My investigations into the art of mould ejection continues.
Imagine a part like a drinking glass, the wall thickness is perhaps 3mm. Lets say for arguments sake that you are going to push the part off of the core by pushing against the 3mm rim of the glass at 12, 3,6 and 9 o'clock positions (4 pins).
Due to the rim being only 3mm I assume I would use a 2.5mm pin so there is no clearance issues.
When the part is ejected there would be 4 small complete circle marks from the ejection pins on the rim.
Now rather than using tiny 2.5mm pins that are awkward to drill ream etc (drill wandering etc), you choose to use say 8mm diameter pins. Rather than pressing all of the pins against the rim, only a crescent of each pin would contact the part rim, but will probably have a much larger surface area than the small pins in complete contact.
Now the problem is that the pins when the mould is clamped together, rather than being in free space, are now against the cavity shutoff surface with only a crescent poking over the edge into the cavity.
Is this an accepted practice?
I've seen it done many times. I never do it though.
Are there pin length accuracy problems?
Yes, but it is not out of the range of a skilled moldmaker.
Will there be leaking flashing problems if the pin is not the correct length and perfectly square on its end?
Yes, but it depends on the material.
Does the ejector plate/assembly have to be different to allow for the cavity (A side) plate pushing the pins back?
No, but the return pins have to be more acurate. I would also spring load the ejector system.
Could the pins be sprung loaded to allow for a small error to ensure a good seal? or would spring loaded pins be pushed back by the plastic injection pressure?
Yes, and No.
Sorry for the bombardment of questions.
many thanks for any replies.
OK first off, If you cannot drill an ejector pin hole and hit a 3 mm wall you probably don't have any rights to attempt it, and I question your ability as a mold maker. That being said, I suppose use of ejector pins on this application would not be ideal way of ejection. The larger pin idea.. not much better. I call this the "Toe nail" method. It's probably OK for a prototype or other small run application., but if thats the case I'd probably run the pins into the bottom of the cup shape. It might not be automatic (without opperator) and there's a chance of the part not falling off the pins when ejecting even with spring loading. I would use an air popett valve or ejector connectors to the ejector plate and maybe use mutiple knock out strokes if your press allows this. You may be able to run it automatic. The best way is to design a three plate mold using a stripper plate rather than ejector pins. Also, might try a stripper ring bolted to ejection. I call this the "poor man's 3-plate" system.