tailieunhanh - Coatings of Polymers and Plastics Part 4

Tham khảo tài liệu 'coatings of polymers and plastics part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 64 Stretch Injection Molding Mechanics. Injection molding represents a straightforward approach to material conversion. The primary materials used by the process are thermoplastics in the form of pellets granules or powder but some partially reacted thermosets are also used as a medium. Figure 4 is a representation of the basic process schematic. Raw materials are heated through a combination of shear work usually induced by the mechanical action of a reciprocating screw and external heating to reduce their viscosities to a point that flow into the closed mold can be accomplished through the application of external pressure. During the filling phase material is forced into the mold cavity under high pressure until the part is filled. Because plastic materials expand when melted they will begin to contract when they enter the relatively cooler environment of the mold. To compensate for this additional material is pushed into the mold during the packing phase. The material is then allowed to cool and solidify Fig. 4 Injection molding cycle. Injection molding is a complex process that produces intricate parts inside a closed mold under high pressures. It is probably the most capable and versatile of all plastic processes. Copyright Marcel Dekker Inc. All rights reserved. Marcel Dekker Inc. 270 Madison Avenue New York New York 10016 Plastics Processing 65 during the holding phase. During injection the mold is clamped together to offset the pressure of the material flowing into it. Most production molds incorporate temperature controls that manage the cooling and subsequent solidification of material once it is in the mold cavity. When the material has solidified the mold can be opened and the part ejected or otherwise removed. The essence of injection molding therefore involves understanding the shear flow behavior of materials under varying conditions of temperature and pressure when melted and being able to exercise precise control over these variables at any .