I do galvanic etching on 20 & 22 gauge copper sheets, often 6" x 4" which is bigger than an iron. Toner transfer is one of the etch resists I use. I have had some successes (and some not) using a electric pancake griddle to bring the heatsoak the metal and then use the iron to apply the toner resist. Set the griddle to about 350 and use the cotton setting on the iron.
Some tips:
- Use a paper towel between the griddle and the metal to keep the metal from slipping around. It won't burn for the temps used.
- Prevent bubbles forming in the paper during the ironing process by running the metal & paper pattern through a laminator several times first then do the griddle & iron method.
- burnishing with the iron is important to ensure adhesion of the toner. The laminator helps but its just not enough adhesion for my etching process.
- If you get it too hot the toner will slip and smear. :-(
- I use the laminator recommended by pcbfx.com from several years ago. Just noticed they have a different recommendation these days.http://www.pcbfx.com/main_site/pages/start_here/laminator_info.html
Good luck!