My first injection mold and the Rabit HY-4 injection molder

[Robin Hewitt]

Let me go through the theory so that anyone here who is not au fait with sprues can keep up.
The mould is in two halves which bolt to two plates in the moulder. The plates are called platens.
There is a fixed platen which you can move by handscrew to allow for different height moulds.
There is a moving platen which winds back when the moulding operation is complete so you can get the part out.
The fixed platen has the ejector pins coming out through it to free the part when the mould is open. So it is very important that the part stays with the fixed platen on opening or everything is going to go horribly wrong.
Coming up through the middle of the moving platen is the injection nozzle which has a whole mess of molten plastic behind it and some kind of piston arrangement that is going to force it out. It's a bit like a hot bicycle pump.
So, the mould closes, the nozzle connects, the plastic is injected. It has to get to the heart of the mould to form the part.
The initial pipe that carries it will end up full of cold plastic. The plastic in that pipe forms the sprue.
If you are still reading, well done, you will now appreciate the problem, the sprue is moulded on the moving platen side but it must stay attached to the fixed platen side when the mould opens. Obviously anything that helps it slide out is good.
First thing you do is make the sprue using a tapered hole and polish it inside.
Next thing is to make the injection nozzle big and hot so the mould never manages to chill it. If the plastic strings a bit when the nozzle retracts that is good. If it strings too much you might need a puff or air to freeze it.
If the sprue connects to runners rather than directly in to the part you can fit a sprue extractor to the fixed platen side of the mould. A sprue extractor is a conical dent widening in to the mould with a monster ejector pin at the bottom to force it out once it has done it's bit. You have to make sure that the whole sprue comes out. If the sprue breaks and leaves anything of itself behind then we have to hope the machine can detect it.
If you are still reading you should be up to speed.
The European nozzle is usually hemispherical and goes in a hemispherical sort of dent. There is no flat end to the nozzle because that would mould a nail head on the top of the sprue and make it hard to pull. That is why I don't understand why this machine seems to have a flat.
This machine has a relatively tiny nozzle diameter so is there a real risk of it freezing? Perhaps it is supposed to freeze, pull the nail head and the remelt it? Perhaps the nozzle is floating and the conical cutout is to guide it? Obviously it must work or they would have done it differently, but I am still puzzled.
This needs to be understood before trying to make a mould.