A plastic material sheet for use in vacuum thermoforming is formed of polymer material. Electrically conductive resistance elements, at least a portion of which are situated within sheet, conduct electrical current to heat the sheet. The elements are generally parallel to each other. They may be straight, have a generally zigzag configuration, a generally serpentine configuration, an arcuate configuration, or a grid-like configuration. The elements may each have a section that extends substantially beyond the edge of the sheet. They may be frangible. They may be embedded deeply within the sheet or may be situated proximate a surface of the sheet.