Postforming

A popular forming technique that has provided both performance and cost advantages, principally for long production runs, is applied as the plastic sheet, film, or profile exits an extruder. Upon leaving the die, and retaining heat, the plastic is continuously postformed. 

With this type of in-line system, the hot plastic is reduced only to the desired heat of forming. All it may require is a fixed distance from the die 

Opening. Cooling can be accelerated with blown air, a water spray, a water bath, or their combinations. Examples of postforming techniques are shown in Figs. 5-16 to 5-20. This equipment, like others, requires precision tooling with perfect registration. 

Insufficient heating Mold is too cool Assist is too cool Sheet cools before it is completely formed Too deep a draw Poor mold design Rough or improperly designed mold 

Long parallel molds with extrusion direction parallel Poor mold layout or design 

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An allowance must also be made for postforming shrinkage. Molds should be designed oversize so that when shrinkage is complete the product dimensions will be correct to within the design tolerances. 

Fig- 8-38 Different products that are postformed in line during extrusion. 

This term is used both to describe designs that are etched or engraved in the mold surface and the process of inserting a printed film into the mold, to be produced as an integral component of the finished product. Etched surfaces can be drawn both parallel and perpendicular to a parting line of molds or postforming in an extrusion line.. However, be alert with molds to the fact that parallel to the parting line additional draft is required. A wide selection of patterns is available and new ones can be readily created. 

None of the N2S2 or N3S systems described above is suitable for the postformed labeling of biomolecules (i.e. coupling a chelator to the biomolecule, followed by incorporation of the radiometal into the chelator) because the unfavorable kinetics of complexation or transchelation require relatively harsh conditions (strong acid/alkali, heat) which the biomolecule would not tolerate. In order to combine the advantages of in vivo stability in the amido systems with the better complexation... 

The thickness of such laminates ranges typically from 0.4 to 0.9 mm and they are available in Standard, Flame-Retardant, and Postforming qualities. The last is the most usual—the thinner postforming laminates being suitable for bends of very tight radius. 

Resistance to moisture Flexural strength Flexural modulus Tensile strength Postforming laminate only Postformability Blister resistance Melamine-faced chipboard only Resistance to wet heat Surface soundness Composite boards only ... 

As was made clear in the Introduction to this chapter, the characteristics and properties of laminates depend on a variety of factors before going on to consider the special qualities of flame-retardant and postforming materials a number of the more important general features are reviewed as follows. 

Figure 63 Stages in postforming (a) Profile at edge (b) At the temperature of forming, the laminate is shaped around the profile and bonded simultaneously (c) Surplus material is trimmed away to give a smooth finish...

The process of postforming consists at its simplest of applying localized heat to the surface of a P laminate which has been bonded previously to a substrate with a radiused edge profile (see Figure 63). 

Postforming laminates, unlike some thermoplastic sheet materials, cannot be formed into compound curves—but with appropriate techniques complex multiple internal and external bends ( S , U etc.) can be achieved easily. 

Flame-retardant postforming laminates are slightly less pliable and for these the minimum radius recommended normally is 15 mm. On the other hand, polyester laminates are more flexible than melamine and very tight bends can be obtained in these without cracking or crazing of the surface. 

In recent years postforming compact laminates (typically, 2 to 10 mm thick) have been developed, using within the laminate thermoplastic films to act as slip layers. These products require special techniques for forming. 

Figure 64 Automated machine for postforming (Photo Orama Fabrications Limited)...

There is off-line (for small quantity of products) or in-line postforming. The in-line refers to forming/shaping the extrudate (tube sheet, etc.) just after it emerges from the extruder die but before the plastic has a chance to cool. It provides specialty products with performance and cost advantages. Upon leaving the extruder s die and... 

Figure 5.21 Few examples of the many different postformed shapes and cuts with some showing dies...

Postforming identifies plastic sheet, rods, tubes, and other profile shapes that are formed into different shapes (coils, corrugated tubings, tubular nets, etc. Its major use is inline with an extruder processing... 

Acetal resins may also be fabricated into rod, slab, and other shapes by profile extrusion. Extruded shapes are frequently further machined. Parts fabricated by molding or extrusion are ammenable to all typical postforming processes. 

Postforming emulsion cleaning Oily and mixed Gentle hydraulic High Mixed and inorganic and surfactant... 

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