Sticky urethane blanks,WHY???

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LandfillLumber

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Hello, I did a few casting last night and two came out sticky the other was just fine.I have not changed anything really,same materials,same pigments,same everything really.I don't think its to much pigment as I use about the same every time,HMM.Anyone have an answer???Thank you,Victor
 
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What are you using to cast? If you are using Alumilite, it wasn't comepletely mixed. If it doesn't harden up on its own, put it in a dedicated oven at 200 degrees for a half hour or so and it should harden up.
 
Like Wolftat said, if it is Alumilite, then you do not have it mixed thoroughly or your ratio is off. There is are no other reasons. Use clear plastic cups for mixing and DO NOT ADD ANY DYE until you have it all mixed first. The clear plastic cups will allow you to see when all the streaks are gone, indicating that it is properly mixed. Then add your colorants.
 
Or...if the paint itself is not thoroughly mixed, then some of the chemicals in the paint can rise to the top and not harden because they do not have enough drying agent in them. Stir your paint really good too before using it. Paint will chemically bond with alumilite, but I don't think the actual un-mixed thinners will. I have had that happen. The blank was completely fine all the way through, but some tacky on the top surface from what I believe is my theory above.
 
What type of material were you casting with? And what type of mold? Others have
mentioned Alumilite mixing, but I recently found out that some tin cured silicones
can react with some types of PR, giving you a sticky outer edge.

Specifically, Silmar41 used with a tin cure silicone can come out tacky. US Composites
suggested washing out the mold with denatured alcohol before pouring to help
alleviate this.. and also that using a platinum cure silicone mold will eliminate it.

I found this out after ordering both Silmar41 and TinSil 70-25. They told me that there
might be a compatibility problem.. something about alcohols in the silicone. (I didn't
understand it all) But a post cure in the toaster oven can firm it up.

EDIT .. sorry, I just saw that you said urethane. Helps if I read the whole thing, huh?
But I'll leave the mistake here, since it's still good info.
 
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