Mould Cooling in plastic Mould

One fundametal principle of injection moulding is that hot material enters the plastic mould,where it coils rapidly to a temperature at which it solidifies sufficiently to retain the shaple of the impression.The temperature of the plastic mould is therefore important as it governs a portion of the overall moulding cycle.While the melt flows more freely in a hot mould,a greater cooling period is required before the solidified moulding can be ejected.Alternatively,while the metl solidifies quickly in a cold plastic mould it may not reach the extremities of the impression.A compromise between the two extremes must therefore be accepted to obtain the optimum moulding cycle.

The operating temperatire for a particular plastic mould will depend on a number of factor which the follow:type and grade of material to be moulded;length of flow within the impression;wall section of moulding;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 moulding by minimizing weld lines,flow marks and other blemishes.

To maintain the required temperture differential between the plastic mould and plastic material,water (or other fluid) is circulated through holes or channels within the plastic 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 hottest material will be in the vicinity of the entry point,i.e.the gate,the coolant fluid,however,increase as it passes through the plastic mould.Therefore to achieve an even cooling rate over the moulding surface it is neccessary to locate the incoming coolant fluid adjacent to ‘hot’ moulding furfaces and to locate the channels containing ‘heated’ coolant fluid adjacent to ‘cool’ moulding surface.However,as will be seen from the following discussion,it is not 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(and other fluids) are commercially available.These units are simply connected to the plastic 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 systerm in which theplastic mould is connected to a cold water supply.

It is the plastic mould designer’s responsibility to provide an adequate circulating system with in the plastic mould,and this section covers some of the different ways this can be achieved.In general,the simplest systems are those in which holes are bored longitudinally through the plastic 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 impression (say close than 16mm ) as this is likely to cause a market 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 plastic 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,spruce 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 adjacent to another hole depends to a large extent on the depth of the flow way drilling 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 150 mm deep the flow ways this allowance is increased to 5mm.

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 plastic mould items such as ejector pins,guide bushes,etc,can then be positioned accordingly.