Thermoforming plug materials

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 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.

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. 

In plug-assist thermoforming, a mechanical device is used to help stretch the sheet Into the mold, as shown in Fig. 10.6. Where the plug touches the material, it is cooled, and the material does not stretch much more. The optimal shape for the... 

In principle, thermoforming is quite similar to the parison inflation stage of blow molding. A complication is the use of plugs to assist forming. The physics of the interaction between the molten material and the plug is not well understood and is difficult to simulate. As a result, there are some limitations on what can be simulated today. 

The first part of our study analyzed the processability of thermoformed cups fabricated on a plug-assisted vacuum MTS thermoformer while the second part compared the sheet sag observed for parts processed on an AVT thermoformer The sagging of the sheet was measured as a function of the time spent by the extruded sheet in the oven. A comparison of this sag to the sag obtained through simulations conducted on FormSim a software program developed by the Industrial Materials... 

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