Combining injection molding technologies can help create a low-cost, painless introduction to the use of ceramic components.

Technical ceramics are undeniably valuable for their cost-to-service life ratios, but they have not found the broad usage one might expect because other engineering materials still dominate the market. Ceramics often have a bad reputation outside our industry because people erroneously assume that the materials have poor impact toughness, are too expensive, or are not possible to manufacture due to process limitations.

Few designers actually understand the material, so engineers often design around ceramics as much as possible. But with the plastics boom and the fantastic progress of powder metallurgy (PM), forming technologies have been the driving force for getting materials to market. Ceramic, polymer or PM components can all be easily shaped by cost-effective processing techniques such as injection molding.

Process Basics

Ceramic injection molding generally falls into two types: low-pressure injection molding (LPIM) and high-pressure injection molding (HPIM). LPIM is much less expensive in terms of tooling but is inherently less precise and more labor intensive. LPIM generally consists of mixing melted paraffin wax and ceramic powders under 150°C. These materials are then pumped under low pressure into a chilled mold, often by placing the tools on a block of ice prior to molding, where the wax/ceramic mix freezes in the cavity of the mold.

The molds are assembled by hand, and the molding is generally done at 100 psi die clamping and injection pressure or less. After allowing the part to freeze (usually 5-60 seconds), the molds are pried open and the parts are removed by hand and placed on a tray, where they may undergo a solvent debinding process followed by a thermal debind. At some point in the process, the flash is removed (via fettling), and the parts are prepared for final sintering.

The basic technology was developed in Russia in the 1950s and is widely used today, particularly in China. The process is not material specific and can be used for almost any ceramic material, making it very versatile. The nature of the process is somewhat slow and imprecise, but the cost of the tooling is comparatively low, as is the capital cost of the molding equipment. Many parts that do not require tight tolerances or thick walls are made this way; ceramic thread guides, old-style oxygen sensors and insulators are typical examples. If tight tolerances are needed, post-sintered parts can be diamond machined as long as the component's shape lends itself to machining.

The HPIM of ceramic components is an adaptation of plastic and powdered metal injection molding that was originated in the 1940s in the U.S. and Europe. The process uses conventional plastic injection molding presses with hardened or lined injection barrels and screws or plungers. The presses range from 5 tons die clamping and injection pressure for small parts to up to 250 tons for high-volume, multi-cavity parts. The process produces accurate net-shape parts with little or no need for fettling of the pressed parts. However, the tools are hardened steel and/or tungsten carbide and are very expensive to produce.

In addition, the tools need to be placed in a mold base that can vary in cost from $2500 to $10,000 or more, depending on the cooling system, part complexity and size. Newer mold base designs like the master unit die (MUD) are less expensive, but they are not as accurate, are not automatable and do not generally contain internal cooling systems to freeze the parts.

Because most conventional HPIM mold bases are internally cooled, these presses can produce up to 20 shots per minute of ceramic components (depending on size and ejection complexity). By necessity, the tools are very accurate because any misalignment at these pressures can crush the tooling.

The mix varies depending on the manufacturer's preference and proprietary process, but generally consists of a plastic source (e.g., polyethylene, bakelite, etc.), a wax (often paraffin), a mold release, surfactant and the desired ceramic powder. These materials are mixed at the melting temperature of the organics and are de-aired to eliminate bubbles.

Creating a homogenized mix is complicated and involves several mixing, pulverizing and remixing steps until a creamy, uniform mix is attained. One new mixing technique involves vacuum, high-shear mixing in a one-step process. The materials are pelletized and molded using plastic molding techniques. The resulting parts are hard when they are ejected and can be part of a molding tree of several parts that can then be snapped off.

Next, binder is removed through chemical or thermal processes, or a combination of the two. After these steps, the parts are still quite robust and can be handled if any additional steps need to be performed. The ceramic molded parts are then sintered using conventional ceramic techniques.

HPIM is a high-precision process. However, in many medium- to high- volume applications, the need for hardened or carbide tooling (to prevent tool wear) multiplies the tooling cost, which limits the use of HPIM in applications where the ceramic manufacturer may not be absolutely sure of their final design or volume.

Combined Technique

The ability of ceramic injection molded parts to follow complex design parameters is possible and proven every day. The drawbacks to broader acceptance in all industries are generally focused on the cost and time it takes to create evaluation prototypes. New methods like free-form fabrication have been tried, but the samples are generally not representative of what a production ceramic component will be. Samples of generally round cross-section can be diamond-machined, but this is slow, expensive and not readily scalable.

Instead, a method has been developed to make complex ceramic shapes using an innovative combination of LPIM and HPIM.* The combination of the two processes allows prototype development that can progress at a reasonable tooling cost, especially if design changes are needed along the way that require rapid turnaround for pre-production samples and the first low-volume requirements. High-volume, high-precision production becomes practical when HPIM is vertically integrated into this production process, because all of the uncertainties of design and performance will have been eliminated using the low-cost LPIM.

The unique method fills the gap between what is needed and what is economically feasible. LPIM can be used to relatively inexpensively produce very complex shapes, up to about 20,000 pieces. The aluminum tooling, which can maintain tolerances of ± 1%, can wear out after making 10,000 parts. HPIM does not have these limitations; tolerances can be held at ± 0.001 in./in. on as-molded parts with walls as thick as 0.600 in. or as thin as 0.010 in., but the tooling starts at a few thousand dollars and varies depending on the complexity and number of cavities in a tool.

To solve these challenges, the design usually goes through several revisions using LPIM and is then scaled to high-volume, net-shape production using HPIM. HPIM tools can often produce over 250,000 ceramic parts before any major overhaul is required.

For more information, contact Ceramco Inc. at 1467 E. Main St., Center Conway, NH 03813; call (603) 447-2090; or visit the website at www.ceramcoceramics.com.

*Developed by Ceramco Inc.

Links

• Ceramco, Inc.