An electric carving knife — the kitchen appliance sold for slicing roast meat — is the standard tool for cutting foamFoam is the structural material most soft puppets are built ... More blocks in puppet building, and it is dramatically better at it than anything sold for craft.
Why it works so well
Two serrated blades reciprocating against each other cut foam without compressing it. Every static blade pushes foam ahead of itself, the material springs back after the blade passes, and the result is a wavy cut on what feltFelt is a non-woven fabric made by matting fibres together u... More like a straight stroke. A reciprocating blade removes material continuously and leaves a genuinely flat face.
What it is for
Bulk removal from blocks and laminated stacks. Slicing sheet to thickness. Rough-shaping a skull before finer work. Anything where you are removing a lot of foam quickly.
What it is not for
Detail. The blades are long and the cut is coarse. Features, bevels and fine contour work belong to scissorsScissors and shears are the primary cutting tools in puppet ... More, a long sharp blade, or sanding.
Technique
Let the blade do the work and move it slowly. Pushing forces the foam to compress ahead of the teeth, which is the exact problem the tool exists to avoid. Draw a cutting line first — the blade wanders easily on a long stroke.
Buy a dedicated one
They are inexpensive secondhand, and foam dulls blades. Sharing one with a kitchen is a bad idea in both directions.
The alternative
for most cuts is a long serrated bread knife, which costs less and is more controllable on curves, at the price of more effort.
The hack: cut oversized and refine by sanding. The knife is for getting close fast, not for finishing.