Differential shrinkage and cooling effects

Mould cooling systems

cooling effects

The most effective cooling method for any mould would be to pump the coolant through the cooling channels counter-current to the flow of molten plastic into the mould- This method is adopted for pressure die casting and by using it the temperature difference between coolant and mould is kept as great as possible, on the assumption that the coolant increases in temperature as it progresses through the cooling channels and the material decreases in temperature as it penetrates into the mould. In the moulding of plastics,this method is suitable only for the moulding of thin-walled containers where quantity is of more significance than quality.

Where there is the possibility of differential shrinkage and the consequent warping of mouldings, attention to the direction in which the coolant flows through the cooling channels can help to alleviate,if not altogether prevent,warpage. The general principle is to make the coolant flow through its channels in the same direction as the material flows into the mould. This is less efficient in cooling effect but gives better quality moulding, although at a lower output rate.

Differential shrinkage and cooling effects

Differential shrinkage and cooling effects

In the moulding of a bowl or bucket, or even of a centre-fed flat plate,it is often found that radial shrinkage will be double that in the circumferential direction. Although,as indicated earlier,this may be caused or made worse by indifferent moulding techniques or by machine faults,it is not always possible to prevent it from occurring, and it is then that differential cooling is useful.

The first material to enter the mould is under very little pressure as it flows into the mould channels but,as it proceeds towards the extremities of the mould,it is progressively cooled. The following material is hotter and it begins to pressurize that already there; but as the mould has filled,a skin of solid plastic has formed on each wall which has reduced the gap through which the material passes and so increased the shear rate.

The extra shear stress causes orientation of the molecular structure and the melt is generally too viscous to allow for relaxation of all the stresses which are developed- The result is that the shrinkage of the material becomes much greater in the direction of flow and orientation; that is,in the radial direction,compared with that at right-angles to it,the circumferential direction.
Controlling the temperature of the mould will not completely prevent the above series of events from happening,but it can help to even out the effects- Cooling to the maximum at the point of entry of material into the mould reduces the time needed for cooling in this area which, in turn,reduces the amount of crystallinity in the solid material and decreases the size of the crystals.

This reduces the amount of after-moulding shrinkage. The perimeter of the mould, cooled less quickly because the coolant is at a slightly higher temperature, has the opposite effect. There is also a slight annealing effect which allows a little molecular relaxation and so relieves some of the effects of orientation. To achieve the best results,therefore, all cooling arrangements should ensure that the maximum cooling effect is nearest to the point of entry into the mould. Thereafter,the cooling effect should be as even as possible across the mould,although with large mouldings,it may be necessary to have two,or even three or more cooling systems, each run at a different temperature.
It is sometimes found that the walls of a box-shaped moulding will tend to bow inwards and that this effect can be alleviated by keeping the core cooler than the cavity.

This would appear to contradict what has already been said,because bowing always takes place where there is thicker or hotter material. If one considers how a box mould fills from a center gate in the base,it will be realized that the flow of material is radial in the base of the box and in virtually straight lines up the walls,giving a great deal of shear in that direction.

The major shrinkage will therefore be in that direction and the perimeters of the walls end up bigger than they should be for the size of the base. The restraining influence of the base causes them to bow inwards. If the core is then cooled more than the cavity, the material on the inside of the walls will shrink less than that on the outside and the net result will be that the walls become straight. If the core is cooled too much there will be a tendency for the walls to bow outwards.