Processing, thermoplastics screw

Normal stress differences also have practical relevance for extrusion They assist in centering the shaft in the extruder when processing thermoplastics as the rotation of the screw generates radial forces (from the shaft to the wall). 

The most common cyclic process for thermoplastics is injection molding. Modern machinery is of the reciprocating screw type. In this process, the screw rotates and loads a metered dosage of polymer toward the tip. A ring valve at the tip of the screw prevents backflow during forward movement of the screw during the injection part of the cycle. The molten polymer flows... 

Most thermoplastics processing involves screw machines, whether for extrusion (pipes, film, cables, etc.), blow molding (bottles, fuel tanks, etc.), or injection molding (from cocktail sticks to car bumpers). The processes in all cases involve a powder or pellet raw material being conveyed, melted, and pumped through a die or injected into a mold. 

In principle, the screws used for processing of crosslinking compounds are shorter, and have a deeper-cut feed section, than thermoplastic screws. 

Polybutadiene resins are cured to a thermosetting structure by peroxide catalysts. These catalysts produce carbon-to-carbon bonds at the double bond sites in the vinyl groups. The final product is 100 percent hydrocarbon except where the starting polymer is the —OH- or —COOH-terminated variety. Use of high-temperature peroxides maximizes the opportunity for thermoplastic-like processing, because even the higher-molecular-weight forms become quite fluid at temperatures well below the cure temperature. Compounds can be injection molded in an in-line machine with a thermoplastic screw. 

Unlike injection moulding and most of the other operations, extrusion is a continuous process. Thermoplastic granules are plasticized and melted in a heated barrel and conveyed forward by means of a screw through a shaped die to produce a continuous section. The automotive products are well known ... 

Thermoplastics as well as thermosets and classic elastomers can be processed with screw injection imits. These material types will be discussed in more depth in Chapters 6 and 7. 

In ntosl respects the process is similar to the injection moulding of thermoplastics and the sequence of operations in a single cycle is as described earlier. For thermosets a special barrel and screw are used. The screw is of approximately constant depth over its whole length and there is no check value which might cause material blockages (see Fig. 4.50). The barrel is only kept warm (80-110°C) rather than very hot as with thermoplastics because the material must not cure in this section of the machine. Also, the increased viscosity of the thermosetting materials means that higher screw torques and injection pressures (up to 200 MN/m are needed). 

Processing and Application. Polystyrene and the other thermoplastics would not have been able to enter on their triumphal march if the manufacturers of processing equipment had not been able to provide efficient fabrication methods. A decisive factor was that between 1950 and 1960 the injection molding machine with a screw preplasticizer was developed and extrusion through slit dies to give sheeting or film followed by thermoforming was evolved. 

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