Plug assisted forming

Rapid plug assist forming also requires an increase in the number of vent holes. Vent holes should be at least be located in those areas into which the sheet will be drawn last. In vacuum forming they project downward into a common chamber at the bottom of the mold. In parts where fine detail or textured patterns must be accurately reproduced, vent holes less than 0.5 in. (0.013 cm) apart is usually necessary. 

Thermoforming has close similarities with vacuum forming, except that greater use is made of air pressure and plug assisted forming of the softened sheet. The process is invariably automated and faster cycle times are achieved than in the vacuum forming process. Only thermoplastic sheet can be processed by this method. 

Similarly, the draw angle is less critical with pressure and pressure plus plug assisted forming, where a draw angle of 3°-6° is usual. 

FIGURE 2.39 (a) Vacuum forming, (b) Plug-assist forming using vacuum. 

And although thermoforming is basically a rubbery solid deformation process, the viscoelastic character of the polymer may need to be understood, particularly for the plug-assisted forming process. Computer-aided design programs also may need polymer viscoelastic properties. This may be particularly true for crystalline polymers such as polyethylene and polypropylene when formed above their melt temperatures. This is discussed below. 

Song, W.N., Mirza, FA., and Vlachopoulos, J. (1992) Finite-element simulation of plug-assist forming. Int. Polym. Proc., 7, 248—256. 

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