Airrat,
Thanks for the clarification on those points.
Can you provide me with the name of the Pearl solution you are using. I'll try to dig into it to see what the % of water is in the solution. Typically anything over .1% water in a urethane will cause a visible foam reaction. That doesn't seem like much and it really isn't but as far as the reaction goes, it is a lot.
I'm guessing that the pearlescent solution contains a fairly good percentage of water or is a water base solution. Whatever amount you added to the yellow probably kept the total water content of the yellow below the visible thresh hold in the cured urethane but the larger amount you added in the black pushed you well over the limit of what would be noticed in the finished cast piece. I know the black dye does contain a slightly higher percentage of water than the yellow does which helped it reach the visible point quicker than the yellow as well.
The reason it may not have been as noticeable in the cup was probably due to the thickness that was left in the cup vs the thickness in the mold. The thinner the section, the less noticeable a 100% increase in volume would appear. For instance if you had a coating on the side/bottom of the cup that was .050" and it became .100" that is one thing but to take a 1/2" and make it foam out of the mold to 1" thick, that is something else.
Even though you are getting the pearl in a powder form, make sure to find out the water content %. This info may also appear on the MSDS. But if you have the ability to speak to someone in tech service, I'd highly recommend asking for the % of water in both the powder and the solution. If the solution was water base, see if they have an oil base version of the same thing. That will work much better for you.
JimGo,
Glad to be apart of this forum and we are always glad to help when we can regardless of the material. We're a small company who try to help everyone we run into as if they were family. We've made a lot of friends this way as well as customers. Give us a call anytime, ask for me or talk to someone regarding what you'd like to do. We're not the answer for every application but we'll give you a straight answer up front whether we think we can help you or not.
Thanks,
Mike
Alumilite
800 447-9344