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⑶、Injection of a molding formation as plastic work piece most effective formation methods because may by one time take shape each kind of structure complex, the size precise and has the metal to inlay a product, and the formation cycle is short, may by mold multi-cavities, the productivity be high, when mass productions the cost is very inexpensive, easy to realize the automated production, therefore holds the extremely important status in the plastic processing profession. Statistics have indicated, plastic mold composition all molds (including metal pattern) 38.2%, the plastic product gross weight about 32% is uses in injecting the formation, 80% above engineering plastics product all must use the injection formation way production.
⑷、counts according to the customs, our country in 2000 altogether imported mold 977,000,000 US dollars, in which plastic molding forms altogether 550,000,000 US dollars, occupied for 56.3%,2001 years altogether to import mold 1,112,000,000 US dollars, in which plastic molding forms altogether 616,000,000 US dollars, accounted for 55.4%.From the variety, the import volume biggest is the plastic molding forms.
In compression molding the plastic material as powder or performs is placed into a heated steel mold cavity, Since the parting surface is in a horizontal plane ,the upper half of the mold descends vertically. It closes the mold cavity and pressures for a predetermined period. A pressure of from 2 to 3 tons square inch and a temperature at approximately 350F converts the plastic to a semi liquid which flows to all parts of the mold cavity. Usually from 1 to 15 minutes is required for curing, although a recently developed alkyd plastic will cure in less than 25 seconds. The mold is then opened and the molded part removed. If metal inserts are desired in the parts, they should be placed in the mold cavity on pins or in the holes before the plastic is loaded. Also, the performs should be preheated before loading into the mold cavity to eliminate gases, improve flow, and decrease curing time. Dieletric heating is a convenient method of heating the performs.
Since the plastic material is placed directly into the mold cavity, the mold itself can be simpler than those used for other molding precesses. Gates and sprues are unnecessary. This also results in a saving in material, because trimmed-off gates and sprues would be a complete loss of the thermosetting plastic. The press requires the full attention of one operator. However, several smaller presses can be operated by one operator. The presses are conveniently located so the operator can easily move from one to the next. By the time he gets around to a particular press again, that mold will be ready to open. the thermosetting plastics which harden under heat and pressure are suitable for compression molding and transfer molding. It is not practical to mold thermo plastic materials by these methods, since the molds would have to be alternately heated and cooled. In order to harden and eject thermoplastic parts form the mold, cooling would be necessary.
模具原件具有高效率，高质量，低成本，节约能量和原材料等一系列优点，模具原件具有高精度，高复杂性，高一致性，高生产率和低功耗，其他制造方法是无法比拟的。已经成为工业生产和技术发展的一个重要手段。是现代工业技术的基础。模具是带有形状工具的高容量产品，是工业生产器材的主要过程。现代工业的发展和技术水品很大程度上依赖于模具的工业水品，因此，模具工业对于国家经济和社会的发展具有越来越重要的作用。1989年3月，国务院颁布了“关于当前工业政策决策点”在模具作为机械行业转型的第一个序列，产品和资本溅射作为第二个序列（在大型发电设备和相关输变设备后），建立了模具工业在国家经济中的重要地位。自1997年来，他们具有模具加工技术和设备，并列入“当前国家重点鼓励发展的产业，产品和技术目录”和“鼓励外商投资产业目录”中。由国务院批准，从1997到2000年，80多家专业模具厂拥有70%的增税退税的优惠政策来支持模具产业。所有这些都充分体现了对有关模具行业的关注和支持的增加。世界上每年的模具输出值大约600亿美元，日本美国和其他工业国家的模具输出量超过了机械工具产业值，从1997年开始，中国的出口工艺产值已经超过机床工具产业值。