Pressure Casting In Resin

[MSzwarc]

I've been experimenting with resin casting, and as I mentioned in an earlier forum topic, my results were less than spectacular. Several of you offered suggestions, and Leigh and Quang posted links to websites with some SBS photos of the pressure casting process. I studied these sites, trying to figure out how I might be able to try this process without a huge investment, and it finally came to me: what about a pressure cooker/canner? Here's a vessel designed to take both pressure and heat, and it has a large opening (much larger than a hand-pump sprayer), so relatively large molds can be used. The one limitation is the pressure limit: pressure cooker/canners are usually used to a maximum of 15 psi, and they usually have some kind of pressure relief device built in to prevent accidents . I happened to have a pressure canner up in the attic, so I decided to experiment .

I connected my compressor to the nozzle?? in the lid of the canner (this "nozzle" accepts a weight, which , during cooking, provides the pressure in that the heated air inside the canner has to reach a certain pressure before the weight is lifted, thus maintaining the desired pressure) with a plastic tube. After pouring the resin into the mold, and sealing it in the canner, I slowly cranked the regulator up to 25psi (I assumed--yeah, I 've heard what happens when one assumes --that the manufacturer had provided a safety margin of at least 100%), and left everything sit for the 40 minutes it takes the resin I'm using set up.

Bled the pressure off and demolded. The results were amazing! No bubbles, no foaming. Best cast I've made to date! If anything, the cast showed the flaws in my mold (I need to figure out now how to pull a vacuum on the vessel when making molds). This would seem to be a relatively inexpensive way for the hobbyist to be able to make decent resin castings when needed.

Mike Szwarc

 

[quang]

Mike,

I've heard about casting techniques using a pressure cooker before, studied the photos on the site but never quite figured out how it works.

Still don't, even after your lengthy explanation :lol:

Anyway, I'm glad it worked for you.

Pictures?

Quang

 

[MSzwarc]

Quang,

I'll try to borrow a digital camera and put up some pictures. What I did is really pretty simple, but a bit difficult to describe. I guess that's why a picture is worth a thousand words .

What struck me about the process I used is that decent results were obtained with a pressure of only 25 psi. I don't know what's the best pressure to use, but one of the websites I looked at used 85 psi of pressure. I expect that a higher pressure gives a better result, but I was impressed by the results I obtained using a lower pressure versus using no pressure.

Mike Szwarc

 

[LCoote]

Mike, good to hear you had success, and a cheaper alternative too.
What about pouring the moulds and putting them under pressure the same way, wonder if that would work.

 

[JohnLea]

Mold design and resin formulation still play a large part in the process.

If you pour the molds under pressure they may not be useful as molds without pressure. Voids that were compressed need the pressure to stay that way in the rubber. Conceiveably, a difference in pressure could cause distortion. Of course the resin hardens keeping the compressed bubbles out of the picture.

I use both vacuum and pressure (80lbs).

 

[quang]

John,

I still don't get it!

I ONLY use vacuum (both for mould-making and casting resin).

What is the advantage of pressure over vacuum and what makes you shift from one technique to another?

Not that I want to change anything in my current procedure but I'm curious about the PRINCIPLE behind the pressure casting.

Quang

 

[Richard E]

I still don't get it!

I ONLY use vacuum (both for mould-making and casting resin).

Nope me neither!!!!, on the incredibly rare occasions that I mould and cast, I use the vacuum method. Is there any real noticable difference between the two ?

 

[MSzwarc]

I still don't get it!
I ONLY use vacuum (both for mould-making and casting resin).
What is the advantage of pressure over vacuum and what makes you shift from one technique to another?

Quang,

The only explanation I've been able to find comes from an Alternative Studios article on vacuum chambers:

"Many people think that you can vacuumize Resin, but this is not true.  Resin cures in a minute or two (depending on the temp of the material) and can be removed from the mold in 4-5 minutes.  To be able to pour the resin, put the lid on the chamber, and deaerate, it could take a few minutes.  So, if you want to do solid resin, then pressure is the way to go.  for that, you can get a pressure pot from a local idustrial supplier store."

I know, it's not much of an explanation. It seems to me that it takes about the same amount of time to pressurize a casting as it would to draw a vacuum on one. And anyway, the resin I'm using needs 40 minutes to demold. I'm beginning to think that perhaps pressure and vacuum would both accomplish the same end.

Mike Szwarc

 

[quang]

Mike,

You said it: "Not much of an explanation". And what's worse, misleading at that.

The resin I use has a 'pot life' ('workable' period) of 2 minutes and can be demoulded after 45 minutes.

It takes me 10-15 seconds to pour the resin in the mould(s), close the lid and reach 0 atmosphere vacuum. Not minutes like they say.

I'm perfectly happy with the results we get so far and has no intention to change my technique.

Like I said, I'm just curious about how the pressure technique works and why it's more appropriate in certain cases. I'm still waiting

Anyway thank you very much for your concern

Quang

 

[gordy]

I recall the vacuum cambers at Verlinden they were pretty good sized:

a square box made of 1 1/2" thick plexiglass, roughly 18" x 20" and a plexiglass lid of the same thickness, the lid would swing up on simple hinges that has quite a bit of play in them, the lid would close on a rubber foam seal

they would place the molds in the box close the lid (no fastening) and pull the lever, the vacuum would suck down and it bow the lid inwards and the resin would bubble like boiling out if the cavity, some would come out alltogether and then a second pour was sometimes necessary... no fastening of the lid ws necassary because naturally the vacuum would pull it tight, Verlinden had said once he had a lid blow out and it was quite a shock, i could just imagine!

 

[MSzwarc]

Like I said, I'm just curious about how the pressure technique works and why it's more appropriate in certain cases. I'm still waiting

Quang,

I'm too new at this to know why casting under pressure might be more appropriate than casting under a vacuum, and frankly, I'm beginning to believe that both methods could be used interchangeably. It seems to me that results matter more than technique.

As far as how the pressure technique works, perhaps I can explain my understanding of it. When you mold or cast under a vacuum, the intent is obvious: to remove entrained air. When you remove air from the vessel, any air in the RTV or resin rises to the surface, and is drawn off, resulting in a bubble-free mold or casting. When casting under pressure, the idea is to squeeze the resin with air pressure, so it fills all the voids in the mold, including any spaces formed by bubbles. This forces the bubbles out of the mold, provided proper venting has been provided in the mold, as in vacuum casting. I expect that there may be some compression of entrained air bubbles, or possibly even air forced into solution with the resin, but based I my observation of my castings (the level of resin in the pouring gates is significantly lower than in castings made without pressure), I believe most of the air is actually forced out of the mold by the resin being squeezed by air pressure. If this is, in fact, the case, I don't see why one couldn't make a mold under pressure as well. I'll try it out with my next mold.

I hope this rambling helps to clarify things

Mike Szwarc

 

[JohnLea]

I use both approaches on 99% of my castings.

I started with Alumilite and was suicidal. I could not do anything quickly enough. I now use a tooling resin with a pot life of seven minutes which is plenty of time for me to vac the mix, the pour and put the molds under 80 lbs of pressure.

"Many people think that you can vacuumize Resin, but this is not true."

It is absolutely true. Nothing like expert opinion.

You will get an argument on both sides of the vac/pressure question. Most people I know use vacuum. I started with pressure and added vacuum. If I had to choose one it would be vacuum. Pressure allows me to be slightly less fastidious in the mold design area. One caution though. Both approaches can be dangerous. You can foresee the results of a pressure accident (scary) but you need to see a crumpled rail tank car to know how serious vacuum can be. I have heard of someone getting a hand caught in the crumpling of a jury rigged vacuum chamber.

Quang, I am told that pressure forces the gases into solution much like a diver with the bends. The difference is that the resin cures keeping them there whereas the body allows them to expand when the diver returns to the surface.

 

[quang]

Mike: Your explanation does make sense. Well, sort of. Thanks!

Gordy: You might've been making a description of my own rig. Except that on mine, only the lid is made of plexiglas. The chamber itself is made from 1 cm-thick arc-welded battleship-grade steel. It's built like a tank (and weights like a tank). Not the kind of apparatus that your wife (or any wife) would allow in the house! :lol:

John: I fully appreciate your attempt to enlighten me . But the more I think about it, the more I feel my case is HOPELESS Thanks for trying!


Quang

 

[JohnLea]

Quang. This is definitely a case if it ain't broke don't fix it. Vacuum works for you so you are wise not to change anything.

I got a wild hair a few years ago and had a combo tank made. It will hold a vacuum and stand 125 pounds of pressure. My wife calls it a cannibal pot. (She would fit into it.) I have never used it since I need to get a bigger compressor tank and my system works now for my production needs.

 

[John Long]

I had a friend who used to do pressure casting. He employed a canister used for painting with a spray gun. Evidently, it was the vessel that held the paint. He capped some of the holes, added a pressure relief valve, and a quick connect for the air and voila.

 

[JohnLea]

John,

That is what I use. I run 4 of them right now. You can get them from DeVilbis for around $225.00 or Harbour Frieght for about $80.00.

When I started, I bought a jewelers vacuum set up along with the DeVilbis. Since then I discovered Harbour Frieght and I make my own vacuum chambers out of a 12" PVC joint ($35.00) and 1" Lexan sheets (free)

 

[Jiver]

This is the advantage of pressure casting:

Fysics say that the pressure you put in the pot is equally,everywhere in that pot.
No matter were there are airbubbles,they get comprimised.This means you can use complicated moulds that would be to difficult for vacuum.
As for RTV silicone.NORMAL dry period is 12 h.You must keep your eye on that,since there's always a loss of pressure trough the joint.


Jiver

 

[garyo37388]

Some light on the subject maybe...

Being new to this forum, I thought I'd share my experience on the subject. I've used vacuum casting for 15 years and pressure for 7 years.

The easiest explanation is this:

1) Vacuum does not "suck" air from solution especially thick liquids as most folks seem to think.
2)The vaccum pump creates a localized decrease in atmospheric pressure. This decrease allows trapped air bubbles to become larger and they as they become larger their ability to break the tension of the fluid holding them becomes strong enough to let them float straight up ( gravity works!) and out of the solution. Then the pump can exhaust the trapped air out through the exhaust port of the pump.
3) If there are undercuts that prevent the enlarged bubbles from ecsaping in a vertical straight line, you've created bigger voids by using vacuum. This means vacuums work great for things like molds where the top surface is not critical, and the enlarged bubbles can escape from the large open vertical surface.

Now for pressure:
4) In a pressure system the minute bubbles are compressed....made so small as to be invisible.
5) A good example is a soft drink.... pop the top and thousands of bubbles magically appear. They were there all along, but invisible because the pressure forced them to be small.
6) Pressure must be held till medium has hardened.

Now for my opinions....pressure is better than vaccum in most applications, especially when casting resin into an RTV mold. For mold making, vac is OK.

Now for an unusual solution as hosted bt my DIY friend Joe at MyHeap.com :
http://www.myheap.com/book/chapter-02/spincaster/spincaster.php

I know it's over the top...but it works flawlessly. The medium is held out of the mold cavity until a vacuum is achieved. The resulting bubbles (if theyexist ) have little mass compared to the casting medium which is slung into the mold chamber at 35 miles per hour.

The whole process takes 1.5 minutes.

The machine cost me $500 to build and took 6 months to perfect.

I'll try to post answers to this forum, but if you want fastest response, please use contact form at my website:
http://www.willowproducts.com/index/content/default/contact

Also if you're into wax or want to know more about wax, please explore my website:
http://www.willowproducts.com

 

[Jeff]

I would say be careful using a pressure cooker. The pressure you can force in to one with a compressor is much greater then what some are designed for. I have friends who do casting and have seen old pressure cookers explode. The worst I have seen with a paint pot was a blown seal. Loud and scary but not lethal. The failed pressure cooker threw big pieces of of metal around the shop. If you are going to do much casting get a real pressure pot. Be safe.

 

[garyo37388]

I would say be careful using a pressure cooker. The pressure you can force in to one with a compressor is much greater then what some are designed for. I have friends who do casting and have seen old pressure cookers explode. The worst I have seen with a paint pot was a blown seal. Loud and scary but not lethal. The failed pressure cooker threw big pieces of of metal around the shop. If you are going to do much casting get a real pressure pot. Be safe.

AMEN!
I'd never use a pot that wasn't designed as a true pressure vessel.
and pressure cookers are really designed to hold much pressure and the metal is inferior ( in terms of strength )

In fact I chose Craftsman over chinese knock-offs due to quality concerns.

I use a paint pot and added a soft rubber gasket under the hard one to make sealing easier.

After about 2 year's use it failed and let the air escape with a loud scream.

I replaced gasket and all is fine.

Use common sense, good practices, and remember YOU are responsible for your own safety!