What happens to the air sat in the mould?

[Scooby]

When injection moulding what happens to the air in the mould?

The moulds ive seen have gates, spruce and runners but i dont see anywhere for the air in the mould to escape, will this not give defects in the mould? obviously it doesnt so im interested what allowance i need to make for it, obviously a hot shot traveling down a long thin part will get quite some build up at the bottom the more plastic is pushed in.

[mactec54]

Hi Scooby
If you look very close at the the faces of the mold/mould you will see ground vents these are ground to different depths depending on the material being used, Material will flash out the vents if they are to deep this is one way.
These vents start at around .0005" deep

The ejector pins in the back of the mold/mould also can have a ground flat on them all most to the end of the pin (Like .100" from the end ) the pin has a small clearance in its hole as well (Like .0005" ) this lets the air/gas out of the mold/mould The clearence the ejector pin has also is to do with what material you are using some materials flash easyer than others so you have your clearence to suit the material you are using

They also can add vaccum to the closed mold to evacuate the mold this is the way to evacuate a mold of air/gas on hard to Fill cavitys

There are also small valves that can be made that are open before the shot is pushed in & close as the pressure builds up in the mold/mould

[neilw20]

Sometimes trapped air becomes obvious as burn marks in the plastic.

[Scooby]

Thankyou both for your replies, most helpful. I didnt realise the plastic burns where there may have been a trapped air pocket, thats good to know to look for.

[Stu_M3]

I once made a large Aluminium mould for a 4ft long frame the air escaping from the corners produced so much heat escaping, the aluminium melted at the corners which had to be replaced with steel inserts

[mactec54]

Hi Stu M3
If your mold/ mould had good vents this would not of happened