【正文】 would not be those cavities,a stressrelieving operation should be carried out immediately after stressrelieving temperature as a rule is about 100186。F for____hours per____ inch of thickness.” Every effort should be made to eliminate the invisible source of problems, namely，fabricating cavities can be produced by a variety of processes. The process to be used is of all by the lowest cost at which the cavity can be produced for the desired end result. Other factors include precision of repairability. Frequently, a bination of processes is employed in order to meet all the specified requirements. The most mon processes are discussed in the following sections. Specifically, investment casting may be considered for applications where the number of cavities is greater than six and tolerances of dimensions are in the range of 177。to 2021186。simplifies cutting of a runner and moldings to simplify finishing of parts。length of flow within the impression。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