urethane plastic

[BEEJAX]

Hi all,
I'm trying to cast Smooth-cast 321 into 3" diameter x 6" tall round block that are to be machined into final shape later.
The problem is that when the resin is cured it has whit flecks throughout the casting. The resin is an ivory color  with no pigment added and the white flecks ruin it.
I'm pressure casting at 60 PSI so there are no bubbles.  Doing a double mix method. Could it be caused by the heat in such a large pour? Is there some way I can slow the cure?

Thanks for any help

Brian

[beaker]

Are you premixing both the A and the B components well prior to mixing them together?

[BEEJAX]

Yes, I'm mixing A&B thoroughly before putting them together.....Brian

[Vince Spellane]

Do you have any pictures? That may help someone answer your question.   

[beaker]

Where are you located?  Did the products freeze for any length of time before using them?  Usually the not properly premixed components is the only problem we generally see with this resin

[BEEJAX]

One of smooth ons  distributors  did a similar pour in order to help me figure out what is going on. When they cut the piece in half the interior was a different color than the outside, the inside was kind of gray, mine are gray as well but develop the white flakes after a few days. I would be interested in any result you might get by doing a similar pour. If there is a smooth on urethane plastic product that is ivory in color and can be poured in 3x3x6 blocks and have the interior the same ivory color as the outside I would be very interested. # 322 or task 4?

Thank You

Brian

[Ravenson]

Have you called and talked to the Guys at Smooth on.  I have always found their techs to be very good at answering this type of question.  It sounds to me like a curing issue.  It may take much longer than you think for the Urethane to reach full cure because of the thickness.   You might think about post curing,   I would have to look it up but 150 degrees For 4 hours sticks in my head as the right temp and time range.

Jeff

[BEEJAX]

I have talked to their tech reps all across the country,  they are very helpful, problem is they are not familiar with a situation where the matl. is being cut into. I tried post cure  at 150F for 6 hours as they recommended and got the same result. Also tried silicone molds with same result. Thanks......Brian

[beaker]

Brian,
I just sent you an email... I am trying a couple different things for you... as far as what you see, resins cure by exothermic action relative to their mass and exposed surface area.  A pour as big as yours generates a lot of heat in the core of the material which seems to be why it looks great until you cut into it and discover that the heat generated from the curative process has turned the center into a different color.  I am in the process of testing and cutting into various materials to find one that doesn't change color in the center of a casting as thick as that.  Generally speaking, most people have no reason to cut into a casting and are more worried about surface finish / detail... you are the first I have come across doing this but that is what makes this a fascinating business... see how people use these materials.
Regards,
Beaker

[BEEJAX]

Beaker,
Thanks for all the help. Looking forward to see what you come up with.

Brian

[Ravenson]

Well BeeJax I hope when you work it out you will let us know.  I have never run into the issue you describe  and I admit I an interested it how it comes out.

Jeff

[Doug Bridges]

Yeh, let us all know the results Beaker.

Oh, by the way. Thanks for that tip on the squash casting. It worked great.

[BEEJAX]

Well, I finally found a urethane resin that is ivory in color and can be poured into large molds up to 5" diameter x 5" tall with No color variations inside or out! It is made by Aero Marine Products in San
Diego. 3 minute pot and 7-10 minute de mold. Unfortunately Smooth-On would not agree to a refund or a suitable replacement even though they claim the resin is machinable. The Sooth-On techs were Very helpful, but to their own admission when they poured the same resin I had purchased they got the same color variations on the interior of the cured piece.
Thanks for the help.........Brian

[Tassenderma]

With such a large cast, you must find a way to cool it to 160 deg, which is the recommended temp. Have you tried keeping the mold cool, such as running some water on the mold during curing? Try finding a way to monitor the temp and then cool it as needed. The white flecks are temp related separations in the resin, crystalization. 

[BEEJAX]

I am extremely happy with the Aero Marine resin. Even though it gets pretty hot with the 7-10  minute demold time there are NO collor variations. They were also happy to do a large pour into my mold free of charge to insure there were no issues!!! Awesome company!