Continuous sheet forming extrusion

Plastic materials have low thermal conductivities, hence during heating, even relatively thin sheets can have surface temperatures that are substantially higher than the interior temperatures. Therefore, plastic sheets must be heated to an optimum forming temperature in order to achieve uniform and repeatable drawdown. Figure 4.5 illustrates continuous sheet forming extrusion via the thermoforming process. 

Figure 4.5 Continuous sheet forming extrusion via the thermoforming process...

Most films are produced by extrusion. Thermoplastic materials are heated and pushed through a die to form a flat tube. Warm air is blown into the extruded tube to produce a balloon which is then cut open and laid flat. Calendering can also be employed to produce film. Hot thermoplastic is passed through a series of temperature-controlled metal rolls with progressively smaller gaps to produce a continuous sheet (Figures 3.27 and 3.28). The technique is used to make shower curtains, food wrap films, carrier bags and protective films. The most widely used films are polyethylene, nylons, polypropylene, cellulose acetate, PVC and polyesters. 

Its malleability and ductility make it possible to produce zinc in sheet, strip and plate form by rolling and as rod and wire by extrusion continuous casting is also used. Rolled zinc in sheet and strip form is a well-established... 

Extrusion coating is ideal for rods, tube and wire. The article is passed through a paint reservoir and then out via a die, which leaves only the correct thickness of paint in place. There are further techniques suitable for flat articles in sheet or web form. Knife coating is ideal for very thin coats, especially on continuous paper or plastic webs. The knife is either a metal doctor blade or a curtain of high velocity air (an air knife) directed onto the surface and it removes surplus material applied previously. 

Bulk Polymerization. This is the method of choice for the manufacture of polytmethyl methacrylatel sheets, rods, and lubes, and molding and extrusion compounds Three bulk polymerization processes are commercially important lot the production of methacrylate polymers hatch cell casting, continuous casting, and continuous bulk polymerization. Approximately half the worldwide production of bulk polymerized methacrylates is in the form of molding and extrusion compounds, a quarter is in the form of cell east sheets, and a quarter is in the form of continuous cast sheets. 

Extrusion is a continuous process in which an Archimedes screw, rotating within a heated barrel, melts and pumps the polymer through dies to form continuous shapes such as pipes, profiles or sheets. In addition, extrusion is the basic element in a number of other polymer processing operations. The plastication and pumping functions... 

On the other hand, during continuous processes (e.g. profiles, pipes, sheets, films and foams extrusion) the solidification occurs in gradual procedure enabling not only forming of final shape in longer timescale usually by the help of additional... 

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