Removing bubbles using Vacuum

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Brian Chislett

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Having just watched a TV commercial advertising a small vacuum pump, to be used with transparent plastic bags for food storage, reducing size of clothing for storage etc. I am wondering if anybody has experimented with this system to remove bubbles , when casting, instead of compressing them. The set up looks very cheap. Pop the mould into a bag and remove the air. Would it work?
 
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I have thought about this in the past, though i have not experimented. Because PR is in liquid state, once you start sucking out the air, the PR would bleed everywhere. The only way I could think was to create a mold with a sealed top and a small check valve that would enable the air to escape but not the resin. Even so I think due to the thick consistency of the PR, the vaccum may not have enough power to extract the air.
But like i said, it is what I think but would like to hear other possibilities others may have and create a prototype that works.
 
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A couple of issues I can see with it:

Pressure is used to reduce/eliminate bubbles my making any trapped gasses as small as possible, vacuum is used in stabilizing to remove air from inside the wood to allow the resin to penetrate and take it's place. Using vacuum in casting would cause any small amounts of gasses to become as large as possible, which is the opposite of what you would want. Vacuum is used by some in casting to remove gasses from the individual parts of the resin before mixing...

I'm pretty sure that vacuum pump in those clothes storage bag systems will not pull anywhere near the amount of vacuum that people consider adequate for stabilizing, their main purpose is removing high volumes of air from clothing and such to reduce volume, not pull out every air molecule there is.

Also, I know that heavy duty vacuum bags are used regularly with proper vacuum pumps for things like carbon-fiber/fiberglass molding and wood gluing/bending to shapes, but again, their goal is to take out the majority of the air to use the pressure to hold the materials to the irregular shape molds. I can see a whole lot of mess potential if you were to put something liquid inside and apply vacuum, as the things rarely sit perfectly flat as the bag contracts. The bag will likely also come in contact with the resin (they are rarely strong enough to support a large surface area that is not supported against any amount of strong vacuum. The bag coming in contact with the resin raises issues of any reaction between the resins and the bag, and the likelyhood that the blanks may bond to the bag and destroy it when you remove them...

In the end, as some had said, there are many ways to try and do these things on the cheap, some work out, some don't, and with the group around here, it won't surprise me if someone had tried what you are asking. It also wouldn't surprise me if someone proved everything I think might happen is wrong...
 
There is also a good posibility that the resins can react chemically and melt the bags depending on the recycle code.
It might work if you support the bags away from the resin, you would have enough vacuum to drop the pressure to allow the air to rise to the top. How ever with PR small vibrations from setting the resin on a running tool usually works just as well.
Alumilite will not react with the bag as far as melting it, I am not sure if you could pull all the bubbles from off gassing out of the mix before it set.
 
Using a vacuum pump intended for food storage won't work. I tried it a couple years ago. There just isn't enough vacuum to do anything.

With a real vacuum pump, using a plastic bag instead of a vacuum chamber might work if:

1) you have a rigid mold that won't crush and distort.

2) you have enough head room so that the resin will never touch the bag. Resin can more than double in volume as you vacuum it. You would need plenty of space in a rigid mold.

3) Your bag is strong enough to withstand full vacuum across any unsupported areas

4) You can find a bag with the fittings / ports to allow you to attach it to a real vacuum pump. Most of the ones I've seen are pretty big (used for making parts for boats and airplanes).

All considered, I think you would probably spend less money and waste less time by starting out with a real vacuum pump and vacuum chamber. But only if you want to work with PR (and maybe epoxy). I wouldn't bother with vacuum for alumilite.

Ed
 
I can only comment on polyester resin since I do not and have not ever cast with alumilite or other resins. I also can only comment on tube on clear casting (snake skins and such) since i do not do casting of colored solid blocks or blanks.

Using vacuum to degas needs to be done prior to adding the MEKP and prior to actually pouring into the mold. In the beginning I used vacuum with polyresin for this purpose. I placed the amount of resin in a glass container that was much larger than the actual amount if resin and placed this container in my pressure pot and applied the vacuum. As the air was removed from the the polyresin the volume increased as bubbles grew and finally burst. Without being able to see what was happening I had a couple of issues with spillage until I worked out the size of container to use with the amount of resin i was using. I no longer muse vacuum for casting. The solvent in polyester resin, styrene, will evaporate around 27 inches of Hg so to keep the resin at the correct ratio of chemicals I would keep the vacuum around 25 inches of Hg. Using vacuum and pressure has been discussed at great length here and doing some searching will yield those discussions and makes for some interesting reading.

While pressure casting will help in reducing bubbles pressure will not solve all of many problems that arise when casting. Come to my shop and take a look at the bucket of failures: many cast under pressure; some were degassed with vacuum; and other were case with neither and they failures look much the same. I do wish the science of clear casting were a more exact process! My thoughts on pressure differ from many others on what actually happens. Most of the pressure use discussions mention that pressure will shrink or reduce the size of bubbles so they become invisible. What I think happens is that the air that is in the mix and not dissolved into the mix is actually driven back into solution and the air that is already dissolved into the solution is kept in solution and not allowed to come out and form bubbles. The best way to try to explain this is with a bottle of soda. No bubbles in the soft drink sitting on the shelf. But why not? Carbon dioxide is the gas used to carbonate the liquid. But once the cap is removed and the pressure inside the bottle is released those bubbles we have learn to love in our soft drinks start to come out of solution and do their thing. Put the cap back on and press builds back and the gas escaping solution will slow or stop.

That's my take on the pressure/vacuum debate. I know pressure is needed for casting alumilite and helps with polyresin but I have good success with my (clear tube on)casting with polyester resin using neither. There are several articles in the library that discuss clear casting.

Do a good turn daily!
Don
 
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not sure if you can pull a strong enough vacuum to stabilize, but using a food vacuum system to remove bubbles from PR works, you can use a marinating container that can be bought online or in the little catalog that comes with your food vacuum. They have assorted sizes, one I use is about 10 inches tall and 8 inches in diameter.
 
Thanks for all interesting replies. I was just about to order a pressure pot when I saw the TV advert which got my little grey cells working. I might try casting without a pot using my ultrasonic jewellery cleaner bath to remove any bubbles. Barry Gross has demonstrated that you can cast without any such gagets. Let the fun begin!
 
I have done several clear casts and a few color casts using PR with no vacuum or pressure. have not had any problems. Maybe just lucky.

Watched a demo by Barry Gross and he does tube on casting in 3 pours. less depth of resin for the bubbles to come to the top.
 
I use an ultrasonic cleaner and have for several yrs now. Check the bubbles free article in the library. Actually, there are several casting articles in the library.
Do a good turn daily!
Don

Brian Chislett said:
Thanks for all interesting replies. I was just about to order a pressure pot when I saw the TV advert which got my little grey cells working. I might try casting without a pot using my ultrasonic jewellery cleaner bath to remove any bubbles. Barry Gross has demonstrated that you can cast without any such gagets. Let the fun begin!
Click to expand...
 
Thank youfor the tip Don. I am amazed by the amount of information in the library. By the way, your book is great. I recently ran a pen turning course in the UK. It was great success.I think the ideas given to me from your book made all the difference.
Cheers
 
You're welcome Brian and thank you for the nice comments about the book. I'm glad you were able to use the information and inspire those who attended.
Do a good turn daily!
Don

Brian Chislett said:
Thank youfor the tip Don. I am amazed by the amount of information in the library. By the way, your book is great. I recently ran a pen turning course in the UK. It was great success.I think the ideas given to me from your book made all the difference.
Cheers
Click to expand...
 
I cast with PR resin and vacuum it every time after adding the MEKP. I mix enough to ensure the bottom and sides have been scraped and exposed to MEKP then place under vacuum in my home made vacuum chamber (not a bag). The mixing action of the degassing process is better than I can do stirring and the bubbles are all gone once it meets a boiling point (it actually boils without heat at full vacuum, amazed me first time). I then pour it into my molds and let it sit overnight. After 24 hours I take it out and after an hour or so exposed to air the last of the tackiness is gone. Vacuum chamber is nothing but an acrylic vase with a Plexiglas lid and fittings for tube and valves. It cost about $20 to build and I stabilize 16 blanks in it at a time with 15 inches of headspace for bubbles. It is a great addition to my tools and anyone can make one.
 
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