Casting techniques plastics

Conventional machining operations are used preferably from the same plastic to be used in the product (Chapter 8, SECONDARY EQUIPMENT). Different casting techniques are used that provide low cost even though they are usually labor intensive. The casting of unfilled or filled/reinforced plastic used include TS polyurethane, epoxy, structural foam, and RTV silicone. Also used are die cast metals. 

In another casting technique, that of infection molding, used for intricate shapes or small diam dimensions, the complete item is produced in place by relatively low press-forcing of an exp 1-plastic mixt which is then allowed to cure (Fig 2). 

Different foundry casting techniques are used. Included are plastic-based binders mixed with sand. Various types of molds and cores are produced that include no-bake or cold-box, hot-box, shell, and oven-cured. Usual binders are phenolic, furan, and thermoset polyester. There is the foundry shell casting, also called dry-mix casting. It is a type of process used in the foundry industry, in which a mixture of sand and plastic (phenolic, thermoset polyester, etc.) is placed on to a preheated metal pattern (producing half a mold) causing the plastic to flow and build a thin shell over the pattern. Liquid plastic pre-coated sand is also used. After a short cure time at high temperature, the mold is stripped from its pattern and combined with a similar half produced by the same technique. Finished mold is then ready to receive the molten metal. Blowing a liquid plastic/sand mix in a core-box also produces shell molds. 

Potting involves casting a plastic in a shell that is a container representing a mold cavity. Within the plastic a product could be embedded. Potting is similar to encapsulation except that the shell is not separated from the finished product. It is an embedding technique in which the shell and plastic remains consolidated. 

Extrusion is employed for the composite propellants of plastic properties, and cast technique is used for composite propellants of rubbery properties. 

A doctor blade spreads the suspension on a fixed support (glass, stainless steel, plastic film). For a given suspension, the height of the blade and the speed of the reservoir will determine the thickness of the tape. This casting technique is suitable for the realization of high thicknesses (substrates). 

Large and small shapes may be sUp cast from both plastic and nonplastic mixes by the usual techniques. Precise shapes, such as glass feeder parts, are made in this way as well as large flux blocks. The process requkes the formulation of a sUp of suitably stable character to be poured into a plaster mold to be dewatered. After it solidifies, the mold is removed and dried further before firing. 

The first five of these techniques involve deformation and this has to be followed by some setting operation which stabilises the new shape. In the case of polymer melt deformation this can be affected by cooling of thermoplastics and cross-linking of thermosetting plastics and similtir comments can apply to deformation in the rubbery state. Solution-cast film and fibre requires solvent evaporation (with also perhaps some chemical coagulation process). Latex suspensions can simply be dried as with emulsion paints or subjected to some... 

Poly(vinyl alcohol) is employed for a variety of purposes. Film cast from aqueous alcohol solution is an important release agent in the manufacture of reinforced plastics. Incompletely hydrolysed grades have been developed for water-soluble packages for bath salts, bleaches, insecticides and disinfectants. Techniques for making tubular blown film, similar to that used with polyethylene, have been developed for this purpose. Moulded and extruded products which combine oil resistance with toughness and flexibility are produced in the United States but have never become popular in Europe. 

Casting permits applying different techniques. As an example half or part of the casting can gel. The product such as an ornament is placed on the gelled plastic followed with the final pouring of the plastic... 

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