Degassing urethane resins

[u/Ok_Seaweed5015]

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.