Thermoforming, part processing temperature

This review on vacuum thermoforming can be related to most of the other forming processes. With a vacuum system a sheet is subjected to heat to meet its optional processing temperature, or technique that forces it against the shape of a mold. The hot, pliable material is moved rapidly to the mold (for example, by gear drives) and/or is moved by an air pressure differential, which holds it in place as it cools. When the proper set temperature is reached, the formed part can be removed and still retain its shape. 

Figure 2, 3, and 4 show the different thermoformed parts obtained on the MTS thermoformer for the design of experiments represented in Figure 1. It should be noted that the cups are formed by pushing the mold cavity down onto the plug. It can be seen that the temperature window of processability was different for each of the three resins.

An increasing variety of automotive parts is being made by thermoforming processes. Many of these products are made from rubber-toughened polypropylene, which are relatively stiff and can withstand a high level of physical abuse over a ivide range of temperatures. When such parts are used in high visibility areas, such as wheel arches, air dams, truck fenders, and... 

Thermoforming is a process for forming moderately complex shaped parts that cannot be injection molded because the part is either very large and too expensive or has very thin walls. It consists essentially of two stages elevation of the temperature of a thermoplastic sheet material until it is soft and pliable and forming the material into the desired shape using one of several techniques. 

Molds for the thermoforming process may be made of wood, metal, or epoxy and are relatively cheap. They are provided with vents to allow trapped air to escape and release possible pressure buildup. Temperature control, as we shall see in the following discussion, determines part quality. It is, therefore, crucial that mold temperature is controlled properly. The mold is consequently provided with channels for the passage of the cooling liquid. 

The mold can be heated or used at ambient temperature, depending on mold design and final application. Parts up to 660-950 mm (2-3 feet) X 1.22 mm (0.048 in) thick can be formed in a matter of seconds. Thermoforming can also be used, to some extent, for preforming reinforced TP foam cores for laminating with TS resins. The Advance RP process (developed by Advance USA, East Haddam, CT, USA) is an adaptation of thermoforming for production of RPs. 

The polymer should be dried before processing, and typical melt temperatures are 340 to 425°C. Polyetherimides can be processed by injection molding and extrusion. In addition, the high melt strength of the polymer allows it to be thermoformed and blow molded. Anneahng of the parts is not required. 

Steady state n. A condition of processes or parts of processes in which the state variables describing the process, e.g., temperature, pressure, compositions and velocities of streams, and amounts of materials residing in various process equipment, do not change with time. Most extrusion operations closely approximate the steady state except during startup and shutdown, whereas injection molding and sheet thermoforming are unsteady, intermittent processes. 

Twin-sheet thermoforming. Twin-sheet thermoforming is generally used to produce large, flat, double-walled parts. Nearly all of the thermoplastics can, at least theoretically, be thermoformed. However, the bulk of parts made by this process are usually polystyrene or ABS. The high-est-temperature plastic thermoformed in any significant amount is polycarbonate. 

The thermoforming process generally consists of five steps (1) the material is usually held in a frame of some kind (2) the material is heated to the desired temperature for forming (3) forming is done by vacuum, pressure, and/or a mechanical assist of some kind (4) the part is cooled sufficiently to be removed without distortion and (5) excess material, usually the areas in the clamps, is removed. 

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