Thermoforming

Thermoforming is the heating of a thermoplastic sheet until it is soft and stretchable and then forcing the hot sheet against the contours of a mold by mechanical (plug assist), vacuum, pressure, or a combination of all three. After cooling, the plastic sheet retains the mold's shape and detail. Thermoforming is still a rapidly growing processing method because of the range of products that can be formed and the relatively low cost of required tooling and equipment. Thermoformed products include dinnerware, cups, automotive parts, egg cartons, and blister packaging. There are a wide variety of processes for thermoforming. One-step processes include the following:

Drape forming

Vacuum forming

Pressure forming

Free blowing

Matched die molding

Drape forming, as shown in Fig. 1.29,

involves either lowering the heated sheet onto a male mold or raising the mold into the sheet. Usually, either vacuum or pressure is used to force the sheet against the mold. In vacuum forming (Fig. 1.30),

the sheet is clamped to the edges of a female mold, then vacuum is applied to force the sheet against the mold. Pressure forming is similar to vacuum forming except that air pressure is used to form the part (Fig. 1.31).

In free blowing, the heated sheet is stretched by air pressure into shape, and the height of the bubble is controlled using air pressure. As the sheet expands outward, it cools into a free-form shape as shown in Fig. 1.32.

This method was originally developed for aircraft gun enclosures. Matched die molding (Fig. 1.33)

uses two mold halves to form the heated sheet. This method is often used to form relatively stiff sheets.

Multistep forming is used in applications for thicker sheets or complex geometries with deep draw. In this type of thermoforming, the first step involves prestretching the sheet by techniques such as billowing or plug assist. After prestretching, the sheet is then pressed against the mold. Multistep forming includes the following:

Billow drape forming

Billow vacuum forming

Vacuum snap-back forming

Plug assist vacuum forming

Plug assist pressure forming

Plug assist drape forming

Billow drape forming consists of a male mold pressed into a sheet prestretched by the billowing process (Fig. 1.34).

A similar process is billow vacuum forming, wherein a female mold is used (Fig. 1.35).

In vacuum snap-back forming, vacuum is used to prestretch the sheet, then a male mold is pressed into the sheet and, finally, pressure is used to force the sheet against the mold as seen in Fig. 1.36. In plug assist, a plug of material is used to prestretch the sheet. Either vacuum or pressure is then used to force the sheet against the walls of the mold as shown in Figs. 1.37

and 1.38.

Plug assist drape forming is used to force a sheet into undercuts or corners (Fig. 1.39).

The advantage of prestretching the sheet is more uniform wall thickness. Materials suitable for thermoforming must be compliant enough to allow for forming against the mold, yet not produce excessive flow or sag while being heated. Amorphous materials generally exhibit a wider process window than semicrystalline materials. Processing temperatures are typically 30 to 60°C above Tg for amorphous materials and usually just above Tm in the case of semicrystalline polymers.37 Amorphous materials that are thermoformed include PS, ABS, PVC, PMMA, PETP, and PC. Semicrystalline materials

that can be successfully thermoformed include PE and nucleated PETP. Nylons typically do not have sufficient melt strength to be thermoformed. Table 1.9

shows processing temperatures for thermoforming a number of thermoplastics.

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