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Thread: Vacuum versus pressure
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16th February 2021, 03:52 PM #1GOLD MEMBER
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Vacuum versus pressure
Excuse me please if this question is answered somewhere else.
When trying to expel air bubbles from resin, what are the pros/cons of a pressure pot as opposed to a vacuum pot.
I'm a bit wary of a pressure pot with regards to safety. I've read about vacuum pots causing the resin to bubble up and out creating a mess.
I'm just filling some voids, not making pen blanks.
Thanks for any input.
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16th February 2021, 04:36 PM #2GOLD MEMBER
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OK. So after some more googling......
Pressure pot compresses the air bubbles until microscopic/invisible. So dont release pressures until resin is fully set......
Vacuum pot causes bubbles to expand until they get to the surface and pop. Frothing occurs causing a mess.
Pressure pot for casting resins.
Vacuum pot for stabilising.
But what if you slowly induce the vacuum. Resin could set before bubbles exit. Slow curing resin???? Allow space for froth???
Any comments or experiences. Eager to hear any other ideas, or suggestions.
Thanks
Lyle
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16th February 2021, 10:03 PM #3
You have already come to the correct conclusion, vacuum for stabilising, pressure for casting.
The biggest issue with vacuum casting is that under vacuum the resin is effectively under a much higher temperature, so will overheat the resin and cause it to set much faster and likely crack, in addition to entrapping air being extracted before it sets.
If you are using pure resin casts, you could use vacuum as the air can be extracted, although the temperature issues still exist. But normally us woodturners cast hybrids from resin and wood, and wood contains a lot of air, that takes a long time to get removed (especially our hardwoods in Aus). To get air out of even pen blanks, assuming they are dried properly in the first place (0%MC not just back yard dry), I've had Gidgee for example under vacuum for a number of days, and no slow cure resin takes that long (especially at effectively up to 50C).
Now casting doesn't have to be done under pressure, there are many very thin slow curing epoxies out there, and as long as the bubbles can escape they do. But with complex hybrids where the air can be trapped, the only way to remove them is pressure, or at least reduce them in size such that they are dissolved into the resin or they are so small they cannot be seen.
Hope this helps.Neil____________________________________________Every day presents an opportunity to learn something new
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17th February 2021, 08:26 AM #4GOLD MEMBER
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Thanks.
Hadn't factored in the temp.
So is vacuum actually increasing the curing resin temp? Or is it the bubbles coming out make it appear 'boiling '.
I think I'll rethink the process.
Lyle
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17th February 2021, 09:14 AM #5GOLD MEMBER
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Are the pressure pot kits for spray painting on ebay any good? Do they need modifying???
Thanks.
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17th February 2021, 02:43 PM #6Senior Member
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Just a thought.....If there were air bubbles in the resin or air in the wood and they the mixture was cured under pressure in a chamber, when the pressure in the chamber is released after the resin has cured, what happened to the air that was, and I assume still is, present in the now resin-encapsulated wood? If I drill a hole in the resin/wood block will air be released and if so, how fast or slowly will it escape?
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17th February 2021, 04:22 PM #7GOLD MEMBER
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I use both pressure and vacuum .
I make screwtop boxes out of banksia pods and I need to get resin into all the seed voids .I have a perspex lid which fits my pressure pot when I need to use it as a vacuum chamber.
I place my banksia pod blank in a mould in the vacuum chamber and pour resin in untill it is submerged then slowly apply vacuum, watching to ensure it does not froth over the top . I then release the vacuum and some of the resin will have run into the seed voids . I then repeat this a couple more times, topping up with resin if necessary .I then replace the perpex lid with the pressure pot lid and apply pressure over night .
Ted
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17th February 2021, 07:42 PM #8GOLD MEMBER
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Thanks Ted.
What pot are you using as both vacuum and pressure. Do the lids require modifications. Or have you different lids.
Any chance of photos, please.
This sounds like a process I'd be interested in using.
Lyle
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17th February 2021, 08:23 PM #9
Lyle,
A quick search on Youtube game me this, there is a lot of information out there.
Grab a cuppa and watch a little (I have watched some of these before as I had similar questions)
https://www.youtube.com/results?sear...+resin+casting
Rick
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17th February 2021, 09:00 PM #10GOLD MEMBER
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Lyle
I use a standard 10L spray painting pot with the uptake tube cut off and blocked and valves fitted . There are plenty of utube videos showing how to do it . For vacuum I replace the lid with a sheet of 12mm
(I think) perspex which is tapped to take a"T" piece with in and out valves and a fitting to connect to the vacuum pump .I use a bit of rubber sheeting between the perspex and the rim of the pot for a seal .
Note unless you have a special need , as I do ,you only need to use pressure when casting .
Ted
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18th February 2021, 06:26 PM #11
The pressure reduces the air to extremely small or dissolved into the resin. Once the resin has set, the air cannot expand again, nor can it come out of what was solution. Hope this makes sense.
With Banksia, as they are full of air, your vacuum will be sucking just as much air into the resin as it is removing air from the resin, so really no point. What I have found for banksia, is to cast them vertically starting with some resin in the molds then slowly twist the banksia in and then hold them down, before topping up with more resin. This avoids the seed voids entrapping large bubbles of air.
Using 12mm acrylic lid on your pot will fail under full vacuum, even 20mm will fail for that diameter, and they go with a bang! I've used 20mm structural glass but it all comes down to the diameter of your pot. Vacuum may cause the pot to implode, rather than explode, but the broken parts bounce off the sides effectively exploding anyway.
Take care with pressure and vacuum, both can kill you in a blink of an eyeNeil____________________________________________Every day presents an opportunity to learn something new
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18th February 2021, 07:28 PM #12GOLD MEMBER
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Should all the pots be made to Aus Stds, say 4343??
Are the pressure spray painting kits in AutoPro/SuperCheap any good. What about the fleabay ones.
Any other manufacturer/supplier in Aus?
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18th February 2021, 07:55 PM #13Senior Member
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"The pressure reduces the air to extremely small or dissolved into the resin. Once the resin has set, the air cannot expand again, nor can it come out of what was solution. Hope this makes sense."
Yes, it makes sense but what happens to the air trapped in the wood? As my interest is mainly pens and other small things that may be made out of a timber/resin composite and if the timber is enclosed in cured resin prior to turning what happens to the air trapped in the wood? The same thing would apply to things like Banksia seeds encased in resin and then turned. Any trapped air would be under pressure, retained by the cured resin, would it not?
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19th February 2021, 08:36 AM #14GOLD MEMBER
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You are right. The bubbles are trapped in the (solid) resin.
Equate it to soft drink. The gas (bubbles) inside the liquid are invisible until the pressure is relived then they are visible.
So when the bubbles in resin are pressurised in the resin they'll be trapped and as the resin is solidified then they can't be released.
So now my question is, does this internal pressure cause any issues in future?
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19th February 2021, 01:30 PM #15
Any air trapped in the wood stays there. Resin will only fill voids it can get to. Even true vacuum stabilising doesn't fill larger cracks or voids inside timber, it only fills voids at the microscopic scale (ie cellular level). From a turning perspective, if you hit an internal void not filled in the casting process, you can refill with more resin or something else
No, the pressure only affects the densityNeil____________________________________________Every day presents an opportunity to learn something new
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