What is Burn Marks
If an area of a moulding becomes the oxidized, discolored, or blackened, the first thing to do is to examine the venting of the mould. Air trapped in the mould is heated to a very high temperature as it becomes compressed by the material entering the mould.
It reacts with the hot plastic and brings about oxidation. There may not be sufficient air to cause complete combustion of the plastic to carbon dioxide and water, and the hydrogen component of the material reacts first, leaving a black deposit of carbon.
If the burn marking is allowed to go unchecked, it will eventually cause oxidation or burning of the metal of the mould, and the surface will become pitted. Even if the burning ceases because the air finds a way of escape, the surface finish of the mould may be blemished, and the part will need to be repolished.
Air trapped in a mould may be the result of poor mould design leaving a zone of the cavity not swept by the incoming wave of plastic. More frequently,it occurs because the mould maker seems to do his job too well.
Normally, air escapes at the mould mating surfaces and along ejector pins or stripper plates- If the latter are too well fitting or if the former have been too accurately polished this means of escape is barred. (It is interesting to note that the mating surfaces of moulds are generally the easiest surfaces to polish and therefore get the most attention).
What cause Burn Marks
A simple expedient, sometimes resorted to by a mould maker (and occasionally recommended by the mould designer), is to arrange for a gap of 0 0005 in (0 0125 mm) about 1 in in length (25 mm) to be made at a suitable place. If this were the only vent for a mould of capacity 100 cm3 filled in 1 s the velocity of the escaping air would be about Mach 1 (1000 f/s or 330 m/s).
Apparently, great heat would be developed at such a vent and for this rate of filling, which is not very great, much wider and thicker gaps are needed.
Most plastics will escape very little, if at all, through gaps of 0 0015 in (0 0375 mm) and the time-worn expedient of the cigarette paper between the mating surfaces has much to commend it. It is nearly always desirable that the mating surfaces should be left unpolished, possibly with fairly coarse tool marks, so that natural venting can occur.
Another cause of burn marking is the trapping of air in the nozzle of the cylinder when suck-back or decompression is used incorrectly. This is the slight backward movement given to the screw after screw-back, which helps to reduce nozzle dribble. If the suck-back is too great, the material in the nozzle may be oxidized and will show as a burn mark in the moulding. Its appearance will be different,as if it were ‘under the skin,of the moulding and its position will vary slightly from shot to shot. Normally, it will not have any soot associated with it.
The third type of burn marking occurs because of oxidation or decomposition of the material within the cylinder and is more common with PVC,acetals,and nylon than with other materials- With PVC it appears as a dark streak extending through the moulding,varying in position from shot to shot. The cure is to reduce the amount of electrical heating and to increase the amount of mechanical heating in the cylinder. The best way to do this is to work with a reverse temperature gradient in the cylinder, keeping the rear zone hot,the centre zone cooler, and the nozzle hot. It is also important to make sure that the screw almost ‘bottoms’ at each shot so that no material resides for too long in the front section of the cylinder.
With acetal the first sign of degradation in the cylinder is a darkening of colour from white to yellow or brown. Temperatures should be reduced and the cylinder purged. At the same time, the temperature controllers should be checked to ensure that they are functioning correctly and that heater bands are working properly.
Finally, shear heating in the gate: this is often blamed for burn marks or signs of decomposition around the point of entry into the mould cavity, but these are much more likely to be the result of overheating in the cylinder. Experiments and theoretical work have shown that shear heating giving a temperature rise of up to 20°C can occur in long capillaries at high shear rates. In injection moulding, the capillary, in this case, the gate, is very short and although the shear rate is high, there is probably no net increase in the material temperature the mould filling is adiabatic. The temperature may rise on the compression side of the gate because of the high pressure being applied, but immediately after passing through the gate the material is decompressed, a process requiring the application of heat.
In injection moulding, the capillary, in this case, the gate, is very short and although the shear rate is high, there is probably no net increase in the material temperature the mould filling is adiabatic. The temperature may rise on the compression side of the gate because of the high pressure being applied, but immediately after passing through the gate the material is decompressed, a process requiring the application of heat.