Thermoforming wall thickness distribution

Elden, R.A. and Swann, A.D, Calendering of Plastics, Plastics Institute Monograph, Iliffe, 1971. Rosenzweig, N., Markis, M and Tadmar, Z., Wall Thickness Distribution in Thermoforming Polym. Eng. Sci. October 1979 Vol 19 No. 13 pp 946-950. 

Many problems may be encountered in thermoforming, some of these are related to plasticizers or they can be eliminated by changes in plasticizer type and amount Problems related to plasticizers include die lines or dull spots (plate-out buildup), dark specs and streaks (polymer degradation), excessive sag, distortion after part removal, warpage, poor wall thickness distribution, and part sticking to mold. 

Thermoforming results in characteristic thickness distributions of the part s walls. Once the sheet contacts the mold surface, it sticks and locally cools. The wall thickness distributions are due to hindering and stretching of the hot sheet material. Thermoformed sheet once formed as product is cooled below its softening to freeze is normally performed by conduction via the mold surface. The mold is kept at a constant temperature by cooling or heating. Finally, the thermoformed product is demolded and cut out or trimmed by various means around the formed regions. 

Parts have relatively good wall thickness distribution compared to processes, such as blow-molding and thermoforming. External comers tend to thicken, which can be an advantage in applications where wear is critical. 

Rosenzweig, N. 1983. Wall Thickness Distribution in Thermoforming. Society of Plastics Engineers Technical Papers, 29, 478-482. 

Derive the equation that describes the thickness distribution after thermoforming the constant diameter tubular geometry described in Fig. 6.75. Use the notation presented in the figure. Assume that the sheet stops stretching once it touches the mold wall. 

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