pressure pot vs. vacuum

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Fat Boy

Fat Boy

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Joined
Oct 7, 2010
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86
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Hudson, Fl
I searched the forum for a post and forgive me if i missed it but i did not see one. I am looking in to getting further in to casting and stabilizing but i am not sure what route to go. All i care to do is maybe a few casted feathers or worthless wood blanks and just standard PR blanks. I have found the tutorials on making both set ups and they both appear to be easily attainable. I am just not sure if pressure is for one thing and vacuum is for another or if they both work for everything. any information will be greatly appreciated. Thank you for your input ahead of time.


Also, can you use PR for a worthless wood blank or is it too thick? the alumilite seems to be more watery (but i have not worked with it yet, only seen pictures).


Thanks,

Chris,
hudson, FL
 
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To stabilize = vacuum
To cast = pressure pot (no more than 30 lbs. for most application)

Worthless wood can be used wit both PR and alumilite. And here I would pressurize at 50 lbs. to get into all crevices.
 
Not to jump the thread but I am confused now. Is worthess wood a stabilized process or a cast process? Can it be either? Would you ever use vacuum to do worthless wood?
 
I'm not sure how others are doing it but I usually draw a vacuum for about 5 minutes before adding the pressure. The theory im working with is to draw off any air bubbles I can before compressing the remainder. YMMV
 
snyiper said:
Not to jump the thread but I am confused now. Is worthess wood a stabilized process or a cast process? Can it be either? Would you ever use vacuum to do worthless wood?
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Stabilize: reinforce wood that is unsound but which is "there". I.E. soft, punky, spalted woods. It's not suitable by itself but helped by stabilization it's beautiful. If you have a real void in wood that is stabilized, you will likely still have a void when the process is done. If done properly, you expect that the wood will be stabilized throughout the piece.

Worthless wood: Fill with resin (usually colored) to replace wood that for whatever reason isn't there. Missing wood may be edge of burl caps, knots, rotted away, split, or bug eaten. But universally, the wood is gone. Here you expect/want any voids to be filled. Rarely achieves more than surface penetration- and barely that.
 
The way I think about it is with stabilization you want to get the solution into the wood to make it harder so it can turn without exploding. To do that, you must evacuate the holes and fill them with the solution. Therefore, vacuum for stabilization. It is too hard to push solution into an already filled void with air and air is only compressible to a certain extent.

As for worthless wood, you are just trying to get the material to stick to the outside of the wood and adhere tightly. Maybe a little penetration but not like you are trying to get with stabilization. Besides, pressure also prevents small little bubbles from forming which is a major problem with casting. Vacuum would only make those small bubbles bigger.....

Hope that made sense...
 
Just to clarify something here and please correct me if I'm wrong.

Vaccuum causes the air to be displaced by removing it from the object and in turn the absence of air is displaced if there is something to displace it...i.e. casting medium or stabilizing medium.

Pressure causes the air to be compressed within an object and if the casting medium or stabilizing medium is present it will take the form of the object (internally such as grain structure) and continue to fill the void as the internal air is compressed within.

To say that casting worthless wood blanks under pressure only adheres the medium to the surface of the material is incorrect. I have tested the process of both casting under vacuum and pressure and results have shown that under a vacuum, the absence of air is replaced with the casting or stabilizing medium. When an object is placed under pressure the exact opposite occurs. The casting medium will follow the grain of the wood while under pressure up to the point where the pressure can no longer cause penetration of the casting or stabilizing medium.

Vacuum results in a better process for stabilizing because the medium used in stabilizing can flow through the grain of the wood due to it's low viscosity properties.

Pressure results in a better process for casting because it compresses the higher viscosity to the point that no air bubbles (for the most part) can not be seen by the human eye.

When people here speak of casting without pressure or vacuum, they usually add heat to the process which lowers viscosity making it more fluid. This in turn allows air bubbles to escape from the casting medium. Alumilite though reacts to heat and this causes the medium to solidify faster, there by trapping the air in solution. Not a recommended practice.

Dave
 
I built a pressure pot from HF and never used it. Take that back . I made one pot full and figured out I can buy blanks cheaper. It's very well made and spent more that $125. making it. I will sell for $100. Pick it up. To much money to ship
 
I haven't read through the entire thread so forgive me if this is redundant info.

Vacuum is used to pull air out of all the nooks and crannies of your "worthless wood". When you release the vacuum your casting material (PR or Alumilite) gets "pulled" into these now vacant holes. Vacuum has the added benefit of "popping" any large trapped bubbles in the casting material by expanding them to the breaking point.
((Note that the smaller bubbles also expand but don't pop so if you leave your vacuum in place until the casting material sets you get a very bubbly blank.)

Pressure is used to force the casting material into any nooks and crannies. But it can only go in until the air trapped in the nook compresses to the same pressure as you are applying. So if there are deep pockets you may not get the material all the way in there. Pressure has the added benefit of compressing small bubbles to "nothing" and big bubbles to small bubbles.

Ok, so if you are trying to cast a porous worthless wood blank and/or stabilize a punky blank, your best bet is to apply vacuum which will clear the air out of the nooks and crannies and pop any big bubbles and then apply pressure which will shove the casting material deeper into the nooks and crannies and collapse the remaining small bubbles down to nothingness. Optimally, you want to run through a couple/few cycles of doing this so that the casting material gets pulled/pushed further into your wood with each cycle.

NOW, with alumilite you have approximately 3 minutes to mix thoroughly, pour, move the mold into your pressure/vacuum pot, seal the pot, and pressurize. You really don't have time for both vacuum then pressure. With PR you have time to do both several times. But it STINKS!

Because of the timing and need for vacuum, PR is better than Alumilite for stabilizing punky/porous blanks. For casting solid worthless-wood blanks you can use pressure-only with either and they both work exceptionally well.
 
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I use both for each process. I often stabilize the wood I make our 'Burls & Swirls' blanks with (using vacuum) before casting under pressure.

I also degas resins under vacuum prior to casting under pressure.

The equipment to do both is pretty handy if you plan on getting serious about casting and stabilizing.
 
gketell said:
NOW, with alumilite you have approximately 3 minutes to mix thoroughly, pour, move the mold into your pressure/vacuum pot, seal the pot, and pressurize.
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This is not quite correct. If you are using Alumilite Water Clear, you have a 5 minute pot life. The 5 minutes begin when the resin is mixed. If you are using Alumilite Crystal Clear, you have a 7 minute pot life.

Not a whole lot more than 3 minutes but significant enough. I have timed it myself and find that with Water Clear in a cool shop, you actually have more than 5 minutes. I have never rushed and have even had to walk across the shop and get my air hose and have never had one go off too soon.
 
Okay, I'm just starting to get into this. The way I understand it is that I can use the same modified HF pot to do both casting and stabilizing but I need to use my air compressor to pressurize the pot for casting and my Gast vacuum pump to remove the air for stabilization. Do I understand this correctly? Is there any other equipment I would need?
 
Thank you for the correction, Curtis!!

For all reading, for accurate information on Alumilite, Curtis IS the man!! Trust what he says.
 
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