Sheet Forming Thermoforming

In vacuum forming, for example, the heat-plasticized sheet (either directly from an extruder or preheated in an oven) is drawn against the mold surface by the application of a vacuum from beneath the surface. Similarly, a positive pressure may be used to force the sheet against the mold surface, and where very deep draws are required, mechanical assists, plug forming, are used. In drape forming, the plasticized sheet is draped over a male mold, perhaps with a vacuum assist. 

FIGURE 17.9 Sheet-forming process (a) vacuum forming (b) drape forming. 

A similar technique is used to make sheet molding compound, but requires the addition of several additives, so the discussion is included in Chapter 18. 

Thermoplastic sheet is converted to a wide variety of finished articles by processes known generically as sheet forming or thermoforming. Although the details vary considerably, these processes all involve heating the sheet above its softening point and forcing it to conform to a cooled mold (Fig. 19.9). 

Among the many familiar items made by sheet forming are drinking cups, meat trays, cigarette packs, aircraft canopies, advertising displays, and lighting globes. 

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Polypropylene Resin Injection Molding Fiber Extrusion Sheet Forming Thermoforming... 

Sheet Extrusion and Thermoforming. Sheet for thermoforming and analogous operations is usually formed by extmding the melt through a slot die onto a set of poHshed chill roUs. The sheet is usually approximately 150 cm wide. After rapid cooling, the web is coiled or cut into sheets. Polystyrene, PVC, polyethylene, polypropylene, and filled polypropylene are prepared in sheet form by extmsion. 

The term thermoforming incoroporates a wide range of possibilities for sheet forming but basically there are two sub-divisions - vacuum forming and pressure forming. 

Polypropylene can be extruded into sheets and thermoformed by solid-phase pressure forming into thin-walled containers. Due to its light weight and toughness, polypropylene and its copolymers are extensively used in automobile parts. 

Sheet is thermoformed into a product by one of the forming process. To reinforce or rigidize it, one side that is usually the back surface of the formed sheet receives a spray-up of reinforced plastic (Chapter 15). 

These polyesters disintegrate in compost after six weeks. Their application includes films for compostable trash or shopping bags lamination of paper, magnetic cards, cloth-adhesive tape sheets for thermoforming, extmsion-forming monofilament for fishing lines, woven nets and ropes ... 

Hollow objects can be produced using twin-sheet thermoforming, in which two sheets of plastic are thermoformed and sealed together. This can be done sequentially, with one sheet formed first and then the other added and formed, or simultaneously, with both sheets formed and sealed together in a single step (Fig. 10.11). 

Thermoforming and adhesive bonding or double-sheet forming can be used to make hollow or tubular shapes in many configurations. Twin-sheet thermoforming is a process that is generally used to produce large, flat, double-walled parts. 

Plastic materials have low thermal conductivities, hence during heating, even relatively thin sheets can have surface temperatures that are substantially higher than the interior temperatures. Therefore, plastic sheets must be heated to an optimum forming temperature in order to achieve uniform and repeatable drawdown. Figure 4.5 illustrates continuous sheet forming extrusion via the thermoforming process. 

Figure 4.5 Continuous sheet forming extrusion via the thermoforming process...

Thermoforming results in characteristic thickness distributions of the part s walls. Once the sheet contacts the mold surface, it sticks and locally cools. The wall thickness distributions are due to hindering and stretching of the hot sheet material. Thermoformed sheet once formed as product is cooled below its softening to freeze is normally performed by conduction via the mold surface. The mold is kept at a constant temperature by cooling or heating. Finally, the thermoformed product is demolded and cut out or trimmed by various means around the formed regions. 

Keywords grid strain analysis, Plytron , trellis effect, kinematic approach, finite-element approach (I M), analytical approach, anisotropy, hyperelastic, interply slip, intraply slip, thermoforming, modelling sheet forming, composite laminates, continuous fiber-reinforced composites. 

Friedrich, K., Hou, M. and Krehs, J. (1997), Thermoforming of continuous fibre/ther-moplastic composite sheets . In Bhattacharya, D. (ed.). Composite sheet forming. Amsterdam Elsevier, pp. 91-162. 

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