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G'day all,
Thank you for this great and informative forum. I'm new to this casting caper, and am a little confused about casting under pressure or vacuum. Can someone please explain to this newbie, what situations lend themselves to casting under pressure or vacuum.
Thanks
Ned
Vacuum can tend to form more bubbles than it removes before the resin goes off when casting, so I only use it for stabilization. Care is also needed with vacuum, resin boiling if too much vacuum, and expansion of the liquid resin.
Pressure is used when casting to force the resins into every nook and cranny of worthless wood type casting. Heating the resin slightly also helps with bubbles. PR resins also shrink and the pressure can help making it stay in contact with the WW wood
Hope this helps explain.
Cheers
Neil.. you might have been cutting and pasting and forgot where you were! :p
Pressure compresses the air bubbles. Some say it will compress them so they
can't be seen. (I'm still not convinced of that one) Warming the resin makes it
thinner, so bubbles don't stick around. Imagine shaking a bottle of honey (cool
resin) and then imagine shaking a bottle of water (warm resin) and you can imagine
what the air bubbles will do.
Vacuum pulls the air out of the wood. Vacuum alone will not fill the voids, you need
to release the vacuum so the resin gets sucked in when the pressure is equalized.
Since I don't do stabilizing, I don't use vacuum any more. And since I started warming
the resin so that it gets watery (and any air bubbles just rise to the top) I don't use
pressure either. Next month will be two years since I ditched the pressure pot and
I don't miss it. And I do cast PR with wood.. no pressure and no vacuum.
Here's a pic from the PITH: (he got his pen yesterday, so I can post the pic! :D )
:doh: Guilty :B - I have amended textQuote:
Originally Posted by NewLondon88
well, I could point it out, as I have never made a mistake. :p
I am convinced this is not so. I have put my casting under 60psi and have regularly got pockets of bubbles down the bottom of the mould. With I do I find the best way to go is both vacuum and then pressure, next best option is stir and pour carefully and slowly so as not to create bubbles or air pockets and then pressure.Quote:
Originally Posted by NewLondon88
I was looking around for a thread on safety gear for turning (first time next week!) and found this one. I have some PhD experience with resins and casting for both light and electron microscopy.
1. Gases dissolve more readily in cold liquids than in warm liquids (warm beer/cold beer).
2. Gases compress under pressure. Bubbles seem to disappear. Think of scuba divers and decompression stages to avoid the bends. The problem is that the little bubbles coalesce into big bubbles and they won't disappear by decending another 20 meters.
3. In mixing, don't whip the resin around as that can only incorporate air. Gentle application of vacuum to a liquid resin mix can be done to out-gas the soup.
4. Repeated (important process) gentle application of vacuum to specimens will eventually pull out most gas voids in the structure. Each release allows ambient atmospheric pressure to push the resin into voids a little further. Might have to repeat this step 5-10 times.
I am still trying to figure this mess out as well but this is whhat I do and it seems to be working.
1. I have some 2oz Silicone "pinch cups" that I picked up from amazon to mix in. They are great because you pop them inside out and the old resin comes right out and I dont have to worry about them melting. I pour a small amount of resin in there and add my power. I mix until the powder is disolved. I then pour the rest of the resin in the cup and mix it around.
2. Hot Water - I place a jar of water in the microwave to heat it up. I pour about 3/4" into an aluminum (disposable) Loaf Pan and place my cup of resin in there. I wont take long before you see the bubbles rise to the top. Warming it thins the resin allowing the bubbles that were trapped in the thick resin to raise.
3 Vacuum - I used to then place the cup in my vacuum pot and pull it under a vacuum to remove any and all remaining bubbles. I skipped this step this time, lets see if it causes a failure.
4. MEKP - Add it and stir SLOOOOOOOWLY so you dont introduces any more bubbles.
5. Pour - Pour it into your mold using what ever process you use (IE. let it thicken then pour, pour and let thicken in the mold......etc)
6. Pressure - If casting a meduim such as Wood or Oatmeal I place it under pressure to fource the resin down and around the wood. I dont know science but what I feel happens is the resin displaces the air and is heavier than the air so the air is forced up and the resin down. This will give you a blank that will have all or most of its bubbles at the top of the resin. For this reason I pour a little extra resin so I can easily cut this off.
Again, this is what I do but I am big time still learning and have a lot to learn yet about timing my pours but we will get there, with each others help.
Quick Edit, I dont place my resin under vacuum once I introduce the medium as it will pull the air out the the wood or whatever leaving it in the resin as it starts to thicken. Only vacuum the resin by itself.
Thanks and good luck.
Chris Burgess
Up and over in Texas