Ceramic Technology and Processing, King

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necessary to remember that condoms are expensive, so it is more economical to buy in bulk directly from the manufacturer. Also, condoms come with a variety of coatings, including lubrication, silica dust, and talc, but they should be ordered cleaned and not rolled. There can be a problem with condoms leaking as well. Often, leaking is caused by air pockets that pop. Inspect the condoms to see if they have holes. Vacuum de-airing helps, but it may become necessary to double or triple bag or to change suppliers.

Heat Sealed Plastic Film. Another trick to bag a pressing is to encase it in a bag of heat sealed plastic film. The bag can be sealed around three edges. Slip the pressing into the bag and seal the third side, except for one corner. A vacuum nozzle is pushed into the corner and the bag is evacuated. With the nozzle still in place, the corner is sealed shut while still pulling a vacuum. It can now be isopressed.

Latex Isopressing.6 One can dip a pressing in a latex emulsion forming a rubber bag around the part when dried. However, when the part displaces air by capillary intrusion of the latex, bubbles can form in the film. To prevent this, the part can be sealed first with a polymer in solution. This is similar to a sanding sealer used on wood. Then, the part is dipped in the latex and dried. This will work at room temperature or at 60 °C. It is a good idea to double dip. One can also use rubber cement (office type), other elastomers, or deformable materials that will not crack. There are some advantages to this process: any preformed shape can be isopressed and parts can be isopressed in bulk. When bulk isopressing, the preformed parts are placed in a mesh bag and then dipped into the latex. The bag is like the type used to contain potatoes or onions in the supermarket. Volumes of space can become isolated during drying of the emulsion and will crush in the press. To avoid this, dilute the latex with one part of water to two parts of latex. Because the latex has a pH of 10.3; it is a good idea to adjust the water to this pH before it is added. Dilution lowers the viscosity of the latex, allowing it to drain after dipping. There are latex emulsions for making surgical gloves and latex emulsions for industrial uses. The latter are much cheaper and more satisfactory.

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Dry Bag Isopressing

The difference between wet and dry pressing is that in dry bag pressing the rubber bag is an integral part of the apparatus as shown in Figure 6.26.

Figure 6-26: Dry Bag Isopress. The rubber liner is fastened to the isopress structure. A central core rod will produce a tube.

Dry bag isopressing is especially useful for making shapes such as tubes, spark plugs, and crucibles. It is a very useful production tool, but since the tooling is so dedicated to specific sizes and shapes, it is not commonly seen in the lab. Procedures are to fill the cavity with the press mix, close the top, and apply pressure to the rubber sleeve. The pressure is released and the part ejected, often with an air cylinder.

Forming 175

Check List, Isopressing

•Elastomer bags

•Perforated Al tube support

•Vacuum de-air

•Do not overpress

•Spider to center central rod

•Double bag condoms

•Order clean and flat condoms

•Heat seal plastic film bag

•Latex isopress complex shapes and bulk materials

6.O SLIP CASTING PROCEDURES

It was observed, on a microscopic scale, why slip casting can sometimes produce a higher green density than die pressing, isopressing, or injection molding. A dilute water suspension of alumina was being observed on a slide. Particles were migrating toward the edge of the slide where a crust was forming due to drying. This crust was analogous to a permeable plaster wall adsorbing the water from the suspension. As the particles approached the wall, they streamed in channels and oriented with the long dimension parallel to the channel direction because of fluid shear across the channel. As the channel narrowed, the particles wedged into the wall with the orientation geometry, resulting in dense packing. As one channel became obstructed, others would open and the process would repeat. High green density in slip casting is the consequence of fluid mechanics, which orients the particles and jams them into the casting surface. Under ideal slip casting conditions, green densities up to 60% can be achieved on sub-micrometer slips.

Leading up to this section on casting procedures, several topics were discussed earlier. These include slip preparation, binders, slip storage, and the effects on viscosity of the surface chemistry.

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176 Ceramic Technology and Processing

Plaster

Plaster of Paris is a mineral product made from calcined gypsum in which one half of a molecule of water is left in the structure. If lacking all the water, the material becomes anhydrous and is useless for casting. Number One pottery plaster is generally used for slip casting. There are other grades that are useful for other things, but use Number One for slip casting.

Plaster of Paris will absorb water when exposed. To slow the hydrating process, store plaster in a plastic bag, place the bag in a garbage can, and cover the can with a lid. Plaster will keep for a few months if kept from hydrating. Figure 6.27 shows the effect of consistency on strength, absorption, and dry specific gravity.

Figure 6.27: Plaster Consistency. As more water is added, the plaster strength decreases and absorption increases.

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Consistency refers to parts of water per 100 parts of plaster by weight. For slip casting, a consistency of 60-70 is in the right range. Almost all plaster shops use tap water at room temperature or, control the temperature so that it is always the same. Water fresh out of the tap will not be at room temperature, so a holding tank is a good idea. Deionized water from a holding tank for lab use is good practice because it is consistent. If one uses tap water, filter it with a fine filter. Distilled water is available at the supermarket and is an alternative. Read the label and make sure it is distilled, however. Spring water may not necessarily be distilled water. Dried plaster is porous so that it can absorb water. Figure 6.28 is an SEM photo of a fracture surface on plaster.

Figure 6.28: Plaster Microstructure. Plaster forms clusters of needles with porosity in between. Scale bar 10 μm.

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The structure is comprised of interlocking needles of the plaster. Interstices between the needles are the channels through which the water flows during casting.

Process For Making a Plaster Slurry8

Formula. To obtain a consistency of 67, mix two parts of water to three parts of plaster by weight. This results in 0.065 final pounds of plaster per cubic inch. The formula establishes how much to mix provided the volume of the mold is known.

To avoid clumps, sift the plaster powder through the fingers into the water. Allow the plaster to slake for three and a half minutes. Next, stir with a mixer, at a moderate speed, for one minute. The plaster will start to hydrate. To prevent settling of the plaster particles and to remove large air bubbles, stir the slurry slowly by hand in a lifting motion. The viscosity will increase in a few minutes to where a path made by dragging your finger across the surface will persist. It is now thickened and ready to pour into the mold. If one pours it too soon, the particles will settle. When poured too late, the plaster will not completely fill the mold.

Revised Plaster Process

The traditional slip process has worked for decades, and many plaster shops still use it or a similar version. Some experience with slips suggests a new process where the plaster is mixed at a higher intensity to deagglomerate the plaster particles. This results in a rapid set. High intensity mixing releases the particles from the agglomerates so that they can participate in the setting process. Only the free particles can participate in the set. Plaster concealed within the agglomerates will hydrate of course, but they are not in contact with the fluidity and gelation of the suspension. As these are released, they are free to participate in the set. The conclusion is that plaster is sensitive to the mixing intensity. By working fast, the mold can be poured.

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Vacuum de-airing the plaster removes bubbles and improves its uniformity. De-airing was discussed in Chapter IV. Plaster slurries act just like ceramic slurries in de-airing. There is little problem in high intensity mixing and vacuum de-airing, but the mold has to be ready for pouring as there isn't time for procrastination. Usually, the mold is vibrated during pouring to remove air bubbles trapped on the walls of the form.

Marchant, McAlpin, and Stangle describe a process where the plaster slurry is indefinitely stored hot.9 In this technique, there is no rush to pour the mold, and this would permit batching larger quantities of slurry that have to be stored hot.

Mold Preparation

The primary consideration for a mold is of course the geometry of the part and allowance must be made for shrinkage caused drying and firing. A pattern is made around which and into which the plaster is poured. Materials used for patterns should be impervious: aluminum, plastics, machinable composite, or well-sealed wood. Some pattern shops use a wood fiber composite called Ren Shape. Some pattern makers have computer-controlled, contouring capability so that any shape can be made. The two other parts of the assembly for simple molds are the base plate and the shell. A few types of shapes will be illustrated.

Right Circular Cylinder. The exterior shell can be anything with the right size and shape. A piece of plastic tubing cut off square and split down the side is useful. The base plate can be glass unless it is necessary to fasten the core to the plate with a bolt, in which case a polished chrome-plated, metal sheet of the type used to finish photographic prints (Ferrotype) is useful. Figure 6.29 shows a typical setup.

Here, the core is a cylinder bolted to the base plate. The shell is a length of plastic tubing split down the sides. Masking tape helps seal the splits. The bottom edge of the shell has to be sealed or the plaster will run out. One can use a fillet of modeling clay from a toy store to seal the base, but do not use the type that dries out. When using a glass base plate, the

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core can be cemented in place with a dot of hot melt adhesive or even rubber cement; this is a little risky as it might not hold. Include in the core the threaded hole that is useful for later pulling out the core from the plaster. Depending upon the length, one can use this type of mold for casting crucibles or tubes.

Figure 6.29: Plaster Form, Cylinder. A simple mold for making crucible or tubular shapes.

All of the surfaces in contact with the plaster are coated with a release agent, such as a mold soap solution of lithium stearate. Foundry supply houses carry a variety of products useful in mold preparation, such as release agents.

When the form is set up, the plaster is poured into the mold down one side to prevent entrapment of air bubbles. Overfill just a little bit. After a short time, the plaster surface will roughen due to formation of the gel structure. It is pasty and the top can be screed off flat. Setting is exothermic and the shell will become warm to the touch. After it cools down in a

Forming 181

couple of hours, the shell can be removed and the plaster mold removed from the base plate. Put the bolt back into the core and pull it out. Plaster expands a little bit when curing, and this helps to remove the core that will be free.

It is important to clean up right away. Excess fluid plaster can be scraped into a container and left to harden; this is harmless and can go into the trash. Washing of the tooling requires a good flow of water in order to prevent the drain from clogging. A sump is a good idea for a ceramics lab because it will permit the solids to settle out.

Plaster has salts in solution that will migrate to the drying surface. This is not desirable on the casting surface, nor are finger prints or other irregularities. To prevent this, dry the plaster mold from the outside surface. This can be as simple as placing the mold upside down on the bench. Dry the mold the same way after slip casting to avoid accumulation of binders or surfactants on the casting surface. It takes about three days to air-dry a new mold. One can tell because it will feel warm rather than cold. The corners of the mold should be chamfered and scribed with an identification number.

Flat Plates. A flat ceramic plate can be cast on a plaster block with side restraints, but it will warp when fired due to density gradients across the thickness. A better design is to cast from both sides with the split along the diagonal.

It would be very difficult to remove a plate from the mold when the part is locked into a depression. By splitting the plaster mold along the diagonal, the plate is not physically restrained for removal. Another problem is that there is a large amount of contact area between a plate and the mold, but there are ways of handling this that will be described in a later section. There is also pipe along the center line. Ways to handle this will be discussed later.

Solid Plate. Two molds for making a solid plate are shown in Figure 6.30. The shapes of the molds are seen in the photograph. Note the separation along the body diagonal used for recovering the part.

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Figure 6.30: Flat Plate Plaster Mold. The photograph shows the mold configuration with separation along the body diagonal.

Solid disc. The mold is shown in the next two figures. Because of the shape, it is easier to cast one edge as a disc and green machine the other later. The tooling and procedure for making the mold are a challenge.

The mold in the figures can be made from wood patterns except the shape form that is aluminum. Aluminum is easier to obtain release. The wood parts can be coated with a sanding sealer as long as the wood is porous, sanded, and then coated with three coats of gloss polyurethane. If the wood is not sealed well enough during the plaster cure, it can hydrate to some extent. The plaster mold is made in two steps: first make the lower part of the plaster mold and then cast the upper half on top of the lower half. By casting the two parts against each other, a perfect fit is assured. A coating of mold soap prevents the two halves from adhering to each other and to the wooden pattern. The whole pattern consists of 14 wood parts and