Problem with slush casting.
- Thread starter Tildane
- Start date
robstyle
Master Member
Some materials dont like to bond to itself due to degassing, oils/surface finish or science thermo stuff. Ive seen in some cases a slush coat done thick, allowed time to cure then had the surface scuffed up to better hold a second thicker layer sandwhiched with fiberglass. Not saying thats best for longevity but in a pinch, it would may get a job done. Key word is "job", not for the collector.
Hand slush casting can be very successful with molds/ parts shaped like yours.
I have never heard of casting urethane's referred to as "isocyanate A and B" The Isocyanate side is usually only one side, the "A" component. (Unless in Asia, where I understand the convention is the "B" labeled side.)
If you plan on multiple layers with urethane's, it is a good practice to add addition layers while the layer you are bonding too is still setting up. Not liquid, but not fully cured. You can also use micro balloons or cabasil to thicken additional layers to reduce the number of layers needed to make a suitably thick casting.
There are a number of reasons you can suffer the problems you note:
Could be the urethane. As noted above, some urethane are not suitable for slush casting. I don't know if anyone still makes it, but there was a "model making" urethane that it's components were water thin and it could be be poured into tiny, detailed molds with minimal air bubbles. But the material was crap for anything else, as it was brittle and had a very high shrinkage. This material was the same color as the pieces in you'er pictures.
It is very important to fully, completely, mix, as in, longer than you think...mix the components before you proportion them out. Some urethane's are not homogenized and the chemical components can separate in the can in only a few hours. So making sure they are fully combined before measuring out you components is important. One of the effects of not properly mixing before measuring, is the more material you use, the more "lopsided" the material becomes, as you pour off the chemical that rises to the surface of the can of either your "A" or "B".
Making sure the two sides are thoroughly mixed together. If you have even small amounts of the "A" or "B" not blended when you pour, these will give you "oily" spots in the casting. Also, Do Not wipe the stir stick off on the edge of the cup. The stick can easily trap unmixed component on it's surface, and when you wipe the stick off, this unmixed material will cause the issues noted above.
Cleanliness! Do not use old cups that you have already mixed in. Reusing the storage cup of "A" side or "B" side is OK, but only for a few hours, after that replace it with a new cup. Do not use old stir sticks. Make sure your work area is clean, make sure you component cans / bottles are clean and the lids seal. Not just screwed on OK, but the lids are free of crap and seal. Oxygen and moisture from the atmosphere will ruin urethanes.
I notice you are using ribbed plastic cups, I'd wouldn't use this style of cup. The ridges can easily trap unmixed components and cause the oiling unmixed material problem. We use waxed paper cups. Some people will tell you waxed cups a re bad, but for nearly 40 years of casting materials, the wax is never been an issue. But, it does keep the cup from getting "soaked" and leaking material if left on the counter.
Good Luck
I have never heard of casting urethane's referred to as "isocyanate A and B" The Isocyanate side is usually only one side, the "A" component. (Unless in Asia, where I understand the convention is the "B" labeled side.)
If you plan on multiple layers with urethane's, it is a good practice to add addition layers while the layer you are bonding too is still setting up. Not liquid, but not fully cured. You can also use micro balloons or cabasil to thicken additional layers to reduce the number of layers needed to make a suitably thick casting.
There are a number of reasons you can suffer the problems you note:
Could be the urethane. As noted above, some urethane are not suitable for slush casting. I don't know if anyone still makes it, but there was a "model making" urethane that it's components were water thin and it could be be poured into tiny, detailed molds with minimal air bubbles. But the material was crap for anything else, as it was brittle and had a very high shrinkage. This material was the same color as the pieces in you'er pictures.
It is very important to fully, completely, mix, as in, longer than you think...mix the components before you proportion them out. Some urethane's are not homogenized and the chemical components can separate in the can in only a few hours. So making sure they are fully combined before measuring out you components is important. One of the effects of not properly mixing before measuring, is the more material you use, the more "lopsided" the material becomes, as you pour off the chemical that rises to the surface of the can of either your "A" or "B".
Making sure the two sides are thoroughly mixed together. If you have even small amounts of the "A" or "B" not blended when you pour, these will give you "oily" spots in the casting. Also, Do Not wipe the stir stick off on the edge of the cup. The stick can easily trap unmixed component on it's surface, and when you wipe the stick off, this unmixed material will cause the issues noted above.
Cleanliness! Do not use old cups that you have already mixed in. Reusing the storage cup of "A" side or "B" side is OK, but only for a few hours, after that replace it with a new cup. Do not use old stir sticks. Make sure your work area is clean, make sure you component cans / bottles are clean and the lids seal. Not just screwed on OK, but the lids are free of crap and seal. Oxygen and moisture from the atmosphere will ruin urethanes.
I notice you are using ribbed plastic cups, I'd wouldn't use this style of cup. The ridges can easily trap unmixed components and cause the oiling unmixed material problem. We use waxed paper cups. Some people will tell you waxed cups a re bad, but for nearly 40 years of casting materials, the wax is never been an issue. But, it does keep the cup from getting "soaked" and leaking material if left on the counter.
Good Luck
