Positive thermoforming

H. Matched metal-die pressure forming, although expensive, is a highly positive thermoforming technique. 

In matched mold thermoforming (Fig. 10.9), matched sets of negative and positive molds are used to form the part from the softened sheet. Vacuum is generally applied through the negative cavity, as well. The result is excellent dimensional control, and the ability to form very complex shapes. This method is most often used in thermoforming of foams, due to their tendency to deform if not restrained. 

Coated or laminated structures containing nylon can be heat-sealed into pouches or thermoformed to provide cavities into which hot dogs, sliced meats, and cheeses can be positioned for aesthetic display and sales appeal in the supermarket.P ... 

The system s key concept is a specially designed and patented frame, integrated into the mould, which is specifically designed to hold the film in position during this thermoforming process. This frame can be either two or three dimensional, to better follow the shape and the cavities of the mould (see Figure 3.15). 

Figure 3.16 A patented frame holds the releasing film in position so that when thermoformed over the mould, it is kept under tension and does not form wrinkles.

Thermoforming with matching moulds (a) the heated sheet in position over the negative (female) mould (b) the male is rammed down forcing the sheet into the female mould and (c) the form is cooled and removed from the mould (after Modem Plastics Encyclopedia). 

This system is ideal for thermoformed trays with porous lids and flexible pouches with one porous side. The test has been shown to detect leaks as small as 0.002 in. Guidmt and Medtronic have demonstrated the reliability of detecting leaks in blind tests and have quantified 100 percent of the purposely manufactured leaks in thermoformed trays (Hacked, 1996). In addition, there were no false positive readings in any of the unaltered packages. 

Sandwich heating (two-sided heating) n. A method of heating a plastic sheet for thermoforming in which the sheet is positioned between two radiant heaters. 

PMMA is a thermoplastic polymer commonly employed as the main component of positive resists for electron and UV photolithography [37, 38], as well as an imprintable material for hot-embossing soft lithography [39]. Because of its excellent transparency in the visible spectrum, PMMA is widely used in optical applications, especially as a matrix for nonlinear optical composite materials [40, 41]. The process of thermoforming PMMA sheets has been used to produce a variety of dental pros-theses that include splints, stents, and bases for occlusal rims. 

If the sheet is formed over the male or positive mold and the inner surface of the thermoformed part touches and replicates, the mold is called positive forming (Figure 10.2a). In other way, if the thermoformed part is in the cavities of a female or negative mold and the outer surface of the part replicates, the mold is called negative forming (Figure 10.2b)... 

Figure 10.2 Two basic types of thermoforming processes (a) positive and (b) negative forming...

The tool can be negative (as for the bath) or positive. A negative mould gives an accurate outer shape a positive tool gives an accurate inner shape. Thermoforming is now performed also with matched moulds (see Fig. 7.38). Excellent reproduction of mould detail and dimensional accuracy can be obtained. The moulds are cheap and can be made of wood, metal, plaster, or epoxy. 

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