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Tham khảo tài liệu 'plastic product material and process selection handbook part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 3 Fabricating product 159 Figure 3.8 Action of plastic in a screw channel during its rotation in a fixed barrel 1 highlights the channel where the plastic travels 2 basic plastic drag actions and 3 example of melting action as the plastic travels through the barrel where areas A and B has the melt occurring from the barrel surface to the forward screw surface area c has the melt developing from the solid plastic and area D is solid plastic and 4 melt model of a single screw courtesy of Spirex Corp. In the output zone both screw and barrel surfaces are usually covered with the melt and external forces between the melt and the screwchannel walls has no influence except when processing extremely high viscosity materials such as rigid PVC polyvinyl chloride and UHMWPE ultra high molecular weight polyethylene . The flow of the melt in the output section is affected by the coefficient of internal friction viscosity particularly when the die mold offers a high resistance to the flow of the melt. The constantly turning screw augers the plastic through the heated barrel where it is heated to a proper temperature profile and blended into a homogeneous melt. The rotation causes forward transport. It is the major contributor to heating the plastic via the plastic s shearing action once the initial barrel heat startup occurs. The melting action through the screw is shown in Figure 3.8. 160 Plastic Product Material and Process Selection Handbook The design of the screw is important for obtaining the desired mixing and melt properties as well as output rate and temperature tolerance on melt. Generally most machines use a single constant-pitch meteringtype screw for handling the majority of plastic materials. Most of the energy that a screw imparts to the plastic material is by means of shear. The velocity of the plastic relates to the shearing action between two surfaces moving in relation to each other. These surfaces are the barrel ID and the root diameter of the screw. Until .
