Electric heating device

Abstract

The present invention provides a flexible continuous sheet heater having a high uniformity in heat propogation that can replace existing thin-wire and etched foil heaters at a fraction of the cost of the existing devices. It is relatively inexpensive to produce, can be used in a wet or damp environment, has a constant watt density per unit length, and is so designed that the watt density can be varied within wide limits. The heater of the present invention includes a paper or plastic substrate (12) on which is printed a semi-conductor pattern (typically a colloidal graphite ink) having (a) a pair of longitudinal stripes (14) extending parallel to and spaced apart from each other and (b) a plurality of identical bars (18) spaced apart from each other and extending between and electrically connected to the stripes. A metallic conductor (22) (typically copper stripping) overlies each of the longitudinal stripes in face-to-face engagement therewith, and the conductors are held in tight engagement with the stripes by plastic sealing layer (23) that overlies the metallic conductors and the semi-conductor pattern, and stripe associated with the particular metallic conductor, to portions of the substrate that are free from the printed semi-conductor pattern.

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