Thermoforming, and Casting

This chapter summarizes other polymer-processing operations not previously considered. Included are calendering, thermoforming (or sheet forming), and casting. 

The first of these, calendering, is a continuous process that uses rolls to form a polymer mass into a sheet of uniform thickness (see Fig. 10-1). Calendering, which is analogous to the rolling operations of the metals industries, had its earliest beginnings in the rubber industry and then spread into the manufacture of a variety of polymer-related products, such as linoleum, plastics, and coated papers. 

Thermoforming, or sheet forming, is a very old polymer-processing operation that involves heating a polymer sheet to its softening temperature and then forming it to a desired shape. The cooled formed sheet represents the product. 

Casting forms a given shape by placing a polymer syrup (partially polymerized system) into a form, shape, or mold in which the syrup is polymerized into a solid mass. The polymers used most often in casting include acrylics, cellu-losics, phenolics, and epoxies. 

The calendering operation is essentially a process involving laminar flow with heat transfer in a roll system. Typical design outputs include desired roll dimensions, roll speeds, separation of rolls, temperature profiles in the processed material, roll-suspension force, and required power. 

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Processing. SAN copolymers may be processed using the conventional fabrication methods of extmsion, blow mol ding, injection molding, thermoforming, and casting. Small amounts of additives, such as antioxidants, lubricants, and colorants, may also be used. Typical temperature profiles for injection mol ding and extmsion of predried SAN resins are as follows (101). 

Molds for hand lay-up (as well as others such as spray-up, contact molding, thermoforming, and casting) are usually made of TS polyester or epoxy shell set in a cradle made of a material such as steel angle. They can usually be made in-house, on a model of the product that could be made from an inexpensive material that can be shaped or sculpted, such as plaster, balsa wood, or expanded polystyrene, sealed and coated with a release agent. 

Literature continues to be rather extensive on this subject since the 1930s. A summarization is provided in this section. Products fabricated include sheets, films, rods and tubes, and embedment. Acrylic castings usually consist of polymethyl methacrylate (PMMA) or copolymers of this ester as the major component with small amounts of other monomers to modify the properties (Chapter 2). Adding acrylates or higher methacrylates lowers the heat deflection temperature and hardness and improves thermoformability and solvent cementing capability, with some loss in resistance to weathering. Dimethacrylates or other crosslinking monomers increase the resistance to solvents and moisture. 

In rotational molding and casting techniques, the processor usually adds his own reinforcement. For thermoforming, glass-RTP laminates are sold commercially. Structural foam molding of glass-RTPs and reinforcing molded urethane foams appeared as later developments in the field of RTFs. 

In all materials used in different aspects of cellulose applications in food packaging, the material characteristics including cellulose, plasticizers, etc., and the fabrication procedures like composites, nanocomposites, casting of a film-forming solution, thermoforming, and so on, must be adapted to each specific food product and the conditions in which it will be used such as relative humidity, temperature, etc. Furthermore, edible and biodegradable films must meet a number of specific functional requirements like moisture barrier, gas barrier, water solubility, color and appearance, nontoxicity, etc. 

Foams are versatile both in their applications and in the way in which the materials are made. These include the production of various shapes by casting, extruding, injection molding, thermoforming, and reaction injection molding. Foam versatihty also includes the fact that any plastic material, thermoplastic or thermoset, may be produced in cellular form. 

Thermoforming is one of the major processes used to produce prototype parts that may be made other ways, such as injection molding. Many materials are used to produce molds that are serviceable for a few to a few himdred parts. Traditional mold materials include wood, plywood, hard plaster such as Hydrocal (US Gypsum), and medium density fiberboard. Sprayed and cast white metal are used on occasion. 

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