Mold release

[graemeian]

I want to make a hemispherical part out of acrylic. My concern is which half of the die will the part stick to so I can incorporate an ejector in the die. My guess is that the opened mold will look like the following sketch when opened. Is this correct?
Thanks8

[processtech]

Id be of the same opinion too, when the part cools wont it contract,therefore staying on the core.See what other opinions are given though

[joeprocess]

Acrylic has a low shrink rate and may require higher pressures to pack out which many times causes a part to stick in the cavity vs. shrink around the core. The vacuum created in the cavity will also be higher then on the core side if ejector pins are placed on the core side. The sprue will also hold this part into the cavity side. Can you add an air poppet to the cavity side or a couple spring loaded pins to push the part onto the core during mold opening?

[rickbatey]

You can assist the part with staying on which ever side you make the ejector half, by placing under cuts in certain areas and with running a higher steel temp on the stationary half.
You have not stated if this will be clear, if/how you will bond the two parts together. This data will be required to make the best decisions for molding.
Have you added in some draft to get the part out of the cavity? How round must this part be? Why acrylic? Is PMMA the best resin for this product? Will it just fall out of the mold when ejected or removed by hand/ robot?
I would prefer to mold it as drawn, with draft and possibly adding an air poppet to both sides of the mold to get it open as fast as possible-stuck to the correct side-and off the core with minimal warpage and cooling time.
Rick.

[Suhas]

Would you need a vacuum release? - i don't think so, just asking.
Suhas

[rickbatey]

Suhas, it really depends on the parts strength , how highly polished the mold finish becomes, and even the nominal wall stock. I tend to build a mold based on high speed requirements so that I'm forced to study the part and tooling more indepth. But at my age I probably miss more than I remember!?
Rick

[joeprocess]

If you want the best shot at making this part without adding undercuts and you need to eject on the inside of the part an air poppet or two should work well.