View Full Version : DE-GASSING pot

07-19-2006, 01:39 PM
Does anyone know of any reputable manufacturers of small 16"x18" vacuumforming uunits and de-gassing pots. USA
Thanks Gary

07-19-2006, 03:13 PM
Are you wanting to de-gas a thick pourable liquid.

07-19-2006, 03:23 PM
Actually I am looking to mix two part silicones for vacuum forming, the de-gassing pot is to take out the bubbles at that time.

07-19-2006, 04:02 PM
How do you vacuum form with liquid silicone? Aren't you referring to casting?

Vacuum forming or Thermoforming is typically done with heat softened thermoplastic sheets sucked over or into a mold. Perhaps I'm missing something here.


07-19-2006, 04:38 PM
I think Gary is referring to something mentioned in another thread. I believe the intent is to make a silicone replica of a surface with many parallel grooves and to prevent bubbles being trapped at the bottom of the grooves it was suggested that the pouring be done in a vacuum.

If I am correct then the operation needs either one very big vacuum chamber or one tall and not very wide for degassing the mix and the other not very high but fairly large for the actual pour.

Because the silicone cures fairly slowly I would be tempted to do everything in one operation:
Make a sturdy box around the pattern with sides around six inches high with a port in the side to connect the vacuum source; this will be the vacuum chamber and the sides of the mold.
Get a piece of acrylic about 3/4 inch thick and big enough to cover the box. This will be the top of the vacuum chamber. This thickness of acrylic should be able to handle a good vacuum without bending too far but read the Note!.

Note: The reason for using acrylic is to see what is going on while the silicone degasses. But having the acrylic implode and fragment is very undesirable. If you choose to follow these suggestions it is at your own risk. I would test this setup at a vacuum greater than I expected to use with everything covered in heavy fabric; blankets or big towels acting as a blast mat. I would wear a full face shield and gloves. I would also use the fabric cover during the actual degassing and just have a small hole to see through. You may choose to take other precautions. Again I repeat doing this is at your risk. End of Note.

Make sure the pattern and box are sitting nice and level.
Mix the silicone and pour it over the pattern and let it flow out nice and even.
Put the top on and connect the vacuum source; my guess is it will need at least 15 inches of mercury negative pressure or in other words about half an atmosphere or more.
Apply the vacuum carefully watching the silicone; it will start bubbling and if the vacuum goes too high too fast the silicone can foam up everywhere.
If there is no bubbling the vacuum is probably not high enough, the silicone mix almost certainly has entrapped air..
After bubbling has occurred and the silicone has settled down nicely then very, very, very many many times very, slowly release the vacuum; you don't want to blow silicone everywhere letting the air back in.
Then just let the silicone cure.

07-19-2006, 04:47 PM
I'm sure that vacuum pots are available on the regular market or some kind of pressure vessel. I have a stainless steel chamber from an autoclave that I use. The mixing bowl with the material goes in first and a vacuum is pulled to pull the bubbles out. After that the mix is poured into the mold and vibrated. From there the poured mold is put back in the chamber and is pressurized to reduce the size of any remaining bubbles.

07-19-2006, 05:20 PM
Colin I don't think either of us is missing something! I like to feel I am learning something, that's why I go to these forums and get advice from experienced people. Ain't it great. I appreciate your response.
Geof, thanks for your detailed opinion it was and is definitly an eye opener and I did get a lot out of it.
2much, thank you for your simplicity.
You blokes have all been great! I really have got alot out of each of you. BUT, I still do not have a good supplier for either the vacuum machine or the de-gassing pot. Actually the de-gassing pot I can put together myself.

07-19-2006, 05:48 PM
If you attempt to degass after you have poured, especially on the example shown on your other thread, I'm afraid you'll never get any bubbles that are trapped in the bottom of the grooves out.
Believe me they wont let go. By all means try it for the experience, but the smaller the detail you are trying to mould, the more likely the bubbles will stay put.
At the back of my mind is something about "excess pressure/surface tension" and small bubbles that I did in high school about fifty years ago, and I don't think the laws of physics have changed much.

I built my first 2 litre degassing pot out of a piece of 6" diam plastic drain pipe, a 7"x7" x1/2" slab of acrylic and two layers of painted chipboard as the base.
The gasket to seal the acrylic to the top of the pipe was cast in si rubber, by mounting the pipe inverted onto a glass mirror with the small bits of prieviously cast rubber as spacers. Two cardboard retaining rings sealed to the mirror with bath sealant completed the mould, one inside the pipe and one outside.
Pour enough to give you a U-shaped ring that will fit over the pipe.

The vac pump extraction connection was a piece of metal tubing sealed into the chipboard base.
It's a lot easier when you're starting up, if you have a vacuum gauge. You need to get down to 1"Hg, but that's far enough.
The rubber will foam up and roughly treble in volume, so if you need 100mls of rubber degassed, you need a 500ml container(it will come over the top of a 250ml ).
You'll need a separate container for the rubber to mix the rubber/catalyst, and to pour from, and you'll leave about 10% in it, so allow for that in your calculations.
Dont be tempted to scrape the sides of the mixing vessel for the last drops, you'll just get air into it.
Unless you've got a deep section, then you could use that as a back fill.

Just one thing on vacuum pouring. You only need to get a continuous layer over the surface. Once you have that, it usually doesn't matter if there is air in the upper layers, so you can always top up a mould with non-degassed rubber.

Best of luck
(I made si rubber moulds of carved picture frames over a period of about twenty years)

All you've got to do is look around your workshop/house, and use a bit of grey matter.
You'll be amazed at what you can build if you have a clear idea of what you are trying to achieve.

Edit - Now I've read you last post, I can see you're on your way :D

07-19-2006, 05:53 PM
For the vacuum pump, I would recommend using a piston or a diaphram style of pump with a vacuum regulator or vacuum switch. They may take a little longer to pull a vacuum but they will pull down further compared to a liquid ring or rotary vane. Use a reserve tank like you would use a tank on an air compressor. Grainger or McMaster-Carr will have pumps.

If you can find it, LEXAN is a stronger alternative for acrylic sheet. If you are worried about it shattering, get the laminated version used for bullet proof windows.

07-19-2006, 05:54 PM
A company that I used to make small silicone molds for used a professional cake mixer to mix the 2 part silicone and while mixing the made a clear plexi-glass plate for the top with the appropriate gaskets.

This worked but you have to find a way to cool the motor of the mixer as it gets really really hot.

once the bubbles are out you just have to be carfull on the pour method so as not to introduce more bubbles (because as geof said they wont go anywhere without alot of help)

07-19-2006, 11:56 PM
This really is a great little forum, thank you all! And thanks John for helping on this thread too.
Okay what have I learnt from you all.
a. De-gassing first is the only way to go.
b. pump s/b piston or diaphram.
c. lexan is the best plastic sheet.
d. A pro cake mixer to mix the 2 part silicone.
e. When mixing Silicone has a tendancy to expand three fold.
f. if you have a vacuum gauge. You need to get down to 1"Hg, but that's far enough.
g. Separate container to mix the rubber/catalyst, and one to pour from.
h. Dont scrape the sides of the mixing vessel for the last drops, you'll just get air into it.
i. You only need to get a continuous layer over the surface. Once you have that, it usually doesn't matter if there is air in the upper layers, so you can always top up a mould with non-degassed rubber.
j. plate should be clean i.e. no finger marks.
The silicone I will be using has the viscosity of 10 weight oil
curing time of 15 hours
working time of 2/3 hours
2 hours @ 150f to bring up the hardness to shore A 77 durometer
0 shrinkage even at temperature of 600f.
I have enclosed a pic of how I see placing the de-gassed silicone on the plate
1st pulled over the surface of the plate with a squeegee. 2nd. place a wall around the plate, then add top layer.
Should I vacuum the mold? If so for how long.
Also the mold needs to be bonded to an aluminium plate,When?
Please let me know if you see any corrections needed on what I have written.
Great day all GAry

Stop thinking in terms of limitations and start thinking in terms of possibilities'

07-20-2006, 03:04 AM
So long as your only moulding the pattern you have described, a slow pull with the squeegee looks ok. Too fast and you'll have a lovely bubble along each of the grooves. :nono:
If you go to any more complex surface with detail at right angles to the "pull" direction, I think you'll have a lot of problems.
The usual method is to put a temporary spacer under one corner of the pattern, and then pour the liquid rubber into the opposite corner. Remove the spacer and then allow the rubber to flow at its own speed across the surface, pushing the air out as it goes.
If time becomes a problem, then move on to the "vacuum" casting method - details supplied if requested ! :)
(I'll try and draw up some pics later - breakfast is on the table)