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Tham khảo tài liệu 'modern plastics handbook 2011 part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 5.112 Chapter Five Melt a b Figure 5.72 Fountain flow. a Top view 166 b side view.167 Instead a melt stream shoots straight into the cavity only stopping when the stream contacts the end of the cavity. The remaining melt then fills the cavity as fountain flow. As shown in Fig. 5.73 166 this jetting produces as weld line within the cavity. The jetting is not aesthetically pleasing and the weld line is not as strong as the surrounding material. While jetting can be reduced or eliminated by reducing the velocity of the melt front the effects of injection velocity are very complex and difficult to predict.166 Typically enlarging the gate and runner reducing gate land length and promoting mold-wall contact by locating the gate so that the flow is directed against a cav- Processing of Thermoplastics 5.113 Figure 5.73 Jetting.166 ity wall are more effective in eliminating jetting. In contrast when cold melt is slowly injected into the cavity the melt freezes as it flows into the cavity. While this cold flow produces poor surface and part properties it can be eliminated by increasing the injection speed melt temperature and or mold temperature. Rapid injection can also cause degradation of the polymer at the gate. Gate blush a discoloration outside the gate can be accompanied by a reduction in part properties particularly impact strength.168 Dieseling occurs when air trapped in the cavity burns. This produces discoloration or burning of parts particularly at the end of the fill and is often accompanied by short shots. Reducing injection speed increasing melt temperature and increasing gate size all tend to reduce or eliminate gate blush whereas remedies for dieseling include reducing the injection velocity cleaning or enlarging mold vents and redesigning venting in the mold. Since hydraulic pumps maintain a constant or controlled flow of hydraulic oil the flow to a particular hydraulic cylinder is regulated by either a flow control valve a proportioning valve or a servo .
