Plug-assist

We use plug assisted thermoforming when we wish to make thick-walled products or thin-walled, deep draw products. The principal steps of the process are illustrated in Fig. 16.2. The first two stages are similar to those of vacuum forming. Once the sheet has been clamped, a plug made from a material with low heat conductivity is thrust downwards into the cavity,... 

Figure 16.2 Schematic diagram of plug assisted vacuum forming ...

Why would we use a plug assist process instead of a vacuum forming process ... 

FIGURE 18.14 Steps in plug-assisted vacuum thermoforming. Initially vacuum is applied and the plug is pulled into the sheet pushing it into the mold. Further vacuum pulls the sheet against the walls of the mold. Finally, vacuum is turned off and the plum and formed product removed. 

The addition of plug assistance usually improves the control on wall thickness, hence in terms of control the general list below applies. 

Figure 3.62 Plug-assist thermoforming using vacuum.

Figure 3.63 Reverse draw thermoforming with plug-assist and vacuum.

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. 

Plug assist is used principally when the process is likely to lead to undue variation in the product wall thickness. Plug assist supplies essentially a selective or localized stretch that is related to the specific demands of an individual mold cavity. It is likely to be beneficial when the draw ratio of a product feature is high, and when the product includes edges, corners and other features where excessive stretch and thinning is likely to occur. Plug assist is preferred for plastics with high enthalpy and low thermal conductivity such as polypropylene sheet formed in the solid phase. 

This process does not use vacuum or compressed air to form the heated sheet. The forces necessary to shape the uniformly heated sheet are applied by mechanical or manual stretching, bending, twisting, compressing, stamping, and/or other devices. Mold components can be used such as mechanically activated undercuts, side pulls, plug assists, etc. 

Thermoformed (Coextruded Plug-Assisted Twin-Sheet) Fuel Tanks Show Promise, MP, Apr. 2000. 

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. 

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