Plastic processes thermoforming

The main heat transfer processes that are come across in plastics processing are conduction, convection and viscous heating, with radiation only playing a role in thermoforming. Most products are much thinner than they are wide, so only one-dimensional heat flow (Fig. 5.2) will be considered. The heat flow direction is along the x axis, perpendicular to the surface of the product there are planar isotherms perpendicular to the x axis. The heat flow Q is considered across an area A of the isothermal surface. 

Styrenic Resins Styrenic resins are thermoplastics prepared by firee-radical polymerization of styrene alone or with other unsaturated monomers. The properties of styrenic resins vary widely with molecular structure, attaining the high performance level of engineering plastics. Processed by blow and injection molding, extrusion, thermoforming, film techniques, and structural foam molding. Used heavily for the manufacture of automotive parts. 

Compressed air thermoforming is a technique borrowed from sheet steel processing its main use in plastics processing is in plastic cup production. Machines are also coming into use for the production of engineering components, such as computer covers. A flexible-pressure pad is used to press the hot sheet into the mold cavity, with the aid of an air cushion maintained under pressure. Compared with vacuum forming, this... 

PLASTICS PROCESSING DATA HANDBOOK Table 5-3. Guide to Thermoforming Processing Temperatures (°F). ... 

The thinnest nominal walls are created by the stretching processes thermoforming and blow molding. In each case, the material is first processed by some other method before it is shaped. Thermoforming uses extruded sheet which is then heated and stretched over a tool. Blow molding uses an extruded tube or injection moldment which is then blown into a mold. Plastic thins as it is stretched, and therefore, the effect of this thinning must be carefully considered in the part design. 

Blow molding, until the last few years, was the main plastic process utilized to produce a hollow object. In the past few years, other plastic processes, such as rotational molding and twin sheet thermoforming, have evolved with technical achievements and plastic raw materials improvements to where today, they can compete with blow molding for many uses as toys, gasoline tanks, holding tanks, etc. 

G. Gruenwald, Thermoforming A Plastics Processing Guide, 2nd ed., Technomic Publishing Co., Lancaster, Pa., 1998, Figure 2.2. 

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