When explaining the moulding fault commonly known as ‘jetting’ to part-time students from the plastics moulding sector, I usually prefaced my remarks by saying that ‘jetting’ was not a fault that you should see these days. Then one of the students appeared the next week with a classic example of ‘jetting’.
When a polymer melt is forced through a small gate at high pressures it can squirt the whole length of the cavity, rather like a fire hose. The melt then concertinas on itself in a squiggle as the cavity fills. When the cavity is packed out you can still see where the layers have failed to weld together completely. There is a visible flow pattern and a mechanical weakness.
Avoiding jetting
To avoid jetting, mould geometry and process conditions have to be altered to induce laminar flow within the cavity, filling from the gate end as an unfolding melt front rather than an uncontrolled jet of melt.
Reducing overall injection speed will help. However, the best approach is to profile the speed from slow to fast, to establish the laminar flow initially but still filling the cavity as quickly as possible. Increasing the melt temperature is another option.
Of course the possibility of jetting should have been addressed at the mould construction stage. Larger gate sections will minimise jetting. However, the best solution is to locate the gate so that the melt is directed across the cavity rather than along the length, thereby quickly establishing an unfolding melt front. Flow deflectors and dummy pins have also been used to avoid jetting. A tab gate works in a similar manner.
Injection moulding simulation analysis at the design stage is geared to alert you to the possibility of jetting. Jetting should be a thing of the past but it still happens.
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