Multicolor thermal printer
This invention relates to a multicolor thermal printer comprising a frame and a carriage that can move along a printing support, the carriage carrying a thermal print head having heating elements and an inking ribbon unwound from a delivery spool and wound on a take-up spool. 
     Thermal printers of this type are known comprising an inking ribbon having zones of various colors, for example black, yellow, cyan and magenta, arranged longitudinally in a predetermined sequence. When it is desired to change color, it is necessary to advance the inking ribbon to the next zone of the desired color. There results a rather long waiting time and a considerable loss of unused ribbon. 
     Other multicolor thermal printers are equipped with an inking ribbon comprising three or four bands of vertically superposed colors. Guide forks in this case make it possible to raise or lower the ribbon to place the band of the desired color opposite the heating elements. Printers of this type also exhibit the drawback of an excessive consumption of inking ribbon. 
     The purpose of this invention is to create a multicolor thermal printer that makes it possible to limit to a minimum the consumption of inking ribbon while assuring a fast change of the color to be printed. For this purpose, the printer according to the invention is characterized in that the carriage consists of two parts, a first part carrying the thermal print head, the second part supporting several independent inking ribbons located opposite the thermal head at different levels of height, the carriage comprising a color selection mechanism that makes it possible to move the second part in relation to the first part so as to place one or the other of the inking ribbons opposite the heating elements. 
    
    
     The accompanying drawing represents diagrammatically and by way of example, an embodiment of the printer that is the object of the invention. 
     FIG. 1 is a top view of the printer. 
     FIG. 2 is a top view of the lower part of the carriage. 
     FIG. 3 is a bottom view of the upper part of the carriage. 
     FIG. 4 represents the carriage in side view along straight line IV--IV of FIG. 1. 
     FIG. 5 is a section along V--V of FIG. 2. 
     FIG. 6 is a section along VI--VI of FIG. 2. 
     FIG. 7 is a front view of the cassette for the black ribbon. 
     FIG. 8 represents a feature in section along VIII--VIII of FIG. 3. 
    
    
     The thermal printer comprises a frame 1 having side walls 1a and 1b, a platen 4 (FIG. 4) and a carriage 2 carrying a head 3 for printing by thermal transfer. Carriage 2 is made up of a lower part 5 and an upper part 6 hinged to lower part 5 along a hinge pin 7 consisting of two bolts 7a and 7b. 
     Lower part 5 is mounted to slide on a stationary cross bar 8 by small wheels 9, 10 and on a universal bar 11 housed in bearings 12 of side walls 1a, 1b of the frame. Carriage 2 is driven by a motor 13 that works with a drive belt 14 fastened by two rivets 16 to the base of carriage 2. This belt 14 is stretched by a belt tensioning roller 15 mounted on side walls 1b. 
     Thermal printing head 3 comprises a vertical row of heating elements 3a (FIG. 4) each connected to a control unit, not shown, that, thanks to a printing signal, makes it possible to have an electric current flow in the heating elements to cause the thermal transfer of ink from an inking ribbon to the paper 20 to be printed at the place considered, under the influence of temperature and pressure. 
     Thermal head 3 is carried by a support part 21 mounted to pivot on lower part 5 of carriage 2 thanks to two bolts 22 (FIGS. 2 and 6). An operating lever 23 is fixed relative the support part 21, and subject to the action of a spring 24. This lever 23 pulls print head 3 in the direction of platen 4. A cam 25 mounted to pivot on one of side walls 5a of the carriage is fixed to a toothed wheel 26 and makes it possible to separate head 3 from platen 4 against the action of spring 24. The rotation of cam 25 is controlled by universal bar 11 on which is mounted to slide a toothed wheel 27 angularly fixed relative bar 11 and meshing with toothed wheel 26. A motor, not shown, fastened to the frame makes it possible to drive universal bar 11 and thus to control the placing of thermal head 3 against platen 4, when printing must be performed, or to separate it in the opposite case (FIG. 6). 
     Upper part 6 of the carriage consists of a support plate carrying four inking ribbons 30 to 33 (FIG. 1). Black inking ribbon 30 is contained in a cassette 44 in which the ribbon is unwound from a superposed upper delivery spool 42 to a lower take-up spool 43 (FIG. 7). This cassette 44 is held on part 6 thanks to a blade spring 29. 
     The three ribbons 31 to 33 corresponding to the colors yellow, cyan and magenta are unwound from delivery spools 34 to 36 and wound on take-up spools 37 to 39. These spools are located on three levels of a height increasing from the front to the rear of carriage upper part or plate 6 (FIG. 4) so that ribbons 30 to 33 are at four different heights on ribbon guides 40 and 41, the black ribbon being located highest. By pivoting plate 6 around shaft 7, the height of the front of the carriage upper plate 6 is varied such that a selected one of ribbons 30 to 33 can be placed opposite the heating elements. For this purpose, the machine comprises a color selection mechanism 45 (FIG. 5) comprising a motor 46 fastened to side wall 5b of the carriage lower part that drives a toothed wheel 49 which, in turn, meshes with a toothed cam 47 mounted on a shaft 48 solid with upper plate 6. Motor 46 is controlled by a central control unit, not shown, and thus makes it possible to determine the choice of colors to be printed by rotation of cam 47, whereby the upper part 6 is selectively pivoted about the axis 7. 
     Drive mechanism 50 for inking ribbons 30 to 33 comprises a cord 51 stretched between the two side walls 1a, 1b of frame 1 thanks to a spring 52. This cord 51 is wound around a roller 53 mounted on plate 6. Roller 53 works with a free wheel device, not shown, with a coaxial toothed wheel 54 to operate a synchronous drive belt 55 when carriage 2 moves from left to right in the direction of writing. Roller 53 freewheels when the carriage moves from right to left. 
     Synchronous belt 55, in turn, works with four toothed drive wheels 56-59 intended to drive take-up spools 37 to 39 and 43. Hubs 60 of these spools fit on drive shafts 61 (FIG. 4) each connected to one of toothed wheels 56-59 by a friction clutch 65. The latter consists of two disks 66, 67 one solid with one of toothed wheels 56-59 and the other with one of drive shafts 61. A spring 68 urges the two disks 66, 67 against one another. The tension of synchronous belt 55 is adjusted thanks to a belt tensioning roller 69. 
     The hubs of the delivery spools are mounted on shafts 70 (FIG. 8) each of which comprises an helical spring 71 one end of which is solid with shaft 70 and the other end of which rests against plate 6. This spring 71 provides a certain braking and return force so that inking ribbons 30 to 33 are constantly stretched. The braking force of springs 71 is greater than the friction force imparted by disk clutches 65. Thus, when print head 3 is disengaged from platen 4, none of ribbons 30 to 33 is moved. On the other hand, when print head 3 presses one of ribbons 30 to 33 against paper 20 and platen 4, the ribbon in question is advanced while carriage 2 moves. The sum of the friction force of clutch 65 and of the force holding the ribbon between head 3 and platen 4 is therefore greater than the braking force applied by helical spring 71. Thanks to this construction, only the ribbon actually used for printing is unwound.