Plug-assist forming using vacuum

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

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. 

Straight cavity forming is not well adapted to forming a cup or box shape because as the sheet, drawn by vacuum, continues to fill out the mold and solidify, most of the stock is used up before it reaches the periphery of the base, with the result that this part becomes relatively thin and weak. This difficulty is alleviated and uniformity of distribution in such shapes is promoted if the plug assist is used (Figure 2.39b). The plug assist is any type of mechanical helper which carries extra stock toward an area where the part would otherwise be too thin. 

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,... 

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

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. 

In general only the simpler materials can be thermoformed on cylinders using vacuum (e.g. PVC, polystyrene, PVdC coated PVC). Platens using pressure forming, particularly with plug assistance, not only offer more uniform blisters but can utilise the more complex materials, e.g. Aclar/PVC, polypropylene, coextrusions (Figure 13.4). 

Plug-assist vacuum forming (Fig. 10-11) is used to overcome thinning problems. Because the plug drops, this technique is also known as drop forming. 

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