The rubber extruder machine is screw reactors, and extrusion is a series of processes which includes mixing, forming, puffing and drying. Single and twin screw extruders are the two main types of extruders based on screw configuration. Twin screw extruders can be co-rotating or counter-rotating depending on direction of rotation. The screws help in conveying and mixing of raw materials, and finally transform the dry mix into plastic melt, which is forced through a die. In general, there are three different zones where structural transformations take place (Fig. 12.1). The first one is the feeding section, where the raw materials are fed to the extruder. At this stage initial mixing occurs, with or without water. In the second part, which is called the compression section, pressure is built up due to increase in temperature by either heating coils or friction. The final transformation takes place in the metering zone, where the dry mixture forms a homogeneous viscoelastic material. The transitions in this zone control the structure, texture, colour and flavour of the product. Expansion occurs right after this stage at the die exit due to temperature and pressure drop (Eastman et al., 2001). Table 12.1 summarizes some features of both single and twin screw extruders.
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