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RTV casting tools...looking for advice

5358 Views 16 Replies 7 Participants Last post by  rkapuaala
I'm looking for recommendations regarding what I should be looking for to develop a casting capability using RTV molds. I have the mold master...so I'll need to be able to make the mold and then make the parts with the mold. I'm not interested in what materials to use...yet. I know RTV comes in a variety of "qualities". But this request is about the tools needed.

I think I need three "tools":

a. Paint pot, in which to do the mold making or part making

b. Vacuum pump
[*]to pull air out of the RTV when making the mold from RTV and [*]When using the finished mold, to pull air out of the resin after it's poured/forced into the mold [/list] c. Presure pump to force the resin into the RTV mold

Is this correct? If so, then what are the specs I'm looking for?
[*]How many inches of vacuum dose the vacuum pump need to pull? [*]How much capacity does the vacuum pump need?...How fast does it need to pull the air out of the paint pot....10 seconds...or 10 minutes? [*]Same for the pressure pump....how much pressure does it have to put out....how fast does it need to pressurize the paint pot? [*]I've found paint pots up to 2.5 gal in size. What size is used by most modelers? Where does one find one in larger sizes...say 5 gal? [*]What gauges are needed on the paint pot to measure vacuum or pressure. What kind of valves? Where does one get the valves. [/list] I'd LOVE to see a photo or drawing on how one of these things is set up. Does anyone have one?

Lastly...some questions related to size. If I wanted to cast a 3' long passenger car side, how does one size this stuff up? What kind of paint pot would work and how much bigger do the pumps need to be.
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You will need a vacuum pump that will pull at least 29 hg and you will need the meter to read it.
You will need a compressor that can deliver at least 110 psi so that your paint pot can get up to at least 60 psi for compressing the resin.
You will need some large mixing bowls and some small plastic - not glass measuring cups that can hold at least 16 oz of rtv (only for rtv)
You will need some small measuring cups at least 8 oz for mixing the resin
Note the sizes are dependent on the size of your castings.
You will not a paint pot that is at least 3 gallons and rated for 80 psi
You will need an assortment of brass plugs and couplers to convert the paint pot to a pressure/vacuum pot
last, you will need some patience to make a lot of mistakes.
I have tried pressure treating my molds and for some reason that does funny things to the rtv causing bubbles and what not.
The best thing I've when mixing the rtv is
1. Mix it in a container that is 2X the size of the amount you need.
2. Vacuum the mixed batch of rtv till its starts to go flat
3. Carefully pour the mold
4. Vacuum the mold for about 17 minutes or longer if it looks like you got a lot of air in the rtv during the pour.
The best thing I've found when pouring the resin
1. Mix no longer than 10 seconds
2. Pour quickly but evenly into the mold so that you have a continuos stream that does not get backed up and threaten to over run your containment area
3. Get the pressure on right away and let it stay for at least 20 minutes on a warm day and 30 minutes on days below 60.
4. Do not pour in temps below 55 f or you'll be sorry
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You can't have one with out the other. The vacuum pump draws out all the air from the mold. Without it, even the smallest air bubble can cause pimples on the casting once it is submitted to the compression chamber. I have heard of people who can just use the vacuum pump for both mold making and for casting, but I haven't figured out how to make that work for me. If I submit resin to a vacuum pump it foams up really bad, almost like insallation foam and in many cases blew out the mold. This even happens to me if I pre vacuum parts a and b. The most I can vacuum I can have on the casting is 22 hg and that hardly pulls out any bubbles before it starts to gel.
Craig can pressure treat his RTV, but for some reason when I try this and pull the mold out of the compressor it gets huge warts. I assume that is because the pressure only reduces the size of any air trapped in my rtv and once the mold cures and the pressure is removed, the soft mold can't contain the air bubbles and they expand to normal size again.
If you're doing okay with ambient, I would just stick to that. The only reason I have the vacuum pump is that I like doing vacuum form laminations from time to time and because I sale some of the things I make to defray the cost. So the mold and the casting have to be perfect.
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I have a 3 gal sears paint pot, and it holds 29 hg just fine. Sorry for the confusion thats 25 hg not 29 which is when you should start seeing the rtv expand,,, again this is also dependent on elevation. I'm at 90' above sea level where I live. For me the 17 min is critical. Any less I have experience inconsistencies on Air bubbles in the mold. Any more and you're just pumping the gases created by the catalyst.
One thing I didn't mention is that you also need a 3/4" 12x12 piece of acrylic of plexiglass for the vacuum phase and some 12X12 polypropelene rubber 1/2" for a gasket on the glass. You need to drill a hole in the center of the glass (slowly and with a good wood backer) so that it can recieve a nipple for the suction.
If you like, I can post an image of my rig so you can have something visual to relate to.
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No problem, I'll get some pics in the morning and posted them hopefully by noon if the wife doesn't figure out something else for me to do.
Well, sorry it took so long Mike, but I swear the faster I do something for her the more she finds for me to do. Its like she's got a built in idea for how long she wants to torture me :) "How longs this gona take, Honey; I got some things I need to do in my shop".
"Oh not long"
"I'm done"
"That was quick,,, you should have time to do this...." Jeeze!
Anyway enough griping about domestic bliss,,, here's the images I promised and some caps.

This is my vacuum pump. Its a Ghast double action pump. I have a water filled vac guage mounted to it because it vibrates too much to get an accurate reading on a regular guage.

You'll note that to the left (our right) of the gauge there is a shutoff valve. Thats to release air slowly from the chamber when you are ready to take the mold out. You don't want too much air rushing in and blowing back into the rtv.
Also not that there is modeling clay around all the joints. This is a little unorthodoxed I guess, but it works for me. I turn the vac pump on, heat the clay with a blow dryer and any leaks in the joints is filled with clay. The clay doesn't dry out and is easier to remove than pipe dope, so I like using it on something I'm expecting to die within a year. This is a used pump I bought on ebay and it has lasted almost 2 years now. I have a back up on a shelf just waiting for a turn at being abused.

This is the 3/4" plexiglass chamber cover I was taling about and my coupler. Nice thing is you can use the same connectors you use for air hoses and even the same hose. I don't know the physics behind it, but I guess round objects stand up good to a vacuum. That yellow hose is just regular air hose, nothing special, I bought it at orchards. Note the neoprene gasket, which was also bought at OSH. It is cut in the center to allow not only viewing of the project, but to keep the vacuum from pulling it down. I use rubber cement to keep it fixed to the plexy glass, it makes it quicker to throw the lid on the chamber and I'm less likely to misalign the gasket and ruin the air seal.

I made this vacuum chamber out of 3/4" birch ply and wrapped in 10 mil visqueen (sorry, I can pronounce it but [email protected]#ned if I can get close to spelling it).
Its for taller pieces. One of the disadvantages of a 3 gal pressure pot is the bottom is round, so you have to put a little platform inside to level the project. This raised the project closer to the top. When I have taller molds to vac, I use this chamber. You need a second piece of 3/4" acrylic, or ply and another gasket for this chamber because its opened at both ends.

This is my whole setup (or hole as I sometimes refer to it when my wife parks her car in the garage and wedges me inbetween the large pile of wood to the right).
The air hoses hanging down to your left are hanging from an air pipe over head that leads to my compressors. For ease of maintenance all the couplers and nipples are the same type.
Thats it, hope this helps,,, if anything you can see it doesn't need to look beautiful to work.
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I use teflon in addition to the clay. Teflon tape is pretty good, but I was getting some vac leaks with just the tape.
You need to reinforce the inside of the box with 3" wide pieces of ply. The best way to describe it is, two sides fit inbetween to wider sides. The sides that are inbetween get reinforced sort of like a rabbat joint, only without the tooling that might cause a disasterous delamination under the vacuum.
I use both pots interchangably sometimes simultaneously. They are both pressure and vacuum pots. The only difference is in which lid I put on them.
I vacuum molds and pressure castings. Usually when I do a run, I use both pots for pressure. It allows me to produce more pieces in less time.
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