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Resin Addict Forum • View topic - whats the correct pressure lvl?
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whats the correct pressure lvl?

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whats the correct pressure lvl?

Postby Greenstuff » Sun Aug 18, 2013 8:03 pm

Hi Guys' I'm new here, new to making moulds/resin miniatures, and also new to forums! I bought myself a vacum pump with a 10l chamber, having no problems with moulding, and I understand the process/why of removing bubbles from from the silicone, the main problem I am having is when I am casting my resin into the moulds (using supersil 25 305) using the vacum and chamber. how do I prevent the bubbles from evacuating the resin from the mould? i.e. I place the filled mould into the chamber (oh, and yes, I have put tape over the top of the mould) switch on ease the pressure as far as I can to -30, but before then whoops the resin has overflowed and hence when the pressure is reduced, I am getting partial results and half the miniature? I know this seems daft, and perhaps it's a very simple solution, would have anything to do with temperature? would appreciate any thoughts.
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Re: whats the correct pressure lvl?

Postby mangozac » Mon Aug 19, 2013 8:35 am

Hi Greenstuff, welcome to RA!

Sounds like you're off to a good start with your set up and I have good news for you: your problem is easily fixed!

When you apply vacuum to silicone or resin, it initially expands in volume as the air trapped in the liquid expands. This will cause it to overflow out the top of a mould box or mould. So when using vacuum in your casting technique you need to create a reservoir in the top of the mould that the resin can expand into without overflowing.

The ideal technique is to make this reservoir part of the mould. I usually make a block with tapered sides, as you can see in the pictures in . Note that normally the reservoir just looks like a part of the sprue. You want the reservoir to be at least roughly 2x the volume of the resin parts (3x for silicone).

If you already have moulds that are overflowing there is a trick we learned from some where you wrap some wax paper around the mould so that it creates a reservoir in the top. It's fiddly though so I only recommend this on moulds you've already made and are having problems with.
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Re: whats the correct pressure lvl?

Postby Greenstuff » Tue Aug 20, 2013 1:02 am

ok, thanks for your advice, I'll give that a try.
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Re: whats the correct pressure lvl?

Postby mangozac » Tue Aug 20, 2013 8:06 am

No worries. Let us know how you get on!
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Re: whats the correct pressure lvl?

Postby Greenstuff » Thu Aug 29, 2013 11:11 pm

Ok, thanks for the advice.... your reply got me thinking, having already have made the mould, I was a little reluctant to make another.. so, I taped some brown parcel tape around the top of the mould making a kind of funnel for the resin, allowing it to bubble up without spilling over, then by easing the vacuum the resin expanded allowing the bubbles to surface without spilling over, I gave a few sharp releases to force the resin down, in total about 2 mins of vacuuming on-off (quickly). Left it for about an hour, came back and hey presto, it worked! my first success. Now I tried this again a few times, had reasonable results. 3 perfect from 6 tries, I'm beginning to see that it will be from understanding of regulating the vacuum, that I will have better results. Thanks. I will insure the mould I make has a larger reservoir, but I'll still use my brown tape method as well. :mrgreen: Brilliant, thanks again for your advice.
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Re: whats the correct pressure lvl?

Postby mangozac » Fri Aug 30, 2013 8:02 am

Nice work! I think someone else suggested that idea here too - I forgot about it!

Learning to cast is a lot of trial and error, so I'm glad that the failures you are having aren't discouraging you!
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Re: whats the correct pressure lvl?

Postby johncast » Fri Aug 30, 2013 8:16 am

Hi Green stuff

Welcome to the forum. Escaping resin from the mould when vacuuming , big problem.

As mentioned in the other posts , make a good tapered sprue area on the bottom of each item you are moulding so the rein can bubble away in this cavity whilst under vacuum.

Taping around the top of the moulds is also Ok but if the moulds are square it is hard to do , as most masking tapes or similar don't stick to the rubber.
Some of my moulds have had the same problem , to keep the resin from running all over the place and well contained , I made a cardboard frame to sit around the opening of the cavity, about a inch high, make sure it sits nice and flat around the sprue . I pour in the resin ,place the frame around the sprue and on top place a small , metal weight , fishing sinker etc on the top of the frame , this helps to seal the frame into the mould a little , vacuum the mould as usual and all the resin can blow up all it likes and will only be contained in the frame . When the vacuum is released the resin just flows back down the sprues into the mould . When the frame gets a build up of resin just make a new one.

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Re: whats the correct pressure lvl?

Postby Greenstuff » Fri Aug 30, 2013 5:18 pm

Ok, Thanks for that JohnCast, I'll try that also. I'm concentrating on making moulds now, No problems with mixing, degassing, and pouring, I make my mould box from Foam board, allowing myself enough space around the miniature to allow strength whilst cutting open. Thats the tricky bit, I have come up with a method of imitating a plastic sprue that you would find on a commercial model kit frame, this helps as a guide when splitting the mould, and the flow of the resin around the more delicate parts of the miniature. It's a bit fiddly when cutting inside the sprue. But i guess practice makes perfect. I have to get this right as I intend to sell my miniatures, and need to reach a high standard. thanks all. is there any way of posting a photo? I could share my results.
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Re: whats the correct pressure lvl?

Postby mangozac » Mon Sep 02, 2013 9:20 am

If using a vacuum casting process why do you need the sprue surrounding the model? The sprue at one is is pretty normal and works for everyone else.

We'd love to see your results - to post a photo you'd need to upload it to an image host (like or ) and then post a link here. Alternatively you can email it to me (mangozac@ the address of this website) and I can post it for you ;)
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Re: whats the correct pressure lvl?

Postby Blindhorizon » Tue Sep 03, 2013 9:04 am

I look forward to seeing your miniatures that you cast. Some quick questions about how you prep your mold before you pour your resin. Do you use baby powder/talc in your molds? I use this method and it gives fantastic results, any one method can work good but if you use a variety of methods you can get the best possible result. I have been working with the idea of using a rosin bag filled with baby powder to just tap it against the molds to apply the powder. Just a thought as i ramble a lot :)

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