Thermoforming parameters

Thermoformability is a property required by the many sheet materials used in the thermoforming industry. These properties are unique for the specific forming methods used, and are best determined by actual thermoforming tests on smaU-scale equipment. The softening or drape temperature of the material, residual stress in the sheet from its manufacture, and its melt strength and viscosity are important parameters relating to this use. 

In an experiment, ferrite powders were dispersed in linear polyacrylamide, the gel was crosslinked, and the swollen gel compressed. This process resulted in particle orientation. Industrial processes, such as extrusion, injection, compression, and blow molding, fiber spinning, and thermoforming induce orientation due to flow. Typical parameters which control fiber orientation in these processes include ... 

There are methods for estimating the pressures necessary for forming and the optimum temperatures. However, these techniques are based on constant strain rates and constant temperatures, neither of which exists in real thermoforming applications. For that reason, the final processing parameters are usually determined by experimentation. 

Thermoforming is an open process, which means the process is sensitive to environmental changes and therefore sometimes unstable and difficult to control. Because of its instability, the thermoforming process has developed more like an art than a science [9]. The settings of process parameters and the development of new thermoformed products is only based on experience [10]. Little attention has been paid to thermoforming in terms of scientific imderstanding, which makes the process critical for complex applications compared to injection molding. 

Finally, both polymers, TPS and activated NR, were mixed in the same internal mixer previously employed for the activation of NR. The blend, after being completely mixed, was removed and introduced in square-shaped moulds. The mould with the blend inside was then placed in a hot-plates press (Figure 26.9) where the vulcanization and thermoforming process was carried out at the same time. Production parameters snch as temperature and pressure were set up to 2.5 MPa and 150 °C for 8 min 30 s." ... 

With regard to methods of fabrication, all processes in Table 1.1 that are applicable to unfilled, unmodified thermoplastics can also be used for discontinuous systems (with the exception of expandable bead molding). In addition to thermoforming, hot stamping of reinforced thermoplastic sheets mostly containing randomly oriented continuous or discontinuous fibers is used for the production of large semistructural parts. Fillers can also be used in the thermoset processes in Table 1.1, often in combination with the primary continuous fiber reinforcement. The content and inherent properties of the additive, as well as its physical/chemical interactions with the matrix, are important parameters controlling the processability of the composite. 

Several general parameters affect thermoformed product design. 

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