Re; Air bubbles in polyurethane castings

From: markschanze (
Date: Fri Jul 24 1998 - 08:07:03 EEST

To eliminate all air bubbles in urethane, epoxy, silicone, and acrylic
resins (or anything) cast or injected into silicone molds, these rules
Slow cure systems (12 to 14 hour or longer cure times) = 30 psi minimum of
pressurized atmosphere.
Fast cure systems ( less than 4 hours cure time) = 80 psi minimum of
pressurized atmosphere.
Yes I'm talking about sticking your resin filled silicone mold in an
appropriately rated pressure pot (like used for painting and don't blow
your head off) for the duration of the cure.
In slow cure resin systems this will actually drive the air from the resin
and into the silicone mold where it dissipates into the pressurized
atmosphere. In fast cure resin systems the 80 psi is the minimum amount of
pressure required to overcome the thermal expansion of the entrapped air
due to the 150 to 190 + degree exotherm associated with crosslinking
thermoset polymers. Fast cure urethanes become rigid too fast and do not
allow the rules of general physics to eliminate the air bubbles from the
system. So you have to out pressure them.
Remember when using fast urethanes the heat and pressure of the cure will
effect the dimensional outcome of the part. Cure the mold at the
temperature of the exotherming resin and at the appropriate pressure. If
you are chasing + or - .002 tolerances like most of us.
Any questions?
Mark Schanze
Molding Technologies Manager
CIVCO Medical Instruments
319 656 4447

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