Hi guys. I need a bit of help for one of my projects. I'm building a vacuum forming table for forming and molding sheets of plastic. The table is built, but now I'm having problems with a suitable heat source. I tried using four 250 watt infrared heat lamps to heat up my plastic, but they don't get hot enough.

What I'd like to do is build an oven box that is 2 ft long x 2 ft wide and 1 ft high and use nichrome wire as the heating element. I need the box to reach 350 deg. F. What size diameter wire do I need, how much should I use, and what kind of electronics are involved? I'd like to use AC as the power source. Any advice would be much appreciated, thanks!

 

Stringing bare nichrome is pretty dangerous. You need to attach it to ceramic standoffs, but it sags and carries the line voltage. You would just do an on-off control with a thermostat. You would want a heavy gauge - 18 or so to minimize sag.

I wonder if a big industrial heat gun wouldn't work better. The volume of the box is not that large. We used one to sweat a j pole antenna made from 1 1/2" copper pipe, so they do put out the heat. Just use one to blow hot air into the box.

 

This might sound a bit obvious - but have you considered converting an ordinary kitchen oven for your purpose?

They're about the right size, and have all the wiring and insulation already in there. If you can get a convection oven, you will be stamping out mouldings like nobody's business. Plus, parts will be readily available.

A "roll your own" version will likely eat up a lot more power, and much will be wasted in inefficiency. Not the kind of thing you really want.

 

What worked well for me was to go to my local pottery supply house. They had the appropriate ceramic bits for holding the wire, and they had the wire itself. I used a 400 watt 120V element and an ordinary (500 W) lamp dimmer. (Lamp dimmer is from the hardware store, not the pottery supply house.)

Wrap the element more closely around the edges, and more loosely in the center, or the plastic will heat up too rapidly in the center and not rapidly enough around the edges. My box is 6" deep rather than 12". A sheet of fireplace screen keeps the cat out of it.

As noted, these can be quite dangerous! Never use such a thing around fools, nor allow a fool to use one.

 

Thanks for the advice. It'll be great to get an even heating of my plastic finally.