Changing materials

[Ein]

Last time I got screwed up big time by Alumilite and since I found a local distributor where I can easily drive to in order to buy what I need, I decided to make the change to Smooth-on.
Any hints or tips? I bought RTV and Resin. Mold max-10 and Mold max-15, Smooth-cast 300 and Smooth-cast 305. Also their own black die.
I think I read somewhere that someone used Alumilite die on the Smooth-on resin because it was better. Is it really?
At least on the RTV it's half the price of Alumilite, which is excellent since silicone is the most expensive part of any project.

[paulson games]

I pretty much use smooth-on all the time now. I use the OMOO 25 silicon which works great and sets up fast, and I usee the smooth cast 300 resin, and sometimes the 305 depending on what I'm casting.

I do use the allumilite black dye as it's great stuff and goes a really long way. You only need about 3 drops to color an entire gallon of resin and I've been using the same 6 oz bottle for more than a year now.

[Hatemonger]

Do you vacuum the OOMOO stuff or just use it as mixed?

- H8

[paulson games]

I only use pressure (60PSI) for my casting and mold making, the vaccum doesn't really seem to deliver any better results. I make all the molds at 60PSI as well otherwise you get dimpling on the molds.

I let them cure under pressure for 2 hours, and then usually give it an extra hour or so to firm up further before removing it from the mold. If you pull it out while it's still somehwat liquid or sticky large air bubbles can leech back into the edges of the mold and mess it up, the silicon can also stretch and distort the mold if you try and take it out too early. You can depressurize very slowly and that won't tend to happen but I've found it's just safest to leave it under pressure the whole time.

When makingf two part molds I use micromark rubber to rubber release to keep the faces from sticking.

I use the spray on mold release that smooth-on sells (Mann) and talc (Johnson's baby powder) to make sure there's no bubbles in the choke points and it works quite well. I'd tried the vaccum system but I honestly don't see any differance in how the casts turn out.

If you do use vaccum you'll need to get the 305 resin, the 300 has too short of a pot life (2 min) which doesn't allow for mixing vaccum and pressurizing. The 305 has about a 7 minute pot life so it'll give you enough time but you still need to move quickly.

[Ein]

Thank you very much.
The OMOO 25 is a 1 to 1 mix ratio, isn't it?

[paulson games]

Yes the OMOO is a 1:1 mix , same with their smoothcast resin. Helps a lot as there's no annoying mixing ratios to mess up.

[Hatemonger]

I'm a little confused about this vacuum/pressure thing. Up until reading this board, everything I saw about casting led me to believe that you vacuum the rubber, then pressure the resin. The idea of doing the same thing to both seems strange.

Maybe this is just old school instead of state-of-the-art, or maybe I misunderstood from the beginning, but I thought the process was:

1) Mix the rubber and degas under vacuum. This is mostly to draw out bubbles introduced during mixing. Once all the air bubbles surface and pop, you can stop the vacuum (should be only ~5-10 minutes depending on rubber)
2) Pour the mold. With a brushcoat for undercuts and small voids, and a careful pour, you should introduce little to no extra air.
3) Allow rubber to cure (24 hrs or whatever).

4) Mix resin. Since pot life on resins is typically much shorter (for quicker turnaround if nothing else), there is not usually time to vacuum.
5) Pour resin into mold, knowing there are inevitable bubbles in the mix.
6) Apply pressure to the filled mold, which shrinks bubbles down to (hopefully) near-invisible size.
7) Leave pressure on while resin cures. Since the cure time is quick, this is also a short process, and since the resin is harder, it will 'freeze' the bubbles in place after the pressure is relieved.

It seems to me that with pressurizing the rubber, you're just distorting the bubbles instead of removing them, and I would think if you don't re-pressurize to exactly the same PSI every time, you're going to get variations in how the bubbles appear (or don't) on the castings. So what am I missing?

- H8

[Lane]

Casting is as much art as science.

Mold making:
I vacuum the RTV before pouring and sometimes after the mold is poured. This method should produce a mold that will not dimple or pimple the casting surface at any pressure. The second vacuum does not work if there are any voids as they can cause huge bubbles or explode the master.

If you do not have a vacuum chamber pressure works as well. Take precautions to avoid bubbles in the poured rubber then pressure cast the mold. Any voids in the model will tend to fill in with RTV but since openings tend to be small the master is still easy to remove. The pressure shrinks any bubbles and produces a smooth surface on the casting if cast at the same pressure. casting at a different pressure can allow any surface bubbles to shrink/ expand.

Casting:
Pressure is the most common method of controlling bubbles.

Vacuum is sometimes used with long set time resins between mixing and pouring. I do not know of anybody casting under vacuum, see note below.

Clear resin tends to have a slightly longer cure time, 3-5 minutes vs 1-3 minutes, so there is time to vacuum. This is important since even tiny bubbles can cloud the casting.

Note: time under vacuum should be kept short. The lower pressure allows chemicals to boil off at much lower temp than normal, extended vacuum can change the behavior your RTV or resin.
When degassing the RTV watch for a large rise then fall in volume, sometimes you wll see a second rise as well. In general 30 seconds under full vacuum is enough. If it takes more than a couple minutes to draw full vacuum you can reduce this time.
Same applies to degassing resin, keep it short. Most likely you will not see a rise in volume, just a foaming at the surface.

[mangozac]

H8, you'll find that through experimentation many people come up with many varied techniques, and these techniques will change over time. I should probably update the tutorials on here to reflect my current technique at some stage.

Indeed it makes more sense to vacuum the silicone before (and even after) pouring, however with the hobby crowd it's more common to use pressure only. The bubbles are generally small enough that they are unable to expand once the silicone is brought back to room pressure.

I use a combination of vacuum and pressure for small parts, but only pressure for larger parts.

Hope that helps.

[Lane]