Things You'll Need
Foam board -- more commonly called foamcore -- is used for a variety of art projects, from model building to mounting artwork. Just as it is used for art, there is an art to cutting foamcore correctly. Cutting incorrectly results in shredded and bumpy-edged foam board that looks unprofessional. You can always elevate the quality of your artwork with good craft techniques and similarly, you can deflate creative and talented artwork with poor craft techniques.
Prepare your workspace.
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Mark the foam board where you want it cut.
Using a metal ruler, cover the foam board part you want to keep and cut into the part you are going to discard.
Cut into the foam board with a utility knife breaking through the top surface of paper. Do not use too much pressure.
Continue cutting until you are clearly through the foam board. This is the trick: Use multiple cuts with minimal pressure.
While cutting, slice into the foam board at an angle that you are going to retain. This undercutting keeps the top viewable edge as clean as possible.
Do not consider the manufacturer's edge of the foamcore to be true or the corners to be square. Always cut all your edges yourself.
Exercise caution when crafting with utility knives and changing blades.