Printhead cartridge

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

The present application is a national stage filing and claims priority under 35 U.S.C. § 119 of International patent application Serial No. PCT/EP01/05594, filed 16 May 2001, and published in English and Great Britain Serial No. 0011916.4, filed 17 May 2000, the content of which is hereby incorporated by reference in its entirety.

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.

FIG. 1shows 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 comprises20identical cartridges (shown in more detail inFIG. 2), each of which is made up of two 92 mm on wide printheads1mounted to a common support2. Each printhead1comprises a 70 mm wide array3of 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 printhead2at its upper end. The array3of ink jet nozzles on each printhead1has an effective printing density of 180 dpi. The two printheads1are interleaved on the support2such that the nozzles of the array3of one printhead are offset relative to the nozzles of the array3of the other printhead by half the distance between adjacent nozzles. In this way, one printhead1is able to print pixels between the pixels printed by the other printhead1. This gives an effective print density for the whole cartridge of 360 dpi.

The printer comprises a chassis4, to which each cartridge is mounted in a precise location. The chassis4comprises an outer, rectangular frame5across which run a plurality of horizontal bars6to which the cartridges are mounted. The bars6are perpendicular to the direction of movement of the substrate (the z-direction) when the printer is in the position of use. On each bar6, adjacent the location of each cartridge is provided a stop7which limits the movement of the cartridge in the direction along the bar6(the x-direction). The support2of each cartridge is urged against the respective stop7by a resilient member8represented schematically inFIG. 1by a spring.

As shown most clearly inFIG. 2, each cartridge straddles a bar6, with one printhead1on either side of the bar6. The lower surface of the support2engages with the upper surface of the bar6to 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 bar6to 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 printheads1is greater than the width of the array3of ink jet nozzles that they carry.

The arrangement of cartridges shown inFIG. 1is 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 nozzles3to operate correctly their temperature must be carefully controlled. The temperature of the nozzles3is controlled by a respective heat sink9provided in thermal contact with the outer surface of each of the printheads1.

Referring toFIG. 3, the heat sink9is made from commercial grade copper and comprises a 18×30 mm solid block10which extends 10 mm from the back surface of the heat sink9, and in which are formed two tapped15holes11for the connection of a coolant water supply (not shown). The block10is formed integrally with a 70×30 mm bottom plate12which has milled therein a channel13which defines a relatively flat fluid passageway of cross-section 10×1.2 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 sink9on either side of the block10.

The channel13is closed by a back plate14which is soldered to the bottom plate12. The bottom plate12is detailed such that it can be soldered to the back plate14with plumbing solder to form an effective water seal around the channel13. During the soldering operation great care is taken to ensure the flatness of the bottom plate12is maintained, so that good thermal contact can be made with the array3of nozzles, when the heat sink9is fitted with the closed end of the fluid pathway flush with the backplate of the printhead1, which is in itself in good thermal contact with the nozzle array3.

The channel13is in fluid communication with the holes11in the block so that coolant water can circulate therethrough to remove heat from the array3of nozzles. The lead-in to the tapped holes11is such that the flow is not greatly impaired compared with the channel13. The cold side of the coolant water is fed directly to the end of the channel13with 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 temperature difference has been found to be even smaller than the calculated value, which is based on pessimistic assumptions.

As can be seen fromFIG. 1, the heat sink9according 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 array3of nozzles is accurately controlled.

In summary, a printhead cartridge for a multi-printhead ink jet printer includes an array3of ink jet nozzles and a heat sink9for controlling the temperature of the nozzles3. The heat sink9is made up of a copper block10and thinner copper wing regions12extending from the block10parallel to the array3of nozzles. A passageway11for coolant water is formed in the block10and extends into the wings12as a thinner channel13. The block10and the wing regions12are mounted in thermal contact with the array3of ink jet nozzles. The heat sink9has the advantage that it allows the cartridges in the printer to be stitched together to cover the full width of a substrate.