By Craig Bisgeier
(more photos at the bottom of the page)
I have been casting models in resin plastic and rubber molds for some time now, but there have always been frustrating pitfalls in the process along the way. One recurring problem was the trapping of air bubbles in the molding rubber, which can make parts of your mold softer and prone to distortion when even light pressure is applied. Worse yet, if an air bubble gets trapped near the part, ugly warts can show up in your castings. Because the molding rubber is expensive, it really hurts when one of your molds ends up getting a flaw like this.
While there are methods to help reduce the impact of trapped air bubbles in the rubber like vibrators or painting a skin coat on the part before pouring the full mold, they are not foolproof and never seem to completely get the job done. The only foolproof method, I had learned was to place the mixed rubber into a vacuum chamber and pull all the air out of it. A ‘De-Aired’ mold is solid through and will not distort even when placed under high pressure. I decided a couple of years ago that to move to the next level in my molding and casting work, I was going to need a vacuum chamber.
I started off by procuring the most important (and expensive) element, a vacuum pump. I first tried a Venturi pump, which is supposed to generate a vacuum in a vessel by using the venturi principle. Sounds great, but in reality it is only good for a few inches of Mercury (negative pressure is measured in Inches of Mercury, with 0” being normal pressure and 30” being a complete vacuum). Not nearly enough for de-airing rubber.
I searched many sources for a good vacuum pump, and eventually I got a RobinAir vacuum pump on Ebay for about $125, used from a pawn shop. These pumps are usually used by air conditioning service persons and are supposed to be of very good quality. Unfortunately the seller didn’t drain the oil out of it and when I got it out of the box it was slathered in oil. It took a lot of cleaning but eventually it was no longer slick to the touch and attracting all the dust in the basement to make a furry greasy coating on itself.
I eventually bought a quart of air pump oil, the proper connecting hose and fitting from an air conditioning repair place a few towns over. The eventual price of all the parts and accessories was about $175-180, and that’s just for the pump. And I still didn’t know if it even worked. Electrically the motor ran, but was it sucking, and would it suck enough? I was truly in for a pound, but more expenses were coming
A while passed before I got the interest back in trying to complete the project. I still needed to build a vacuum chamber to use with the pump. I did some online research and got a lot of information from the people at the Casting list on YahooGroups. I also spent some time looking at Alumilite’s vacuum chamber, and determined that I could build something similar fairly easily. The Alumilite chamber costs over $200, and I figured I could make one for less than half. And if I’d been a little more selective, I could have.
I determined the Alumilite chamber was made from an 8” PVC pipe coupling from the pictures, and two pieces of lexan polycarbonate plastic for the ends. “No problem”, I said, and went out to buy the needed parts. Well, an 8” coupling costs $33, and a 1 foot by 2 foot chunk of ˝” Lexan plastic is $50! I got them, but I needed to let my credit card cool off a bit afterwards. I’ve recently decided that a 6” coupling at about $20 less would have been fine, and a smaller piece of Lexan would have saved me another $20 or so. Live and learn. Unless you need to vacuum a lot of rubber at once, make a smaller chamber and save money.
Then came the plumbing bits. Some pressure pipe fittings I already had because I have several compressors around, and the vacuum gauge was a gift from a friend who wanted to help, but the rest had to be purchased. I only spent about $20 in parts for this, but if I’d had to buy it all it would have been close to $50 or more, depending on the gauge. In all I used a triple-tap, two male-male and one female-female connectors, a ball valve, the gauge and a flare fitting for the vacuum pump hose, all Ľ” NPT. That completed the collecting of almost all the parts for the chamber.
My friend who donated the gauge borrowed a tap from work the day were ready to make final assembly of the chamber and brought it over. We made one hole in the side of the PVC coupling and tapped it out, then carefully threaded in the pre-assembled plumbing into the side of the coupling. A pair of ‘O’ gaskets were cut from some neoprene-like shelf liner sheets I bought at Home Depot, as we were finally ready for the first test. One lexan end was placed on the workbench, an ‘O’ gasket placed on it, then the PVC coupling. Another ‘O’ gasket, and the top end of the cylinder was set in place. The oil level in the pump was topped off, and the hose connected between the chamber and pump. We were ready. I put on my safety glasses and took a deep breath.
I flipped on the switch on the pump, and opened the valves on the pump and the chamber. Immediately the pressure in the chamber started dropping, and within 30 seconds was down to 27”. A few seconds later it got all the way down to 29-1/2”, pretty close to a total vacuum and more than enough to de-air molding rubber. We left the chamber under vacuum for a few minutes and closed the valve, and disconnected the pump. It held the vacuum with no noticeable leaks. I was very pleased that everything had worked out so well, and nothing had imploded!
But the real test was to de-air some rubber, which was what this whole thing was built for. We made ready a mold box and quickly mixed up a batch of rubber, without the usual caution used trying to keep air bubbles out. We stuck it in the chamber and turned on the pump, and in less than a minute the mixed rubber rose in the cup, broke as all the suspended air bubbles grew and broke apart, then collapsed back in the cup. We cheered and disconnected the pump and let the air back in, then took the cup out of the chamber and poured the rubber into the mold. It was amazing, not a single air bubble to be found during the pour.
Was it worth all the trouble and expense? Time will tell, but it will definitely improve the quality of my molds, and that’s a good thing. I don’t have to worry any more about hidden air bubbles ruining my molds before they even get used. For the moment I’m enjoying using it and the results are great.
Use this link for good information on the casting process and quality resins and RTV's:
The Alumilite Home Page
This is a link to an excellent three-part tutorial on the casting process:
Dan Perez Studio's Moldmaking and Casting tutorial