we can try to visualise the flow of the melt into the cavity form. The material is initially directed down towards the bottom of the part and will then fill the cavity upwards from this point.
Any air in the cavity will be forced upwards and escape via the split line of the tool until the melt reaches the split line. Once the melt continues beyond this point, it seems that there is no exit path by which the air can escape. This means that the air may become trapped in the fixed half cavity and result in burning of the moulding.
Note that the actual fill pattern will depend on the gate size, speed of injection, injection pressure, tool temperature, and so on. To accurately simulate the most likely fill pattern a computer simulation is preferable.
In this case, however, we have identified the possibility of air entrapment and, if this possibility exists, we should do something about it before the event, and to counter this it is necessary to provide for a route for the air to escape.
The solution kills two birds with one stone. If we extend the core pin upwards into the fixed half cavity insert, we will give extra support to the pin, preventing any tendency for it to deflect because of nonsymmetrical melt pressures. By extending the locating hole for the pin upwards to the top of the insert we also provide an escape route for the air. This ensures the air will exhaust along the top of the fixed half cavity and the cavity retaining plate.
This method works well for moderate injection speeds, but extra provisions will have to be made if high injection speeds are used. This can be achieved by grinding a flat channel approximately 0.03–0.05 mm deep along the cavity retaining plate as shown. The width of the channel is not critical and can be any width within reason. As soon as possible this vent should be opened up to allow the air to exhaust and expand into a larger space, because the air and gases being forced out of the cavity are very hot and can reach very high temperatures. By opening up the vent, the gases will be allowed to expand rapidly and thereby cool rapidly.
With high injection speeds it may also be necessary to grind small flats on the core pin where it locates in the top insert to allow the gases to escape more easily.