Vacuum vs. Pressure? - Banksia Nuts

[Matt Estlea]

Hi all,

I recently made these salt and pepper mills for my Mum using banksia nuts. Can you tell which is which? :wink: (Image attached)

I had a bunch of problems with getting epoxy into the holes. The main one being voids getting exposed as I was turning the pieces.

I'm looking to invest into a pressure pot or vacuum chamber to make jobs like this easier, but struggling to work out which will be better for the job. Does anyone have any experience using both pieces of equipment and which would you recommend? I have a budget of about £200.

Cheers in advance!

 

[marcros]

I saw your YouTube on this. I liked the result, but felt your frustration.

I would suggest casting them- ie make a surround, using an unthickened resin and then pressure. Drill and turn afterwards.

 

[--Tom--]

House of resin do a lot of this and would be worth phoning to ask.

 

[SVB]

A useful read

https://www.artmolds.com/pressure-pot-vacuum-chamber

 

[Garno]

This is not very helpful to you sorry.
They look absolutely stunning
Garno

 

[Trevanion]

I don't have any experience of this kind of thing, but I've heard more good things about pressure pots than vacuum chambers. I know some people like to use a vacuum chamber first and then a pressure pot after.

Peter Brown has some excellent videos on casting, I'm sure you've already seen them.
[youtube]ShSxUg4Yyws[/youtube]

 

[Dalboy]

Pressure pots are to reduce the size of the bubbles in resin by compressing them so small they can not be so easily seen. If you were to use vacuum pump it would try to extract the air out of the wood and when the resin sets it will be full of bubbles.
Vacuum pumps are really used to stabilize wood where the pump sucks out the air from the wood while it is submerged in the stabilising fluid and when the pump is turned off the fluid replaces the air, the stabilising fluid will not set like resin until it is baked.
This is just a basic explanation so more research is needed beyond this.

 

[Fitzroy]

From a purely theoretical view point. What you are trying to achieve is elimination of gas volume. Starting at one atmosphere pressure:

Drawing a vacuum of 0.1 atmosphere all air will expand by 10 fold, assuming you can remove the air, then when you let pressure back in the epoxy will then be driven into the substrate. Approx 10% of original air volume will remain.

Submerging substrate in epoxy and increasing pressure to 10 atmospheres will drive epoxy into substrate reducing volume of air in substrate to approx 10% of original.

A cheap vacuum pump (£70) off amazon will draw 1/1000th of an atmosphere with ease. A rig to increase the pressure to 1000 atmospheres will cost you an arm and a leg and be incredibly scary to be anywhere near.

Theoretically it is much easier, and cheaper, to remove the air than squeeze it into the corner where it’s no longer noticed. I can see why a vacuum chamber to remove to air followed by a pressure chamber to force the epoxy in would be the gold standard.

Fitz.

 

[Inspector]

Fitz the problem with the application of vacuum to polyester, epoxy and polyurethane resins is that they can foam when the air in it expands and the air in the wood can’t get past so bubbles into the resin and gets trapped. Plus the components of the resin may boil off in the vacuum. None of which will be corrected when the pressure is applied.

Vacuum can be used to degas some resins before casting. The primary use is, as already stated, for stabilizing with specific products like Cactus Juice. After it is heat cured, casting with resins reduces the air bubbles because the voids in the wood are filled.

Think of the pressure keeping the air dissolved in solution much like a scuba diver’s blood has dissolved nitrogen in it. The resin gets the bends. :wink:

 

[Fitzroy]

Fitz the problem with the application of vacuum to polyester, epoxy and polyurethane resins is that they can foam when the air in it expands and the air in the wood can’t get past so bubbles into the resin and gets trapped. Plus the components of the resin may boil off in the vacuum. None of which will be corrected when the pressure is applied.

Vacuum can be used to degas some resins before casting. The primary use is, as already stated, for stabilizing with specific products like Cactus Juice. After it is heat cured, casting with resins reduces the air bubbles because the voids in the wood are filled.

Think of the pressure keeping the air dissolved in solution much like a scuba diver’s blood has dissolved nitrogen in it. The resin gets the bends. :wink:

More than understand, phase behaviour of multi component mixtures at varying temperatures and pressures is a major part of my job.

Mine was entirely a thought experiment having zero experience of epoxy use at any pressure. Should have been much clearer about that. And my last statement should have said, ‘could’ be the gold standard.

F.

 

[rafezetter]

A possible question to check on; is cactus juice hard enough to turn on it's own, or do pen turners add another step after?

If it is, then that might be all that's needed?

If Matt wants coloured versions then I guess he'd have to go down the pressure route.

Also, are there vaccum pumps that draw air slowly? Would that reduce / eliminate the foaming?

 

[Dalboy]

A possible question to check on; is cactus juice hard enough to turn on it's own, or do pen turners add another step after?

If it is, then that might be all that's needed?

If Matt wants coloured versions then I guess he'd have to go down the pressure route.

Also, are there vaccum pumps that draw air slowly? Would that reduce / eliminate the foaming?

Ideally (not essential)a vacuum pump and cactus juice would be the first stage on a nut, then add resin and put in a pressure pot.

Any vacuum pump will allow air to be drawn in slowly if the set up is done correctly it is all about having the correct control valves included in the system