Foam is an amazing prototyping tool - you don’t have to go far out of your way to create models with realistic-looking finishes to convey a concept. For this project, a foam CNC router was used to cut out a shape of a candleholder which was coated with filler, Gesso primer, and spray paint. (This was only a prototype for display - foam is highly flammable and should not be used near an open flame.)

Materials
Insulation foam
Access to 2Bot (foam CNC)
Sand paper (course or fine depending on the step size of the CNC)
3M water-based contact glue
Filler
Gesso
Paint brushes and/or sponges
Spray paint (stone texture)

Step 1: Solidworks Model to CNC

The Solidworks model (in stl format) is cut out of the 2 inch thick insulation foam using the CNC router (in 2 halves) using a step size of 0.03 inches. For the purposes of model making – foam can be shaped in various other ways (by hand, hot wire, etc).

Step 2: Sanding and Gluing

The two parts of the model are sanded slightly to remove the CNC “steps” from the surface. They are then glued together using 3M water-based contact glue. The glue is applied (using either a spray gun or sponge brush) to both sides, allowed to dry, and only then the two parts are attached to each other.

The model is sanded again to make sure the two parts fit together seamlessly (the CNC router was bumped when the foam was flipped over to cut the opposite side, so one side came out much thinner while the other much thicker. The sanding allows for a smooth transition between the two).

Step 3: Filler

Next, 2-3 coats of filler (with drying time of about 30 min in between) are applied to make sure that the foam is completely covered and sealed with filler. Oil based spray paints eat away at foam so any pinhole could potentially destroy your model if you’re using those paints. The filler can be applied using a sponge brush or your fingers. The filled model is sanded in between coats to create a smooth surface for the Gesso.

Step 4: Gesso

The Gesso is applied in 2 coats as well – again with a combination of fingers and paint/sponge brushes. The model is sanded for the last time to create the desired smooth finish.

Step 5: Spray Paint

About 2-3 coats of spray paint are applied in a ventilated spray booth with about a two hour drying time in between. The stone spray paint takes a bit longer to dry than most others so it’s really important to wait for it to dry before applying more.