Chip Sawdust

Making and turning resin castings

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33 minutes ago, Woodenskye said:

I will leave it to you to research, experiment or do whatever you need to get the answer you want.  

Why pull all the air out just to put air back in when working with somewhat tight timeframes.  Most people just use pressure, it works, why complicate.  KISS method works for me. 

Gas in liquids doesn't really work that way. If you remove trapped air with a vacuum applying pressure will not cause the air to magically jump back into the liquid. Applying the pressure would increase the amount of dissolved air but that is going to be a constant directly related to the surrounding pressure. If anything it does take a good portion of time for the gas to re-dissolve so in theory if you pull a vacuum, you will be able to remove a good portion of the dissolved gas. When pressure is re applied it could take longer for the gas to dissolve back into the liquid than it would take for the compound to cure. I don't understand how applying pressure to the liquid would introduce trapped air. I also don't see how increased pressure would help release the trapped air. Boyle's law after all follows that there is no way to reduce the volume of air to zero by increasing the pressure.

If dissolved air is causing the problem than applying any pressure will yield worse results than removing all pressure. This is not the case as then there would never be a recommendation to use a pressure pot.

I don't have a pressure pot or vacuum chamber. I also don't know where you'd search for this. I"m just thinking back to the fluid dynamics I've learned.

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Drew, your right in what you have learned, but I think that is part of the problem, your being so analytical that you will probably never get an answer that will satisfy you.  Doing both is unnecessary in my opinion.  

Again KISS method has worked best the few times I have done this.  Mix resin, pour in mold, put in pressure pot, add compressed air, let sit for couple hours, take out and let it finish cure and then do what you need to.  

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Its not dissolved air but the tiny bubbles and a few bigger ones from mixing. If  your mix has enough set time vacuum first and then pressure works, but may be messy if it foams over the edge of your mold. When stabilizing attention has to be paid to how fast you introduce the vacuum or it will boil up to the point resin gets into the hoses and damages the pump. Not saying this will happen with thicker two part resin, but you would need to be able to watch the mold as you apply the vacuum and possibly build the sides of the mold high enough to handle this problem.

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22 hours ago, Gary Beasley said:

The resin casting turning cloudy may have been the result of moisture if alumilite was used. Its very picky about moisture in the air and in the wood, for that reason alone stabilizing wood before casting gives greater success as it leaves the wood very dry if done right. Otherwise you will want to do everything you can to completely dry the wood before it goes into the resin.

My shop isn’t what anyone would call climate controlled so this is a concerne for me. I have sprayed lacquer a lot in my shop and no moisture effects, so in the summer since we are often at 30% or so it shouldn’t be a problem, i hope? In the winter I think my humidity in the shop is a bit higher. I guess the only way to know is to start small and see what happens, taking whatever non-scientific measurements I can (I don’t have a precise hydrometer, just a cheap “decorative” one of questionable accuracy. 

Would baking the wood in the oven for a few hours at some low-ish heat be useful for drying? I have no other way I can think of to do it with what I have at home. Ah, maybe the wife will let me spring for an autoclave! :)

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To the whole vacuum/pressure discussion, I think the only benefit of vacuum would be to draw resin into voids in the wood. Let that cure, then continue on with the rest of your mold/resin project and since the wood is now “sealed” the benefit of the pressure pot is to “make bubbles smaller.” I think we’ve all seen the video of the guy who compared atmospheric curing to pressure pot curing and the clarity of the latter being substantially better. 

Does that sound reasonable? 

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11 minutes ago, Chip Sawdust said:

My shop isn’t what anyone would call climate controlled so this is a concerne for me. I have sprayed lacquer a lot in my shop and no moisture effects, so in the summer since we are often at 30% or so it shouldn’t be a problem, i hope? In the winter I think my humidity in the shop is a bit higher. I guess the only way to know is to start small and see what happens, taking whatever non-scientific measurements I can (I don’t have a precise hydrometer, just a cheap “decorative” one of questionable accuracy. 

Would baking the wood in the oven for a few hours at some low-ish heat be useful for drying? I have no other way I can think of to do it with what I have at home. Ah, maybe the wife will let me spring for an autoclave! :)

Humidity does factor in, but also temperature is a big factor.  For drying wood, you could also use a toaster oven so you don't upset the wife.  Drying the wood will help with the moisture before casting.

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On 2/1/2020 at 5:47 AM, Woodenskye said:

Humidity does factor in, but also temperature is a big factor.  For drying wood, you could also use a toaster oven so you don't upset the wife.  Drying the wood will help with the moisture before casting.

Heheh.... we have a toaster oven in our guest apartment but our daughter is moving back in soon, so that’s in use. I think I should stick to harder, less porous wood to start with. My wife is pretty tolerant of my weird projects, so using the oven wouldn’t cause trouble unless she had plans for it :) 

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I have some pretty dry cherry, what I hope will be a burl when I cut it. There are three or four branches that came together in one place on the trunk so I have high hopes. But I’ll probably not start out with that, opting instead for something I can experiment on. I don’t know yet what that will be though. I’m open to suggestions. Most of the wood I have is 4/4 and so on. I may slice a piece of African mahogany off a 12/4 piece I have and try a small project first. Or white oak, that’s pretty close-grained stuff. 

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