Thermal printing apparatus with tensionless donor web during printing

Abstract

A thermal printer includes motor control apparatus for variably controlling the rotational speed of a take-up spool used to wind-up an expended dye-bearing donor web after passage through a print zone. The donor web is moved through the print zone at a substantially constant velocity by a rotatably driven print drum which cooperates with a thermal print head to unwind the donor web from a supply spool. The motor control apparatus operates to rotate the take-up spool at a speed inversely proportional to the instantaneous take-up spool diameter, whereby the expended donor web is accummulated at a rate slightly slower than the rate at which it is payed-out of the print zone during the printing operation. Preferably, the desired rate of rotations of the take-up spool, which gradually decreases with the number of prints made, is determined by the use of a shaft encoder mounted on the support shaft of the supply spool. During the production of thermal prints, the output of such shaft encoder is used to control a variable speed motor which appropriately decreases the rotational velocity of the take-up spool to maintain a zero tension in the web between the print zone and take-up spool. By eliminating tension on the donor web during printing, certain tension-produced artifacts in the thermal print are avoided.