Shell molding process

The shell-molding process, introduced in the United States in 1948, is an important market for phenoHc resins. In the original process, dry sand and powdered resin (6—8%) are blended. However, because of the high binder content and the difficulty in obtaining a uniform mix, precoating methods were developed. 

F Molding processes, all thermoplastic processes Chemical apparatus, typewriter cases, bags, luggage shells, auto trim... 

Anon, New Process , Industrial Bull, A.D. Little, Inc, Cambridge,Mass (1950) 4) PB Report 105131, Plastic Bonded Shell Molding Simplified US OffTechServices, Washington 25, DC (1951)... 

IR-7734, Present Status of the Shell-Mold or "C Process of Precision Casting of Metals ,... 

Molding processes include compression molding, resin transfer molding (RTM), injection molding and reaction injection molding (RIM), autoclave molding, and several types of specific processes such as shell molding, which is used in foundries. 

As a final example of modeling thermoset processing, the process called the shell molding or Croning process will be analyzed. This is just one of many processes found in a variety of industries (automobile, electronics, sports goods, furniture, etc.) that involve the cure of thermosetting polymers. The selected example is a typical process used in foundries. 

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. 

A good mold made of RP will produce hundreds of moldings, with a minimum amount of maintenance. It consists of a shell, reinforced as necessary, often mounted on a light timber or metal frame. Once a suitable master pattern has been prepared, it is possible to produce many RP molds from it, easily and at low cost, using the contact molding process (but in reverse). Patterns can be made from timber or metal, or using plaster on a timber framework. They must be accurately... 

Electroformed molds are produced by a process derived from standard electroplating. The metal (usually nickel) is first dissolved and then reassembled electrolytically around a model. By this means a very dense non-porous metal shell is formed, exactly conforming to the three-dimensional contours of the model and, at the same time, able to reproduce fine surface detail, such as textures or engraving. After forming, the shell (which may be up to 10 mm thick) is removed from the model and engineered into a finished mold by various methods, according to the molding process in which it will be used. 

Coinjection n. A process similar in its results to coextrusion but accomplished by modifications of the injection-molding process. By means of various nozzle and valving arrangement, two or more materials, can be injected either simultaneously or sequentially to form an article with an outer shell of one material with certain desired properties, the shell filled with another material to attain other desired properties such as reduced cost. Coinjection, like coextrusion, basically means that two or more different plastics are formed into a composite or laminated structure. 

Shell molding n. In metal foundries, a process of casting metal objects in thin molds made from sand or a ceramic powder mixed with a thermosetting-resin binder. Some authors have misused the term by equating it to plastics processes such as dipping and slush casting. 

An early move toward automation and mass production was the invention of preform machines that sprayed chopped-glass strands onto a three-dimensional air screen that approximated the shape to he molded. The preformed glass mat was placed in a matched metal die in a compression molding press and liquid resin was added to form the product. Today, resin transfer molding (RTM) and resin injection molding (RIM) preform die-molding processes are used to mass-produce small and mid-size parts with two finished sides, notably automobile bodies, truck cabs, and even small boat shells. 

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