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【正文】 itioned on opposite sides of the mould?s centreline serve to balance the mould. 五、 Mould cooling One fundamental principle of injection molding os that hot material enters the mouls, where it cools rapidly to a temperature at which it solidified sufficiently to retain the shape of the impression. The temperature of the mould os therefore important as it governs a portion of the overall molding cycle. While the melt flows more freely in a hot mould, a greater cooling period is required before the solidified molding can be ejected. Alternatively, while the melt solidifies quickly in a cold mould it may not reach the extremities of the impression. A promise between the two extremes must therefore be accepted to obtain the optimum molding cycle. The operating temperature for a particular mould will depend on a number of factors which include the following:type and grade of material to be molded。length of flow within the impression。wall section of the molding。length of the feed system, etc. It is often found advantageous to use a slightly higher temperature than is required just to fill the impression, as this tends to impreove the surface finish of the molding by minimizing weld lines, flow marks and other blemishes. To maintain the required temperature differential between the mould and plastic material, water or other fluid is circulated through holes or channels within the mould. These holes or channels are termed flowways or waterways and the plete system of flow ways is termed the circuit. During the impression filling stage the hottert material will be in the vicinity of the entry point, i. e. the gate, the coolest material will be at the point farthest from the entry. The temperature of the coolant fluid, however, increases as it passes though the mould. Therefore to achieve an even cooling rate over the molding surface it is necessary to locate the ining coolant fluid adjacent to?hot?molding surfaces and to locate the channels containing?heated?coolant fluid adjacent to ?cool?molding surfaces. However, as will be seen from the following discussion, it is not always practicable to adopt the idealized appreach and the designer must use a fair amount of mon sense when laying out coolant circuits if unnercessarily expensive moulds are to be avoided. Units for the circulation of water and other fluids are mercially available. These units are simply connected to the mould via flexible hoses, with these units the mould?s temperature can be maintained within close limits. Close temperature control is not possible using the alternative system in which the mould is connected to a cold water supply. It is the mould designer?s responsibility to provide an adequate circulating system within the mould. In general, the simplest systems are those in which holes are bored longitudinally through the mould plates. However, this is not necessarily the most dfficient method for a particular mould. When using drillings for the circulation of the coolant, however, these must not be positioned too close to the impression say closer than 16mm as this is likely to cause a marked temperature66variation across the impression, with resultant molding problems. The layout of a circuit is often plicated by the fact that flow ways must not be drilled too close to any other hole in the same mould plate. It will be recalled that the mould plate has a large number of holes or recessers, to acmodate ejector pins, guide pillars, guide bushes, sprue bush, inserts, etc. How close it is safe to position in a flow way adjacent to another hole depends to a large extent on the depth of the flow way driolling required. When drilling deep flow ways there is a tendency for the drill to wander off its prescribed course. A rule which is often applied is that for drillings up to 150mm deep the flow way should not be closer than 3 mm to any other hole. For deeper flow ways this allowance is increased to 5 mm. To obtain the best possible position for a circuit it is good practice to lay the circuit in at the earliest opportunity in the design. The other mould itens such as ejector pins, guide bushes, etc. , can then be positioned accordingly. 六、 Designs CAD/CAM Although CAD/CAM manufactures and suppliers are addressing the challenges mold disigners face when using software, these designers are still grappling with a number of issues. Kevin Crystal, senior quality engineer with The Protomold Co. (Maple Plain, MN)a rapid injection molding panyreports that the greatest challenges he faces are with file translation and creating drafted surfaces. IGES, STIP, etc. are imperfect tools as implemented by various CAD packages, ??he explains, Some packages create horrible models, some are soso, and some are pretty reliable , but none that I know of are perfect. Also, draft cause subtle changes in faces, and if you don?t understand what is happening one of two things will happen:your CAD package if it?s good will till you itcan?t create the draft, or your CAD package woll create geometry that has internal faces, hairline cracks or other defects. ?? Adequate training is a problem that Jack Mason, general manager Unity Mold LLCa provider of precision molds used for molding ezotic materials such as stainless stiil and ceramic, and fixtures associated with the various secondary processesfaces. He also finds retraining as a result of equipment/software evolution a challenge. Excellent technical support tops the trio?s list of what to look for in a supplier. AccuMold?s Holby also wants uptodate information and his phone calls returned in a timely manner. Mason of Unity Molds also wants his response time quickand qualified. Although not specifically a CAD/CAM challenge, offshore cimpetition is probably the number challenge faced by moldmakers todayespecially offshore suppliers. As a result