Split Mould Casting Technique

[mangozac]

Pack your bags indeed and come enjoy some milder weather up here

[Fleafa]

I have mostly used two-part moulds until I recently started experimenting with one part split moulds.
I am getting bubbles quite alot where I didn't before. I guess injecting the resin through a channel which pushed the air out of the parts as it filled them was why. Still, I see most people on here just using one part split moulds and not having the same problem.

Do I need to pull a better vaccum? I can only reach about -25". I also wonder how far the resin gets towards setting in the time taken to pull the vaccum. Even casting at 40-50 psi after squeezing and vaccuming doesn't help.

[mangozac]

What is the pot life is the resin? -25 isn't a great vacuum, so the combination of that and a short pot life could be giving the issues.

Actually would you describe the issue as bubbles or air pockets? It's an important distinction between the two: bubbles are the very small individual bits of trapped air which occur from insufficient vacuum and/or pressure. Air pockets are due to bad flow in the mould.

The squeeze as you pour technique should sort out air pockets, but maybe you need to cut some air channels into the mould? If you're using vacuum and 50 PSI I would be surprised if you're getting bubbles, unless again the pot life of the resin is too short.

[Fleafa]

More like air pockets I guess. Will post pics when I am back at home.
Pot life is very short but I don't know how to slow urethane curing.
I use a fast cast 2 parter. Could I use less of one part, so it's not 1:1?

[mangozac]

Pics will help a lot!

You can try sticking the unmixed resin in the fridge for a bit before casting as that retards the curing, however ultimately it's best to switch to a different product. For example the 4PU resin that I use has a 4 minute work time and a 25 minute demould time (although demould time is more like 45 minutes for tiny, thin model parts). 4 minutes seems to be an optimal work time.

If it's curing too fast then your pressure step is probably having limited effect.

Still, like I said before I would expect the squeeze technique to get rid of most air pockets. You much have an odd layout. Show us some pics - slicing some new air channels will probably be the answer.

[blind pig]

What Mangozac said.

Try dusting the inside of the mould with talcum powder instead of your usual release agent and see if that improves the evacuation of air pockets. You just need a very light dusting, then shake/brish out the excess. It really does help to make the edges as smooth as a babys' bum.

[edit]

Oh, and I'd just try pressure, and not vacuum to cure the resin under. Vacuum will just make any trapped air pockets bigger.

[Fleafa]

I'll try with just squeezing and pressure. Pot life and demould are similar to your resin mangozac.
Layout is fairly standard for pauldrons.





UPDATE:

Dusted with slate filler, squeezed and pressurised; we have a winner!
I had already cut little channels across the bottom but I think they were redundant with this method.
Added small amount of filler to colour for clarity. Thanks for your help guys and girls!

[factor40]

Those look great! I understand the idea of split moulds but in the case of those shoulder pads, once you cut around the outside of them, how do you address cutting down through the center mass of the mould? How do you maintain an even line between the two parts as you cut?

[mangozac]

[Deadmeat30]

I find the wonky lines help line up the 2 halves. But mangozac is right, you break the top block off, then cut down. Shoulder pads are easy to do. You just make the cut at the back, down the straight edge.

[mangozac]

[Fleafa]

I feel the same. Ensures less slip with the halves locking in to one another.
Mould release is wax based, can't remember name right now.

[Seb]

In all my small part moulds, I used sections of wavy and indented splits. Aided lining up greatly.

Lovely result on the shoulders!

Seb

[Lane]

I thought that was standard practice.

[mangozac]

Heh well there you go it must be! But perhaps isn't obvious to the beginner...