Splay marking and Staining

Splay marking

This fault,which appears on a great many mouldings,shows an area, generally near the gate, which has an uneven, lined appearance.It may be reminiscent of the grooves on a gramophone record or it may appear simply as an irregular pattern of lines,roughly in a half-circle round the gate.

The reason for splay marking is slow pressurisation of the last part of the mould. As the back pressure in the mould builds up,the rate at which material flows through the gate is reduced and the molten plastic does not have a low enough viscosity to give good conformity to the mould surface.

To overcome the defect it is important to fill the mould rapidly at one attempt. If the machine runs out of power before filling is complete ,the last material can enter the mould only at a very much slower rate. Pressure will gradually build up in the hydraulic system and will be transmitted through to the material entering the mould,forcing a little more in.

The material already there will have cooled, and conformity to the mould shape cannot be obtained. When splay marking occurs, therefore, the first action should be to check that the hydraulic system will enable filling to take place at the correct rate and in the time that can be allowed. If it is,and splay marking still persists, the sprue and runner system should be examined for any points where pressure and flow could be lost during filling and,if necessary,made larger or more direct. The land length of the gate should also be made as short as possible.

Splay marking is particularly liable to occur in mouldings made in acetal and this may be due to its somewhat higher melt viscosity. As already mentioned, however, the fault has been observed when small mouldings in many other materials are produced on hand machines or on low-power machines. There is little that can be done to improve such mouldings, and it must be remembered that their performance may be impaired Any moulding showing a discontinuity of surface finish is suspect and in aggressive environments may be expected to crack or break down completely.

Staining

Occasionally,a highly polished mould surface suddenly takes on a dull, discoloured appearance and this shows up on the moulding. There can be several reasons for this: decomposition of an additive,or the separation or blooming of a pigment (that is,the migration of the pigment to the surface of the plastic),or simply slight oxidation of the material, not sufficient to cause a burn mark but liberating a little gas which reacts with the mould surface.

The cure for this fault depends on the material being used. If there is a percentage of re-work, there may be contamination,for example,with mould release agent. There might also be a quantity of slightly decomposed or depolymerised plastic. Dry-colouring pigments may have become damp,or there may be

Dry-colouring pigments may have become damp,or there may be preponderance of fine,dusty material. With PVC,decomposition causes the liberation of some free hydrochloric acid that can quickly stain a polished mould. All these matters need to be considered and eliminated in turn until the cause is discovered. It is nearly always a material fault, but sometimes,by altering conditions slightly so as to reduce the possibility of decomposition, a cure can be effected.

 

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