Thermoforming vacuum

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. 

The foam sheet is clean, bright in appearance, has excellent cushioning properties, and is nonporous. The foam is extruded as a sheet and is subsequently vacuum thermoformed into the desired shapes for packaging, etc. (Chapters 5 and 7). Use includes low temperature insulation in freezers, coolers, and other types of refrigerated rooms auto, truck bodies, and railroad cars refrigerated pipelines and low-temperature storage tanks for liquefied natural gas. They find usage as a replacement for molded-paper-pulp board in meat and produce trays and egg cartons. 

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. 

The general mle for vacuum thermoforming is that it should have a minimum inside radius (IRi) equal to the starting gauge (IRi > Ti) increasing to 4 times the starting gauge (Ti)(IR2 4Ti) at its thinnest... 

In female thermoforming, the outside radius is determined by the mold. For vacuum thermoforming it would be quite large, equal to the... 

Figure 4.49 Schematic representation of straight vacuum thermoforming in three steps (a) a thermoplastic sheet is clamped to the mold, is heated well above its glass transition...

FIGURE 1.9 Plug-assisted vacuum thermoforming. (Reprinted by permission of the publisher from Greene, 1977.)... 

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. 

Thermoforming includes the extmsion of sheets, thicker than 0.25 mm, followed by forming a reheated sheet in an open-face mold by pressure, vacuum, or both. Sheet of less than 0.25 mm thick is therm oformed in-line, and filled and sealed with contents such as processed meats, cheeses, and pastas. 

Thermoforming of extmded sheet is achieved by warming the sheet to its glass-transition poiat and coUapsiag it over a mold shape (21). The coUapse can be effected by vacuum suction or pressures appHed on the sheet above the mold. This technique is used to make trays and profiled advertisiag signs. Both single- and multilayer sheets can be used. 

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

A variation of thermoforming which does not involve gas pressure or vacuum is matched die forming. The concept is very simple and is illustrated in Fig. 4.53. The plastic sheet is heated as described previously and is then sandwiched between two halves of a mould. Very precise detail can be reproduced using this thermoforming method but the moulds need to be more robust than for the more conventional process involving gas pressure or vacuum. 

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... 

We use plug assisted thermoforming when we wish to make thick-walled products or thin-walled, deep draw products. The principal steps of the process are illustrated in Fig. 16.2. The first two stages are similar to those of vacuum forming. Once the sheet has been clamped, a plug made from a material with low heat conductivity is thrust downwards into the cavity,... 

---