Apparatus for priniting graphic images on sheet material having an ink web cassette with constant web tension

摘要

An apparatus (10) for printing graphic images on sheet material (S) has a thermal print head (30) with a linear array of heating elements pressed into engagement with an ink web (W) overlying the sheet material on a platen (32). The platen (32) extends in the lengthwise direction of the print head (30) across the width of the sheet material (S), and is rotatably driven to in turn drive the sheet material with the ink web (W) relative to the print head. A removable cassette (34) having a predetermined length (L) of ink web (W) bearing a printing ink is mounted adjacent to the print head (30) with the ink web interposed between the sheet material and the print head for printing the graphic images on the sheet. A supply spool (36) carrying the ink web (W) is rotatably mounted in the cassette (34), and a take-up spool (38) is also rotatably mounted in the cassette for receiving the ink web (W) from the supply spool upon passage between the platen and print head. The take-up spool (38) defines a first overall diameter (D1) without receiving the ink web (W) from the supply spool (36), and a greater second overall diameter (D2) upon receiving the predetermined length (L) of ink web, wherein the second overall diameter (D2) is no more than 10% greater than the first overall diameter (D1). A spring-wrapped clutch (42) is drivingly connected between a take-up motor (40) and the take-up spool (38) to impart a constant torque ( tau C) to the take-up spool (38) for winding the ink web (W) onto the spool during printing operations and to thereby maintain a substantially constant tension (Tw) within the ink web.