Short mouldings and Scuffing

Short mouldings

Short mouldings and Scuffing

The causes of short mouldings are many and varied and are sometimes difficult to diagnose. It is,for example, perfectly easy to flash a mould and still obtain a short moulding. On the basis that the same property,too low a melt viscosity for the material,will probably be a cause of flashing and that it also ensures easy mould filling,the combination of both faults should be impossible.

The failure of a mould to fill is brought about by a combination of factors, the most important of which is the build-up of back pressure in the mould before it has completely filled. Commonest among causes of this build-up of back pressure is air or gas trapped in the mould.

In the event of complete trapping of air,that is, it cannot escape at all,a burn mark is inevitable if some mould filling has taken place. However, air trapped in the mould but being slowly vented may not cause oxidation but may prevent the material from reaching the limit of its travel in the mould before it solidifies.

Thus,short mouldings may develop into burn-marked mouldings if the vent becomes completely blocked. This can be the result of a new mould bedding down— new moulds need just as careful ‘running in’ as new cars—or by grease or dirt entering the vent. In this context, it should always be remembered that the steel of which the mould is constructed is elastic, and under the high pressure during locking, it can be compressed quite considerably; in fact,enough to close small venting gaps.

Back pressure is also built up in the mould by solidification of material as it enters and flows through the mould channels. It should be reiterated that if the gap between the walls of the mould is going to be reduced by over 50 percent during filling, there will be a strong possibility of producing a short moulding.

In these circumstances, the operator probably increases the injection pressure. A full moulding may then be made,but it will contain residual stress near to the gate. On reflection, it becomes obvious that what is needed is more rapid filling, requiring,as previously indicated, a hydraulic system capable of delivering the required amount of hydraulic fluid in the time allowed for filling.

By now, it will be appreciated that some criticism of the standard >fa^flt- finding chart is justified. To indicate the various expedients to avoid short mouldings without discussing the underlying questions can lead to a trial-aiKt1 error situation in which the operator is virtually enjoined to ‘try everything and see if it works,.

Eliminating short mouldings, therefore, involves the proper energy being available in the machine hydraulics,the material being in the correct state of fluidity,and the mould filling without causing back pressure of the kind that blocks the way through it. If short mouldings are made, and all the obvious things such as feed setting,temperatures, and power requirements are in order,then, one must look further for the cause and its cure.

A broken,or ineffective back-flow stop valve can cause starving of the mould by reducing the available pressure and feed. Adjusting the feed to compensate will not give a complete answer because the leakage along the screw will be reducing the effective filling rate and short mouldings will result.

Malfunctioning of heaters and temperature controllers may sometimes cause short mouldings,but this should easily be diagnosed. A blocked injection flow control valve should similarly be noticed because the injection rate will have fallen off. A blocked nozzle should also be quickly diagnosed.

It is also assumed that runner and gate systems have been properly designed and executed. In a new mould it may be necessary to increase gate dimensions, but this should always be carefully carried out- A well-tried mould should need the gate altered only for a different grade of material.

Altering a mould by putting in two or more gates rarely has a beneficial effect and sometimes causes more problems than it solves. The general principle of gating in only one place applies to nearly every possible situation.

A technique that can be of great help in assessing how a mould fills is that of deliberately producing short mouldings. Having set the appropriate filling rate and checked that the gate size and all temperatures and pressures are correct, a series of short mouldings is produced,adjusting the feed control to give the varying degrees of shortness. As the trial progresses,a study of the mouldings will indicate exactly how the material is flowing and whether,for example, additional venting will be required for complete mouldings to be produced.

If the mould-filling calculations are correct and the machine conditions are set accordingly,the temperature of the mould will make little difference to mould filling and the making of short mouldings except in special circum¬stances. An example of such a case concerns the moulding of a sheath on a solenoid,the material being glass-filled nylon. The mould in this case needed to be at between 80 and 90°C to mould a complete sheathing.


Sometimes when a mould opens,the vertical wall of the moulding rubs against the wall of the cavity and receives a scufT mark in the direction of opening. The most obvious cause of this is that the mating surface of the cavity has become “hobbed”,possibly by too rapid closing of the mould,and the edge is burred. If the mould has not been damaged and scuffing occurs, it may be caused by excessive packing of the mould during injection or by the draught angle on the mould walls being too small. With excessive mould packing, the cure is to Adopt the correct mould filling procedure and to reduce the hold-on pressure a册 time (or possibly to dispense with hold-on altogether). Scuffing caused by over-packing will nearly always be associated with residual stresses in the moulding, which can cause warping or breakdown in service.

‘The draught angle for moulds to enable articles to be produced free from scuffing depends on the shape of the component and the material. As little as 0-5° has been used satisfactorily for polypropylene,and sometimes no draught angle at all can be permitted. For materials such as ABS and acrylics, a more generous draught is required. Even if the draught angle is adequate, there may be some difficulty in extracting mouldings without causing scuffing if the mould has been polished at right-angles to the direction of mould opening.