What is it? Large thermoplastic sheet moldings are economically made by thermoforming. In vacuum thermoforming a thermoplastic sheet, heated to its softening point, is sucked against the contours of a mold, taking up its profile; it is then cooled, solidifying against the mold. Drape thermoforming relies partly on vacuum and partly on the natural sag of the hot polymer to form the shape. Plug-assisted thermoforming augments the vacuum with a compression plug. Pressure thermoforming uses a pressure of several atmospheres to force the hot polymer sheet onto the mold. Male or female molds are possible and – for vacuum thermoforming – can be machined from wood, polymer foam, or from aluminum (for larger batch sizes).
Shapes Curved thin-walled shapes.
Design Notes The low pressure in vacuum forming gives poor reproduction of fine details; pressure forming, using higher pressures, gives sharper features but is more expensive because wooden molds cannot be used. The surface of the sheet in contact with the mold tends to mark, so the tool is usually designed with the finished side away from the mold. Colored, textured or pre-decorated sheet can be molded, reducing finishing costs.
Technical Notes Thermoforming is used to shape thermoplastic sheet, particularly abs, pa, pc, ps, pp, pvc, Polysulphones, pbt, pet, foams, and shortfiber-reinforced thermoplastics. The maximum depth-to-width ratio of the molding is in the range 0.5 to 2. Inserts can be molded in. The process is able to cope with a very large range of sizes from products as small as disposable drink cups to those as large as boat hulls; and it is economic for both small and large batch sizes. It gives products with excellent physical properties but the starting material is more expensive (sheet rather than pellet). The product has to be trimmed after forming, and sheet scrap cannot be directly recycled.
The Economics Both the capital and the tooling costs of thermoforming equipment are low but the process can be labor intensive.
Typical Products Appliances, refrigerated liners, bath tubs, shower stalls, aircraft interior panels, trays, signs, boat hulls, drink cups.
The Environment No environmental problems here.
Competing Processes Injection molding (for large batch sizes); rotational