Ceramic Technology and Processing, King

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screen. The mix has to be damp and with a binder. Both the amount and viscosity of the binder solution are critical and determined for each material. The binder solution has to stick particles together to form the granule, so it seems that a viscous liquid is preferable. The same process can, of course, be done with a lab screen by pressing it manually through with a plastic or rubber scraper. While it depends on the particle size, the screen is in the range of 40 to about 60 mesh. As the mix is pressed through the screen, it forms granules consistent with the mesh size. This can now be dried, usually aired or pan dried.

Some rotating mixers are capable of granulating the mix. Granulation occurs at a slower speed than mixing with the speed being a critical factor. Problems are with forming lumps or a loose powder. The equipment manufacturer can probably help with finding the right conditions and binder types.

Usually, this is more appropriate for pelletizing than granulation. Rotary drums can be used where the dry mix is sprayed with the binder solution while it is tumbling. Just at the right point, the mix will start to ball up, sometimes making pellets and less often making granules. After drying, the mix is screened to narrow the particle size distribution.

Check List, Granulation

•Use of a granulator

•Screening

•Binder type and amount critical

4.0DIE PRESSING

Die pressing is a very widely used process for forming ceramics. It is suitable for both fine and coarse grained ceramics. This section is divided into eight parts: drying, granulation, screening, die manufacture, basic shapes, cavity fill, pressing procedures, and problems with pressing.

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Die Manufacture

Die tolerance, which is the gap between the punch and the die body, is tighter for fine press mixes than those for coarse mixes, which seems reasonable. Usually, 0.001" is common for fine mixes, 0.002" is common for coarser mixes, but sometimes this can be greater. For lab applications, dies are usually relatively small and can be manually moved. Usually, lab dies are made from hardened steel, while production dies sometimes are faced with cemented carbide. Durability is not ordinarily an issue in lab tooling as the number of parts is limited. Both metallurgy and machining will be discussed.

Metallurgy

One can use a variety of tool steels for dies. The best source of information is the die machine shop. There are two requirements for the metal: hardness and elongation to fracture. Hardness determines the die wear resistance, and is usually between 55 and 60 Rockwell C. Since the die wear mechanism is abrasion, harder metal wears at a lower rate. Hardness is limited by brittleness. There is a relationship between hardness and brittleness: harder is more brittle. Some elongation to fracture is needed to give the metal some forgiveness. At Rockwell C 65, the metal is said to be glass hard. With no give, the die may break, and this can be hazardous and costly. When one needs a very hard die, an outer shell of soft steel can be shrunk fit on forming a safety ring. The press should have a safety shield, regardless. The elongation to fracture should be around 10 to 15%. ASM International Metals Handbook gives this information and the heat treatment necessary to attain the desired properties. 1

Not everyone realizes that the modulus of elasticity does not change appreciably as the steel is hardened. While it becomes harder, it does not become stiffer. Multi-component dies are often fastened with bolts. Always use high strength bolts or cap screws. The tensile strength of these is between 150,000 and 170,000 psi. The cap screws are marked on top as to the strength class; six markings are the high strength ones. Cap

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screws with a recessed head result in a flush surface and are easier to handle. Stainless steel galls are more difficult to work with. Some stainless steels can be hardened, but they are not commonly used because of the tendency to gall. Anti-galling compounds are available. These contain copper and aluminum flakes in grease. This is not the sort of thing one gets into the press mix. If it is necessary to use stainless, use the compound or the die will be ruined. The compound can be found at an auto parts store. Now, it is not necessary to know all about metallurgy to ask the machine shop to make a die. If one asks the right questions at the machine shop, they will be a little more careful with making the die. Shop people like to talk to knowledgeable clients as it enhances their own status and gives them an opportunity to contribute. These craftsmen/women have impressive skills.

Machining Dies

The shop will use milling machines, drill presses, lathes, or other metal cutting tools to rough cut the die shape while the steel is still soft. They will cut a little fat to finish the die to tolerances after it has been hardened. Machine tools usually produce surfaces that are planer, right circular cylinders, or conical. Well-equipped machine shops can produce a variety of other shapes especially if they have computer-controlled, contour facilities. After hardening, the die parts are ground to the final tolerance and finish. We should discuss a few topics on machining.

Grinding Square Corners. A grinding wheel cannot reach into a square corner, as shown in Figure 6.3. Part A of the sketch shows where the edge of the wheel jams up on the corner. Part B shows how this can be relieved by a groove at the corner. These surfaces often are alignment surfaces and have to be flat and square. Another case is machining an inside cone, as shown in Figure 6.4.

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Figure 6.3: Grinding to a Corner. The grinding wheel will jam in the corner unless relief is provided.

Figure 6.4: Grinding a Conical Surface. Relief must be provided at the apex to prevent jamming.

This is a similar problem. The grinding wheel cannot reach into the apex and has to have a run-out space in order to grind the conical surface properly.

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Bolt Positioning. We have placed three bolts at 120 degrees around a bolt circle. There is a better way, where the bolts are placed such that the die body can only fit one way, always the same. One way is shown in Figure 6.5.

Figure 6.5: Offset Bolt Positioning. Offset fasteners provide only one assembly configuration.

This is only one example of a foolproof design. There are many variations of the same principle. Design the assembly so that it can only go together one way. Electronics designers do similarly when designing connectors. A similar thing can be done with plates. Consider the front and back plates of a die as shown in Figure 6.6. The bolts are placed a little off center so that the plates can be assembled only one way. It is also helpful to mark the corners for a visual reference.

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Figure 6.6: Offset Plate Bolting. Offset bolts are designed so that the die is assembled only one way.

Die Parts Positioning. This is another example of foolproof design. Rectangular dies are fastened with bolts that are not precision positioners. Pins can be added and placed so that they fit only one way and with precision. Figure 6.7 shows such an example. Looking back at Figure 6.3, the properly ground flats will assure that the mold is assembled rectilinearly, and the pins assure that it is aligned laterally.

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Figure 6.7: Positioning with Pins. Pins are a precise way to provide alignment.

Fastening to Shafts. Parts can be threaded onto shafts. If properly done, the thread will not unscrew during use. There are both right-handed and left-handed threads. Use a thread where the torque on the shaft will tighten the thread. One can also use jam nuts to hold the part in place. Devices such as cotter pins or castellated nuts can prevent the thread from unscrewing. When there is not much space, one can use tapered pins as the fastener through a close fitting sleeve. Figure 6.8 shows such an example. Tapered pins result in a flush surface that can be a safety advantage, as well as conserving space. They, when selected at the appropriate size, can also act as shear pins that limit the stresses on the shaft.

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Figure 6.8: Fastening a Shaft with Tapered Pins. Fastening is secure and compact with pins.

Strength. Holes and notches are stress risers and will weaken the die. Whenever one can, it is better to put a radius on inside corners and position the holes where they are not in the high stress areas. A mechanical engineer can calculate the stresses in the die design under load. For lab dies, this may not be necessary as it is easy to over design and to avoid the problem.

Machine Shops. These can be internal or external. Internal shops often have their own priorities, especially if they are also doing work for production. Additionally, if the project is not perceived to have a high priority, the project will be placed at the bottom of the list. These priorities are capricious much of the time, and it can be a battle to get the work done. However, there are several alternatives. When it is critical to assign your job to an outside machine shop, assign a machinist specifically to your project. The outside shop is far better than the other choices as the outside machine shop does not get paid for the job until they furnish the part correctly. Money is a very powerful incentive.

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The other advantage is that there are choices for selecting particular skills. Not every shop has all of the same skills and it is worth while to shop around.

Finish. Hardened steel dies are machined oversized, heat treated, and ground to finished dimensions. Faces on die punches should be polished to a reflective finish, especially for fine-grained, press mixes. It is also better to have the inside surfaces polished. There might be some difficulty in finding a shop to do this work locally. There are advertisements in the Bulletin of the American Ceramic Society for this work.2 A superfinish is not a reflective polish, but instead is a lapped finish that is not the best choice. One can coat die surfaces with many materials by applying many processes. Electroplating with chromium is common, but one cannot plate the inside unless the die is dismantled. Plating the outside does slow rusting a little bit. Punch faces can be plated with hard chrome, a reflective finish, provided the steel is previously polished. There are a variety of CVD coatings to coat the die face. CVD coatings are applied at elevated temperatures. CVD materials have a high modulus deposited on a lower modulus substrate.

Basic Shapes

Laboratory dies usually have simple shapes. Some of these shapes and the die setup in the press are discussed here.

Right Circular Cylinders

The die for making this part has four components, as shown in Figure 6.9. With the top punch out, the press mix is poured into the die cavity and leveled. There are two ways to meter out the amount of press mix: by volume or by weight. When metering by volume, the mix fills the cavity and is screed off the die top. Then, the lower punch and mix are

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lowered by removing an additional shim (not shown) and the top punch is then inserted.

Figure 6.9: Cylindrical Die Assembly. The assembly includes a die body, two punches, and shims.

A fill by weight is done by pouring the mix into the die body and leveling it. Leveling is accomplished by vibration, tapping, or with a leveling tool, which is preferred. A leveling tool is rotated in the die to spread the mix out evenly with a blade. The blade is gradually raised to where no more mix is plowed. This is shown in Figure 6.10.

The blade edge should be a knife edge so that the mix is not compacted during leveling. When leveling in a rectangular die, always start from the end and work toward the center to avoid packing the mix at the ends. This is useful whenever the part shape is critical. Packing will lower the firing shrinkage locally and distort the part.