You can opt to use a special tool such as a hot knife or dremel tool to cut curves in foam core, but with a sharp razor knife and a slow, steady hand, you can do the job just as well. There are times when it is necessary to cut a perfect curve or other nonlinear shape into foam board. When the need arises, the best tool you have available is a razor knife. Beyond that, patience and steadiness are required.
Trace the Curved Line
Cutting a curve starts with accurately drawing the line you want to cut out. If it needs to be a perfect curve, a stencil or some other guide is the best way to draw it. After the line is drawn, you will score it and cut through the foam board.
Scoring and Cutting
Very steadily, trace the curved line you drew in pencil with the razor knife. Move confidently over the line so your hand does not waver, but don’t try to go too fast. Your goal for the first pass is to lightly score the upper layer of paper. Go over the line a second time. It will be easier because you have cut a slight notch into the paper. On the second pass, completely score the paper. The third cut should penetrate through the foam core. If you go slowly, the blade will remain along the line. Finally, the fourth cut is meant to score the bottom layer of paper and complete the curve.
After making the cut, it may help to lightly sand down any rough edges so the curve is perfectly smooth.