Splash marks,mica marks,silvering

Splash marks,mica marks,silvering

This fairly common fault is easily recognized by the streaky,silver appearance of the moulding, nearly always radiating from the gate area.

Cause of the splash marks

The cause is the vaporization of volatile matter in the advancing front of material as it enters the mould,or sometimes within the mould itself. Volatiles can be the products of decomposition or depolymerization of the material or may be due to the absorption of water by the plastic before moulding.

The volatile material entering the mould condenses on the surface, forming liquid droplets. As the material enters the mould,the liquid droplets are carried along, forming the streaky pattern. The same effect is obtained if the mould is cooled to below the dew-point of the moulding shop and atmospheric moisture then condenses on it,producing the water droplets.

How to deal with splash marks

The cure for splash marks is,therefore to eliminate all volatile matter. If the material contains moisture,it should be dried; if decomposition or depolymerization are causing the volatiles, the cylinder temperature should be lowered, and if moisture in the mould is to blame, its temperature should be raised.

Materials most likely to be affected by moisture are those that contain hydrophilic groups in their molecular structure. Among the commonly used plastics, acrylics,nylons,and cellulosics are the most hydrophilic and therefore the most prone to splash marking. However,owing to the presence of finely divided carbon (carbon black), some outdoor weathering grades of polyolefines have a tendency to absorb moisture, and others,such as ABS and polycarbonates, tend to attract sufficient moisture to cause splash marking by adsorption of moisture on to the surface pores of the granules themselves.

Manufacturers normally supply material in the dry state. Only rarely is the moisture content too high to permit satisfactory moulding,but whereas a small quantity of moisture (less than 0-1 per cent) can often be tolerated in nylon or acrylic material, no moisture at all can be tolerated in ABS and polycarbonate if splash-mark-free mouldings are to be made. These must be dried immediately before use and put hot into the feed hopper. Nylon and acrylic should be stored in air-tight containers or polythene film sacks and re¬work should be kept dry.

Decomposition

Decomposition as a cause of splash marking is not easily recognized unless the purgings from the machine nozzle are examined. If there is any tendency towards bubbling or frothing, the cylinder temperature should be lowered and a check made to ensure that there are no hot spots. Excessive mechanical heating developed during screw back can also be a cause of overheating and should be suspected as the cause of splash marking if any variations in screw- back time have been observed prior to the fault developing.

Some moulders believe that splash marking is caused by shear heating in the gate,basing their assumption on the fact that splash marks nearly always radiate out from the gate. With a well-designed gate system this is extremely unlikely for the reasons already discussed in the section of this chapter dealing with burn marks.

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