Sink marks and voids

Sink marks and voids

Sink marks and voids are two manifestations of the same fault, and their complete eradication is extremely difficult. Diagnosis is easy. All plastics take up more volume when molten than when solid,and where there is more volume there will be more shrinkage; in other words, thick sections shrink more than thin ones.

In any part of a moulding where there is a sudden change in the section which locally increases the volume of material,either the surface will be drawn in as the material shrinks,giving a sink mark or,if the surface has become hard enough not to sink; the shrinkage will take place within the body of material causing a void. Overcoming this problem is not simple.

increase the injection holding time

One method of reducing or preventing sink marks and voids often recommended in manufacturers,literature is to increase the injection holding time (plunger dwell). This may have some effect,particularly where thick mouldings are being made using a large gate, but there is a danger that the pressure will not be evenly distributed throughout the material in the mould and that some parts will be more highly stressed than others. Thus,a moulding may seem to be perfectly free from blemish,but would break down in service under adverse environmental conditions. This technique demands considerable care in making sure that the holding pressure is not so great as to cause undue stresses.

reduce the temperature of the material in the cylinder

Another recommended method is to reduce the temperature of the material in the cylinder (on the basis that cooler material shrinks less). This can have only a limited effect because it is extremely unlikely that a reduction of more than a few degrees could be tolerated,bearing in mind that,moulding should always be carried out at as low a temperature as possible.

The average volume change on heating for crystalline plastics is about 1 percent for every 10°C of temperature change (about half that figure for amorphous materials). If a reduction of 10。C could be tolerated, this will make a difference in shrinkage of only 1 percent, when the effect being considered is the result of a 14-18 percent change in volume.

Furthermore, the lowering of temperature would also cause a reduction in the ease of filling because the viscosity of the melt would be somewhat higher. Consequently, more pressure would be needed, bringing with it the possibility of greater residual stresses.

Rapid mould filling with the correct amount of feed at the lowest possible temperature is the most effective way of reducing sink marks and voids in the great majority of cases. The main exceptions are very thick sections injected through very large gates,using low-viscosity materials (with the proviso above).

Although these faults are difficult to eradicate completely,it is sometimes possible to make a virtue of necessity and to design into a component a feature that will actually make use of the fact that a sink mark may occur. Thus,if a thicker section is desirable it may be possible to make an indentation in the moulding at,or near,the place where a sink mark may occur.

If the design of a component renders it virtually impossible to mould without either sink marks or voids, the former should generally be chosen. Voids are always likely to provide centres where notches can occur, thus reducing the impact strength of the moulding.

By adjusting conditions, voids can often be changed into sinks marks,and so long as the sink mark is not detrimental to appearance,it may be less harmful to performance than a void. It may be possible to mask sink marks by designing in styling beads or changing the contour where the sink mark is likely to occur. In this event,the styling should not be of such a nature that it turns a sink mark into a void.

Sink marks and voids are perhaps the most difficult faults to deal with satisfactorily. They are basically a problem of design,but if they have not been considered at the design stage,the only advice that can be given to the moulder is,” do the best you can”.

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Author: ceq12 1222