THERMAL TRANSFER PRINTER

Abstract: In a thermal printer, the sheet material 6 to be printed is feed by endless transport belts 2 which travel at a speed substantially equal to the peripheral speed of a relatively small platen roller 20, the sheet material 6 being thereby transported in contact with the circumference of the platen roller, between it and the transfer medium 8 and printing head 1.

Description: This invention relates to thermal transfer printers, and in particular, to a paper feed mechanism for use in a thermal transfer printer. In a conventional thermal printer the material to be printed, usually paper, is wrapped on a platen roller or drum, which is rotated to carry the paper past a thermal printing head in contact with a transfer medium. A disadvantage of such thermal printers, is that the circumference of the drum must be great enough to accommodate the length of paper to be printed, so that a drum capable of accommodating A3 paper would have to have a diameter of at least 160 mm. Consequently, the printer as a whole is inevitably large. Furthermore, because the surface of the drum is less sharply curved as the diameter is increased, a large drum comes close to the thermal head over a larger area, and as a result, electronic components, such as integrated circuits for drivers, installed on the ceramic substrate surface of the thermal head, may accidentally come into contact with the surface of the paper drum or platen. This in turn made it necessary to increase the area of the ceramic substrate so that such components could be placed in regions well clear of the printing region to avoid contact with the drum, with the disadvantage that the thermal head becomes larger and more expensive. The object of this invention is to provide a paper transport mechanism with a platen of reduced size, not conditioned by the size of the material to be printed, and thereby to reduce the size and cost of the thermal printer. US-A-4388628 describes a thermal transfer printer with a thermal printing head, a feed mechanism for conveying a thermal transfer medium past the printing head, and a transport mechanism for conveying sheet material to be printed past the printing head in contact with the transfer medium, including a platen, at least one flexible elongate driven member, and gripping means attached to the elongate member(s) for gripping the material to be printed and transporting it between the printing head and the platen. Specifically this document discloses a multi-colour thermal transfer recorder in which the output medium is transported by a belt provided with a clip but without practical details. The present invention resides in a thermal transfer printer with a thermal printing head, a feed mechanism for conveying a thermal transfer medium past the printing head, and a transport mechanism for conveying sheet material to be printed past the printing head in contact with the transfer medium, including a platen, at least one flexible elongate driven member, and gripping means attached to the elongate member(s) for gripping the material to be printed and transporting it between the printing head and the platen, characterised in that the transport mechanism comprises a plurality of said elongate members spaced laterally and parallel, and the gripping means comprise a bar extending across and attached to the plural elongate members. Because the platen does not have to be large enough to accommodate the entire length of the sheet to be printed, it can be made relatively small in diameter with a correspondingly pronounced curvature of the surface to enhance clearance between the platen and the printing head, so that the area of the printing head can be reduced by placing ICs closer to the printing region. The length of the flexible transport members for the sheet to be printed must be long enough to accommodate the sheets to be printed; however, away from the platen, the sheet material can be transported along a path of various shapes, possibly serpentine, instead of along the conventional large-diameter circular path, and this path can be adapted to the structure of the printer, enabling the overall size of the printer to be reduced. Preferably, the platen is a driven roller, and the said flexible elongate member(s), is driven at a speed substantially equal to the peripheral speed of the platen roller. Preferably, the or each flexible elongate member is an endless belt or the like, for example a toothed belt similar to a timing belt. The printer may have a single elongate flexible transport member wide enough to accommodate the sheet material to be printed. Alternatively, the printer may have a plurality of relatively narrow flexible elongate transport members spaced laterally and parallel to each other, and the gripping means may comprise respective grippers on the said members, or a gripper bar extending across and attached to more than one of the said members. The invention will be further described with reference to the accompanying drawings, in which: Figure 1 is a schematic side view showing the main parts of a thermal printer with a conventional paper drum, Figure 2 is a side view showing the main parts of a thermal printer embodying the present invention, and Figure 3 is a plan view of the main parts of the thermal printer of Figure 2. Figure 1 is a side view of a transfer section of a conventional thermal printer such as shown in Japanese patent laid open 1983-142887 Koho. In the figure, a paper drum 30 is covered by rubber etc. and provided with clamp 7 at one part. This clamp is configured in such a way that the edge of the material 6 to which ink is to be transferred, that is wound around drum 3, is clamped down. The material 6 to which ink is to be transferred is for example sheet paper, and its length is shorter than the circumference of drum 30. A thermal head 1 for heat transferring the ink of transfer material 8 onto material 6 is pressed against drum 30 by a drive mechnism that is omitted in the drawings. Transfer material 8 is, for example, ink ribbon; many color inks are coated in the case of color transfer printing. After the material 6 to which ink is to be transferred is clamped by clamp 7, first of all heat transfer of ink of the 1st color is carried out by thermal head 1 while the material 6 is being wound onto drum 30. Then, thermal head 1 is released from drum 30, and drum 30 rotates in the reverse direction opposite to that of arrow A and returns to its initial position. At this time, the next color of transfer material 8 is brought to the transfer section of thermal head 1. While thermal head 1 is pressed once again against drum 30, drum 30 is rotated once again in the direction of arrow A. As a result, the next color of transfer material 8 is heat transferred onto sheet material 6. In this way, color printing by heat transfer for a single sheet of material 6 is completed, by three reciprocating actions in the case of three-color printing. A conventional thermal printer that used a drum had problems in that the diameter of the drum had, for example, to be at least 160 mm in a case where A3 size material for thermal transfer was used, and the entire device became large. Furthermore, as the radius of curvature at the part where drum 30 contacts thermal head 1 increases when the diameter of the drum increases, electronic parts such as ICs for drivers that are installed onto ceramic substrate surface on thermal head 1 may sometimes contact the outer surface of the drum. There were problems in that the area of the ceramic substrate had to be increased in order to prevent that, and that the thermal head became expensive. Figures 2 and 3 show, schematically, a thermal transfer printer embodying the present invention. The thermal printer comprises a thermal printing head 1 which in operation generates heat in accordance with an applied image signal. Opposite the printing head is a platen roller 20, effecting contact between sheet material 6 to which ink is to be transferred by the printing head, a web of transfer material 8 fed between the sheet material 6 and the printing head, and the printing head itself. In operation, the transfer material 8 is fed in the direction of the arrow C from a feed roll 8a to a take up roll 8b, and the sheet material 6 is transported in the direction of the arrow B, while the printing head is activated to cause thermal transfer of an image from the transfer medium head to the sheet material 6. The platen 20 is a roller which is rotated in the direction of arrow D corresponding to the transport direction of the sheet material 6, by any suitable drive means. The platen roller serves primarily to ensure intimate contact between the sheet material 6, transfer medium 8 and printing head 1. It is not positively attached to the sheet material 6, and the latter is wrapped on the platen roller only over a fraction of the circumference of the roller. The circumference of the roller is substantially shorter than the maximum length of sheet material to be printed and the roller has a correspondingly small diameter and acutely curved surface. The sheet material 6 is transported past the printing head by a transport mechanism comprising a pair of spaced parallel endless toothed timing belts 2 which pass around and are held taut by three pulleys 3a, 3b, 3c, one of which may be coaxial with the platen 20. Suitable driving means (not shown) are provided for driving the endless flexible belts 2 at identical speeds, in the direction of the arrow B, corresponding to the direction of rotation D of the platen roller. The belts are driven at a linear speed which is substantially equal to the peripheral speed of the platen roller. A gripper bar 7 extends at right angles to the endless belts, and is attached to these so that it is transported by the belts in the direction of the arrow B. As can be seen in Figure 3, the spacing between the endless transport belts is somewhat greater than the length of the platen roller. The belts and platen roller may be driven by separate drive means, controlled to ensure the correct relationship between their respective speeds, or they may be driven by a common drive, through suitable transmission means to ensure the correct relationship between the speeds of the belts and the platen roller. The thermal head is held in contact with the transfer medium, sheet material and platen roller by a suitable drive mechanism, only during the actual thermal transfer printing operation, the printing head being released at all other times. In operation, the sheet material 6 to be printed is fed from below between the platen roller and the thermal head, by a feed mechanism which is omitted from the drawings, and its leading edge is gripped by the gripper bar 7 and thereby attached to the endless transport belts 2. When thermal transfer commences, the transport belts start to travel in the direction of the arrow B, and the platen roller 20 rotates in the direction of the arrow D. The sheet material 6 is thereby fed past the printing head. Simultaneously, the transfer medium 8, in contact with the sheet material 6, travels in the direction of the arrow C between the printing head and the sheet material 6, while ink is transferred from the transfer medium 8 to the sheet material 6 by the action of the thermal printing head, in accordance with the thermal image generated by the printing head, corresponding to the applied signals. When the transfer of ink to the sheet material 6 is completed, the thermal printing head is retracted in the direction of the arrow E, away from the platen roller, thereby releasing the pressure on the transfer medium 8 and the sheet material 6. When printing of the sheet 6 has been completed, it can be released from the gripper 7 and fed out of the printer. In the case of multi-color printing, after printing of the first color on the sheet 6 has been completed, and the thermal printing head has be retracted, the endless transport belts continue to travel in the direction of the arrow B, round the guide pulleys, until they and the sheet material return to their initial positions, at which they stop temporarily. The printing head is then again advanced to press transfer medium and sheet material against the platen roller, with the transfer medium now carrying ink of the second color to be printed, the transport belts and platen roller are again driven, and the sheet material 6 is printed a second time, using the second color ink. The printing operation is repeated as many times as necessary, for example three times for three-color printing, and four times in the case of four-color printing. When printing has been completed, the sheet material 6 is transported away from the platen roller, released from the gripper 7 and discharged from the printer. Preferably, during the actual heat transfer, the peripheral speed of the platen roller is controlled to be slightly faster than the transport speed of the transport belts and hence the speed of the sheet material 6, so that during the thermal transfer operation the sheet material 6 is conveyed mainly by friction with the platen roller.

Claims: 1. A thermal transfer printer with a thermal printing head (1), a feed mechanism (8a, 8b) for conveying a thermal transfer medium (8) past the printing head, and a transport mechanism for conveying sheet material (6) to be printed past the printing head in contact with the transfer medium, including a platen (20), at least one flexible elongate driven member (2), and gripping means (7) attached to the elongate member(s) (2) for gripping the material (6) to be printed and transporting it between the printing head (1) and the platen, characterised in that the transport mechanism comprises a plurality of said elongate members spaced laterally and parallel, and the gripping means comprise a bar extending across and attached to the plural elongate members. 2. A thermal transfer printer as claimed in Claim 1 in which the platen (20) is a driven roller, and the said flexible elongate member (2), is driven at a speed substantially equal to the peripheral speed of the platen roller. 3. A thermal transfer printer a claimed in Claim 2 in which during the thermal transfer the peripheral speed of the platen roller is slightly faster than the speed of the elongate member. 4. A thermal transfer printer as claimed in Claim 1, 2 or 3 in which each flexible elongate member (2) is endless.

Inventors: NARUKI,KENICHI C/O MITSUBISHI DENKI K.K., UNE, RYUZO C/O MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1988
Main classification: B41J13
IPC codes: B41J13, B41J11, B41J2