Ceramic Technology and Processing, King

Forming 213

and cracking. Always set the part on an absorbent or mesh structure so that air can reach all surfaces of the piece. This is also true for oven drying. A fine, expanded, metal mesh works well. Depending on the shape, one can turn the part from time to time to help attain even drying on all surfaces.

Binder Burnout

After drying, the binder remains in the body and is burned out during the sintering cycle. The initial heating ramp is slow (0.5 °C/min. for sensitive materials).

Binder burnout for injected molded parts is another story. The binders are thermoplastic and are present in large amounts. There has to be sufficient liquid present during molding to make the compound fluid.

Wax injected parts have a low viscosity fluid when they are reheated. The part will slump. To get around this, the part is saggered and packed in an absorbent material. Clay is sometimes used. Once a little of the wax is removed from the surface, the part could be self standing.

Injected molded parts with a polymer vehicle have a much more viscous liquid when the polymer melts. It is not common practice to sagger and pack these materials. When the polymer is heated, it expands, and then gives off a large volume of gas. Since the fluid is viscous, the escape of the decomposition gases is difficult. However, this is done in large volumes every day, indicating that the problems are solvable.

Types of Dryers

Ceramic parts can be dried in many ways, depending on the properties of the ceramic and the equipment available. A few types will be discussed.

Air Drying

For this, one just sets the part to be dried on the bench. This is often a suitable method. There are two problems with this method,

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however. First, the part has to be set on an open mesh or absorbent material so that it dries evenly. The second problem is that, if there is dust in the air, the part should be loosely covered.

Oven Drying

This is the most common laboratory drying method, using an electric oven. The oven has a temperature control, temperature indicator, expanded metal shelves, and a vent. The expanded metal shelves allow drying to take place on the bottom of the part. Do not set parts on an impervious shelf as they will not dry evenly. It is good practice to place thermometers at different places in the oven to see how much variation occurs; there will be some. One also has to think about air circulation in the oven. Usually, the heaters are on the bottom so that the heated air can rise through the setting. The temperature control is often at the top of the oven some distance from the heating element. If an air blockage in the oven occurs because it was overloaded or the shelving blocks air flow, the heater will overheat, ruining the ware and possibly ruining the oven.

Some ovens have a blower or fan to circulate the hot air. This is a good practice.

Humidity Driers

Humidity driers control both the relative humidity and the temperature. By controlling both, the rate of drying of the parts is controlled. Humidity is gradually reduced as the parts dry. Temperature can also be controlled along with humidity. This makes humidity drying the preferred method if parts warp or crack when dried by other methods. The equipment is more expensive and not usually seen in the lab. This is partially also true because lab parts are generally small. Large parts that have low permeability are candidates for humidity drying.

Forming 215

Microwave/Dielectric Driers

With this method, the part is heated more uniformly as the microwave energy penetrates the body and couples with water thus increasing the temperature. Since heating is more uniform, drying tends to be more uniform. There are two snags. As microwave energy is adsorbed by water, the intensity of the field diminishes. For a thick part, the center receives less energy than the exterior. The second snag is that the water vapor still has to escape or the part could burst. Microwave drying has not been popular for general laboratory use. It is popular for drying films or fibers on continuous production lines.

REFERENCES

1.Metals Handbook Ninth Edition. Am. Soc. for Metals, Metals Park, Ohio.

2.Bulletin of the Amer. Ceram. Soc., Editorial and Subscription Offices, P.O. Box 6136, Westerville, OH 430866136.

3.W. Kingery, Ceramic Fabrication Processes. Cambridge, MA: Cambridge Technology Press of Massachusetts Institute of Technology,1958.

4.Robert A. Thompson, "Mechanics of Powder Pressing: I, Model for Powder Densification," Bull. Am. Ceram. Soc., 60[2]237-243(1981).

5.Robert A. Thompson, "Mechanics of Powder Pressing: II, Finite-Element Analysis of End Capping in Pressed Green Powders," Bull. Am. Ceram. Soc.,60[2]244-251(1981).

6.Alan G. King, Method for Isostatic Pressing of Formed Powder, Porous Powder Compact, and Composite Intermediates. U.S. Patent 5,244,623 (1993).

7.E. L. Whiteside, "Quality Control in the Plaster Mold Shop," Bull. Am. Ceram. Soc., 45[11]1022-1026, (1966).

8.Carl Pletcher, Personal Communication.

9.David D. Marchant, James L. McAlpin, Tim Stangle, "Investigating Plaster Properties," Ceramic Industry, December(1994).

10.J. T. Jones, Personal Communication.

11.Frederick F. Lange, U.S. Patent 4,624,808 (1986).

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12.Ogbemi O. Omatete, Alan Blier, C. G. Westmoreland, et al, "Gel Casting Alumina," Ceram. Eng. Sci. Proc.,1991, 12[9-10] 2084-90.

13.Karl Frank Hens, “Advanced Tooling and Molding for PIM,” 1995 International Powder Injection Molding Symposium, July 19-21, 1995, State College, PA.

14.Alan G. King, S. T. Keswani, "Adiabatic Molding of Ceramics," Bull. Am. Ceram. Soc.,73 [9] (1994).

15.C.E. Weir, from P.W. Bridgman data, "System H2O PT diagram Between 0 and 10,000 bar”; Fig.1915 in Phase Diagrams for Ceramists, Edited by M.K. Reser, Bull. Am. Ceram. Soc. (1964). Redrawn by the authors.

16.F. A. Cantalope, R.I. Frost, L.M. Holleran, "Method for extruding thin walled honeycomb structures," U.S. Patent 3,919,384 (1975).

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acrylic emulsions are strong binders. Recently, Rhome and Haus introduced a new acrylic binder with very high strengths. They claim that parts can be machined with single point tools, but they hesitate when discussing drilling. Comparative green strengths are shown from their data in Table 7.1.

Table 7.1: Comparative Green Strengths

One commercial example of green machining is making spark plugs. A blank is isopressed and then ground by a form-grinding wheel. This process is very fast and produces an accurate and uniformly dense body.

2.0 LAPPING FIXTURES

An easy technique uses lapping fixtures for green machining a part. Design the fixture so that it has a large area in contact with the lap and is hard, thereby increasing its wear resistance. As the green ceramic is soft, it will lap to dimensions in a short time. Laps are often metallurgical types with an 8 inch D wheel and a Silicon Carbide paper on its surface.

Fixtures for Lapping Flats

Figure 7.1 depicts a typical fixture. The fixture has five parts: a base plate, two wear plates mounted on the base, and two thinner wear plates for lapping the obverse side flat and parallel. Flat-headed screws

Green Machining 219

hold the wear plates on the base plate. Slotted holes in the wear plates provide lateral adjustment. The part is placed on the base and snugged up by the lateral adjustment. The fixture is held lightly on the lap when it is flooded with a coolant. (For water soluble binders, use kerosene as the coolant. Bisqued parts use water.) Let the fixture float on the lap as it is not necessary to use much pressure. Quickly, the green part will grind down to the plane of the wear plates. Now install the thinner wear plates, turn the part over, and repeat the process. Both sides are now flat and parallel. It is just like using a surface planer in wood working. With another set of wear plates, the edges can be squared up so that the part has all four sides flat, parallel, and square, just as would be done on a joiner in wood working. Next, square up the ends.

Figure 7.1: Lapping Fixture, Flats. Wear plates fix the depth of lapping. Different thickness plates are used to lap flat and parallel.

Lapping Ends

To square up the ends, use a different fixture, with the same principle but with a different shape, as in Figure 7.2.

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Figure 7.2: Lapping Fixture, Ends. A simple square block with a corner cutout is useful for lapping ends.

The fixture is simply a block of hardened steel with a notched corner. The part is held in the notch by hand with the fixture floated on the lap. Press the part to the lap with a finger. Direct the lap rotation so that it presses the part into the notch. Now, the other end can be lapped to length. If careful, tolerances of 0.001" are attainable. It turns out that this fixture found a major use in grinding; with metal-bonded diamond laps, the ends of glass thermal expansion specimens, which were lapped square and to length. Whenever the fixture wears out of square, it is remachined.

Chamfering

Often, one needs to chamfer the edges of the part with a fixture as shown in Figure 7.3. Two angle blocks that one can move to different

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spacings are fastened with set screws. Control the width of the chamfer by the spacing between the blocks. One can quickly grind the portion that protrudes. A similar technique was used to chamfer ceramic-cutting tools by holding the corner up against a diamond wheel. In this case, set the width of the land by the cross slide position of the grinding machine.

Figure 7.3: Chamfering Fixture. The chamfer is made by grinding away the exposed corner.

Holes

A good way to cut a hole in a part is with a core drill as shown schematically in Figure 7.4.

Core drills will be discussed in the following section on tools. Drill speed is dependant upon the diameter of the drill. The periphery speed is kept as a constant and, for this application, fairly low. One can also use metal tubing as a core drill with an abrasive grain and a coolant. Grain is fed into the cut during drilling. Chances are that the cut will not be as clean as that from a diamond drill, but if cut fat, the cut can be cleaned with a mounted point.

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Figure 7.4: Core Drill. A core drill mounted in a drill press is used for green machining holes.

Cylinders

It is possible to green machine cylinders on a lap. Figure 7.5 is a fixture for lapping cylinders.

The fixture has two steel blocks that are adjustable up and down and locked in place to set the diameter of the cylinder. Fasten two hardened wear plates to the blocks with flat head screws. When the plates wear, they can easily be reground. The right block is on a slide that, with horizontal hand pressure, holds the part in place by way of centers ground into the ends of the part. The centers have to be in line, which can easily be done with temporary shims. One way to do this is to grind them by hand using