Ceramic Components Produced By Low Pressure Injection Molding
Oxide ceramics cover a vast field of materials. The most general classification of ceramics states, if it is not made of metal or is not of organic origin (plastics) - it's ceramic. The most commonly used ceramic seems to be alumina (A12O3). It's also known as sapphire, corundum and aluminum oxide. In terms of tonnage, most alumina, besides being the raw material for aluminum smelting, is used in products such as dinnerware, sanitary ware, refractories for steel and glassmaking furnaces, as insulation in nearly all kinds of other furnaces, and in spark plugs. A very small fraction is highly refined materials used to produce alumina ceramic components.
Zirconia (ZrO2) is a newer material with remarkable strength properties and rather peculiar electrical properties, which is beginning to emerge as a favorite construction material for components, in addition to its already established use as oxygen sensors. It comes in so-called partially stabilized form. The stabilization refers to the retention of the high temperature crystal structure, and the stabilizing agent is typically either Y2O3, MgO, CaO or CeO2. Without the stabilizing agent present, zirconia would literally fall apart upon cooling from the firing cycle. This has spawned a multitude of designations such as Partially Stabilized Zirconia (PSZ), Tetragonal Zirconia Polycrystals (TZP), and Yttria Stabilized Zirconia (YSZ) to name a few. Some are trade marks, some are abbreviations, but they are all zirconia containing a certain concentration of one of the above mentioned stabilizing agents. Zirconia is useable to much higher temperatures than alumina, but the raw material cost is 20 to 30 times that of alumina.
One thing is having access to all these materials with their exceptional properties, another is to have components made from them. This is where low pressure injection molding comes in. Most engineers and designers who have worked with ceramic components have experienced the frustration and expense to have a prototype made that actually worked, only to find out that production of the part would be prohibitively costly, or not feasible at all. The reason for this is that prototypes are generally machined from either tube, rod or plate stock at a usually quite high cost. As all machining on fired ceramics require diamond tooling (it is actually grinding), it is both expensive and time consuming, and making many parts this way is almost as expensive as making just a few. With low pressure injection molding, it is comparatively easy to make any shape and, if needed, to impart higher tolerances by grinding a few critical dimensions. Because the tooling cost when processing parts with low pressure molding is much less than when using high pressure methods, the economics of moderately sized production runs are much more favorable.
The process consists of taking a suitable ceramic powder and mixing it with a wax while heating it. The wax melts and the ceramic powder is homogeneously dispersed forming a moderately high viscosity fluid. The tool is made from aluminum, and the cavity has been scaled up with an appropriate shrink factor. The wax/ceramic mixture is injected into the tool using air pressure, and the injected mixture freezes in the tool, thus forming the green part. This step is the most important feature of low pressure molding. With any other high pressure process (except for isostatic pressing) a density variation is introduced in the part, the density being highest at the gate. This, in turn, leads to differential shrinkage in the part during firing, and to non-uniform tolerances throughout the part. This is avoided with low pressure molding where a noncompressible liquid is injected and frozen after the tool has filled completely. After the tool is disassembled, the green part can be extracted. It then goes through a proprietary de-waxing process, after which it gets fired to achieve the desired density. Depending on the tolerance requirements, the part is now either finished, or it may need additional grinding to achieve the required dimensions, and/or characteristics.
Ceramic components are used in a vast variety of products. The main reason for using oxide ceramics is one of the following: its high temperature resistance, corrosion resistance, abrasion resistance, and, in the case of alumina, its electrical insulation properties. Five major groups of products have been identified, where ceramics are commonly used. They are: welding equipment (nozzles, swirl rings and spacers), heat treating equipment (element hangers, feedthroughs and insulators), instrumentation (DTA/TGA equipment, IR emitters, plasma nozzles), lighting (high intensity reflectors, lamp bases) and medical equipment.
Only one off-the-shelf product is available from Ceramco, and that is fasteners. In stock are hex head bolts (and nuts) 1/4-20 to 0-80 up to 3" long (1" long for sizes under 6-32) made from 99.85% alumina or 3 mole% Yttria Stabilized Zirconia. These fasteners are standard hex head, but any head configuration is offered at no additional cost. All other product is custom made to customers' specifications. One of the special advantages of this process is its ability to mold threaded parts. Both inside and outside threads are easily molded into any part. Another feature is the ability to produce small holes and thin walls.
Shapes can be manufactured in the following materials:
- Alumina, 99.8% to 90% alumina, fully dense (A998 to A90)
- Partially Stabilized Zirconia (Y, Mg, Ca, Ce stabilized), fully dense
- A9468 (70% dense, 94% alumina ceramic, mullite bonded)
- A9968 (70% dense, 99% alumina ceramic, mullite bonded)
- MUL6 (70% dense, 72% alumina mullite ceramic)
- Cordierite (70% dense Mg, Al, Silicate)
- Specials (MgO, TiO2)
Maximum dimensions should not exceed 4" for dense material and 6" for nondense material. The wall thickness cannot be more than 0.25" for dense alumina, or 0.20" for zirconia. For non-dense material the wall thickness can be 0.40". Minimum dimensions are 0.020" wall and 0.005" diameter.
As molded tolerances are a function of the actual dimension. For dimensions up to 0.5" the tolerance is +/-0.003", from 0.5 to 1" it is +/- 0.007", and above 1" it is +/- 1%. Due to firing distortion molded OD's and ID's can have an out of round condition of up to +/- 1/2% depending on the shape. Camber is usually better than 0.005 inch/inch, again as a function of shape.
Threaded components are made to the following specifications: male threads will have a pitch diameter 0.001/0.002" under the minimum allowable according to Class 2. Female threads will have a pitch diameter 0.001/0.002" over the maximum allowable for Class 2. Due to shrink factor fluctuations, it is not recommended to engaging any thread for more than 10-15 pitch lengths, as stack-up tolerance on the pitch may make the thread "bind up". The above stated tolerances can all be improved by after-machining of the molded blank. Automatic computer controlled grinding equipment is used for ID/OD grinding, flat grinding, ultrasonic milling and free abrasive lapping. The company can impart tolerances up to 50 millionths, but a more practical limit is 0.0005".
Whether one or a few prototypes are desired, this process is still more expensive than machining, provided the part can be machined at all. If 250,000 pieces/year and up are needed, automated processes such as dry pressing or iso pressing are the more economical, if these processes can make the detail and features you want.
However, if a continuous series of parts are needed in the hundreds or thousands, and the flexibility of incorporating future design changes is desired without having to start all over, this is the process to use. The tooling, being made from aluminum, is typically only 5 to 10% of the cost of traditional injection mold tooling. The time to make a tool is also considerably less, and the cost to change certain dimensions or features is negligible. The part cost decreases rapidly with rising quantities, and large quantities can be made both quickly and inexpensively by designing multicavity tooling.
The key to economical parts lies in the tolerancing. It is worth the effort to scrutinize your print and only apply tight tolerancing where needed. The guidelines are given above.
Ceramco has been making parts for 11 years. The process used is proprietary. Components are not sold and bought. Everything is made from well characterized raw material powders. Our only business is to make ceramic components, and we are not part of a bigger conglomerate. QC department is fully qualified according to MIL-I-45208A.
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