One fundamentak 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 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 entremities of the impression.A compromiise netween the two extremes must therefore be accept 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 by moullded;length of the feed system,ect.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 moulding by minimising weld lines fllow marks and other blemishes.
To maintain the required teperature differential between the mould and plastic material,water is circulated through holes or channels within the mould,These holes or channels are termed flow-ways or water-ways and the conpletesystem of flow ways in termed the circuit.
During the impression filling stage the hottest material will be in the vicinity of the entry point,i.e.the gate,the collest material will be at the point farthest from the entry.The temperature of the coolant flued,however,increase as it passed through the ,pould.Therefore to achieve an even cooling rate over the moulding surface it is necessary to locate the incoming coolant fluid adjacent to ‘hot’ moulding surfaces and to locate the channels containing ‘heat’ coolant fluid adjacent to ‘cool’ moulding surfaces.However,as will be seen from the following discussion,it is mot always practicable to adopt the idealized approach and the designer 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 are commercially 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,and this can be achieved.In general,the simplest systems are those in which holes are bored logitudinally through the mould plates.Howver,this is mot necessarily the most efficient mothod for a particular mould.
When using drilling for the circulation of the coolant,however,these must mot be positioned too colse to the impression as this is likely to cause a marked temperature variation across the impression,with resultant moulding 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 recalled that the mould plate has a large number of holes or recesses,to accommodate ejector pins,guide pillars,guide bushed,sprue bush,inserts,ect.
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,When drilling deep flow ways theres is a tendency for the drill to wander off its prescribed course.A rule which is often applied(is tat for drillings up 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 5mm.
To obtain the best possibke position for a circuit it is good practice to lay the circuit in at the earliest opportunity in the design.The other mold items such as ejector pons,guide busdes,ect.,can then be positioned accordingly.
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Feed system of mould It is necessary to provide a