Principle of thermoforming

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.

A simple approximation of the thermoforming process is based on a mass balance principle. To illustrate this concept, let us consider the thermoforming process of a conical object, as schematically depicted in Fig. 6.25. 

With viscoelastic models used by an increasing number of researchers, time and temperature dependence, as well as strain hardening and nonisotropic properties of the deformed parison can, in principle, be accounted for. Kouba and Vlachopoulos (97) used the K-BKZ viscoelastic constitutive equation to model both thermoforming and parison membrane stretching using two-dimensional plate elements in three-dimensional space. Debbaut et al. (98,99) performed nonisothermal simulations using the Giesekus constitutive equation. 

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. 

In principle there are two techniques to process porous substrates with thermoforming. A novel method was shown by Giselbrecht et at [23], who processed pre-ion-beamed foils with thermoforming. Later the formed structures were etched to achieve porous microstructures. The advantage of this method is the easy process setup. 

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