Degassing Silicone rubber

[paul01]

I've searched the net for the processes involved in degassing silicone rubber with no real luck. I basically wanted to know:-
-what is degassing
-how is it done
-what equipment is needed
-why it should be done as opposed to simply pouring the rubber over the kit master.

Any tips would be greatly appreciated...

[Blappy]

-what is degassing

It is the process of removing the air bubbles from a mixed batch of rubber.

-how is it done

You mix your rubber as you usually would. You then place that tub of mixed rubber in a pressure pot. You seal up the pressure pot, attach your vacuum pump and run in for five to ten minutes. You meed to make sure the container you have your mixed rubber in has at least three times the volume of the rubber. When the rubber if vaccumed it will expand a LOT!

-what equipment is needed

-Rubber
-Containers to mix it in
-mixing stick to mix it with.
-Pressure pot
-Vacuum pump
-Air line to connect said pump to pot
-Parts to pour rubber onto.

-why it should be done as opposed to simply pouring the rubber over the kit master.

If you do not do this and then you pressure cast your parts any bubbles in the molds will cause bumps to form on your castings. The bumps are caused by the bubbles in the mold near the part surface being crushed and resin getting into them. It can also form little beads on the castings as well for the same reason. So if you ever plan to pressure cast with these molds you haver to vacuum the rubber.

[Sparky]

Here's the write up I did. I tried to inlcude all the issues I've worked through so far with the process:

http://www.kc6sye.com/advancedcasting.doc

And here's the slide show I gave, I've added some movie clips on this page, if you've never seen something as thick as silcon RTV rubber boiling at room tempeture it's worth the download time, the pink moldmax 30 is 3 meg and oomoo 25 is a 4 meg clip.

http://www.kc6sye.com/advancedcasting_k.html

[SolitaryMan]

Great info on vacuuming guys. I just got a large pressure pot and need to de-gas some silicone rubber.

I would appreciate if someone could direct me to a supplier or give me a link to a pump that will suffice.

I have new to de-gassing and need to get a pump before I can test it. I plan on testing it on a fluid hair gel with a lot of bubbles.

Thanks a bunch!!

[Sparky]

Go to ebay do a <a HREF="http://search.ebay.com/robinair_W0QQfro ... Z1QQfsopZ1" target="_blank">search for Robinar</a>, and look through the pumps. Based on my limited experience (I've only bought one pump) just do a buy now, to get the thing into shipping. All the bidings I saw/did ended at our above the buy now price. However the season is ending (these things are normally used to service airconidtioners) so the bid option might land you a deal. There are lots of the 6cfm two stage pumps, that are either new or factory reconditioned. . .

[SolitaryMan]

Thanks Sparky. I wasn't sure what to look for under the product specs to determine if it does the job. The Robinair two-stage isn't a bad deal at all.

I just need to be re-assured that it will do the job and pull air out of silicone rubber no problem.

Also, can I use it when casting resin for the same purpose?? I imagine it will do the job and quite a bit faster with the resin.

Thanks again for the info.

[Sparky]

You can only vacuum the resin/mold. To pressure cast any compressor will do. Even the introductory pasche I got for my first airbrush will do 40 psi. It might take a bit of time to fill a paint pot. I use the vacuum method for small detail bits in open face molds to help get the air bubbles out. If you have a sheet of 100 parts of more you don't have time (with respect to the resin starting to set) to poke/coax the air bubbles out of the part.

The biggest problem if ran into is you need to plan for a big reservoir in the mold for the resin to bubble up into. Also I ran into the mold deforming then the resin gets thick enough so that the mold can't draw the resin in. Got some weird casting of a ship that looks like its imploding. . .

[paul01]

i'm watching this on ebay and i'm not sure if it is suitable for degassing silicone rubber...

http://cgi.ebay.com.au/ws/eBayISAPI.dll ... %3AIT&rd=1

can anyone tell me if it is suitable?

[Sparky]

Not sure how much vacuum that can pull before it explodes. . .vacuum chambers explode when <A HREF="http://www.kc6sye.com/advancedcasting_o.html" target="_oops">things go wrong</A>.

The paint pot will work if you some plexi. . .mechmonkey hooked me up with some thick stuff from scrap. But I've been using the thickets stuff they had at home depote:
http://www.kc6sye.com/advancedcasting_c.html