Forming processes plug-assisted

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 ... 

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. 

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. 

Process steps for plug-assisted negative pressure forming are exemplarily shown in Fig. 1. Relevant process steps are described subsequently in more detail. 

Figure P.3 Plug-assist reverse draw forming process. (Dow Chemical Company also Schwartz, S.S. and Goodwin, S.H., Plastics Materials and Process, van Nostrand Reinhold, New York, 1982, p. 637)...

Molds may be either male (raised) or female (recessed). Female molds are preferred, with plug assist process. A female mold will provide a thicker flange area, sharp definition on the outside area, and easy ejection. A male mold is usually cheaper to construct and provides for a deeper draw. Disadvantages include a tendency to web, form a weak flange area, and provide an extremely difficult release. 

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. 

Figure 10-16 gives data for plug-assisted sheet forming of polystyrene. Analyze the overall process to the best possible degree of engineering reliability. 

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