New to vacuum pressure casting - check list

[smtk]

This week will be my first time vacuuming rtv and pressure casting I just wanted to run through the list to make sure I haven't forgotten a step;

- Masters to be filled with resin to prevent implosion
- Measure and mix RTV with a squirrel paint mixer
- Vacuum RTV for 30seconds no more (I cant see inside the vacuum chamber so will have to time it)
- Vacuum mould with poured RTV for 30seconds (will it rise? ie do i need to build up the mould box)
- remove mould allow to set

Repeat for 2 half

- clean mould
- add release agent and talc
- mix resin
- pour
- slowly pressurise to 40psi
- allow to cure

De mould for perfect cast..

I do plan to get a clear 12mm Polycarbonate lid cut for my pressure pot for the vacuuming but for now it'll just have to be timed.

[Anvils Hammer]

Whats a sqirrel mixer? I just use a metal stick..

30 seconds is nothing like enough unless you have the worlds largest vacuum pump..

it could take up to a couple of minutes for teh chamber to get right down to vacuum, I would allow at least 2 minutes, if you have a vacuum gauge, 30 seconds minimum at full vacuum.

To stop spillage, ensure your rubber in in a continer at least 5X its volume or more. so for 200ml of rubber you want at least 1L. Ice cream tubs are great.

Same deal with degassing the mold, degas for at least a couple of minutes at full vacuum to allow all the bubbles to rise to the top. so maby 3-4 minutes total.

You will need at least an extra inch of mold box above the level of the rubber for a small ish mold, as even rubber which is degassed will still "boil" quite a bit.

Good luck,

AH

[smtk]

ok so 2mins on the rtv

and a squirrel mixer is a vortex paint mixer very quick to mix


A guy from another forum recommend using one

[blind pig]

[smtk]

fill void spaces with resin check
resin leaking all over details check...

[paulson games]

I'd suggest using 60 PSI if possible. I'd worked at 40 PSI previously but found that it doesn't do the job in some situations. I've had far fewer issues with bubbles at 60 PSI.

Honestly I don't even mess with degassing my silicon, so long as the mold is created under pressure it works fine without the degassing step.

[smtk]

My main issue now is how to fill the styrene masters

resin leaked everywhere, so what are my options?
Vaseline over the outside of the master to seal it and then resin
Plaster
PVA + some kind of filler like sand
Milliput

I'm real sad about the resin leaking onto all the surface details I just got to hope it can be picked off. I tried to mop some of it up be its hard to tell how much was left on the model ./sniff

[Anvils Hammer]

you should have got the resin off while its still liquid!

Resin will "flake" away from plasticard if you pick it with a scapel.

Some photos of what you are doing would be helpfull.

The best material for filling is definately milliput. cant go wrong with that stuff.

AH

[smtk]

Here's a photo of the masters build from styrene sheets made into fancy boxes (the nose is milliput) red part shows the leaking areas



I made holes in the master and filled them with resin (first time). I was using a new brand today and it was far more liquid than smooth-on. I was trying to work fast before the resin kicked I was about 3/4 of the way way through when I noticed the resin seeping out the joins.

I poured out as much resin as I could and mopped the rest up with paper towel. The resin is clear so it wasn't easy to tell if I got it all. The cure time is 1 hour and my house and garage are 20mins walk apart so I left and I'm just about to walk back there and see whats what

[smtk]

Well it think it all turned out ok.

Today I picked off a side of each box and then filled the cavity with millput. Seems it got lucky and the resin didn't mix properly so it could be scooped out and scrubbed off with tooth brush.

Its worth mentioning the 0.5mm styrene also seem to be softened by the resin. Styrene is terrible at reacting with pretty much everything in my experience.

[wormwood]

ABS is also available in sheet form, and is a little less vulnerable to chemical reactions than styrene.

[mangozac]

Hi SMTK I'm sorry to hear about all of your problems. I guess the lesson here is that parts need to be constructed in a different way when you know from the start you plan to pressure cast them. Opening up and filling with Milliput will certainly be a lot safer than injecting the resin. But just imagine if you'd tried moulding the hollow parts - if they didn't implode they would have filled with silicone through the small hole that the resin leaked out of!

Styrene does have a tendency to react with stuff, but that's what makes it so versatile. In many cases you want that reaction in order to create a good bond. For example, when priming the solvents in the primer lightly break down the outer surface of the styrene so that the primer bonds to it extremely well.

Have you started the moulding yet?

Oh and unless you're mixing really large amounts of RTV I wouldn't use the squirrel mixer. I'd use a chipmunk mixer instead....

.. haha I'm a comedian!
Actually I wouldn't use any kind of motorised mixer as they tend to introduce a lot more air bubbles. I know we're pressure moulding and everything but starting off with more of them just increases the chance of problems.

[smtk]

I'm still waiting for an end cap (dam imperial measurements mix up again) and its +1 to -5 degrees here so I cant cast anything atm. I'm using my garage (for safety) but its a walk away and un-heated. While I do have a space heater I'm reluctant to leave if on if I'm not there.

[paulson games]

[smtk]

ack oh well that model is going in the bin

I finally got my vacuum/pressure system all working. But I discovered the styrene on the model has gone all mushy from coming into contact with resin (I'm guessing it reacted badly to the catalyst) add to that the new silicone I'm using is hard to mix right (gooy silicone yay) and I don't know if my vacuum pump is really do the job right, soft parts of the model have collapsed whilst it seems some of the resin catalyst still trap inside the model messing with the silicone and working for my garage sucks I'm going to cut my losses.

Before any one mentions pressure casting my system wont hold 60psi for 24hrs and I can't sit there for topping it up.

I think it's just too much hassle to save on top of the wife's negativity about everything to do with models.