Process Capabilities

All thermoforming techniques are stretching processes. The stretching of a flat sheet of plastic material into a larger shape results in a larger surface area and a corresponding reduction in the sheet's original thickness. Unfortunately, the thinning down of the sheet's original thickness is not uniform.

A hemisphere (Figure #1) would be the ideal shape for thermoforming.

A hemisphere formed in a female die (Figure #2) would be the thinnest at the apex of the part and thickest at the open end.

The same hemisphere drape formed over a male die (Figure #3) at the open end of the part and its maximum thickness at the apex of the hemisphere.

In both cases, that portion of the sheet that first contacts the relatively cool die stretches and thins out the least. This is due primarily to the fact that the material that contacts the die first begins to cool and becomes stronger and therefore resists further stretching and thinning out.

A five-sided box of the type shown below (Figure #4) is a more difficult shape to form due to the greater amount of stretching required to reach the square corners.

The forming of parts of this shape becomes increasingly more difficult and results in greater thinning of the sheet as the sharpness of the corners becomes more pronounced as in (Figure #5) .

This situation resulted in the traditional practice of providing generous radii on the corners of square thermoformed parts. A good inside radius has traditionally been considered to be four times the original sheet thickness (Figure #5 A) , with a minimum inside radius being equal to the original sheet thickness (Figure #5 B). Consequently, thermoformed parts have always been typified by relatively larger corner radii, obvious draft angles and smoothly blended contours. The corner radii that are practical with Thermo Pressure Forming are reviewed in detail on Page 10.