注射成型工艺英文文献和中文翻译(16)
Enough draft must always be provided, especially in deep-draw moldings, such as long containers. Reverse draft or even parallel sides should be avoided wherever possible. The draft allowance for EVA moldings should be generous, as they tend to be stickier at part ejection temperatures. Normally, a 3- to 5-degree draft significantly assists in part ejection. On shallow draft molds, the use of stripper rings and/or air ejection may be necessary. Draft angles of less than one degree are not recommended and should be avoided unless dictated by part requirements.
Clarity
High melt temperature and low pressure are necessary to eliminate flow marks in molded parts. Clarity of molded parts also can be improved by lowering the mold temperature, especially in thin walled sections when higher MI resins are used. This reduces the size of the crystals formed which, in turn, reduces the light diffraction.
Polypropylene random copolymers have greater clarity than PP homopolymers. The clarity of random copolymer resins can be further enhanced by the addition of clarifying agents. Optimum clarity for PP articles in the nominal 0.050 in. thickness range is obtained at melt temperatures of about 430°F (220°C) and mold temperatures about 50 to 80°F (10 to 25°C). Generally, high injection rates also enhance clarity. Highly polished tools are necessary for highest clarity.
Gloss
Surface gloss of the molding is affected by resin properties, the condition of the mold and molding conditions. The higher the MI or MFR of the polyolefin resin the greater the gloss of the molding. Further, higher density polyethylenes give higher gloss than lower density resins.
Highly polished molds are one of the most important factors for obtaining high-gloss parts. For polyethylenes, a warm mold gives better gloss than a cold mold. Gating also may contribute to obtaining a highgloss part. Restrictive gating produces higher gloss because it keeps the temperature high as the melt is injected into the mold cavity. Highest gloss for polypropylene resins is obtained with a cold mold and a fast injection rate.
注射成型工艺从聚烯烃颗粒在重力作用下进入塑化/注塑机的注射单元开始。在温度和压力作用下聚烯烃树脂融化并流动，压力迫使熔体注入模具并文持到型腔内的熔体冷却固化。当制件温度充分降低到材料分离温度时，模具打开制件推出。
这一完整的过程被称为一个成型周期。从熔体被注进模具型腔到开模这段时间称为合模时间。总注塑成型周期由注射时间、保压时间、推出时间和合模时间组成。
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