PRINTHEAD CARTRIDGE

Abstract: A printhead cartridge for a multi-printhead ink jet printer includes an array (3) of ink jet nozzles and a heat sink (9) for controlling the temperature of the nozzles (3). The heat sink (9) is made up of a copper block (10) and thinner copper wing regions (12) extending from the block (10) parallel to the array (3) of nozzles. A passageway (11) for coolant water is formed in the block (10) and extends into the wings (12) as a thinner channel (13). The block (10) and the wing regions (12) are mounted in thermal contact with the array (3) of ink jet nozzles. The heat sink (9) has the advantage that it allows the cartridges in the printer to be stitched together to cover the full width of a substrate.

Description: PRINTHEAD CARTRIDGE The present invention relates to printing and in particular to printing using a multi-printhead ink jet printer formed from a plurality of printheads and which is wide enough to print across the full width of a continuous substrate. A multi-printhead printer may be used in so-called "drop-on-demand" ink jet printing to print on to a continuous substrate, for example to print directly onto packaging in a production line. In this case, the printer may be arranged opposite a transport mechanism for the substrate, such as a part of the production line. Such printing is particularly attractive in the production of packaging because it is possible to package items from the same production run into packaging with a different appearance without stopping the packaging line. Thus, for example, the packaging printer may print packaging in one language for the first hundred units and then switch to print packaging in a different language for the next hundred units. Alternatively, the printer may switch from printing the packaging bearing one customer's trade marks to printing those of another. In either case, it is not necessary for a continuous production line to stop for the outer packaging to be changed, and this saves time and therefore money in the production process. A multi-printhead ink jet printer comprises a very large number of densely packed nozzles through which ink is ejected onto the printing substrate to form the printed image. The spacing between nozzles can be around 140 microns to give a pixel density of 180 dpi. It is important that the ink ' jet nozzles are accurately located relative to each other, as a very small misalignment of even one nozzle can produce a noticeable effect on the printed image. In order to achieve a desired print density, the ink jet nozzles may be interleaved, i.e. one row of nozzles may be arranged to print pixels between the pixels printed by a second row of nozzles. If it is desired to print in colour, separate nozzles are provided for each of the different coloured inks and the location of these nozzles must be coordinated with the required degree of accuracy. Many different coloured inks may be used for a full colour industrial printing process, and even in simple situations several colours may be used. A multi-printhead ink jet printer can be made up of a plurality of printheads in the form of cartridges which fit together to form the whole printer. Such printheads are available from XaarJet Limited of Cambridge, United Kingdom. Such a multi-printhead printer has the advantage that failed ink jet printheads can be replaced without replacing the whole printer. In order that the whole printer is wide enough to cover the width of a desired substrate, the printheads are "stitched" together, so that the printheads overlap in the direction perpendicular to the direction of transport of the substrate . Thus, in a multi-printhead printer the printheads must be accurately aligned to ensure acceptable printing results. In addition, it is desirable for the printer to be relatively compact in order to fit into standard production lines. For ink jet printing, it is important that the temperature of the ink at the ink jet nozzles is controlled carefully to ensure reliable printing. Often, the printhead is run at a temperature sufficiently higher than ambient that cooling of the nozzles is not required. In other situations, it is not possible to operate at such temperatures and some form of cooling or heating is required. Furthermore, the temperature profile across an array of ink jet nozzles should also be relatively uniform. However, in a multi-printhead printer where the cartridges are stitched together, there is very limited space in which to locate a device that is able to regulate the temperature of all the ink jet nozzles. The present invention seeks to provide an arrangement for the temperature control of a stitched printhead cartridge . According to the present invention, there is provided a cartridge for a multi-printhead ink jet printer, the cartridge comprising at least one array of ink jet nozzles and a heat sink for controlling the temperature of the nozzles, wherein the heat sink comprises a block of thermally conductive material having formed therein a passageway for a thermally conductive fluid, and wing regions of thermally conductive material extending from said block substantially parallel to said array of nozzles, and wherein the block and the wing regions are in thermal contact with the array of ink jet nozzles and the wing regions have an extent in a direction perpendicular to the array of nozzles which is smaller than the extent of the block in the same direction. In accordance with the invention, the heat sink for the printhead cartridge has thin wing regions and a thicker block. The thin wing regions allow the cartridges to be stitched while maintaining the required temperature control . The thicker block allows a connection to be made to a coolant circuit of thermally conductive fluid, and increases the overall thermal capacity of the heat sink to maintain thermal stability. The heat sink may be made of any suitable material and in the preferred embodiment the material is copper. Similarly, the thermally conductive fluid may be any suitable fluid, and is water in the preferred embodiment . The passageway for the thermally conductive fluid may extend into the wing regions of the heat sink, in which case the passageway may be thinner in the wing regions than in the block. An embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings in which: Figure 1 shows schematically a plan view of a multi-printhead printer according to an embodiment of the invention; Figure 2 shows schematically an enlarged elevation of a single cartridge of the printer of Figure 1 viewed in cross-section along the line A-A of Figure 1; and Figure 3 shows schematically the heat sink of the cartridge of Figure 2 in elevation (Figure 3a) and plan (Figure 3b) views. Figure 1 shows a multi-printhead printer according to an exemplary embodiment of the invention, which is intended for printing onto a continuous web of material such as paper or cardboard. The printer comprises 20 identical cartridges (shown in more detail in Figure 2) , each of which is made up of two 92 mm wide printheads 1 mounted to a common support 2. Each printhead 1 comprises a 70mm wide array 3 of 500 ink jet nozzles at its lowermost end, and contains an arrangement of miniature valves for controlling the ejection of ink through the nozzles. An ink supply (not shown) is connected to the printhead 2 at its upper end. The array 3 of ink jet nozzles on each printhead 1 has an effective printing density of 180 dpi. The two printheads 1 are interleaved on the support 2 such that the nozzles of the array 3 of one printhead are offset relative to the nozzles of the array 3 of the other printhead by half the distance between adjacent nozzles. In this way, one printhead 1 is able to print pixels between the pixels printed by the other printhead 1. This gives an effective print density for the whole cartridge of 360 dpi. The printer comprises a chassis 4, to which each cartridge is mounted in a precise location. The chassis 4 comprises an outer, rectangular frame 5 across which run a plurality of horizontal bars 6 to which the cartridges are mounted. The bars 6 are perpendicular to the direction of movement of the substrate (the z- direction) when the printer is in the position of use. On each bar 6, adjacent the location of each cartridge is provided a stop 7 which limits the movement of the cartridge in the direction along the bar 6 (the x- direction) . The support 2 of each cartridge is urged against the respective stop 7 by a resilient member 8 represented schematically in Figure 1 by a spring. As shown most clearly in Figure 2 , each cartridge straddles a bar 6, with one printhead 1 on either side of the bar 6. The lower surface of the support 2 engages with the upper surface of the bar 6 to locate the cartridge in the direction perpendicular to the surface of the substrate (the y-direction) and the inner surface of one printhead engages with a lateral surface of the bar 6 to locate the cartridge in the z-direction. In order for printing to be possible across the full width of the printer, adjacent rows of cartridges are "stitched", i.e. arranged with an overlap in the x- direction. The overlap allows the whole width of the substrate to be covered continuously by the ink jet nozzles even though the width of the printheads 1 is greater than the width of the array 3 of ink jet nozzles that they carry. The arrangement of cartridges shown in Figure 1 is intended for four-colour (yellow, magenta, cyan, black) printing with a respective row of three cartridges stitched with the adjacent row of two cartridges for each colour. Thus, the four colours are printed by respective rows of cartridges sequentially in the z- direction. For printing with a greater number of colours, it is necessary only to increase the number of rows of cartridges . To increase the width of the printing, it is necessary to increase the number of cartridges in each row. In order for the ink jet nozzles 3 to operate correctly their temperature must be carefully controlled. The temperature of the nozzles 3 is controlled by a respective heat sink 9 provided in thermal contact with the outer surface of each of the printheads 1. Referring to Figure 3, the heat sink 9 is made from commercial grade copper and comprises a 18 x 30 mm solid block 10 which extends 10 mm from the back surface of the heat sink 9, and in which are formed two tapped holes 11 for the connection of a coolant water supply (not shown) . The block 10 is formed integrally with a 70 x 30 mm bottom plate 12 which has milled therein a channel 13 which defines a relatively flat fluid passageway of cross-section 10 x 1.2 mm. The total thickness of the bottom plate is 2.5 mm and it is the bottom plate that forms the thin wing regions of the heat sink 9 on either side of the block 10. The channel 13 is closed by a back plate 14 which is soldered to the bottom plate 12. The bottom plate 12 is detailed such that it can be soldered to the back plate 14 with plumbing solder to form an effective water seal around the channel 13. i , During the soldering operation great care is taken to ensure the flatness of the bottom plate 12 is maintained, so that good thermal contact can be made with the array 3 of nozzles, when the heat sink 9 is fitted with the closed end of the fluid pathway flush with the backplate of the printhead 1, which is in itself in good thermal contact with the nozzle array 3. The channel 13 is in fluid communication with the holes 11 in the block so that coolant water can circulate therethrough to remove heat from the array 3 of nozzles. The lead-in to the tapped holes 11 is such that the flow is not greatly impaired compared with the channel 13. The cold side of the coolant water is fed directly to the end of the channel 13 with a fluid pathway that passes over the warm return path. The heat sink is supplied with 2.6 litres/minute of water at the minimum operating temperature for the ink and at a pressure of greater than 1 bar. It has been calculated that the maximum heat sink temperature, at the end of the thin wings regions, is approximately 5°C higher than the water temperature, assuming a required thermal dissipation of 40 W. In practice, the measured ! i temperature difference has been found to be even smaller than the calculated value, which is based on pessimistic assumptions . As can be seen from Figure 1, the heat sink 9 according to the invention allows the cartridges to be stitched to cover the entire, width of the substrate. Nevertheless, the heat sink ensures that the temperature of the array 3 of nozzles is accurately controlled. In summary, a printhead cartridge for a multi- printhead ink jet printer includes an array 3 of ink jet nozzles and a heat sink 9 for controlling the temperature of the nozzles 3. The heat sink 9 is made up of a copper block 10 and thinner copper wing regions 12 extending from the block 10 parallel to the array 3 of nozzles. A passageway 11 for coolant water is formed in the block 10 and extends into the wings 12 as a thinner channel 13. The block 10 and the wing regions 12 are mounted in thermal contact with the array 3 of ink jet nozzles. The heat sink 9 has the advantage that it allows the cartridges in the printer to be stitched together to cover the full width of a substrate.

Claims: Claims 1. A printhead cartridge for a multi-printhead ink jet printer, the cartridge comprising an array of ink jet nozzles and a heat sink for controlling the temperature of the nozzles, wherein the heat sink comprises a block of thermally conductive material having formed therein a passageway for a thermally conductive fluid, and wing regions of thermally conductive material extending from said block substantially parallel to said array of nozzles, and wherein the block and the wing regions are in thermal contact with the array of ink jet nozzles and the wing regions have an extent in a direction perpendicular to the array of nozzles which is smaller than the extent of the block in the same direction. 2. A cartridge as claimed in claim 1, wherein the passageway for the thermally conductive fluid extends into the wing regions of the heat sink. 3. A cartridge as claimed in claim 2, wherein the passageway is thinner in the wing regions than in the block.

Inventors: BAXTER WILLIAM RONALD STUART, BAXTER, WILLIAM, RONALD, STUART
Applicants: BARCO GRAPHICS NV, BAXTER WILLIAM RONALD STUART, BARCO GRAPHICS N.V., BAXTER, WILLIAM, RONALD, STUART
Application year: 2001
Main classification: B41J2
IPC codes: B41J2, B41J29