Can anyone give me some guidance on what I need in the way of a vacuum pump for "de-airing" a resing casting? All I've been able to determine so far is that I need one that will pull 29 inches of mercury/-1 Bar of vacuum. Is that enough? Too much? What type of vessel do I use to hold the resin? Do I need to modify something or buy them ready built. I'm looking for help on capabilities, brand names, anything really. I'm a "newbie" on this technique. TIA.
Ed
I did hear of a simpler solution using a pressure cooker and a compressor re-wired to suck instead of blow...
Chris
As far as I am concerned, the air that is in the mold cavity when you pour the resin is of more concern than the air dissolved in the resin. Most bubbles in parts, in my opinion are due to air trapped in the mold by poor mold design. The mold should have adequate height, especially in the pour riser, to create some hydrostatic pressure head, the pour riser should NOT enter the top of the mold cavity, it should enter at the bottom (j-shaped riser) and there should be a vent tube for every local high spot in the cavity. If you design the mold properly, you will get good castings even without pressure or vacuum. This is not to say that pressure or vacuum don't have their place, but proper mold design is a simple and cheap thing to perfect first.
There is only one reason to de-air resin: If it is very thick and the bubbles won't rise on their own. If this is the case, the resin had better have a really long working time so the de-airing process can actually be accomplished.
The best way I've found to get rid of surface bubbles is to coat the mold with talcum powder. Just dump some in, whack the mold around until you are satisfied the powder has come in contact with the entire surface of the mold cavity, then dump out what's left. You just want a very light coating, like it almost isn't there. Make sure no pockets of powder are stuck in corners or recesses. For most resins, this will do the trick and you won't have any surface bubbles. If this doesn't work, the resin you are using is thicker than most and you need to use a pressure chamber. However, a pressure chamber is useless unless you have de-aired the rubber before pouring the mold. Since you obviously don't have a de-airing chamber, your molds are not de-aired, so you're stuck. Try the talc. It cheaper than buying a lot of equipment. If it doesn't work, switch to a resin that is more viscous than what you are using.
One last suggestion. If the talc doesn't work, try again. Use the talc and once you've poured the resin in, dump it back out and let the mold sit for about 10 seconds. The put the resin back in. You're only trying to defeat the resin's surface tension, so in putting the resin back in the possibility of more bubbles forming is very unlikely.
Scott
The last time I vacuumed resin was in fact the first and last time I will ever vacuum resin. It came out of the mould in the blink of an eye and made a big mess. I vacuum my silicone rubber moulds but pressurise resin. If you construct the mould correctly you don't really need to vac or pressurise it .(as per other posted reply) My vac/pressure pump is an old refridgeration compressor ,I just change over the hose over from the suction side or the pressure side. 29inches Hg is easily obtainable.
I bought a new Chinese vacuum pump that does 0.5 mbar absolute and 6 CFM for slightly over 200 euros. To de-air silicone rubber you need 30 mbar absolute or less in my experience, so this pump will do very nicely.
The vacuum chamber can be an old pressure cooker, something welded together, or a polycarbonate desiccator. But be careful! An implosion can be as dangerous as an explosion. Take a peek at the implosion photo titled 'A sharp piece of wall/side punctured a water bottle':
- become a member of YahooGroups 'casting' list; resin casting is one of the main subjects in that group.
- 'Secrets Of Expert Mold Making & Resin Casting' by Karl Juelch is about to be reprinted and the probably the best book on the subject.
Rob
My models:
Ah, that may be with thick resins. Personally, all the resins I use (urethanes) are very thin (so called water pour- maybe they are not as thin as water, really, but pretty close) and so do not need to be de-aired. Is there some advantage to these thicker resins? I also do not use the fast setting resins as I want plenty of working time, to mix well and pour carefully.
Even with the thin resin I use, the main bubble problem I have is with air already in mold when I pour. It can get trapped in mold, especially if I do not design pour sprue and vent risers well enough.
I have a vacuum chamber that I used successfully for a dozen years. Since I can't do casting anymore (that stuff is toxic and I got slightly contaminated) I'll sell it. It's worth about $200, I'll sell it for $150 plus actual shipping cost (whch may be expensive, this sucker is heavy!).
Anybody interested drop me a line.
Tom
D> > There is only one reason to de-air resin: If it is very thick and the
Don- I use water-thin(ish) resin as well, but there have been some occasions where I've used a clear urethane resin that was really thick. It definitely needed to be de-aired and it had the working time to make that possible.
Other than that, I stay away from thick resins.
Scott
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