Copying a Profile Using Liquid Urethane Foam

I needed to copy the basic shape of a part of my car so I could work with it without damaging the original. It's part of the rollbar which is covered in leather. If I had a fancy scanner and 3D printer, I could have easily done this. Instead, I did it the manual way using aluminum foil and liquid urethane foam.

Roll bar
Liquid urethane foam comes in two parts that are mixed together in equal measures. The amount of rise from the foam is dependent on the weight per cubic foot. The range of densities goes from a very light half pound per cubic foot to a very hard and dense sixteen pounds per cubic foot and even more. The more dense the foam, the harder it is and the more abuse it will take to damage it.

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
Be sure to have the mold ready to accept the liquid foam before measuring out the foam parts. Once mixed, it will start rising within a minute. Being prepared will remove the excitement from the process.

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
Foam will stick to virtually anything but it will not stick well to plastic film. The plastic film is a reasonable release agent for things that are not too oddly shaped.

Copy compared to the original
After the foam has cured completely and cooled to room temperature, I checked the copy to the original. The copy had the proper curvature and width. To finish the process, the copy requires some cutting and grinding. I used a multitool to cut away excess material and a die grinder to get the curves right.

The finished roll bar copy
I used four lb. foam for this project. It's still a bit fragile so to beef it up so I can use it, I'll add a couple of layers of fiberglass and epoxy resin. Urethane resin will melt this foam so only use epoxy for finishing.


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