Thermoforming and Vacuum Forming

Forming represents a relatively low cost method, utilizing male or female molds, to which a heated extruded polymeric slab is attached, so that the 

I) Attachment II) Heating III) Performing vacuum and cooling IV) Ejection of final product. 

The power requirement depends on the geometry of the roll, speed and gap, as well as on polymer viscosity. In spite of an initial high capital investment and a relatively long time to reach a steady-state, calendering offers an apparent advantage for the fabrication of sheets at a high rate with excellent quality. A mathematical analysis of the flow process in a calender has been developed for both Newtonian and non Newtonian liquids, including viscoelastic behavior, whereas, in simple cases an analytical solution is possible. 

Through approximations a numerical solution by finite elements is derived, providing very accurate information about the field of temperatures, pressures and the effect of velocity on thickness and output. Contrary to extrusion, there appears to be almost no orientation. The main products are sheets and fabrics for upholstery (printing and decorating), conveyor belts, and flooring. 

PP can be thermoformed in two distinct phases in the solid phase below its crystalline melting point and in the melt phase above its crystalline melting point. Both techniques have advantages and disadvantages. 

Crystallinity, the orientation of the chains and the clarity depend on the forming temperature. The shrinkage of PP thermoformed articles is complex and is a function of sheet crystallinity, orientation, thermoforming conditions and orientation arising from stretching of the sheet. Hence, a reliable figure for shrinkage is not possible. 

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Pressure forming refers to a type of thermoforming process in which pressures greater than atmospheric are used to force material against the contours of a mold. Deep draw PS deli containers and PP barrier food containers are product examples. See Thermoforming and Vacuum Forming. 

Food packaging, cups and dairy containers from thermoformed sheets, vacuum-formed packages... 

All the above processes can also be used in a form fill seal process with, for example, thermoforming and cold forming blister packs. Rommelag type bottle pack systems use an extrusion process where the container is formed by either blowing or vacuum (smaller sizes). Immediately after this, containers are filled, and the pack is sealed (welded) by using the residual heat in an extension to the main body of the container. 

There are two main types of thermoforming, (1) vacuum forming and (2) pressure forming. In vacuum forming, the plastic sheet is heated until it becomes soft and pliable, the heater is removed and the mold is rapidly evacuated. The reduction in pressure on the mold side allows deformation of the sheet, by atmospheric pressure, into the mold cavity. On contacting the mold, the sheet cools rapidly and retains the shape of the mold. The force available to deform the sheet is limited to that provided at atmospheric pressure and this type of thermoforming is thus limited to thin gauge, low-modulus polymers. 

Nylon films are used in lamination or coated form to ensure heat sealabiHty and enhance barrier properties. The largest uses are as thermoforming webs for twin-web processed meat and cheese packagiag under vacuum or in an inert atmosphere. Other uses include bags for red meat, boil-ia-bags, bag-in-box for wine, and as the outer protective layer for aluminum foil in cookie and vacuum coffee packages. 

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

The chassis is made from a 1/4 in. sheet of either ABS or TPO vacuum formed into a tub and reinforced with reinforced pultruded glass fiber-TS polyester plastic tubing. The hood and other products are being made from a 20 mm thick sandwich of thermo-formed PPO-alloy skins, glass fabric infused with thermosetting vinyl ester, and a urethane foam core. The bumper and front fascia is thermoformed from a polyolefin elastomer sheet with an UV-resistant cap layer of DuPont s Tediar PVF film. The dash and... 

FIGURE 18.14 Steps in plug-assisted vacuum thermoforming. Initially vacuum is applied and the plug is pulled into the sheet pushing it into the mold. Further vacuum pulls the sheet against the walls of the mold. Finally, vacuum is turned off and the plum and formed product removed. 

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.

Techniques commonly used to manufacture advanced composites from prepregs, which are available in unidirectional fiber or fabric form, include compression molding, vacuum-bag autoclave molding, stamp molding, thermoforming, and on-line (in situ) consolidation [3]. In compression and vacuum-bag autoclave molding techniques, the prepreg is cut to the dimensions of the structure, and the fiber directions are oriented to obtain the required... 

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. 

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