Talk about mould cooling
One fundametal 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 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 two extremes must therefore be accepted to obtain the optinum 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 advantagous to use a slightly higher temperature than is requried 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 temperature 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 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 coolest material will be at the point farthest from the entry. The temperature of the coolant fulid,however, increases as it passes through the mould. Therefore to achieve an even cooling rate over 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 surface. However, as will be seen from the following discussion, it is not always sense when laying out coolant circuits if unnecessarily expensive moulds are to be avoided.
Units for the circulation of water are commercially availabe. these units are simply connected to the mmould 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 responsiblility to provide an adequate circulating system within the mould. In general, the simplest system 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 impression (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 recalled 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 depneds 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 150mm deep the flow way should not be closer than 3mm to any othe hole. For deeper 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 mould items such as ejector pins, guide bushes,etc. can then be positioned accordingly.