Upholstery foam may be too thick to cut through all at once with scissors or a utility knife. Whether creating rounded shapes or straight edges, carving knives are the ideal tools for the tasks.
Plotting the Cuts
Before shaping the foam, it helps to have a basic template for your cuts. For mostly straight edges for foam creations such as sofa cushions, measure and cut out a paper or card stock template first. Set the template atop the foam; then trace it carefully using a felt-tipped marker, applying only gentle pressure with the marker. Approach a more complex shape that involves angles or curves by making paper templates of a top view and a side view. Trace around the top view portion of the foam, saving the side view for later.
If the foam shape you wish to create is wider at the bottom or middle than at the top, make a template for the widest portion of the foam instead, so you can carve the narrower areas down to size later. Think of the foam as if it is a block of wood or stone for carving complex shapes.
If cutting two items from the same piece of foam, keep them separated a bit, since you may lose a bit of foam or make a slight cutting error as you slice through the foam.
For square or rectangular foam projects, line your template up with factory-cut straight edges on the foam to reduce the number of cuts you have to make.
Cutting the Foam
Use either a sharp carving knife or an electric carving knife to cut through the foam -- in either case, the knife blade is long enough to cut through the foam with a minimal number of slices. Craft knifes and utility knives also cut foam, but their blades are too short to efficiently cut through thick foam.
Step 1: Make the Straight Cuts
Cut the foam along one of the straight lines you've marked on the foam, if the project has straight lines. If none of the lines are straight, cut along the line as much as possible, cutting slightly outside the line if the area also has notches or sharp curves or angles that can be addressed later. Keep the knife blade vertical while cutting to ensure you aren't carving an angle into the foam. Cut out the basic foam shape with the knife blade held vertically.
If using an electric knife, allow the knife to do the work, guiding it gently and slowly through the foam. If using a regular carving knife, use gentle sawing and slicing motions to cut through the foam without compressing it.
Step 2: Cut Details From the Top
Shape the foam with the knife to create curves and angles as needed while keeping the knife vertical.
Step 3: Tracing the Side Views
Place a template along the side of the foam cutout if your foam project requires additional angles through the depth of the foam. Trace the template with a marker; then cut the foam along the lines. For a design that isn't symmetrical along the side, you may need to repeat the process several times. Save the design with the most cutting or angling for last, since that side will be smaller or narrower than the others.
Step 4: Final Cuts
Trim up the foam as much as possible using the electric or manual carving knife to reach the desired shape. If the shape requires fine details, use the tip of a craft knife or utility knife to make small slices and cuts.