Thermoforming plug forming

Figure 7.83 Schematic illustration of (a) vacuum-forming and (b) plug-forming variations of thermoforming. From Z. Tadmor and C. G. Gogos, Principles of Polymer Processing. Copyright 1979 by John Wiley Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

Fig. 19.45. Thermoforming with pressure bubble snepback, The damped, heated sheet is sealed across a pressure box. Air pressure applied beneath the sheet forms a large bubble. When the bubble reaches the proper height, a plug is forced into it under constant air pressure. When the male plug doses on the pressure box, higher pressure beneath the sheet and vacuum behind the plug create a uniform draw around the plug form, (Courtesy Modern Plastics Encyclopedia, 1369. ...

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 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. 

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. 

Figure 7.10 Thermoforming (1) female mould, vacuum, no prestretching (2) female mould, pressure forming with male or plug, prestretching (3) male mould, vacuum, with pressurised air prestretch...

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). 

Thermoforming consists of forming a preheated plastic sheet or film into a desired shape against a mold surface. Forming can take place pneumatically or mechanically. In the first case, a differential air pressure is used, either vacuum on the mold side, compressed air on the other, or both. In the second case, the preheated sheet of material is pressed into the mold using either a preform, a plug, or by a combination of both. Viitually all thermoplastics can be thermoformed. However, because of limitations in the material characteristics, some can only be used for simple shapes with small draw ratios. 

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