Warping and Weld lines


Mouldings warping after removal from the mould are very common and are always the result of frozen-in stress; differential shrinkage or,with filled materials,filler orientation.

This is nearly always a processing problem,but it can also be caused by a machine failure or incorrect component design.

Anything that prevents injection at the correct rate will contribute to warping.

Thus,the non-return valve on the screw not seating and allowing backflow along the screw flights prevents proper injection and causes warping.

In fact, a moulding which would have warped,except that it was restrained by a strong rim, such as the base of a bucket or bowl,will have a tendency to break down in service.

Expedients such as differential mould cooling and jig cooling are only partial palliatives.

The only real cure (given correct component design) is correct injection and mould filling.

Both the two expedients mentioned will certainly produce unwarped mouldings, but the question is: how long will they retain their shape?

For example,,a component for use at elevated temperatures may be free from warping when the first made,but after a few weeks or months of use at its normal working temperature it may warp sufficiently to become unusable.

When warping occurs, the injection rate should first be checked. Short mouldings should then be made by the method described in the section on Short Mouldings,to determine how the mould is filling.

Moreover, then,having ascertained that filling is uniform and that material is reaching all extremities of the moulds at the same instant,mouldings should be made at the same (correct) injection rate with exactly the correct amount of feed.

In nearly all instances, this method will prove satisfactory. If the material does not flow uniformly in the mould, it will be necessary to ease some of the channels or, with a deep-drawn symmetrical article,to true up the mould parts to make sure that all the flow paths are of equal thickness.

If warping is still a problem,even with this moulding technique,the mould may need to be altered so that several pin-point gates can be used to cause virtually undirectional flow during filling.

On account of the cost of modifying a mould, this should be thought of only as a last resort if all else fails.

Weld lines

Weld lines occur in mouldings where two or more streams of material meet and fuse together.

At best, the weld may be a small,barely discernible mark; at worst,the component breaks at the weld line. They are difficult to eradicate and are not caused by any deficiencies in material, machine,mould,or process,but are inherent in the design of the component.

Modifications to the mould and the use of a freer flowing grade of material are ways in which the effect of weld lines can be reduced, but the chief factor involved is finding the optimum processing conditions.

About the mould,it is seldom advantageous to feed from two or more gates.

This may assist concentricity but will almost certainly produce a weld line at right-angles to the line joining the two gates.

Side gating, where there is a single hole in a component,will nearly always produce a weld line at the side opposite to the gate.

When a moulding has several holes,a diaphragm gate in the largest will help to reduce the effect,as will over- flow wells beyond the weld line, but each smaller hole will produce its own pattern of weld lines.

Mould temperature control helps to reduce the effect and using the maximum feasible temperature will often almost eliminate them.

However,with crystalline plastics,a high mould temperature not only increases the crystallinity but also increases the size of the crystals,and it is possible to end up with a brittle component having even less strength than one with weld lines.

The most successful way of tackling the problem of weld lines consists in trying to arrange so that the two streams of material meet while their leading fronts are still capable of fusing together.

It involves a suitable combination of mould and cylinder temperatures,rapid injection and mould filling and making sure that there is adequate mould venting.

Obviously,anything that hinders the flow of material in the mould increases the severity of weld lines, and sometimes even the judicious application of a little mould release agent will help by lubricating the mould surfaces.

It also helps if polishing marks on the mould surfaces are in the direction of the flow of material.

As with warping and short mouldings,the deliberate making of short mouldings will help in showing why the weld lines are developing. 

Occasionally, weld lines can be made to form in positions where they are unobtrusive or where fracture will cause the minimum of inconvenience.

This involves careful control of the injection rate,bearing in mind that the injection flow control valve may become warm during the moulding programme and alter its characteristics.


The formation of windows—areas of amorphous material in an otherwise largely crystalline mass—has already been discussed,and all that needs to be said further is that they will not appear where plasticization is thorough and complete.

They may be produced in small machines being used for the making of mouldings of about the maximum capacity of the plasticizing cylinder,in which case a longer time cycle will generally effect a cure.

Windows should be avoided because they provide areas of discontinuity and have the same effect as notches in reducing impact strength.

Witness marks

Witness marks are always associated with badly mating mould parts. This may be caused by the mould not locking properly and allowing flash to form and accentuating the mark.

They are sometimes masked by the inclusion of a styling bead and,of course, are less apparent on patterned mouldings than on highly polished ones.

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