Re; Air bubbles in polyurethane castings

Date: Wed Jul 29 1998 - 23:01:50 EEST

Dear collegues:
First of all, what can we call bubble-free castings?
In my opinion, if you don't see any bubbles in transparent parts or on the
surface of the opaque parts
after sandblasting, even through a magnifing glass, it's bubble-free.
But if you see bubbles, what could be inside?
     My answer is:
1. Carbon dioxide ( by-product )
2. Air
3. Vacuum
     My explanation is:
1. Balance of chemical reaction in urethanes extremely dependable to amount
of water vapors absorbed
by part B and external pressure, for example:
     a. H2O = 0,0 % --- Urethane set without CO2 --- Atm. pressure
30" Hg
     b. H2O > 0,0 % --- Urethane set with CO2-gas --- Atm. pressure
30" Hg
     c. H2O > 0,0 % --- Urethane set without CO2 --- Ext. pressure
> 10 psi
             ( if you see a lot of tiny bubbles on the surface of casting -
that's it )
2. Air bubbles
     a. entraped into casting;
     b. entraped between urethane and the mold surface ( open bubbles on
the castings )
3. This problem exist in some water clear systems with short working time,
high viscosity and relatively
high volatile content, when you use high vacuum to degasing mix.
     My suggestions for hand casting process in job shop environment:
     1b. Try to use urethanes from another manufacturers. Different tipes
have a different stability of
reaction. In my practice this difference could be as much as 1:100. You can
lose some properties, but eliminate bubbles. Otherwise use pressure vessels or
urethanes in original pakages once.
     1c. In my practice (shortest working time 2.5 min) 30 psi in pressure
vessels is good enough for
95% of all works, when isn't, we use slow version of urethane systems. If you
added a dye and have a problem, try to use x3 concentrate instead of regular.
     2a. To difficult to advice something without details. Solution is
somewhere in the flow control, mold design, temperature of urethanes and so
on. At least, you can reduse air bubbles size, but not eliminate
them in 30 -100 psi range.
     2b. You didn't make a vent in this place.
     3. Try to encrease pressure, if it fails - use static mixer or
centrifuge degasing or try another urethane systems.

     Alex Ivanov, Chemical Engineer
     Dialact Corp.
     45979 Warm Springs Blvd, #2
     Fremont CA 94539
     510-659 7910
     510-659 8464 fax

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