Pressure casting resin

modelrealms · Post by **modelrealms** » Mon Nov 10, 2008 9:31 am

Ok, when using a pressure pot how do you prevent the resin from squeezing out of a 2-part mold? and please dont say strap the 2 molds together as this would distort the casting.

Blappy · Post by **Blappy** » Mon Nov 10, 2008 9:33 am

Backing Boards.

You can see some molds set up here on my bench

http://i247.photobucket.com/albums/gg15 ... ing_1a.jpg

Gouf · Post by **Gouf** » Mon Nov 10, 2008 10:09 am

Blappy wrote:Backing Boards.

You can see some molds set up here on my bench

http://i247.photobucket.com/albums/gg15 ... ing_1a.jpg

Exactly! If you use the board it applies even pressure. If you try to use a rubber band or some such thing it will distort.

modelrealms · Post by **modelrealms** » Mon Nov 10, 2008 10:17 am

I will run some test castings today, I picked up 4 pressure pots at 54.99ea at Harbor Freight. And bought a 2hp 8 gallon air compressor for 99.99

http://www.harborfreight.com/cpi/ctaf/d ... mber=95386

modelrealms · Post by **modelrealms** » Mon Nov 10, 2008 10:20 am

I've used backing boards on larger molds that wont fit into a small pot and that works fine but haven't tried anything under pressure using backing boards only 1 part open face molds.

modelrealms · Post by **modelrealms** » Mon Nov 10, 2008 10:21 am

Blappy what size pressure tanks are you using???? that blue one looks huge! and where can I get one

Blappy · Post by **Blappy** » Mon Nov 10, 2008 10:53 am

warrensgc wrote:Blappy what size pressure tanks are you using???? that blue one looks huge! and where can I get one

That is GORT. He is 27" tall and 14.5" in Diamatre. I think it is about 26 gallons

http://i247.photobucket.com/albums/gg15 ... e_pots.jpg

I stumbled on him and the other two large pots by accident. They were in the back of a local air supplier's warehouse. They had no idea where they came from as they were there when they took the company over.

I got them for $50.00 each plus $100.00 a piece for the seals.

I have to idea where to get pot that size. Look for industrial paint suppliers maybe? I think a pot that size is about $1000.00 or more new.

thunderbearr · Post by **thunderbearr** » Mon Nov 10, 2008 11:30 am

Make sure the backing boards are stiff enuf to not warp when you use the rubber bands to strap them together.

I use the SIG 1/4 Scale rubber bands, should be available at your LHS.

If not, get them to order them or you can get them direct from Sig Manufacturing.

Rob

Kylwell · Post by **Kylwell** » Mon Nov 10, 2008 12:12 pm

27 inches... now I know who to cast up my 1/72 Monster barrels when I get around to mastering them (or machining the molds, not sure which).

Heh, GORT... cool.

Sparky · Post by **Sparky** » Mon Nov 10, 2008 2:40 pm

Per Blap's suggestion I put in a standing search on ebay and finally got a big pot. I don't think I have pics of it. The stirring handles got get hacked off still though. For now its vacuum degassing only.

DX-SFX · Post by **DX-SFX** » Wed Nov 12, 2008 12:59 pm

Try a firmer rubber too.

Jeff · Post by **Jeff** » Thu Nov 13, 2008 12:01 pm

I have used packing tape to close molds and it has worked pretty well for small molds.

modelrealms · Post by **modelrealms** » Thu Nov 13, 2008 3:50 pm

I have my pressure system setup! out of 10 pots I hooked up 2 for testing. I made my air line out of gas pipe and 2 valves to open and close the air way. I'm going to make a test mold and try degassing it then I will try to pressure cast at 60psi some resin. The part I'm going to do a test with is the strut for the TIE Defender master.

Sparky · Post by **Sparky** » Thu Nov 13, 2008 3:56 pm

Are you using the HF paint pots? You shouldn't run those hotter than 40psi, that's their run rating, they tolerate higher peek pressure, but you don't want to run them all the time hotter than 40. 40 will give you crystal clear castings with Smoothon's crystal clear resin.

Larger bubbles are vent and relief issues and should be handled in there, as collapsing them will only result in warped or distorted parts.

modelrealms · Post by **modelrealms** » Thu Nov 13, 2008 6:14 pm

Talked to a rep at smoothon today he said the resin manufacturer recomends 60psi, I am using the HF paint pots and they have a peak of 80psi, so your saying I can still get good quality castings with 40psi?

Stu Pidasso · Post by **Stu Pidasso** » Thu Nov 13, 2008 6:39 pm

I use mine at 35-40 psi for smaller parts, and it works just fine.

Blappy · Post by **Blappy** » Thu Nov 13, 2008 7:28 pm

I run at 45-50 psi. I used to run at 60 until I realized it was not necessary.

WARNING! SCIENCE CONTENT!

What you are trying to do is defeat atmospheric pressure (about 15psi). So if you go double atmospheric pressure you really only need about 30 psi.

Read about atmospheric pressure here
http://en.wikipedia.org/wiki/Atmospheric_pressure

PetarB · Post by **PetarB** » Tue Dec 09, 2008 9:22 pm

My 10L pressure pot arrived yesterday. I managed to hook it up and have found a bit of a pressure leak, nothing huge, a couple of PSI over half an hour. I've got some plumbers tape to go around the valve ends. Hopefully that should sort it out, but I'm open to any other suggestions, if anyone has any.
I'm going to do my first pressure cast this evening, I'll give it a go at 45PSI.
What kind of resin volume shrinkage occurs? I guess what I'm asking is do I need to build in much of a pouring plug into my moulds to account for the liquid resin volume decreasing once the pressure is put on, or is that not necessary?

Sparky · Post by **Sparky** » Wed Dec 10, 2008 12:37 pm

Shrinkage due to resin cure is noted on the resin product sheet. Usually the faster the cure the greater the shrinkage factor. I don't think a pour stub will help with this, this shrinkage occurs through the cure cycle (after the resin is hardend enough to be unable to draw in more resin from a plug.

Shrinkage due to collapsed air bubbles though is a function of the size of the pocket. As long as you have a pour stub and a good pour position that should take care of the problem. Castings done with a piece of material rolled down on the back of a mold though have no relief and any large pockets of air will induce warping in the castings.

PetarB · Post by **PetarB** » Wed Dec 10, 2008 8:41 pm

Thanks Sparky, I knew about general shrinkage, but I wanted to know about the air bubble component - thanks for your data.
In anycase, casts were done, and shrinkage due to collapsed airbubbles was negligible.

DX-SFX · Post by **DX-SFX** » Thu Dec 11, 2008 7:30 am

I've used backing boards on larger molds that wont fit into a small pot and that works fine but haven't tried anything under pressure using backing boards only 1 part open face molds.

Sorry, a bit more science. It sounds like you're working under a misconception. The mould will be no more likely to leak at normal pressure than it will under high pressure. You're not pressurising the resin but leaving the exterior of the mould under normal pressure like injection moulding. The whole thing will sit within a pressurised enviroment. The only thing squeezed will be the air bubbles. The problem of backing boards holding the two halves together is a problem independent of whether the casting process is in a pressure pot or not.

Sparky · Post by **Sparky** » Thu Dec 11, 2008 2:36 pm

Here's what I've observed:

If you roll a flat back on a mold and leave it on the desk the mold will cure with the large air pockets in the casting and any micro bubbles on the surface detail of the castings, no warpage.

We have all gotten parts in resin kits that didn't sit flat against each other, and looked tweaked, this is most likely caused when the part is cast in an open face mold with a flat back rolled over the compartment. When an air pocket is introduced either by a large under cut or by the act of rolling the back down, then pressure cast, the air in the mold compresses under the force, and mold silicon flexes inward as a result.

Now air pockets right at the edges of an open face mold, where the backing material may be slightly warped from previous roll & peal cycles, my actually get relief but this is a side effect from the back material not laying flat. So if the resin hadn't been squished out while rolling the back down, then you'd have a casting that needed to be back sanded, which was the issue one was trying to eliminate when rolling the backing down in the first place.

I also believe there air bubbles created due to the cure process, when older resin is used, the resin parts A and or B absorbs moisture from the air when you are dispensing. This moisture definitely causes the cure time/pot life of the resin to decrease. It kicks faster, either the water creates even more heat or its another chemical reaction but you will get more bubbles spontaneously forming in the casting. Pressurizing during casting and leaving the casting till it reaches cure keeps these bubbles minimized.

I've pulled castings too soon form the pot and seen all the zits pop up on the casting, some bad enough to warp the casting as well.

Sparky · Post by **Sparky** » Thu Dec 11, 2008 6:19 pm

PetarB wrote:My 10L pressure pot arrived yesterday. I managed to hook it up and have found a bit of a pressure leak, nothing huge, a couple of PSI over half an hour. I've got some plumbers tape to go around the valve ends. Hopefully that should sort it out, but I'm open to any other suggestions, if anyone has any.
snip

I wanted to through this out to those converting their pots:

1) You should probably replace the hard rubber gasket with a new silicon gasket. Spray the pot lid/lip (prior to pour) and the gasket (after cure) with mold release. This will help you peel out the gasket later and peel off dripped resin (respectively). This also helps the pot clamps in that you won't need to tighten them so drastically to get a good seal.

2) There is this non drying pipe thread sealant I got on a whim, well I had gotten this other stuff that didn't work at all. The new stuff is RectorSeal's T plus 2 (I have it in a tube not a can):
http://www.rectorseal.com/index.php?sit ... uct_id=213
It has worked so well I think I can fix one of the leaky pot lids I've been stuck with. I've used it without plumber's tape and it works super.

If you can't find it near by PM me.

DX-SFX · Post by **DX-SFX** » Thu Dec 11, 2008 7:33 pm

I think there's some confusion over the term backing boards. I'm referring to two part moulds being squeezed together by the use of two rigid boards either side as per Blappy's photo in the second post and the original question. I think Sparky is referring to laying a piece of flat silicone on top of an open mould. The former assumes the mould halves are brought together before the resin is poured and any large airbubbles are burped out before dealing with tiny air bubbles in the mix or caught in detail. If there's a pour stub, this will act as a small reservoir to feed additional resin in as the tiny bubbles shrink.