Thermoforming
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Thermoforming is a manufacturing process for thermoplastic sheet or film. The sheet or film is heated between infrared, natural gas, or other heaters to its forming temperature. Then it is stretched over or into a temperature-controlled, single-surface mold. Cast or machined aluminum is the most common mold material, although epoxy and wood tooling are sometime used for low volume production. The sheet is held against the mold surface unit until cooled. The formed part is then trimmed from the sheet. The trimmed material is usually reground, mixed with virgin plastic, and reprocessed into usable sheet [1]. There are several categories of thermoforming, including vacuum forming, pressure forming, twin-sheet forming, drape forming, free blowing, and simple sheet bending.
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[edit] History
Thermoforming is one of the oldest plastics processes. Baby rattles and teething rings were formed of camphorated cellulose nitrate or celluloid™ in the 1890s. The industry did not grow substantially until the 1930s, when the development of cellulose acetate and acrylic provided the industry with formable sheet. The earliest roll fed thermoforming machines were developed in the late 1930s in Europe.
[edit] Thin and thick gauge thermoforming
There are two general thermoforming process categories. Sheet thickness less than 1.5 mm (0.060 inches) is usually delivered to the thermoforming press in rolls. Thin gauge roll fed thermoforming applications are dominated by rigid or semirigid disposable packaging. Sheet thickness greater than 3 mm (0.120 inches) is usually delivered to the forming press cut to final dimensions and stacked on pallets. Heavy or thick gauge, cut sheet thermoforming applications are primarily used as permanent structural components. There is a small but growing medium gauge market that forms sheet 1.5 mm to 3 mm in thickness.
Heavy gauge forming depended on convection oven heating of the sheet and draping of the sheet over male, female or paired molds. Aircraft windscreens and machine gun turret windows spurred the advance of heavy gauge forming technology during WWII. Heavy gauge parts are used as cosmetic surfaces on permanent structures such as automobiles, refrigerators, spas and shower enclosures, and electrical and electronic equipment.
[edit] Engineering
Thermoforming can fabricate thin walled parts with large areas, using relatively inexpensive, single-sided tooling. Its deficiencies – variable wall thickness, added cost of sheet and trim regrind, and extensive trimming – have been offset by the ability to economically produce a few, thick walled parts or very many thin walled parts.
Thermoforming has benefited from applications of engineering technology [3]. Infrared radiation is the primary method of heating sheet. The softened sheet is arithmetically treated as a rubbery elastic membrane as it stretched mechanically or pneumatically into or onto a cooled single surface rigid mold. Finite element analysis is used to predict local wall thickness of the formed sheet. Heavy gauge sheet is trimmed with multiaxis routers adapted from the woodworking industry.
[edit] Industry
The more than USD10 billion North American market has traditionally been ¾ thin gauge and ¼ heavy gauge. In 2003 there were about 150 thin gauge thermoformers in North America. Sixty percent formed proprietary products. Thirty percent were custom formers and 10 percent were OEMs with in-house forming capability. There were nearly a dozen thin gauge formers having annual sales of at least USD100 million. The largest had annual sales in excess of USD1,000 million. There were about 250 heavy gauge formers in North America. Nearly all were custom formers. Only two or three heavy gauge formers had annual sales of more than USD100million. The largest had annual sales of about USD140 million [2].
[edit] References
- J.L. Throne, Understanding Thermoforming, Hanser Gardner Publications, Inc., Cincinnati OH, 1999.
- The Industrial Thermoforming Business: Review and Outlook, Plastics Custom Research Services, Advance NC, 2004.
- J.L. Throne, Technology of Thermoforming, Hanser Verlag, Munich, 1996.
- Florian, J., "Practical Thermoforming", 1996, ISBN: 0824797620
- Gruenwald, G, "Thermoforming: A Plastics Processing Guide", 1998, ISBN: 1566766257
