Vacuum vs. Pressure

[flyrod]

Rookie question....

When do you use Vacuum vs Pressure when making a blank? Seems they would both do the same in my tiny mind...

 

[frank123]

Don't know about pressure since I haven't used it yet, but I use vacuum on PR resins to remove air that would otherwise make bubbles by sucking it out of the resin before it sets.

Alumilite -PU- sets too fast for vacuum so I think most people use pressure to sort of shrink the bubbles to unnoticeable size. Gonna get a pressure setup someday and try it.

 

[underdog]

I've never cast a thing in my life, so take this with whatever grain of salt you like...

....but a process described by someone here, has always struck me as being the best of two worlds.

It goes something like this:

Pour casting material into your forms, place them inside your vacuum chamber, which sits atop a vibrating table, and pull a certain amount of vacuum for a period of time, and then when that's done, apply pressure to compress whatever air is still there.

It made sense to me. But what do I know?

 

[Sylvanite]

Whether you use 2-part resin (such as Alumilite) or "catalyzed" resin such as Silmar 41, you will introduce small bubbles when mixing. The bubbles might not be visible in your casting, but will become apparent when it is turned.

Vacuum makes the bubbles larger, so that they will float up to surface and pop. If the bubbles originate on the edge of an embedded object, they will have a greater chance of dislodging. After degassing, the resin is returned to atmospheric pressure before it gels.

Pressure makes the bubbles smaller, so that they will be invisibly small in the hardened resin. It must be applied from the time the resin gels (or before) until it solidifies.

Either or both can be used, as the processes compliment each other. I've used both on some of my Polyester Resin (PR) and Alumilite Clear casts.

Vacuum alone works well on low viscosity resin such as straight Silmar 41 (provided you pour before it gels). Some people have also had success thinning PR (with heat or with styrene) without vacuum.

Viscous resins such as Alumilite Clear or thickened PR require pressure -- vacuum alone is not sufficient. I also use pressure on Alumilite White and Alumilite Black.

I hope that helps,
Eric

 

[its_virgil]

Vacuum is used before casting and pressure is used after the cast is poured and while it is curing.

I cast only polyester resin so I can't speak for alumilite. But, I don't see why alumilite couldn't be placed in a vacuum chamber prior to mixing the two parts together.

The vacuum will cause the resin to bubble and if placed in a mold chances are it will bubble up and spill out of the mold. It has happened to me.

When I was using vacuum I placed the uncatalyzed resin into a jar much larger than needed. A cup of resin was placed in a quart jar. The jar went into the vacuum chamber and vacuum was applied.Too much vacuum on polyester resin can cause the styrene to evaporate. I think the styrene evaporation happens at 27 in Hg so I would keep vacuum gauge close to 25.

After using vacuum the MEKP was added and gently stirred. The PR was poured into the mold and placed into a pressure pot. I used 35 psi. So, what does the pressure do? Some seem to agree that pressure shrinks the bubbles. But, I have done the math and don't really adhere to this thought. (I could be wrong) My thoughts are that pressure forces what air is left back into solution so it can't form bubbles.

So, vacuum and pressure both so the same thing in the end. They help get rid of air but in different ways. One pushes it out of solution and the other pushes it back into solution.

If you search the site for "vacuum and pressure and PR" you will get enough info to keep you reading for hours.

Do a good turn daily!
Don

Rookie question....

When do you use Vacuum vs Pressure when making a blank? Seems they would both do the same in my tiny mind...

 

[switch62]

I've done some rubber mould making and hobby casting, but not pen blank casting. Having read all I can, this is what I've found.

Vacuum is used to degass the resin, either before adding catalyst and mixing, or immediatley after. As the bubbles of air form the resin can foam 2-3 times it's volume. Like opening a fizzy drink.

Pressure will initially shrink the air bubbles and then they will actually dissolve into the resin. Once the resin sets the air becomes trapped (dissolved) in the resin. Again fizzy drink, you don't see all the bubbles when the bottle is unopened because the pressure keeps the gas dissolved. Once you open the bottle (release the pressure) all the bubbles appear.

It's all part of the same coin. Lowering the pressure will release any gas that is already dissolved in a liquid. Increasing the pressure will cause any gas to dissolve into a liquid.

Heating the resin helps because the air can't dissolve as well, heat the air and the bubbles become bigger. Also a thick resin will thin out a bit letting the air escape more easily. The reason you see water boiling is that the heat drives out all the dissolved air. First you get a lot of large bubbles and then smaller and smaller until there is no air in solution.

Vibration/ultrasonics literally shakes the air out by producing alternating high and low pressure waves in the liquid.

I'd say that either vacuum or pressure will work equally well, provided that the resin doesn't set before all the bubbles have escaped/dissolved. I think that pressure is more common, as most people will have a shop compressor but not a vacuum pump. Also pressure won't make the resin foam and empty from the mould.

 

[jbswearingen]

I've done some rubber mould making and hobby casting, but not pen blank casting. Having read all I can, this is what I've found.

Vacuum is used to degass the resin, either before adding catalyst and mixing, or immediatley after. As the bubbles of air form the resin can foam 2-3 times it's volume. Like opening a fizzy drink.

Pressure will initially shrink the air bubbles and then they will actually dissolve into the resin. Once the resin sets the air becomes trapped (dissolved) in the resin. Again fizzy drink, you don't see all the bubbles when the bottle is unopened because the pressure keeps the gas dissolved. Once you open the bottle (release the pressure) all the bubbles appear.

It's all part of the same coin. Lowering the pressure will release any gas that is already dissolved in a liquid. Increasing the pressure will cause any gas to dissolve into a liquid.

Heating the resin helps because the air can't dissolve as well, heat the air and the bubbles become bigger. Also a thick resin will thin out a bit letting the air escape more easily. The reason you see water boiling is that the heat drives out all the dissolved air. First you get a lot of large bubbles and then smaller and smaller until there is no air in solution.

Vibration/ultrasonics literally shakes the air out by producing alternating high and low pressure waves in the liquid.

I'd say that either vacuum or pressure will work equally well, provided that the resin doesn't set before all the bubbles have escaped/dissolved. I think that pressure is more common, as most people will have a shop compressor but not a vacuum pump. Also pressure won't make the resin foam and empty from the mould.

Actually, this part is false. The water boils because the water molecules closest to the heat source become vapor. They then expand, forming bubbles. Because they're lighter than the liquid, they rise to the top.

A container of water will boil until it is completely gone if heated long enough.

The rest of what you said is good science.