resinmodeller
New Member
Okay cool 
starts getting a little warm around the 20-30 secs so will experiment
starts getting a little warm around the 20-30 secs so will experiment 
Vacuum Chambers and Pressure Pots... Need some Advice :)
- Thread starter resinmodeller
- Start date
I've made molds like that, but using much thinner layers of thixotropic (also called "brushable") RTV. My molds made that way are completely unsuitable for pressure casting because they have a LOT of entrapped air bubbles. I don't imagine it would be quite the same with a pourable RTV since thixo is so much thicker. However, when you are layering RTV most manufacturers (Smooth-On included) recommend applying each layer while the previous one is still tacky. They say if you let it cure fully between layers there is some risk of delamination. I've never had any problem getting new silicone to stick to cured silicone, especially when I fail to get parting agent on part of a two-piece mold, but that's what they say in their technical bulletins and such.
Also, sorry, I didn't get any pictures of my casting gear this weekend, I was working on another project. Maybe tomorrow or the next day.
Here are some shots of the results different techniques yield:
I made a couple of sample molds and some castings for them to use as instructional aids for a casting class I'm planning to do at my local makerspace, Novalabs. The molds are simple one-piece block molds, both of the same part.
First is a poured RTV mold made of RTV that has not been de-aired under vacuum. You can pretty easily see where air bubbles were, leaving voids:
Then, I cast some resin into that mold using a simple pour. On the left outside edge, you can see where an air bubble in the RTV resulted in an unwanted protrusion on the casting. You can also see how air entrapped in the resin has created pits in the casting.
After that, I cast some resin into that mold and let it cure under pressure. The final part shows significant distortion and odd protrusions caused by air bubbles inside the mold collapsing from the pressure. But at least there are no pinholes to fill, right?
Next, I made a second mold. This one was made using vacuum to de-air the RTV. It is an obviously superior mold compared to the other. While I haven't made any molds using pressure instead of vacuum, I think results would probably be similar.
I made a casting not using any pressure. It has obvious pinholes that would need to be filled. Also, note that some smaller details such as the bolt heads failed to fill and would need to be re-worked.
Then I did a pour into that mold, and let the resin cure under pressure. This is the kind of casting I am looking for. No pinholes, no unfilled detail, no distortion.
Finally, just to illustrate the point a bit better, I made two castings using clear resin. The one on the left was done under pressure, the one on the right was done without pressure. It isn't really easy to see in the photo (mostly due to my lack of camera skills), but the pressure-cast part is significantly more clear. The middle piece is there just to give the camera something to focus on.
Lastly, here is a shot of my pressure chamber - a paint pot. I think it is 2 gallons, it has been a long time since I bought it. It is rated for 80PSI.
A closer look at the connections. In the foreground you can see one of the openings has been capped off. Moving up on the left side, you can see the pressure relief valve that came with the pot. I personally would never use nor suggest a pressure pot that didn't have a relief valve. This one opens at about 70 PSI, but I usually only go up to 40 or 50 PSI anyway. Sometimes up as high as 60. Above that is a shut-off valve that I almost never use. I just leave the compressor hooked up in case there is a slow leak. To the right of the valve is the air hose fitting, and above that the pressure gauge. When the resin has had time to set, I just unhook the hose and let the pressure blow out of the air fitting. If you apply pressure and then release it rapidly when the resin is still liquid, the air bubbles trapped in the resin will expand rapidly, splashing resin all over the inside of your pressure pot. Or, uh.. so I have, uh.. been told. By people. People more foolish than me. Who would do that sort of thing. Because I wouldn't.
I hope this is helpful.
Eric
I made a couple of sample molds and some castings for them to use as instructional aids for a casting class I'm planning to do at my local makerspace, Novalabs. The molds are simple one-piece block molds, both of the same part.
First is a poured RTV mold made of RTV that has not been de-aired under vacuum. You can pretty easily see where air bubbles were, leaving voids:
Then, I cast some resin into that mold using a simple pour. On the left outside edge, you can see where an air bubble in the RTV resulted in an unwanted protrusion on the casting. You can also see how air entrapped in the resin has created pits in the casting.
After that, I cast some resin into that mold and let it cure under pressure. The final part shows significant distortion and odd protrusions caused by air bubbles inside the mold collapsing from the pressure. But at least there are no pinholes to fill, right?
Next, I made a second mold. This one was made using vacuum to de-air the RTV. It is an obviously superior mold compared to the other. While I haven't made any molds using pressure instead of vacuum, I think results would probably be similar.
I made a casting not using any pressure. It has obvious pinholes that would need to be filled. Also, note that some smaller details such as the bolt heads failed to fill and would need to be re-worked.
Then I did a pour into that mold, and let the resin cure under pressure. This is the kind of casting I am looking for. No pinholes, no unfilled detail, no distortion.
Finally, just to illustrate the point a bit better, I made two castings using clear resin. The one on the left was done under pressure, the one on the right was done without pressure. It isn't really easy to see in the photo (mostly due to my lack of camera skills), but the pressure-cast part is significantly more clear. The middle piece is there just to give the camera something to focus on.
Lastly, here is a shot of my pressure chamber - a paint pot. I think it is 2 gallons, it has been a long time since I bought it. It is rated for 80PSI.
A closer look at the connections. In the foreground you can see one of the openings has been capped off. Moving up on the left side, you can see the pressure relief valve that came with the pot. I personally would never use nor suggest a pressure pot that didn't have a relief valve. This one opens at about 70 PSI, but I usually only go up to 40 or 50 PSI anyway. Sometimes up as high as 60. Above that is a shut-off valve that I almost never use. I just leave the compressor hooked up in case there is a slow leak. To the right of the valve is the air hose fitting, and above that the pressure gauge. When the resin has had time to set, I just unhook the hose and let the pressure blow out of the air fitting. If you apply pressure and then release it rapidly when the resin is still liquid, the air bubbles trapped in the resin will expand rapidly, splashing resin all over the inside of your pressure pot. Or, uh.. so I have, uh.. been told. By people. People more foolish than me. Who would do that sort of thing. Because I wouldn't.
I hope this is helpful.
Eric
Oh, and just in case I haven't yammered on enough... A shot of my vacuum "chamber" (an aluminum stock pot) and lid. I bought it on eBay, and have been very happy with it. It is far superior to the one I built myself years ago. The stock pot is roughly 11" (28cm) tall, with a 5/32" (4mm) wall thickness. The lid is 1/2" (12mm) acrylic. The orange hose goes to a ball valve, and then to the vacuum pump. When i have enough vacuum, I close that valve and shut off the pump. When I need to release the vacuum, I open the ball valve on the lid.
I just caught this thread and this is the best explanation I've seen on using vacuum chambers and pressure pots. I've only done really simple small molds so I haven't needed this yet, but I'm going to try and buy a good setup soon.
You might want to check out Gattaca's thread on muti-part mold making as well. http://www.therpf.com/f9/how-mold-cast-prop-part-221034/
resinmodeller
New Member
Cheers everyone on the assistance so far Just browsing online for UK companies that can help stock me up with a vaccum chamber and a pressure pot What's the best/cheapest way of DIY said you used a stock pot? lol Things I've seen are like £300, a stock pot is like £10 haha
Just browsing online for UK companies that can help stock me up with a vaccum chamber and a pressure pot What's the best/cheapest way of DIY said you used a stock pot? lol Things I've seen are like £300, a stock pot is like £10 hahaIf you go with a stock pot, make sure it has a good thick wall. If I recall correctly, when I bought mine it was a couple hundred dollars including the acrylic lid with the inlet, valve, and hose. The pressure pot shouldn't be too difficult, any industrial tool supplier should have them.
If I were starting over, I'd probably start with the pressure pot and compressor (which you can also use to power an airbrush) and see what kind of results I was getting from that. I'd only spend the money on the vacuum chamber if pressure wasn't giving me satisfactory results.
If I were starting over, I'd probably start with the pressure pot and compressor (which you can also use to power an airbrush) and see what kind of results I was getting from that. I'd only spend the money on the vacuum chamber if pressure wasn't giving me satisfactory results.
robn1
Master Member
That's my recommendation also. It depends on the nature of the parts you're making, but pressure alone may be enough. If not then you can get the vacuum equipment. The pressure pot should work okay for vacuum as well, you'll just need a clear top for use with the vacuum pump.
I usually de-air my RTV, pour it into the mold box, then put the mold box into the vac chamber for a second draw. The RTV doesn't expand as much the second time, so I've never had any issue with overflow or anything. I usually have to do some clean-up the finished mold after it cures because the RTV that touches the mold box walls when it expands leaves a sort of flash.
But, like I said before... Based on Gattaca's testimony and demonstrated results I'm going to try using pressure for both procedures just to simplify things and free up my vacuum pump for a vacuum former.
But, like I said before... Based on Gattaca's testimony and demonstrated results I'm going to try using pressure for both procedures just to simplify things and free up my vacuum pump for a vacuum former.
TK7713Gra30
Active Member
I run a figure manufacturing business and vac cast daily. From my experience quality that can be achieved by vacuum casting is no different than that of pressure casting, providing you allow feeds for the air to escape. Admittedly the feeds need to be larger in vac casting but if done correctly and in discreet areas no damage is done to the piece, the simpler the piece the less feeds usually.
Bubbles added to the pour can and are dispersed, providing you have feeds and outlets at the correct locations. As with anything, it is not the actual moulding process that give good results but the prep in mould making and correctly spruing the piece.
Excellent detail can be obtained and minimal moulds lines if any, I do 1 piece moulds.
The rubber is vacuumed before mixing then in the mould.
I also vac the resin before mixing and then in the mould.
The bonus of vac casting is that the mould can be removed before full cure, which is of great use if using a slow set resin, I understand pressure casting needs the piece to remain in the pot until cured, although I don't pressure cast.
My resin has 7 minute pot life which is plenty of time to get all the air out, my pump vacs to -30 in 7 seconds.
Demould is approx 2 hours and so pressure casting would take up pot time.
The aim at the end of the day is for a high quality and detailed cast
These pieces are vac cast
Bubbles added to the pour can and are dispersed, providing you have feeds and outlets at the correct locations. As with anything, it is not the actual moulding process that give good results but the prep in mould making and correctly spruing the piece.
Excellent detail can be obtained and minimal moulds lines if any, I do 1 piece moulds.
The rubber is vacuumed before mixing then in the mould.
I also vac the resin before mixing and then in the mould.
The bonus of vac casting is that the mould can be removed before full cure, which is of great use if using a slow set resin, I understand pressure casting needs the piece to remain in the pot until cured, although I don't pressure cast.
My resin has 7 minute pot life which is plenty of time to get all the air out, my pump vacs to -30 in 7 seconds.
Demould is approx 2 hours and so pressure casting would take up pot time.
The aim at the end of the day is for a high quality and detailed cast
These pieces are vac cast
TK7713Gra30
Active Member
Island Scientific in the UK for Edwards pump and chamber. It is where I got mine and going strong. The Edwards are some of the best
Not cheap but high quality
Not cheap but high quality
TK7713Gra30
Active Member
Just one extra thing I have found. If moulding and casting in a vacuum, the release of vacuum during the moulding and casting is just as important and should be done at least once during the mould stage and twice during casting. This sucks the rubber and resin into the mould, or around the master if at the moulding stage. Then the vacuuming is started again. I never send a piece out with a single visible air bubble. Holding pieces up to the light also show if air has been trapped and they do not go.
The weather can also have a major affect on casts, recently in Devon my 7 min pot life went to 1 1/2 minutes. The resin should have specs on temperature min and max. PM me if you would like info as I am uk.
The weather can also have a major affect on casts, recently in Devon my 7 min pot life went to 1 1/2 minutes. The resin should have specs on temperature min and max. PM me if you would like info as I am uk.
cavx
Master Member
Hopefully THIS VIDEO will be of use. Sorry for the bad camera work, my 9YO shot it on my tablet. It is a bit of a long winded process (and the music stops well before the video ends), but the pour results are awesome and no way could I achieve the results I am getting without the vacuum chamber. I have tried the "high pour" method which is supposed to pop the bubbles in the mix as they fall. All this did for me was make a mess with more product going everywhere except into the mold.
I have only been casting parts in clear elastomer for a short time and today did some polyurethane which seems to have also worked out well. This is actually my 2nd vacuum chamber (first one was 2006) and where I could not get proper results from using it and had lots of fails.
I have found that using a 2 litre ice-cream container works well with room for the product to expand. I am using a 3CMF vacuum pump that I bought off evil bay for about $100 (brand new) and had the chamber made up by a local plastics workshop. All up the rig cost me about $400. Hoses were actually the most expensive part.
I had watched many videos about degassing silicone rubber, but could not find too many on degassing the casting medium. Back in '06 I had many fails with resins 'going off' in the chamber. Too much catalyst or bad batch of product? Who knows. Today's results are looking promising and the pour is currently setting up now.
I don't have a pressure pot, so can't contribute anything there.
I have only been casting parts in clear elastomer for a short time and today did some polyurethane which seems to have also worked out well. This is actually my 2nd vacuum chamber (first one was 2006) and where I could not get proper results from using it and had lots of fails.
I have found that using a 2 litre ice-cream container works well with room for the product to expand. I am using a 3CMF vacuum pump that I bought off evil bay for about $100 (brand new) and had the chamber made up by a local plastics workshop. All up the rig cost me about $400. Hoses were actually the most expensive part.
I had watched many videos about degassing silicone rubber, but could not find too many on degassing the casting medium. Back in '06 I had many fails with resins 'going off' in the chamber. Too much catalyst or bad batch of product? Who knows. Today's results are looking promising and the pour is currently setting up now.
I don't have a pressure pot, so can't contribute anything there.
Similar threads
