Postformings

Various methods can be used for postforming products after the hot plastic melt leaves the extruder die. Examples are netting products that are flat to round shapes, rotated mandrel die makes perforated tubing, spiral spacer web around a coated wire or tube, varying tube or pipe wall thickness, and different perforated tubing or pipe pattern. 

Many advantages exist in coextrusion. The different materials used in the coextruded structure meets different performance requirements based on their combinations. A single expensive plastic could be used to meet performance requirements such as permeability resistance. 

The machines used for molding TSs are basically the same system as in molding TPs. Temperatures differ, as does the design of the screw. Unlike TPs that just melt in the plasticator and solidify in the cooled mold, the TSs melt in the plasticator and cure to a harden state in the mold that operates at a higher temperature than the plasticator. 

The review in coextrusion also applies with coinjection providing similar advantages. Two or more injection molding barrels are basically joined together by a common manifold and nozzle through which melts flow before entering the mold cavity by a controlled device such 

The usual coinjection with two or more different plastics is bonded/ laminated together. Proper melt flow and compatibility of the plastics is required in order to provided the proper adhesion. Some of the melt flow variable factors can be compensated by the available plasticator and mold process control adjustments. 

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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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