THERMOFORMING

Thermoforming is the art and science of forming commercial products by heat-
ing plastic sheet to a softened, pliable state, pressing the sheet against a cool mold, holding the formed sheet against the mold until rigid, and trimming the formed part from the web or skeleton surrounding it. Nearly all unfilled or un-reinforced thermoplastics are formed in this manner on conventional equipment. Newer forming technologies are used to form filled and reinforced thermoplas-tics and certain thermosetting polymers. In general, thermoforming is used when large surface area-to-wall thickness parts are needed, when rapid evaluation of product designs are sought, when very high production rates of thin-walled parts are desired, and when a few to a few hundred thick-walled parts are needed.

Although commercial thermoforming, sometimes called vacuum forming or swedging, was not developed until the 1870s, when cellulose nitrate was first cut into thin sheets, Egyptians, Pacific natives, and American Inuits formed naturally occurring tortoise sheet and tree bark or natural cellulose into bowls and boats long before then . In the 1870s, cellulose sheet was formed using metal molds and steam as the heating and forming medium . The earliest products were baby rattles, toys, mirror cases, and hairbrush backs. In the early 1900s, piano keys were drape-formed over captive wooden cores. In 1930, Fernplas Corp. patented a bottle fabri-cated from two thermoformed halves. Relief maps for the U.S. Coast and Geodetic Survey were thermoformed of cellulose acetate in the 1930s. The first automatic roll-fed thermoformer was sold by Clauss B. Strauch Co., in 1938, to manufacture cigarette tips and ice-cube trays. The heating, bending, and shaping of plastic sheet were taught in high school industrial art courses in the late 1930s .

The Second World War accelerated interest in thermoforming, with the demand for cast poly(methyl methacrylate) fighter/bomber windows, gun closure and windscreens .
By the mid-1950s, thermoformed blister packages and food containers of polystyrene were found in most grocery stores. In 1962, approximately 77,000 t of plastic was thermoformed in the United States. By 1998, approximately 2.9 million metric tons of plastic were thermoformed in North America . This is a sustained annual growth rate of about 10% over nearly four decades. An additional 4.55 million metric tons are thermoformed worldwide. The total world market is estimated to have a value of about US$ 35,000 million.

Thermoforming is typically bifurcated into thin-gauge thermoforming and heavy- or thick-gauge thermoforming. As seen in Table 1, thin-gauge thermo-forming uses sheet 1.5 mm or less in thickness, with its primary products be-ing packaging containers. Typical disposable products include blister packages, point-of-purchase containers, bubble packages, slip sleeve containers, auto/video cassette cases, hand and power tool cases, cosmetic cases, meat and poultry con-tainers, unit serving containers, convertible-oven food serving trays, wide-mouth jars, vending machine hot and cold drink cups, egg cartons, produce and wine bot-tle separators, medicinal unit dose portion containers, and form, fill, and seal (FFS) containers for foodstuffs, hardware supplies, medicine, and medicinal supplies.

Heavy-gauge thermoforming uses sheet 3 mm or more in thickness, with pri-mary products being permanent or industrial products. Typical products include equipment cabinets for medical and electronic equipment, tote bins, single and double deck pallets, transport trays, automotive inner-liners, headliners, shelves, instrument panel skins, aircraft cabin wall panels, overhead compartment doors, snowmobile and motorcycle shrouds, fairings and windshields, marine seating, locaters and windshields, golf cart, tractor, and RV shrouds, skylights, shutters, bath and tub surrounds, lavys, single- and double-wall shipping containers and pallets, storage modules, exterior signs, swimming and wading pools, landscap-ing pond shells, luggage, gun and gulf club cases, boat hulls, animal carriers, and seating of all types.

There is a growing but very still limited market for products formed from sheet between about 1.5 mm (thin-gauge) and 3.0 mm (heavy-gauge) thickness. Usually, products of this thickness are either too expensive to be disposable or too thin to be industrial or permanent products. One major application is in the manufacturing of very large volume drink cups (1/2 L or more).

Currently thin-gauge thermoforming accounts for about three-quarters of all sheet formed, in both tonnage and dollar volume. Thin-gauge thermoforming companies tend to be very large with broad spectra of products. Furthermore, companies that manufacture products may also do in-house thin-gauge thermo-forming for the packages for these products. Heavy-gauge thermoforming compa-nies tend to be small with narrow product lines. As a result, there are many more heavy-gauge thermoforming companies than thin-gauge thermoforming compa-nies. In 2001, it was estimated that there were about 500 heavy-gauge thermo-forming companies and less than 200 thin-gauge thermoforming companies in North America .

As outlined in Table 1, there are substantial differences in the characteristics of these two thermoforming categories. In addition to the sheet thickness crite-rion, there is a difference in the way the sheet is presented to the thermoforming machine. Thin sheet is usually delivered in rolls of up to 3000 m in length, weigh-ing up to 2300 kg and having diameters up to 1.5 m. The sheet is fed continuously into the thermoforming machines that are usually called roll-fed machines. Thick sheet is usually guillotine-cut to size and palletized. The individual sheets are then loaded manually or pneumatically into the thermoforming machines, known as cut-sheet machines.

Thermoforming is a competitive technology. In thin-gauge it competes with paper, paperboard, plastic-coated paper and paperboard, paper pulp, expanded polystyrene foam, aluminum foil, and roll-sheet steel. It also competes with plastics extrusion, compression molding, stretch-blow molding, injection molding, and injection-blow molding. In heavy-gauge, it competes with injection molding, rotational molding, blow molding, fiberglass-reinforced polyester resin spray-up molding and lay-up molding, compression molding, sheet compound molding, bulk compound molding, sheet metal forming, and metal die casting.

When compared to other technologies, thermoforming offers many advantages: there is a wide variety of polymers from which to choose; molds are singlesided and are thus less expensive than injection molds; the time from concept to final part acceptance is usually quite short; there are many available mold materials; aluminum—the mold material of choice—is lightweight, has a high thermal conductivity, is relatively inexpensive, and is easy to machine and cast; processing temperatures are low; processing pressures are very low; mold detail replication is good; part surface area-to-wall thickness is extremely high; and there are many excellent trimming techniques.

However, thermoforming has some serious limitations. Among others, the polymer of choice may not be extrudable or may sag too much during heating in the thermoforming machine; there is additional cost in producing sheet; the un-used portion of the sheet—the trim, web, or skeleton—must be recycled to keep sheet costs reasonable; because of the end-use of the product (medical, pharma-ceutical, foodstuffs), recycling of the trim may not be acceptable, it may not be possible to stretch the sheet sufficiently to achieve the desired part shape, part wall thickness is not well-controlled or predictable, and is not uniform across the part; wall thickness cannot be changed locally through design; surface texture may be required on both sides of the part; the part performance criteria may required reinforced or highly filled polymers; the part tolerance, edge radii, and draft angles may be unacceptably tight for the thermoforming process; and there may be other processes that are more economically attractive.