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1、Lesson 12 Mould cooling,One fundamental principle of injection molding is that hot material enters the mould, where it cools rapidly to a temperature at which it solidifies sufficiently to retain the shape of the impression.,The temperature of the mould is therefore important as it governs a portion
2、 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 compromise between
3、 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 moldin
4、g; 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 improve the surface finish of the molding by minimizing weld lines, flow marks and other blemishes.,To maintain the required temperatu
5、re 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 flow-ways or water-ways and the complete system of flow ways is termed the circuit.,During the impression filling stage the hotte
6、st 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 through the mould.,Therefore to achieve an even cooling rate over the molding surface it is
7、necessary to locate the incoming 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 approach and the d
8、esigner must use a fair amount of common sense when laying out coolant circuits if unnecessarily expensive moulds are to be avoided.,Units for the circulation of water (and other fluids) are commercially available. These units are simply connected to the mould via flexible hoses, with these units th
9、e moulds 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 designers responsibility to provide an adequate circulating system within the mould. In general,
10、the simplest systems are those in which holes are bored longitudinally through the mould plates.,However, this is not necessarily the most efficient method for a particular mould.,When using drillings for the circulation of the coolant, however, these must not be positioned too close to the impressi
11、on (say closer than 16mm) as this is likely to cause a marked temperature variation across the impression, with resultant molding problems.,The layout of a circuit is often complicated by the fact that flow ways must not be drilled too close to any other hole in the same mould plate.,It will be reca
12、lled that the mould plate has a large number of holes or recesses, to accommodate 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 drilling required.,Whe
13、n 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
14、 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 items such as ejector pins, guide bushes, etc., can then be positioned accordingly.,Notes, It is often found advantageous to use a slightly higher temperature th
15、an required just to fill the impression, as this tends to improve the surface finish of the molding by minimizing weld lines, flow marks and other blemishes.,使用比充模要求稍高的温度注塑比较有利，这样生产的塑件熔接痕少、流痕不明显，其他缺陷也较少，因此，提高了塑件表面质量。 句中的it 作形式主语代替不定式to useto，as引导的是结果状语从句。, Therefore to achieve an even cooling rate o
16、ver the molding surface it is necessary to locate the incoming coolant fluid adjacent to hot molding surfaces and to locate the channels containing heated coolant fluid adjacent to cool molding surfaces.,因此，为使塑件表面获得均匀的冷却速率，冷却通道的入口应开设在高温塑件附近，受热后冷却介质温度升高，出口开设在低温塑件附近。,句中的to achieve an even cooling rate over the molding surface作表示目的的状语，it是形式主语代替两个不定式短语。,
