Thermoforming of Plastic Film and Sheet
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Thermoforming is the process of heating a plastic material in sheet form to its particular processing temperature and forming the hot and flexible material against the contours of a mold by mechanical means (e.g., tools, plugs, solid molds, etc.) or pneumatic means (e.g., differentials in air pressure created by pulling a vacuum or using the pressures of compressed air). When held to the shape of the mold and allowed to cool, the plastic retains the shape and detail of the mold. Because softening by heat and curing by the removal of heat are involved, the technique is applicable only to thermoplastic materials and not to thermosets.
Examples of thermoformed products are plastic or foam dinnerware, cups, meat and produce trays, egg cartons, refrigerator liners, computer housings, interior and exterior automotive parts, blisters for packaging, and countless others. These common and often taken-for-granted products are not usually thought of as the result of detailed tooling design, precise controlled heating and forming, expert material technology, and trimming/finishing operations. Clearly, the thermoforming process is an important link in the plastics industry.
The basic process as it is known today has been developing for over 40 years, but historians have found that the Pharoahs of Egypt developed a type of thermoforming by softening tortoise shells with hot oils to form food utensils. During World War II, with the development of thermoplastics, thermoforming was used to produce aircraft canopies and domes, as well as relief maps.
During the 1950s, thermoforming began to expand into new areas—acrylic windshields, outdoor signs, blister packaging, and refrigerator door liners.
Exceptional growth occurred during the 1960s, as thermoforming became one of the major plastics fabricating techniques. High-volume markets for individual-portion food packaging, blister packaging, and appliance housings were developed.
The 1970s brought an emphasis on high-volume, high-speed processing machinery—and viable competition with the pulp and paper industry resulted.
Advantages of thermoforming over most other methods of processing plastics include lower tooling and machinery costs, high output rates, the ability to use predecorated plastic sheet, and good-quality physical properties in finished parts.
Its disadvantages include the need to begin with sheet or film rather than less costly basic resins, trimming material used to clamp sheet for forming, and the problem of trim scrap reclamation.
KeywordsPlastic Film Mold Temperature Syntactic Foam Zinc Stearate Sheet Stock
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