How do I degas a mold?

[raildogg]

How does one degas a mold if one does not have a vacuum pump or expensive chamber? Does the mold need degasing or just the resin itself?

[Greg Wann]

Richard, I sent you a PM.

When it comes to mixing a batch of silicone the amount of silicone and the amount of activator mixed with the silicone is quite relevant. Make certain you have a good accurate scale and that you mix the silicone per instruction. I would suggest that you mix small batches to make certain that is does allow all the bubbles to escape. It would be a good idea to mix a small amount and pour it over your part to allow a thin skin of silicone to cure to insure that you don't get a bubble forming next to your part. If you mixed a big batch, a big batch for me is 300 ML plus the 30 plus ML of activator that I add, it would be too thick to allow all the bubbles to escape. Bubbles would be trapped in the silicone because it cured to quickly. The more activator you add the quicker it cures.

If you are thinking about a pressure pot go to a Goodwill store and find a stovetop pressure cooker to convert to a pot. Make certain your finished mold actually fits inside of it when it is finished. You can cut the corners off to make it fit if you need to. I like these because the lid has about a 1/8th turn to lock it closed and slam the liquid resin inside with about 50 pounds of pressure. Those big pressure pots with those fiddly things that need to be pulled over a latch and tightened down are too time consuming. There are resins that do have a pretty lengthy cure time. Go to a Harbor Freight store and buy a vacuum pump. Be certain to get the extended warranty. The best deal for a vacuum chamber might be the one that Allumilite sells or build your own as I have done myself. Once you have some silicone and some resin on hand you can make all kinds of cool stuff with some time and patience. I use SMOOTH-ON TASK 8 resin. Once cured it will withstand a fair amount of heat and will be mostly chemical proof. Here is a link to a board I post some of my resin work on.

http://smcbofphx.proboards.com/index.cgi?board=rs

[my66s55]

I have found that by using Smooth-On's Mold Star found here: http://www.smooth-on...1341/index.html, that I have no need to degas the mold. I then use Smooth-On's Smooth Cast 305 found here: http://www.smooth-on.com/Urethane-Plastic-a/c5_1120_1209/index.html in a pressure pot at 30 plus psi and get great casting. Both are 1:1 mix ratio which eliminates the need for a scale.

[raildogg]

This is the part of this casting idea that cofuses me. Is it pressure or vacuum that I would need. I plan on building some exotic big rig trailers, Schnable and special heavy duty types. They have mad tires man ,and to buy them would render the project undoable, the costs would be prohibitive to say the least. So I will experiment with an Luminite kit I'v had for a longish time, unopened, still factory sealed and then move on to real stuff from Smooth-On. Their stuff seams interesting. I am a retired Master Pattern Maker for the sand casting industry and have used two part urethanes and casting epoxies so I'm not a total noob. But, I'm used to have all the right equipment on hand in the shop, which I do not have at home. I like the pressure cooker idea though, but you'er right self gassing resin is the ticket. More comments and help is needed.

[Greg Wann]

You would need the vacuum chamber to de-air the liquid silicone before pouring it in the mold box. You need the pressure pot to crush bubbles in the resin once you have poured it into the mold. I only make two part molds. Since you have experience in this kind of work you will see it's neccessity.

I do forget that there are 1 to 1 silicones. What is important to me about silicone is what it costs per pound to ship it to my door. I buy 44 pound kits that are under 10 bucks a pound. I started using SMOOTH-ON 300 resin. My hot water is enough to make the stuff give and bend to become deformed much too easily. Alcohol rubbed on it will damage the surface. After a club member bought a kit from another resin caster and had it in his trunk during a 30 minute cross town drive to a build session it warped beyond repair. That got my attention. I found the TASK 8. I'm very happy with it. I made a special plumbing tool out of this stuff. It sets all day in a closed Dodge Caravan at work in the hot sun. It is not affected at all by that kind of heat. When I first made it and left it in the van I grabbed it up and gave it a squeeze and it was still hard as a rock. I find this as a great advantage for someone traveling to a model show. All those dedicated hours to a project only to have it melt out of shape. BIG BUMMER!

[my66s55]

If you are making tires search resin tires in the resin section of this forum for ideas.

[raildogg]

O.K. I have a compressor, is there an accessory to change it up for vacuum? Details please

[raildogg]

My message box is empty? I thought WoodWorkerJoe? had a venturi I could use is that real. Anybody use one?. I am short on diposable cash so I'm looking for a one stop aparatus to use in my endevors in resin casting. What do you say guys am I fooling myself or does my idea have a chance?. I just emailed them but I still want your input you guys seem savy to me.

Edited July 26, 2012 by raildogg

[Greg Wann]

I have a 30 some year back ground in plumbing and those years have been spent at two major universities, Purdue and ASU in Tempe, AZ. In all my years I don't think I have seen a unit that could do Pressure and vacuum, but some pump may exist. I'm not sure if a autoclave could be used as a vucuum tank and pressure pot by switching valves around between pumps or not. I did build a vacuum chamber that works just fine but I have still not taken the time to build a frame so that the lid can be forced tight to hold the 40 PSI I need to cast. You might look at Ebay to see some scientific equipment that might be available. It would still cost a good chunk of change though. You might get on a science forum and ask. I just remembered that Harbor freight sells some kind of gizmo that you hook up to a air compressor that is somehow supposed to create a vacuum.

http://www.harborfreight.com/air-vacuum-pump-with-r134a-and-r12-connectors-96677.html

You would still need a tank of some sort.

[Chief Joseph]

My message box is empty? I thought WoodWorkerJoe? had a venturi I could use is that real. Anybody use one?. I am short on diposable cash so I'm looking for a one stop aparatus to use in my endevors in resin casting. What do you say guys am I fooling myself or does my idea have a chance?. I just emailed them but I still want your input you guys seem savy to me.

A typical cheap venturi valve like the ones sold for auto A/C service require a huge volume of air pressure to make them work. The one I have from Harbor Freight will only pull 27"Hg with my 5.5hp compressor (6.4cfm @90psi) and I'm not going to buy a bigger compressor just to see if it will hit 28" because it's slow and loud. Gast makes some little air pumps that have both in and out ports for vacuum and pressure and some of them can pull 29"Hg and push 50spi but they are expensive. I have a cheaper Gast pump that that I use for certain casting applications and it will pull 25" and push over 50psi but it needs a rebuild. My current vacuum pump for degassing rubber is a single-stage Robinair clone that pulls 29.5" and was just over $100 a few years ago.

Far and away the cheapest (and safest) way a beginner can start making quality molds and parts is to get a paint pressure tank and convert it for casting with some standard 1/4" fittings and hook it to a nice utility compressor. Molds can be pressure-cured in the tank to eliminate air bubbles (masters have to be solid, though) and will work just fine when used to pressure-cast parts. When it comes time to move up to a setup with pressure and vacuum, you can build a vacuum chamber fairly easily or you can buy a dessicator or a commercially-produced vacuum chamber. My vacuum chamber is made from a section of 10" water main pipe with 1/2" polycarbonate plates for the top & bottom.

[raildogg]

O.K. I'm getting it now. I guess I'm a little hard headed,LOL. First I'm trying the good self gassing resin from SmoothOn then I'll just have to build my own setup.

[Mike Kucaba]

If you don't get/use vacuum to de-air the rubber,Mix small quantities at a time and pour in the mold box. Works best with a rubber that has a long cure time. Pouring in a small stream from a few feet above the mold box helps too.

[Art Anderson]

This is the part of this casting idea that cofuses me. Is it pressure or vacuum that I would need. I plan on building some exotic big rig trailers, Schnable and special heavy duty types. They have mad tires man ,and to buy them would render the project undoable, the costs would be prohibitive to say the least. So I will experiment with an Luminite kit I'v had for a longish time, unopened, still factory sealed and then move on to real stuff from Smooth-On. Their stuff seams interesting. I am a retired Master Pattern Maker for the sand casting industry and have used two part urethanes and casting epoxies so I'm not a total noob. But, I'm used to have all the right equipment on hand in the shop, which I do not have at home. I like the pressure cooker idea though, but you'er right self gassing resin is the ticket. More comments and help is needed.

It's the mixing of the rubber with the catalyst that stirs in air bubbles into the RTV. Some will rise to the surface, of course, but not all. Some have mentioned that it's all right as long as no airbubbles are right on the surface of the master, but that's only partially true. Bear in mind that virtually all catalyzed RTV rubber shrinks ever so slightly as it cures; the really good quality stuff generally no more than 1/10 of one percent (not enough to affect the fit of parts) which also will compress any air bubbles in the rubber slightly. However, if an air bubble of any size larger than say, the head of a straight pin is within as close as 1/64 inch to the surface of the master (particularly in a large mold such as that for a body shell, that air bubble will expand slightly once the master is removed from the mold itself, forming a raised spot on the surface of the mold cavity itself. When this happens, the resulting casting can, and generally will, have a slight "depression" at that spot in its surface, almost as if a BB was shot at it. It's for this reason that professional resin casters will use a vacuum pump and chamber to draw out those air bubbles. In order to do this, they use vacuum pumps capable of pulling a mechanically perfect vacuum, which is expressed as 29" of mercury. At that vacuum, the liquid rubber will rise, like a nice souffle', with a huge head of foam on top of the rubber, which as the vacuum decreases, will "fall", as the air bubbles begin bursting, and the rubber settles down to the point that it looks like a thick soup boiling on the top of your stove. Once the air bubbles die down to few enough to count, the rubber is about as degassed as it will get, and once the vacuum is released, those airbubbles get crushed down by atmospheric pressure to insignificance. That ends the problem of air bubbles in the mold, trust me on that one. Used vacuum pumps can be found a lot of the time, being sold off by retiring HVAC techs (refrigeration and A/C techs), and also at a lot of colleges and universities who replace them with newer ones for their various scientific laboratories. (I got mine at the University here back in 1989, just for the asking--it had been produced in 1940, rebuilt in 1959, and hadn't been used since the late 1960's, but that would be a pretty rare thing to have happen). Vacuum chambers are often made from VERY heavy glass, almost like a punch bowl, that is an inch or more thick, with a very close-fitting heavy glass lid--break that lid, and the chamber itself is toast, due to it's having been fitted exactly to that glass lid. I made a new lid for mine with 1" thick clear Lexan, with a thickwall Lexan tube 3/4" OD, and used a length of clear PVC flexible tubing between that and the vacuum pump (mine had a "nipple" on it, smooth brass with a rounded lip around the end, which captured that PVC tubing perfectly air tight. But your experience may vary depending on what manner of vacuum pump you can find.

As for pressurizing: This is done with resin once poured into a mold. Polyurethane resin is just thick enough consistency that when one mixes the catalyst in it completely, air gets stirred into the resin. Putting the filled mold into a pressure pot and pumping it up to say, 75psi will crush those airbubbles to insigniicance, as the air bubbles formed at normal atmospheric pressure. This must be done within the "pot life" of the mixed resin, in order to work, so working quickly is essential.

Campbell-Hausfeld makes 2.5 gallon pressure tanks for use with power paint rollers, these are probably at least in the $125 range or so (I paid perhaps $75 apiece for the 5 that I used in production line resin casting, starting back in 1989). As for the air source, a decent shop type air compressor, with at least a 3-gallon tank works the best with these, along with the appropriate pressure hose and fittings, AND good, reliable pressure gauge. Those run a fairly wide range of prices, but you need not buy the most expensive one.

Note that no professional resin casters I know of try to degas Urethane resin by vacuum--that actually can release more gasses than just the air that got mixed into the liquid, and the vacuum process will take longer to do than the pot life of the resin--usually 90-120 seconds at room temperature. That would leave you with a foamy mess rather than a solid casting.

Hope this clears up these two issues at least a little bit.

Art Anderson