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Thread: Resin shrinkage
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13th August 2020, 09:22 PM #1Senior Member
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Resin shrinkage
I have been clear casting for some time using a pressure pot to reduce the risk of air bubbles. I am using a clear casting resin (30ml resin/10 drops of hardener) as recommended by the manufacturer and pressurise to 58 PSI. I am using tube in silicone moulds. The shrinkage exposes the ends of the tube when set. I fill the moulds to the maximum level. Just wondering if the pressure is too high which may be causing the shrinkage. I would appreciate any advice which will assist in overcoming this problem.
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13th August 2020, 10:26 PM #2Senior Member
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14th August 2020, 01:18 AM #3GOLD MEMBER
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If there is minimal room between the end of the tube and the moulds when you pour, the high pressure collapses the silicone to some extent, exposing the tube. Just for grins try casting a single blank at just 10 or 15 PSI and see if you still have the issue. If there are no bubbles as well then you don't need to use such a high pressure. If a few increase the pressure a little. Another "trick" to try to reduce the bubbles is to warm the resin, add the MEKP, and gently stir the resin to mix. The less air you mix in the fewer bubbles you have to deal with.
RSD when you use vacuum on the resin it can boil off some of the volatile compounds changing the properties of the resin. It also makes the bubbles expand and if there isn't sufficient time for the bubbles to float away before the resin starts to set and cure you'll have more and larger bubbles. Pressurizing keeps the bubbles in solution so the don't form. Much like a diver having nitrogen bubbles in the blood. Great as long as they are deep but take the pressure off with a rapid assent and the bubbles expand and they have the bends.
Pete
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14th August 2020, 10:32 AM #4
You are using Polyester resin that shrinks. Not much you can do with that resin, but you could use an Epoxy or Polyurethane resin instead, that have far less shrinkage and not as brittle too
Neil____________________________________________Every day presents an opportunity to learn something new
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14th August 2020, 10:38 AM #5
Because casting uses pressure not vacuum. Vacuum is used for stabilising. Yes vacuum can be used to remove bubbles, but timber (for hybrid blanks) is full of air and takes ages to get removed, so in vacuum the resin goes off long before the bubbles are removed. In pure resin, it does work, but the effective temperature is a lot higher so can cause temperature issues (ie cracking and heat can actually cause bubbles). Lastly the OP is casting with a tube in, so vacuum can draw the air from inside the tube, into the resin.
Neil____________________________________________Every day presents an opportunity to learn something new
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14th August 2020, 11:33 AM #6Senior Member
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I'm horribly confused now! On youtube I often see them forming bowl blanks of wood and epoxy - but after they pour the epoxy into the mould with wood in it they put it into a vacuum pot for curing - but are you saying that they should be pressurising it instead?
If you had access to a large vacuum chamber and a large pressure chamber would you put a river table in the vacuum chamber or the pressure chamber?
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14th August 2020, 11:38 AM #7SENIOR MEMBER
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Most guys that make large resin tables will remove bubbles prior to the pour, so either vac or pressure will work. Once poured they'll just hit any surface bubbles with a torch.
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15th August 2020, 07:21 PM #8
Removing bubbles from resin for a river table before casting is a waste of time. The majority of bubbles in a river table came from the wood, too much heat, or whilst pouring/mixing the resin. As river tables don't fit in a pressure pot, best way is to ensure timber is sealed, resin is warm but a low exotherm so it doesn't get too hot, hit the poured resin with a heat blower and/or misting spray of metho that will pop bubbles.
Hybrid blanks are best cast under pressure NOT vacuum. As I previously said, wood contains a lot of air, so under vacuum you would end up with honeycomb resin under vacuum. When you say you've seen cast blanks put into pot, they are usually referring to a pressure pot, not a vacuum chamber. Pressure pots are solid and run off a compressor, whereas vacuum chambers are usually clear run with vacuum (for stabilising using a stabilising resin like Cactus Juice).
River tables are not cast under either vacuum nor pressure as they are just too large to fit. The pressures involved with casting and stabilisation (pressure and vacuum) need to be very strong and are not cheap, if they fail, serious injury can occur even death (pressure vessels go off like a bomb)Neil____________________________________________Every day presents an opportunity to learn something new
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21st August 2020, 07:09 PM #9SENIOR MEMBER
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