How to Make a Hot Knife Cut Foam

Foam is often used to insulate roofs and to fill minor cracks in external walls.

Foam is a type of insulating material made from polyurethane. When using foam to insulate a roof or inner wall, you may need to measure and cut the foam to fit your residential specifications. In order to cut your foam, you will need to use a heated knife. Foam melts when heat is applied to it, and the application of a heated carving knife will cut through the material accordingly. The task is made a little tricky because the melting action ensures that the knife cuts through the foam very quickly, just like a knife does through warm butter, and so there is little room for error when working with such a potentially delicate substance.

Things You'll Need

• Kettle (brought to boil)
• Bucket boiled water
• Sharp craft knife
• Foam material
• Black marker pen
• Measuring tape

Instructions

1. Place at least 3 inches of water in your kettle and bring it to the boil. If you do not have a kettle appliance you can simply boil the water in a pan, but be sure to boil enough water so that you can heat the tip of your cutting knife with ease.

2. Heat the knife tip. Once your water has begun to simmer, rest the craft knife so that the tip is in the boiling water, and leave it there for approximately five minutes to heat up to the maximum temperature. Do not dunk the whole knife in the water, because the handle will become too hot to cut with, even if the handle of the knife is made of wood, plastic, or another non-conductor.

3. Measure your foam material for cutting. While the knife tip is heating up, measure out where you plan to cut your foam material. If the insulating foam is not going to be visible, you can mark your measurements using a marker pen for a greater level of accuracy. If you are using spray foam or are planning to cut spray foam, it may be easier to just use your eyes to guide where to cut.

4. Cut the foam. Take your heated knife out of the water. You can dry the tip if you wish to keep the foam dry, but the drying action will only conduct the heat away from the knife tip, so it may be better to simply switch off the heated pan of water and take your knife directly to the foam layer. The knife, if sufficiently heated, will cut through the foam like a knife through butter. Be careful not to accidentally cut carpet or scratch the surface on which the foam has been laid. Once the foam has been cut to your liking, leave the knife to cool on a breadboard and return to the drawer. Alternatively, if your knife cools down and ceases to cut through the foam, simply reheat it again and repeat.

References

• www.horrorseek.com: Making Foam Tools
• Crafters Hot Knife.com: The Crafter's Hot Knife
• Envirofoam Insulation.com: Why foam is better