The principle of color development of molten transparent thermal paper is to coat a layer of opaque wax as the surface layer on the black base paper. After the action of the hot pen, the opaque waxy material is melted and transparent, and the color of the bottom layer of the base paper is revealed. image. The color development principle of metal compound color thermal paper is based on the mixing of heavy metal salts of fatty acids and reducing agents. The fatty acid salts are melted and reacted with reducing agents by heating and heating, so that the heavy metals are reduced and freed to develop color. The color development principle of leuco dye-type thermal paper is based on the mixing of a leuco dye containing an internal alicyclic ring with a color developer. Under the action of heat, the color developer and the leuco dye are heated and melted to react and develop color.