Jump to content

Vacuum degassing results


Recommended Posts

For those of you wondering how my first attempt at silicone mold making (using a vacuum degassing set up) went, it didn't go as well as I hoped.

Here is what I did:

-mixed two part silicone for a full three minutes (careful to scrape the sides and bottom of mixing container) to get as complete a mix as possible
-poured silicone into mold box slowly, using thin stream away from master (letting it build up around master)
-vibrated mold box to encourage complete coverage for one minute
-put mold into vacuum pot and brought vacuum to 29"/HG (took about a minute to fully degass vacuum pot) and turned off pump
-held it at 29"/HG vacuum for full three mintues, then slowly let pressure return
-pulled vacuum again and this time let it sit under a vacuum for several hours

The pot held its vacuum for the several hours but it did slowly loose it over time. I don't think I had the lid on as tightly as I should have.

After five hours of letting it sit, here is what I found:

I noticed right away that the level of the silicone on the sides of my mold box indicated it had only risen maybe 2 to 2.5 times it's original height. I also noticed there were some medium sized bubbles (about 2mm across face) on the surface of the mold which surprised me and indicates to me that I needed to get it under vacuum sooner, for longer. I removed the mold from its box and inspected it further. I noticed the detail of the master it captured was really no better than when I have made molds without degassing. After breaking open the mold I also saw that there are medium and smaller bubbles trapped within the silicone! Kind of bummed at that.

So my main problem is still bubble entrapement. I am going to forget about the vibrating part, put it under vacuum right away after mixing for three minutes and then hold that vacuum for a full seven minutes as some have suggested. I'll do all this is a larger container to allow for expansion. After that I will break the vacuum, pour it into my mold box, vibrate one minute and put the mold back in the pot and pull it under vacuum to leave for 5 hours.

What do you guys think?

One thing I noticed when pulling a vacuum is the guage on the pump would get down to 29" but the pot would show roughly 28". The pots guage is a cheaper vacuum guage so I'm not sure how accurate it is, but could there be that much difference in vacuum between the pump and the pot?

Last but not least, should I keep the pump on the entire time I am pulling my vacuum that first time (seven minutes) or turn it off once it gets down to 29"?

Thanks again guys, Chuck.

Link to comment
Share on other sites

Pretty tough to nail it on the first try. Bubbles in the mold are not a problem, you wont get all of them out,I didnt. As long as they are not on the master its ok. My last molds have bubbles in them , 2/3rds of the mold is mostly bubble free ,the last (top) third is bubbley. These were done without vacuum.

Try vacuuming the silicone before pouring. and do the long distance stream as suggested, straight to vacuum again after that. Cooler temperature is good as well , keep the silicone components in the fridge.

Its not rocket science, easily done ,soon you'll be getting it right.

Link to comment
Share on other sites

Never though about keeping the silicone on the cool side, that makes sense to slow down the reaction!

 

I'll give it a try using my modified method and your suggestions.

 

Stand by for updates.

Link to comment
Share on other sites

Our work cycle is mix rtv for at least 5 minutes if doing it by hand. A paddle in a power drill will help, jst be careful

not to fling rtv all over the place! Into the vacuum chamber, watch the level rise as the bubbles swell, until the foamed rtv collapses. This is where you need a much bigger bucket, as the rtv will swell to 4 times its original volume before the bubbles start to burst. Leave in full vacuum for another 5 minutes. Vent back to atmospheric, and pour, thin stream method, into the mould box. Box back into vacuum chamber and hit it with max vacuum for another 20 minutes. Finally, vent tank to allow the rtv to settle back onto the master, leave at least 24 hours to cure, depending on temperature. Don't let the mould set while in the vacuum, otherwise lbubbles that would be tiny at atmospheric pressure will be swelled up as the rubber sets. Not what we want at all!

 

This should ensure absolutely no bubbles in your mould, but you need as low a vacuum as you can possibly achieve. You can't have too much vacuum..... Or should that be too little?

 

Anyway, moving on... try again, you are very close! Improve the seal on your vac tank, maybe just a smear of vacuum grease on the seal will solve things for you.

 

Next time will be better, if not perfect, honestly!

 

Tim

Link to comment
Share on other sites

Thanks Tim for the info.

 

When I pull a vacuum it takes roughly 1-2 minutes to get the gauge on the pump to read 29"/HG. At the pots gauge it reads 28"/HG. If I keep the pump going I can get it down to just below 30" at the pump which gives me roughly 28.5" at the tank. This is where I shut off the pump and let it sit for the required times under a vacuum.

 

Should I keep the vacuum pump going the entire time the silicone is in the pot or will those burn out the pump?

 

What do you suggest?

Link to comment
Share on other sites

SUCCESS!

 

I did a test run last night using what everyone suggested and it came out perfect! Zero bubbles on the surface and inside and a glass smooth surface capturing everything! Exactly what I was hoping for.

 

Now I just to wait for more silicone. Thanks again everyone, Chuck.

Link to comment
Share on other sites

Excellent Chuck! ref leaving the pump running, yes, we do that, although our pumps are quite big , designed for that sort of use. Without knowing the details of your set up I can' t advise more, but try ift, see if the pump motor starts to get hot, labouring, or smelling bad, in which case stop. Actually, the pump is not moving any air at that point, just working against the back pressure, so it probably isn't under too much strain. But it sounds like you have the basic process cracked! Now for some castings......

 

Tim

Link to comment
Share on other sites

  • 2 months later...

Chuck,

I'm retired now but in a former existence I repaired transport refrigeration equipment and airconditioning on diesel trucks and the reason the vacuum gauge not going all the way to the -30 psi in do to your not being at sea level, its simply a gauge error so just dis guard it as a harmless anomaly related to the same reason water boils at a lower temperature at higher altitudes.

 

I'm glad you got things about sorted out to make a suitable mold. If you have any problems with the resin pouring not completely filling your mold then consider adapting your air compressor to pressurize the pour to 15 to 20 psi. My equipment is a pressure pot used for painting houses which I have adapted to hold bold a pressure or a vacuum.

Link to comment
Share on other sites

Great info. Thats does make sense. Thanks for sharing.

 

I too have a modified paint pot that is able to handle pressure and vacuum.

 

Now if only I could find some time to do some model building!

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...