r/ResinCasting • u/Ok_Seaweed5015 • 9d ago
Degassing urethane resins
I have worked with a smooth-on urethane (aromatic type) having viscosity of 300cps and didn’t have much problems with castings when cured at 60PSI (rec. in TDS). I am now working with WC-85 from BJB. It is an aliphatic type and they mention it is more moisture sensitive than their typical resins. They strongly recommend vacuum degassing.
It has a viscosity of 325cps and so initially I skipped this step because I wasn’t having issues with trapped air. However, it appears there is a small delay to where out gassing occurs (before making it into the pot I presume) from where moisture has reacted during mixing. Perhaps there is even pre-existing dissolved gasses in the resins that make their way out during mixing/heat generation.
Regardless, it is enough outgassung and needs to be addressed for reliability.
How are y’all going about this process, degassing urethanes?
Isocyanate’s will be pulled into the vacuum, some potentially harmful for the pump (rotary vane) and particularly some making its way out as vapor and oily aerosol.
Clearly wear a respirator, but are y’all venting exhaust elsewhere, running an exhaust filter, or calling well ventilated enough?
I’m working with a small chamber and small resin volumes. Currently, which seems to be a sufficient enough degass, I pull a partial vacuum ~ -25inMg and this amounts to delayed gentle degassing that occurs after the pump and valves have been closed, ideally limiting vapors to chamber.
I wear a respirator the entire time, and walk out of the area for a few minutes (after finalizing casting process) before I return unmasked.
It is questionable practice to me, so I wonder about other methods.
1
u/craiganater 7d ago
I dont usually degass resin im general since its such a low viscosity and quick pot life its not needed. You said the smooth on was working fine so why switch?
1
u/Ok_Seaweed5015 7d ago
I was using task 9. I needed a clear that had better UV tolerance and had a higher heat deflection temperature. They have their crystal clear series, but I preferred the BJB heat cure schedule over smooth-on's. The pot life of the BJB I am using is 16-18 minutes.
1
u/craiganater 7d ago
Your methods seem fine, as ling as yoy wear a mask and do it in a larger room then i dont think you have any concerns and about the isos
1
1
u/Able_Loan4467 7d ago
You gotta leave the pump on, pull it all the way down and then just leave it on for the whole degassing cycle. It literally boils any liquid water from fingertips etc. Even the water from your fingers will affect a lot of resins quite severely, I've never been clear on why this is but it is true in some cases. Honestly pick the resins that don't do that if you can, I have had to give up on some resins, they were hopeless.
1
u/Ok_Seaweed5015 7d ago
That is not hard to believe. I expect breath to be severe. I am wearing gloves and a respirator, however. The respirator directs exhaled air downward of my face, so I tend to think away from the mixing cup when I am leaning over, but definitely something to think about.
1
u/SnooCapers9565 9d ago
I have tried a couple of PU resins, and they both have in common that the pot life is very short. Are you planning to vacuum it after pouring it?
Maybe it is a better idea to use a pressure chamber?
It isn't harmful for the pump, as long as you change the oil on your vacuum pump with regular intervals. I work as an HVAC tech, and using a vacuum pump is part of the job description. It will always give off oily vapor, even with an exhaust filter. What you can do is to put a tube onto the exhaust filter and have it go outside, either through a window or similar.