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Degassing polyurethane resin cannot remove myriad of bubbles


Merlin
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Hi,

9cfm pump gets to 28.5inch (sometimes 28.7)  never 29, bought for this purpose, its a good pump.  Javac 231.

7-8 min potlife resin. SG2000L

Resin goes in chamber after mixed B into A, pump on etc , 3 mins elapsed since mix started, I watch loads of bubbles about  2mm dia covering top surface and also filling the drinking beaker down to base, 1inch worth of resin in there. I see thin wisps of something, a gas ? but then I look closer and its gone.

I expect them to rise but they just sit there, some activity of bursting at surface but not much, .

One must not overdo this else resin will boil. you get then loads of bigger bubbles and the resin is getting warmer and sets up with a resin froth !

After a while with nothing happening and not wishing to boil the resin I let air in, so I have resin with lots of small bubbles floating in it, I repeat the degassing and again achieve nothing more than before.

 

THis cannot be used for pouring into mould and I also need to place that into chamber as there are some areas that may trap air I cannot vent easily.

 

what is going on ?

 

room 23 deg humidity 55 my gubbins says. Its a normal UK day, blue sky some grey cloud nuking the sunset, doesnt feel humid as such.

 

I cant seem to degass resin before it sets up, as for time to degass beaker then degass mould as well, impossible. How does one cope with such dual needs ?

 

this is setting up and its not a liquid through which bubbles can rise after 5 mins if not even 4 mins, it has the consistency of a milkshake even at 3 mins.

 

I also need to add in aluminium powder and thats 1:1 by weight at the least, I have tried 1.5:1 for a more aluminium effect and its like paste, impossible to degass, let alone pour, like the remains of choc icing or polyfilla setting on a plate end up with a metal muffin. Yet its sold for adding into ply resins then pouring !

 

Merlin

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I think that resin with this kind of pot life is generally not very suitable for vacuum degassing, it's still not enough time before the reaction sets in. However, your resin may have absorbed moisture, which causes excessive bubble production.

I de-gas only my clear resin which has a pot life of around 15min and doesn't produce much heat on its own.

 

Either mix very carefully and cast without degassing, just into the open mould or use a pressure pot setup.

 

For the aluminum powder, decant and measure your required quantity of the thicker part of the resin first (usually the polyol), mix in the powder, then de-gas just this mix.

Then add the other part, mix carefully and cast.

 

J

 

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  • 3 months later...

I have never had the opportunity to use a vacuum to de-gas, I just pour it and hope but one think I have experienced is bubbling whilst the resin goes off and that is caused by moisture in the resin. I now have my resin cans opened for the minimum time to decant some into a jar and the lid goes back on. I only decant as much as I need. It still bubbles if the resin has been on the bench for any length of time though. I work in centrally heated room so it obviously does not ned much moisture for the resin to do that. This might not apply to you at all but it's all grist to the mill!

 

Simon

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  • 2 weeks later...

Is your resin old? The more times you take the top off the containers, the greater the chance there is of moisture getting into the resin, a squirt of an inert gas into the containers will prevent moisture build up. If your patters have a lot of detail and you are using fast cast resins, you might be better off pressure casting rather that vacuum degassing. You can make a pressure chamber from a cheap pressure cooker, you won't need to degas the resin prior to pouring. Alternativly, try Axson F31 resin - it has a very low viscosity (and a very short pot life so don't try degassing the resin after mixing), short vacuum (not to a full 1 bar) your castings after pouring.

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  • 2 years later...
On 8/15/2018 at 12:17 AM, Merlin said:

One must not overdo this else resin will boil. you get then loads of bigger bubbles and the resin is getting warmer and sets up with a resin froth !

After a while with nothing happening and not wishing to boil the resin I let air in, so I have resin with lots of small bubbles floating in it, I repeat the degassing and again achieve nothing more than before.

 

I think your assumption is wrong: the resin should boil to de-air. At least that is my 25 year experience playing with resins. If your vacuum level is insufficient, nothing useful will happen in your vacuum chamber. I've observed it many times: only when you achieve 1-2 mbar absolute, the resin starts to de-air, and at say 0.5 mbar things are really happening.

 

In my process, I need 1 minute to mix the resin, 1 minute to achieve deep vacuum level, then I let the resin de-air for some 3 minutes, lastly I release the vacuum. At that point I'm 5 minutes into the 7 minutes potlife, enough to take out the moulds and set them nice and square, so the parts are straight after curing. The result: perfect parts without any air bubbles.

 

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Rob

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3 hours ago, Rob de Bie said:

 

I think your assumption is wrong: the resin should boil to de-air. At least that is my 25 year experience playing with resins. If your vacuum level is insufficient, nothing useful will happen in your vacuum chamber. I've observed it many times: only when you achieve 1-2 mbar absolute, the resin starts to de-air, and at say 0.5 mbar things are really happening.

 

In my process, I need 1 minute to mix the resin, 1 minute to achieve deep vacuum level, then I let the resin de-air for some 3 minutes, lastly I release the vacuum. At that point I'm 5 minutes into the 7 minutes potlife, enough to take out the moulds and set them nice and square, so the parts are straight after curing. The result: perfect parts without any air bubbles.

 

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Rob

What resin do you use Rob?

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4 minutes ago, Simon Cornes said:

What resin do you use Rob?

 

Generally Smooth Cast 305. It has a 7 minute potlife, 30 minute demoulding time. Smooth Cast 310 is even slower. But I've used the same process with other resins too, and it works very well as long as the potlife is long enough.

 

Rob

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4 minutes ago, Rob de Bie said:

 

Generally Smooth Cast 305. It has a 7 minute potlife, 30 minute demoulding time. Smooth Cast 310 is even slower. But I've used the same process with other resins too, and it works very well as long as the potlife is long enough.

 

Rob

Where can I get that from?

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On 11/20/2018 at 10:42 PM, Simon Cornes said:

I have never had the opportunity to use a vacuum to de-gas, I just pour it and hope but one think I have experienced is bubbling whilst the resin goes off and that is caused by moisture in the resin. I now have my resin cans opened for the minimum time to decant some into a jar and the lid goes back on. I only decant as much as I need. It still bubbles if the resin has been on the bench for any length of time though. I work in centrally heated room so it obviously does not ned much moisture for the resin to do that. This might not apply to you at all but it's all grist to the mill!

 

This has been my experience too. Older resin, that has been opened multiple times, can have so much absorbed water that it will foam during curing, with the water acting as a blowing agent it seems. I've been able to use it nevertheless, because the vacuum process will boil off most moisture. But basically the resin is at its end at that point, you will have to buy fresh resin.

 

Rob

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1 hour ago, Rob de Bie said:

 

Generally Smooth Cast 305. It has a 7 minute potlife, 30 minute demoulding time. Smooth Cast 310 is even slower. But I've used the same process with other resins too, and it works very well as long as the potlife is long enough.

 

Rob

Hi Rob,

I've got Smooth Cast 305 in my inventory as well, I use it mainly for really small or thin parts. It has one huge disadvantage: it sticks to the moulds and starts to deteriorate them much more quickly than any other resin I use. Even the use of mould release spray (which I don't normally use) helps with that....And this happens with all sorts of RTV brands, even with Smooth-On's on Mold Star range. It's extremely important to de-mould the parts as soon as possible as the longer you leave the cast parts in the mould the more they will stick.

Do you have any experience like that?

Cheers,

J

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9 hours ago, JeffreyK said:

Hi Rob,

I've got Smooth Cast 305 in my inventory as well, I use it mainly for really small or thin parts. It has one huge disadvantage: it sticks to the moulds and starts to deteriorate them much more quickly than any other resin I use. Even the use of mould release spray (which I don't normally use) helps with that....And this happens with all sorts of RTV brands, even with Smooth-On's on Mold Star range. It's extremely important to de-mould the parts as soon as possible as the longer you leave the cast parts in the mould the more they will stick.

Do you have any experience like that?

 

Ha, that's interesting! I never did much series production, but when I did (with 305), I noted that the moulds had a life of 20 castings. After that, a scale-like surface would be visible on the parts, that would get worse and worse.

 

I wasn't expecting more than 30-35 castings, but 20 was less than I expected. I blame(d) it on the long potlife: like you said, the more time the chemically active curing resin is in the mould, the more it suffers. It's the same with epoxy. Therefore I did not blame the 305 perse. But maybe it is to blame?

 

Apart from the scale-like surface, the rubber turns whitish, like the second one on the right. But that is something I've seen for 25 years.

 

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Rob

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14 hours ago, Rob de Bie said:

I think your assumption is wrong: the resin should boil to de-air.

 

Exactly. If you think about that, what are those bubbles that make resin boil? 

I achieved far better results when I increased the depth of the stubs so that the resin does not splash out from forms when it boils and changed the oil in my vacuum pump to achieve lower pressure. I use Symlasta and EasyFlo resins with relatively fast curing time, usually I let it boil for just 10-15 seconds

 

IMG-20210503-190738.jpg

 

 

IMG-20210503-191748.jpg

 

 

 

 

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7 minutes ago, Rob de Bie said:

I found a picture of the scale-like surface texture.

 

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Rob

 

 

I've seen that effect on many a short-run producer, trying to eke out a few more casts before they have to retire the moulds. <_<

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9 minutes ago, Mike said:

I've seen that effect on many a short-run producer, trying to eke out a few more casts before they have to retire the moulds. <_<

 

Yep! I've seen that too. For smooth large parts it can be sanded off, but with small and detailed parts, it's a real problem.

 

It took me a while to understand what's going on. But with a 20-casting limit on each mould, the problem was solved.

 

Rob

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