Deck 4: DIY Heat Bending Composite/PVC Deck Boards

My plans call for some elaborate, curved trim pieces that really defined the style of the deck. There’s some commercial systems for bending boards, but I don’t have access to them, so I made my own strip heater. The boards I’ll be bending are actually PVC, not composite, because I think they’ll bend a bit easier.
This strip heater is super dangerous, and I don’t recommend building it to anyone.

circuit-diagram

For the heater circuit I used nichrome wire connected to normal AC lighting dimmers, directly hooked up to mains current. The dimmers are only rated to 500W each (and get hot quickly when overloaded) so I used 3, with the intent of being able to use the ~1500W available from a normal plug circuit. As it turned out, I think one would have been more than enough, assuming the nichrome had high enough resistance.

There were some unexpected difficulties, for instance the wires expanded enough from the heating that adjacent wires could touch each other, and shorts between wires on different dimmers became dangerously hot very quickly.

Board Heater

The heater was composed of the nichrome wires sewed to a fiberglass strip with nomex thread. The fiberglass strip was stretched in a frame made from scrap 2x4s, and normal fiberglass insulation was used top and bottom. I laid the first board directly on the wires, but (predictably) the wires melted into the board a bit, so after that I used some plywood spacers between the wires and the board.

A digital oven thermometer provided temperature readout – this was a bit problematic because the thermometer’s metal probe sheath could easily short the heater wires.

Bending Form

Once the boards reached 120-130°C, they were transferred to a plywood form to cool. This was a delicate balancing act: too cool and the boards wouldn’t bend to the form, but too warm and the boards would get imprints from the clamps and form pieces.