Worthless Wood, why pressure?

[glycerine]

I'm curious why a pressure pot is used with worthless wood casting instead of vacuum. Wouldn't vacuum be better in that some of the resin would be sucked into the wood and help to stabilize?
I haven't done much casting, and never had the opportunity to use pressure or vacuum, just trying to think through the logic of each method.

 

[wood-of-1kind]

Pressure used to fill 'void' of wood.

 

[GregHight]

Pressure is also much more powerful than a vacuum. Even if you could pull an absolute vacuum, it would only be 14psi less than ambient air pressure. The pressure also compresses any air that is in the wood or casting medium which reduces bubble sizes dramatically.

 

[RDH79]

Man, A day late on learning this. I just tried my first worthless wood cast yesterday. I used just vaccumn. Was out to check them and they are still sticky. Will see what happens.

 

[jleiwig]

Pressure is also much more powerful than a vacuum. Even if you could pull an absolute vacuum, it would only be 14psi less than ambient air pressure. The pressure also compresses any air that is in the wood or casting medium which reduces bubble sizes dramatically.

Not even close to accurate.

In vacuum casting, your worthless wood is placed in your pot and vacuum is applied. The resin will be driven into the mold by atmospheric pressure (perfect vacuum =29.53 in/hg = 1 atm), while the vacuum will also remove trapped air that would otherwise impede the free flow of the resin casting material.

It's generally not used due to it's more complicated than using pressure and most people aren't set up for vacuum. Either method will work well.

All the big stabilization companies use a combination of vacuum, pressure, and heat. A vacuum is pulled in the chamber, then an inert gas is compressed into the chamber followed by a heat cycle. It allows them to stabilize in an hour what would take days for the DIY person to do.

Here is an example of what I used to do with Vacuum infusion when I worked with automotive composites. The resin is drawn into the carbon fiber using vacuum which creates a super lightweight, perfectly saturated part. The only way to get better parts is to use pre-preg and an autoclave, which again uses vacuum and heat to cure the pre-impregnated cloth.

http://www.youtube.com/watch?v=fP9_eWAsuYQ

 

[bitshird]

I can implode a small mammal in my vacuum chamber we use to degass casting investment I know this because we caught a rather large rat in our shop, and euthanize the creature, he exploded all over the inside of the bell jar, a 1.5 cubic ft jar and a 5.5 CFM Welch Vacuum pump, at 6000 ft it would pull to 28.5 in. My wife made me clean every thing and replace the lines from the pump to the table. I've done a couple of worthless wood blanks with just vacuum and so far so good. but we are talking a serious 1800.00 vacuum pump.

 

[PTownSubbie]

I would have thought that pressure would be better. I am no expert but if you put 50psi in your tank, you are putting 3 atmospheres on the blank.

In fact, the pressure will force the resin into the blank. I put a blank in the pot one night with the mold full to the top. I went out the following morning and the mold was half empty. The resin had been forced into the blank and essentially stabilized the blank. I used 70psi pressure and there was no resin inside the tank meaning that if it didn't go into the tank, it went into the blank.

Needless to say, I had to mix more resin and pour a second time to fully fill the blank.

I think either way would work. Someone that has done both should compare which one they believe performed better.

 

[MesquiteMan]

Pressure is definitely better for worthless wood. I tried both vacuum and pressure when I was perfecting the technique before I posted the tutorial. I have a Gast vac pump that will pull 28" +/-here in my location. Vac on its own will NOT get rid of all the air if you are doing a piece with lots of voids and "overhangs".

The best way I describe it when doing demos is to think about scuba diving if you know anything about that. You can take a plastic cup on the surface and turn it upside down and trap it 1/2 full of air. Keep it upside down and descend to 30 feet or so and there is very little, if any, air left in the cup. Don't turn it over and go back to the surface and the air returns.

The same principles apply with WW. In WW casting, there are a lot of overhangs and areas where they air can not just expand and freely go to the surface. If you simply use vacuum and let the resin cure under vacuum, then the air in the "pocket" will expand until it spills out and rises to the surface but the pocket will still be full of air. If you do use vacuum and then release the vac before the resin has set, you may have good results because the air has expanded and then when you release the vacuum, you are in effect adding 14 psi +/- to the resin and the remaining air gets compressed.

That is all fine and dandy but why not just put 50-80 psi on the resin and not mess with the vac? Your air will be completely compressed and all but the very largest overhangs will be completely air (bubble) free.

Remember, Worthless Wood and stabilizing are two totally different animals. Stabilizing using vacuum works quite well. The example that jleiwig gives above is more in line with stabilizing rather than WW casting.

 

[MesquiteMan]

Not even close to accurate.

In vacuum casting, your worthless wood is placed in your pot and vacuum is applied. The resin will be driven into the mold by atmospheric pressure (perfect vacuum =29.53 in/hg = 1 atm), while the vacuum will also remove trapped air that would otherwise impede the free flow of the resin casting material.

It's generally not used due to it's more complicated than using pressure and most people aren't set up for vacuum. Either method will work well.

All the big stabilization companies use a combination of vacuum, pressure, and heat. A vacuum is pulled in the chamber, then an inert gas is compressed into the chamber followed by a heat cycle. It allows them to stabilize in an hour what would take days for the DIY person to do.

Here is an example of what I used to do with Vacuum infusion when I worked with automotive composites. The resin is drawn into the carbon fiber using vacuum which creates a super lightweight, perfectly saturated part. The only way to get better parts is to use pre-preg and an autoclave, which again uses vacuum and heat to cure the pre-impregnated cloth.

http://www.youtube.com/watch?v=fP9_eWAsuYQ

And I am sorry, but GregHight's post is spot on when it comes to WW casting. They way I understand it (I am not a physicist so I may be wrong) everything he says is accurate. Air pressure at sea level is approximately 14.7 psi. Air pressure in a complete vacuum is 0. If you take something to complete vacuum (very difficult to do with normal equipment) and then release the vacuum, the most amount of pressure differential is 14.7 psi. Nowhere near the 80 psi that I can safely get in my pressure pot!

If I am wrong, someone please educate me!!

 

[glycerine]

Wow, thanks for all of the info guys!