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Discussion Starter · #1 ·
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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I have successfully made a few small molds with one simple tool, a paint pressure pot. When I make the mold, I cure it under pressure rather then using a vacuum pump. I went to Sears and bought a 2.5 gallon paint pot, air regulator value, and a quick disconnect for my air line, and a ball valve to open and close the air pressure to the pressure pot. Maybe a total of $100 spent on it? I don't have any pictures of my set up, but I'm sure others will. Have you read Burl Rice's article in GR? Oh most of my molds have been one sided molds, so I'm not to sure how this set up be with 2 or 3 part molds.
Craig
 

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I have some boxcar sides I cast that are about 24" long, so 3 feet is probably possible. These are ambient castings though, not pressure castings. The trick is to make the pattern with shallow detail. And when you pour it, be quick! Any air bubbles on the back get filled with CA & sanded down smooth on the belt sander.

http://burlrice.com/_LS_60ftHiCube/index.php




The best advice I can give you right now is: start small & work your way up. Trying a 3' casting right off the bat would probably have frustrated me to the point of giving up.
 

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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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Hi everyone,

Follow-up question to Mike's question...

I have been casting parts for about two years... simply doing, as Burl calls it "ambient casting". I do not vacuum pump my RTV. Mostly I have one part molds, some "glove" type molds.

My RTV molds seem pretty good, the level of detail matches my original model quite well (e.g. rivets, undercuts, etc). If I get a good pour of plastic, its near perfect match. About 2/3 of the time I get bubbles that I have to go back and fix with small batches of Alumilite (usually not related to mold details).

Based on this... my question, where would I get the biggest bang for my buck: pressure pot for the Alumilite or vacuum pump for RTV? I think historically I have read that degassing the RTV is the best thing, but my experience seems to be that RTV is quite forgiving, and that the resin is the problem area...

Any thoughts?

John.
 

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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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Discussion Starter · #8 ·
OK...I'm back to my questions...about TOOLS. The info and videos were fantastic...and I thank you Richard for being specific on the pumps. The TAP plastic device I saw in the video (thanks Russ) might be a whole lot easier to just buy....versus building this rig...it all depends on the cost.

I've been thinking through the mold making part of the process...

OK...the paint pot. Do you guys use a regular paint pots? At 29" hg, do they hold the vacuum? Paint pots are designed to hold pressure...often 100 psi...so I don't see that there would be a strength issue since 29" hg of vacuum is about -15 psi. Is that thinking right?

Question 2...how long does one have to pump the pot down to -15 psi? This goes to the capacity of the pump...and I know the pumps initially pump air out fast...then the pressure drop speed flattens out. A 2.5 gal paint pot has about 2.5 times the volumn of the TAP system...so I would expect it to take AT LEAST 2.5 times as long to evacuate the cavity to the same pressure using the same vacuum pump...assuming no leaks in the larger seal area. I have a hunch that "expectation" about the 2.5 times has a gotcha I don't understand...and it would take a lot longer. Is my thinking of pump down time correct?
Richard you said suck on the RTV for 17 minutes if you got bubbles in it while mixing the RTV. I am assuming that is AFTER it reaches 22" hg (what you stated later...versus the 29' hg you stated first....I'm a bit confused on that). Is the 17 minutes critical...because it means a vacuum pump that has at least 2.5 the capacity of the TAP rig. Thoughts?
 

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Sorry for getting off on a tangent...

Personally, I have a 2.5 gallon Sears paint pot that I use for pressure casting. I made my own vacuum chamber from a 6" PVC coupling and lexan (never use Plexiglas). It uses a small vacuum pump that I got off eBay - I think it's a Gast pump - and it only draws about 25-26" vacuum. If I had it to do over, I'd get a bigger vacuum pump, and make a bigger vacuum chamber.

I only degas my RTV long enough for it to rise & fall. Then I'm careful to pour with a long, thin stream and that seems to get all the bubbles.
 

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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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Discussion Starter · #11 ·
Richard...woof....a photo would be worth a thousand words. And....you have a rig like I first understood I needed....vacuum AND pressure. Please post a photo.

Burl...thanks. I saw the long thin stream on the TAP video...looks like a winner to me...cause it's simple. Thanks.
 

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Mike, for what it's worth, I've had very little time so far for casting, but I have attempted several times to cast (ambiently) arch-bar sideframes. I'm very happy how my mold turned out, but None of the four castings have been without voids. After reading Richard's post I think this might be due in part to stirring too long, and therefore not enough time to fill the mold before gelling.
I do plan to invest in a vacuum setup ASA(financially)P.
This has been a very informative thread. Thanks for posting, all!
 

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Mike,
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.
 

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Discussion Starter · #14 ·
Posted By rkapuaala on 11/01/2008 2:06 AM
Mike,

"if the wife doesn't figure out something else for me to do."


Hah....same everywhere. Is that possible??
 

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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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Discussion Starter · #16 ·
Thanks Richard....the clay as pipe dope was a sure surprise. I always use teflon tape...and not pipe dope. Another surprise was the 3/4" birch ply box...I would NOT have thought it would be strong enough to handle the vacuum.

Question...is the other paint pot used as a pressure pot...for forcing resin into molds? I see you have both pressure and vacuum present at your work location.
 

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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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