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Silicone mold degassing/pressure? question


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I am about to make some silicone molds and I'm unsure if after degassing the silicone (to get out the air bubbles) if I need to let the silicone set under pressure?

 

If I understand the process correctly, I mix the silicone, put it in a large container (to allow for expansion) and put it under a vacuum to degass the silicone. Once the bubbles are out, I pour the silicone over my master (in a box) and let it set up. It's here where I'm unsure of the process.

 

Do I let the silicone set up at atmospheric pressure or in a pressure pot at say 40 psi?

 

I know I am to cast the resin under pressure, but not sure about the silicone mold itself?

 

Thanks in advance, Chuck.

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Hello Chuck,

 

Atmospheric pressure is enough. Putting it in a pressure pot is not necessary for the silicone molds. You can also, to make your life simpler, prepare a small initial quantity, and paste your master with a thin layer of silicone with a brush. This is where you do not want bubbles to settle in. When this layer is set, you can pour more silicone for the volume and mass, but you do not really care if it's riddled with bubbles then, as long as the skin around the master is bubble-free.

 

Hubert.

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How are you achieving vacuum? My reading suggests that you gave to pull 28 inches of mercury to deguass most silicones. I use one of my neon vacuum pumps to achieve this level of vacuum. I was finally able to cast parts without bubbles. The vacuum is more important than the pressure casting. With my setup I pull vacuum, then open a petcock to slowly return to atmospheric pressure , this forces the silicone/resin into the mold/master.

IMG_5391_zpsbe7e9bec.jpg

 

IMG_5384_zps6df9367c.jpg

IMG_5388_zps12fb1ae7.jpg

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Thanks Hubert.

 

It's my undstanding that if I do not degass the silicon under vacuum, when I go and use pressure for my resin casting, it will cause the trapped bubbles in the silicone mold to collapse and distort my mold. Does that sound right to you?

 

I have also read that if you are NOT going to use pressure for casting your resin you do not need to use vacuum to get the bubbles out of your silicone. Its starting to make sense to me now.

 

I like your idea of brushing on the initial coat of silicone though.

 

Thanks for the input, Chuck.

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I hold the end of our household vacuum hose just over the surface of the silicone and it sucks all the air out to give me a vacuum, right

 

(kidding of course!)

 

I have a vacuum pump and and a modified pressure pot that allows me to pull down to 29"/hg.

 

I like your castings, looks good.

 

Thanks for the info, Chuck.

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Those parts molds were set without vacuuming the silicone , I set the molds on my spray booth ,turned it on and the vibration allowed the bubbles to rise out of the silicone. I did not pressure cast the resin . I mixed the resin , poured it into the mold and vacuumed the air out then. I did a series of 4 ,five minute vacuum/relieve sessions with each pour, that is to say ;

I poured the resin , pulled 28 inches of mercury (less then 1 minute), let off vacuum , let the pot return to atmospheric pressure(3 minutes) ,then repeat 3-4 times. No pressure needed with the proper vacuum and smaller molds. I also cut a hole in the top of the pressure/vacuum pot ,bolted on some plexi so I can see what is going on.

Super happy with my results.

IMG_5472_zps0d909ec0.jpg

IMG_5474_zps3de52308.jpg

IMG_5476_zps8eeecda6.jpg

Edited by krow113
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ya it was a big relief, look closely at the parts ,you can see the slight flash and some 'positive' bubbles.

I did have trouble with the silicone/resin compatibility, my molds for these parts are history, I used the wrong resin for the silicone used. The parts were 'torn' from the molds in the end,even when I worked mold release into the molds with a q-tip. Having limited (none) experience results in a disjointed learning curve.The parts are perfect though, and I already bought more silicone.

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If you can de-gas the silicone when you make the mould, you will get better moulds. Firstly, you will remove any voids against the masters, which will result in not picking up the details or something more significant, when you do your casting. The rubber won't touch all of the master before it sets, in other words. But even if you get complete coverage over the master, if there are still bubbles in the solid rubber mould, they will collapse under your pressure pot pressure and give you bulges on the castings, if they are close to the cavity, or alternatively will swell and give you dimples on your casting, if you degas the resin under a vacuum. They can even break through, and then you will get shiny little balls adhering to your parts. (Smirk... Sorry. How infantile...)

 

If you don't have any vacuum or pressure equipment, try CAREFULLY brushing the initial coating of RTV onto you pattern, to make sure every important detail gets wetted by the liquid rubber. Then, and this is fun, put the mould box on the floor, on some old newspaper, and pour the remaining rubber into the mould box as a thin stream, from maybe shoulder height. The liquid will thin out so much that any bubbles from the mixing will pop out of the stream, before it gets to the box. Filling the box slowly in this way will remove most of the mixed in air, and as the liquid level rises slowly, it should flow around the already-wet master and not trap any more air against it. This method does make your arms ache though, and beware anyone opening a door nearby, because the draft will blow the silicone all over the floor! Hence the newspaper...

 

Resins are best mixed slowly too, so as not to whisk in any more air. You might find using a slow setting resin will give better results as it will give you time to mix slowly, fill carefully, jiggle out any trapped bubbles and top it up, while it is still runny. Using the quick set resins might give you loads more castings per hour, but if they are all full of cavities....

 

Have fun. There is nothing like churning out your own cylinder heads, or wheels, or ordnance, or prop blades; all identical, and all your own work!

 

Tim

Edited by wunwinglow
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Hope this will help, Videos and a book on what you ask. IMHO I mix the RTV vaccum and Vibrate at the same time with this

http://whipmix.com/product/vacuum-power-mixer-plus--combination-unit/

 

http://www.youtube.com/user/smoothoninc Videos

 

http://www.smooth-on.com/howto.php Book

 

Cheers Vandy

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Chuck, make sure you allow at least a five-fold swelling of your mixed rtv rubber before the foam collapses! In other words, if you mix 500gms of rubber, do it in a bucket of at least 3 Kilos-worth size, otherwise the rtv will flood out of the pot, all over the inside of your vacuum chamber! Looking forward to seeing your first castings.

 

Tim

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FWIW, In addition to the good advice on casting from the guys here there is also a forum for guys who do this casting all the time, some professionally. They have long discussions on the topics and really get deep into it. Also they are happy to assist the newcomer to castings.

You can write to them at

casting@yahoogroups.com

You can gets all sorts of info from them, like where to get the equipment and which you should not waste your money on, etc.

One can really get into this casting stuff.

Enjoy,

Stephen

Edited by ssculptor
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