Copying a Profile Using Liquid Urethane Foam
Once mixed, the foam rises very fast, in the order of a few minutes, and then sets to a very hard material in about twenty minutes total. While it is rising, the foam itself exerts very little force on the material constraining it if it is allowed room to rise completely. Constraining the foam as it rises will cause some force on the constraining mold.
The aluminum foil mold I made had very little strength. The slightest pressure could deform it to the point of being useless. Urethane foam is the perfect material to pour into this type of fragile mold. The inside of the mold is lined with Saran Wrap. Nothing sticks to this wrap and it makes a perfect release agent.
|The mold before pouring in liquid foam|
There are a couple of important things to pay attention to when using this substance. Mixing ratio is very important. Use of a measuring cup or scale can greatly improve the consistency of the finished material. Correct mixing technique will assure the finished product is consistent throughout with the entire thing cured to the same density. It's pretty easy to tell when the two parts are mixed sufficiently. When the color of the mixture is uniform and looks sort of like a coffee milkshake it is ready.
|Seconds after the pour|
|About a minute after the pour|
|Five minutes after the pour - Rising complete|
Once the foam reaches the maximum expansion, it starts to harden. At first, it's like marange in that it is soft and gooey. At this point, touching the foam can cause gaping holes to form. It is best to leave it completely alone during the post expansion period until it hardens completely. The foam curing process is exothermic, meaning it generates heat. In large pours, this heat can cause all sorts of problems. It is best to cast this foam in depths not greater than a couple of inches and to let the foam cure and cool before pouring another layer. The next layer will stick nicely to the previous layer without any preparation.
|Foam after removing from the mold|
|Copy compared to the original|
|The finished roll bar copy|