Vacuum to make moulds

[Pinselknecht]

hey, if you would have used pressure you would squeese them together! I am always telling my cutomers: no styro foarm, no air pokets!

That is essential!

[timdp]

[TomG]

Yeah silicone can expand 4-5 times of original volume. So you need use big enough container. After that you have almost bubbles free silicone. Then you can pour it into mold and vacuum again few small bubbles. Still without clear window in the chamber. Expansion this time is only minimal, sou you still need some reserve on molding frame.

[mangozac]

OK guys I'm currently in the process of building myself a dedicated vacuum chamber for making moulds. Which is really good timing as a mould I made (under pressure) yesterday has a bubble in an annoying spot, something I'd like to avoid for the new sets of moulds I'm going to be making soon for Battlegroup Helios ships.

Now the problem I have is that the silicone I use only has a 5 minute pot life, so I don't have time to vacuum it in the container before pouring it into the mould and vacuuming again. It is otherwise an awesome silicone product and the short pot life means that it can be demoulded in 30 minutes.

The procedure I'm hoping to be able to use is as follows:
1. Create the mould box to be as tall as will fit in the vacuum chamber
2. Mix silicone
3. Pour into mould box
4. Place into vacuum chamber and start pump
5. Watch the silicone degas. If it is about to overflow, release the vacuum so that it collapses again.
6. Keep repeating steps 4 and 5 until the silicone stops rising
7. Release the vacuum and put the mould aside to cure.

Do you guys think it will work as expected? Will I be able to vacuum enough in the limited working time I have? The new vacuum chamber will have about half the volume of my pressure pot, so will be quicker to evacuate.

[Nostok]

It is how I have been doing my moulds but then I am using oomoo silicon so have far longer to play with.

If you are making your own vacuum chamber try to avoid having the inlet/outlet directly above where you would place your piece, the "burping" can be a little violent otherwise and I have had silicon spray about the chamber because of the inrush of air on a taller mould.

[mangozac]

Thanks Nostok, that's good advice! I was thinking of putting the inlet off to the side.

So you don't vacuum the silicone in the mixing container? Just in the mould box? How long do you estimate it takes for you to get the silicone to full vacuum?

[TomG]

I vacuum (maximum that vacuumpump can do) silicone after mixing, it takes something about 7-8 minutes. Then pour it into molding frame and vacuum again for 5 minutes and let cure.
I use this big casting chamber with some wooden pieces to make it smaller inside, but have reserve for bigger molds.

[mangozac]

[Nutso]

I have great success degassing silicone after it is poured into a mold box. You don't have to be as careful pouring silicone over your model since it will pull any entrapped bubbles away from the model.

I'm not sure I'd try it with a silicone that has only a 5 minute pot life. Even if the silicone you use has a very low viscosity, you will be hard pressed to get it done in time. Also keep in mind that you'll want a very tall mold box if you choose to degas only once after you have poured it into the mold box. Releasing the vacuum and reapplying vacuum to prevent overflow takes up a long time that you just don't have even with a powerful vacuum pump.

[mangozac]

[TomG]

Or use slower silicone.

[Fleafa]

I've just started experimenting with silicone thinner. As you would expect from the name, it lowers the viscosity of the silicone but there are other effects. It lowers the shore hardness a little as well as slowing the pot life and cure time.
The lower viscosity allows for easier degassing and pouring. Together with the slightly extended pot life, this may be a real help to you.

[mangozac]

Thanks Fleafa, that's interesting. Will have to take a look into it