA has over a threefold increase in flexural strength over high purity (99.99%) alumina and double that of ZrO₂.

Polycrystalline ruby (Al₂O₃ + Cr₂O₃) is a translucent alumina ceramic that gets its distinctive red hue from CrO₃ (about 1% of Al³⁺ is replaced by Cr³⁺). Polycrystalline ruby is inert, biocompatible, and resists wear, heat, and acids. It is slightly harder than high-purity (99.99%) alumina.

Designing for CIM

Designers familiar with plastic injection-molding principles have a head start on designing for CIM. In general, CIM parts tend to have:

• Minimized, consistent wall sections.
• Gradual transitions (Avoid sharp corners).
• Allowances for parting lines, gate vestige, ejector pin marks, and draft angles.
• Designs optimized for minimal mass.
• Complex features.
• Production volumes of >10,000 parts/yr.

The design and manufacture of custom CIM components requires close cooperation between designers and molders. As with many technologies, early supplier involvement carries many advantages, including reduced lead times and lower overall costs. Partnering with experienced CIM vendors also lets designers tailor components to take maximum advantage of CIM processing.

It's important to submit CAD models, sketches, or drawings early in the design process to see if the components are compatible with CIM. That's because it may be more economical to produce simple shapes with traditional manufacturing techniques. In general, the more complex, the more advantage CIM has over other methods. Annual quantities, critical dimensions, function, and allowances for molding features (gate, ejector pins, and parting lines) are other factors that influence whether or not CIM is a suitable manufacturing option.

Based on technical specifications, project complexity, and urgency of the request, quotations may take anywhere from 24 hr to 2 weeks. Quotes should include additional provisions for non-recurring engineering charges, and tooling (both mold tooling and sintering fixtures if required).

Designers looking to experiment with component variations will also be quoted on various prototyping options including machining and rapid-prototype tooling.

Once tooling is complete, initial samples are inspected against the customer's drawing. At this time, dimensions on the drawing may have to be altered to meet the sample or the tooling may be revised. If the tooling is changed, a second set of samples are molded and inspected against the print.

CIM is a very repeatable manufacturing process that is best suited to high-volume components that are difficult, costly, or impractical to produce via traditional techniques. CIM is a net-shape process that offers designers the advantages of ultra-hard materials without the costs associated with other manufacturing methods. Partnering with a well-established CIM firm will allow engineers to take full advantage of the process, while minimizing lead time and costs.