This invention relates to a thermal printer for printing letters and/or graphics on paper by selectively energizing a set of heat emitting elements of a thermal head, thereby melting the ink of an ink ribbon on the spots pressed by the energized heat emitting elements. More particularly it relates to a thermal printer that can selectively and reliably perform printing using a hot release or cold release type of ink ribbon depending on the type of ink ribbon cassette used.
To begin with, the overall configuration of a conventional thermal printer of the above mentioned type will be briefly described by referring to FIG. 5. A long cylindrical platen 3 is horizontally and rotatably arranged between the two side walls 2A, 2B of frame 1 of the thermal printer for holding a printing paper (not shown) wound around it. A long carriage shaft 4 is arranged parallel with and in front of said cylindrical platen 3 between said two side walls 2A, 2B. A carriage 5 is supported by said carriage shaft 4 in such a manner that it can be moved back and forth along said platen 3 by a drive means such as motor (not shown). A thermal head 6 is mounted on said carriage adjacent to the platen 3 so that it can be moved into contact with and away from the platen 3 . A plurality of heat emitting elements (not shown) are arranged on the side of the thermal head 6 that faces the platen 3. A ribbon cassette 7 housing an ink ribbon is removably mounted on said carriage 5 in such a manner that an exposed portion of the ink ribbon and the printing paper wound around the platen (not shown) are tightly held between the platen 3 and the thermal head 6 when the thermal head 6 is pressed against the platen 3.
To print letters and/or graphics using a thermal printer having a configuration as described above, the thermal head 6 is pressed against the platen 3 with the exposed portion of the ink ribbon and a paper on the platen pressed tightly therebetween. Then a set of heat emitting elements of the thermal head 6 are selectively energized according to data provided for the printing operation. This melts the ink in the ink ribbon at spots corresponding to the energized heat emitting elements and transfers molten ink onto the paper as the carriage 5 is being moved across the paper by a drive means (not shown). While there are a variety of printing papers available for thermal printing, OHP (overhead projection) papers, or light transmitting plastic sheets to be used for the purpose of overhead projection (OHP), require a type of ink ribbon which is different from the type of ink ribbon good for popularly available normal printing papers.
FIG. 6(A) shows the construction of an ink ribbon designed for use with normal printing papers. It is a multilayered structure comprised of a substrate 10 of a film of resin material such as polyethyleneterephthalate, an ink layer 12 of a mixture of carbon and resin carried by said substrate 10, and a overcoat layer 13 of a highly viscous material such as polyamide formed on said ink layer 12. FIG. 6(B) shows the construction of another ink ribbon which is also good for normal printing papers. This ribbon has a release layer 11 formed between the substrate 10 and the ink layer 12. The release layer 11 is made of a wax having a low melting point. For printing letters and/or graphics on a normal printing paper using an ink ribbon of the type of either FIG. 6(A) or FIG. 6(B), a set of heat emitting elements selected for a given letter or graphic are energized and pressed against the ribbon and the paper to melt and transfer ink from corresponding spots of the ink layer 12 onto the paper as thermal head 6 moves across the paper. The ink ribbon is released from the paper while the ink layer 12 is still hot. The force bonding the ink layer 12 to the substrate 10 is relatively small in these types of structures. This operation is called hot release mode printing.
FIG. 6(C) shows the construction of an ink ribbon for use with OHP papers. It is comprised of a substrate 10 of a resin film and an ink layer 12A of a mixture of carbon and wax formed on the substrate 10.
For printing on an OHP printing paper using an ink ribbon of the type illustrated in FIG. 6(C), a set of heat emitting elements are energized and pressed against the ribbon and paper to melt and transfer the ink from corresponding spots of the ink layer 12A onto the paper. The ink ribbon is released from the paper after the ink layer 12A has been cooled and solidified. The force bonding the ink layer 12A to the substrate 10 is relatively large so that the OHP paper has a smooth surface after printing. This operation is called cold release mode printing. Since the mode for releasing an ink ribbon from normal printing paper is different than the mode used for OHP printing paper, a conventional thermal printer designed to be used with both normal printing paper and OHP printing paper is normally is equipped with a specifically devised arrangement as illustrated in FIG. 7. Printing only occurs as the carriage 5 and thermal print head 6 move from left to right, as shown in FIG. 7. The arrangement includes a release arm 20 rotatably supported by the carriage 5. The release arm 20 is held in a fixed position on the left side of (behind) the thermal head 6 and moves with the carriage 5, as the carriage 5 and the print head 6 are being moved from left to right for printing. In this arrangement the release arm 20 can be selectively switched by a drive means (not shown) to one of two positions, either to the position indicated by solid lines or to the position indicated by dashed lines. A release pin 21 is provided projecting from a front end of the release arm 20 in such a manner that the ink ribbon 14 passes around the release pin 21 at a position to the left of the thermal head 6, as printing is occurring from left to right.
When said release arm 20 takes the solid line position (FIG. 7), the release pin 21 is located close to the platen 3, so that during printing the ink ribbon 14 moves toward and around the pin 21 which retards the release of the ink ribbon 14 from the printing paper until the spots of the ink layer 12A of the ink ribbon 14 made molten by the energized heat emitting elements can cool and solidify, thereby performing a cold release mode printing operation. On the other hand, when said release arm 20 takes the dashed line position (FIG. 7), the release pin 21 is located within the outline of the ribbon cassette 7 so that the release pin 21 is not touching the ink ribbon 14. In this position the ink ribbon 14 is immediately released from the printing paper as soon as it passes the thermal head 6. As soon as the ink ribbon moves past the thermal head and while the molten area of the ink layer 12A of the ink ribbon 14 still remains in the molten state the ribbon lifts from the printing paper, thereby performing a hot release mode printing operation.
With a conventional thermal printer as described above, referring to FIG. 7, an operator has to look to see if the ribbon cassette 7 mounted on the carriage 5 contains an ink ribbon for normal printing paper or if it contains one for OHP printing paper and, if necessary, has to operate a switch (not shown) to bring the release arm 20 to the cold release mode position or the hot release mode position, whichever is appropriate. However, selection of a release mode for the ink ribbon 14 based on the observation and judgment of the operator or operation of a switch for switching the release arm to the cold release mode or hot release mode positions when conducted by the operator has a high potential for errors. Errors in observation and misjudgment of the type of ink ribbon 14 in position and errors in switching the release arm to the appropriate position for the applicable release mode can result in poor quality printing or unsuccessful printing. If an ink ribbon 14 intended for use with ordinary printing paper is used with OHP printing paper in the hot release mode, the ink after being transferred onto the printing paper presents a surface which bears undulations that give rise to irregular reflection of light, which can hinder the uniform coloring of the OHP printing paper. If ink ribbon 14 intended for use with ordinary printing paper is used with OHP printing paper in the cold release mode, the ink is not transferred onto the printing paper, thereby making the printing operation unsuccessful because the affinity between the substrate 10 and the ink layer 12 is very large in a hot release mode ink ribbon.
If, on the other hand, a cold release mode ink ribbon intended for use with OHP printing paper is used with ordinary printing paper, the molten ink permeates into the paper producing voids in the printed letters and graphics, making the printing operation unsuccessful.