Cracking,Delamination and Distortion

Cracking,Delamination and Distortion

Cracking or Crazing

This fault occurs only when there is residual stress in a moulding which may be affected by an aggressive environment. The term ‘aggressive,may be considered a misnomer for such environments as a low concentration of ozone in the atmosphere or for a seemingly bland material such as Vaseline petroleum jelly. However,both are aggressive to stressed plastics. The only way to avoid cracking and crazing are to produce stress-free mouldings.

Delamination

Like cracking and crazing, this fault is a sign of a stressed moulding in an aggressive environment. It is sometimes seen in domestic hollow-ware which has been subject to household detergents. For example, in using a washing-up bowl,the liquid detergent is nearly always poured into the center of the bowl before water is added. The high concentration of detergent and the sudden rise in temperature as the bowl is filled with hot water,coupled with the frozen-in stress in the base of the bowl cause the surface to peel like the skins of an onion and to show the signs of delamination.

Another common cause of delamination is the excessive use of mould release agents, particularly those based on silicone oils and resins. These should only be used very sparingly, not at all in the moulding of polythene, and if sprue-sticking or other extraction difficulties occur,a check should be made of moulding technique. With a well-designed mould and correct moulding techniques, mould release agents should not be required.
Contamination of one material by another (e.g., polythene and polystyrene) often manifests itself as delamination.

Distortion

By distortion in this context is meant the taking up by a moulding of a shape other than that required. It is difTerent from warping,in that it implies that the shape never was correct. The commonest cause of distortion is that the moulding has not been allowed to cool sufficiently before removal from the mould and that it has been physically pushed out of shape by ejectors or other means. For example,ejector pins acting on a still-soft moulding will stretch the material,or a very soft portion of a moulding prematurely taken from the mould may flow slightly. Although jig cooling, either in the mould or in special jigs after extraction of the moulding,would seldom be recommended, it is important to make sure that the cooling period is long enough to allow the moulding to become strong enough to handle.

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