Patent Description:
In the operation of a continuous ink jet printer, a continuous jet of ink drops is formed. Usually, the drops are deflected in flight so that only some drops are used for printing. Drops of ink that are not required for printing are caught by a gutter and are returned to an ink tank within the main body of the printer. In an electrostatic deflection ink jet printer, this deflection is performed by trapping electric charges on some or all of the drops of ink, and using an electrostatic field to deflect the charged drops so that the printer prints the desired printed pattern.

<CIT> shows an example of an electrostatic deflection type continuous ink jet printer. It has an ink circuit comprising an ink tank, an ink pump, an ink supply path from an ink inlet opening inside the ink tank to the ink pump and an ink return path from the ink pump to the interior of the ink tank. Ink pressurised by the pump flows into an ink feed line through a feed valve. The ink feed line <NUM> extends out of the main body of the printer, along a conduit and into a printhead to an ink gun. If the feed valve is open, ink pressurised by the pump will flow along the ink feed line to the ink gun, and form the ink jet. The ink return path includes a Venturi suction device, For as long as the pump is running, ink will flow out of the ink tank, through the pump and the Venturi suction device, and back to the ink feed tank, regardless of whether or not any ink is flowing through the ink feed line to the ink gun. The Venturi suction device is arranged to use the flow of ink through it to generate suction at suction inlet(s) thereof.

The ink includes a colourant. In most cases the colourant is a dye, or a mixture of dyes, that are dissolved in the solvent. However, in some cases (typically when it is desired to print a pale colour such as white or yellow) the colourant comprises a pigment suspended in the ink, either with or without a dye. In use, the pigment may tend to settle out of the ink, especially if the printer is not used for an extended period. If an excessive amount of the pigment is allowed to settle out of the ink, the ink becomes less opaque and the print quality of the printer output is reduced unless the settled pigment is mixed back into the ink.

Various arrangements are known for preventing pigment from settling out of the ink or for mixing pigment back into the ink after it has settled out. For example, the ink tank may be provided with a mechanical stirrer, which can be magnetically coupled to a drive system outside the ink tank, as disclosed for example in <CIT> and <CIT>. An alternative approach is disclosed in <CIT>, in which the ink tank has a conical funnel-shaped bottom so that any pigment that settled out of the ink tends to gather at the lowest part of the cone. An ink outlet is provided at the bottom of the cone and a pump draws out ink, together with any pigment that has settled out, and returns it to the top of the ink tank. This mixes back in any pigment that has settled to the bottom of the ink tank. The pump may run continuously, and may run even when the printer is not in use.

Document <CIT> discloses a continuous inkjet printer and document <CIT> discloses a drop-on -demand printer.

Claim <NUM> discloses a continuous inkjet printer and claim <NUM> discloses a method of operating a continuous inkjet printer.

In an embodiment of the invention the ink system for a continuous inkjet printer comprises an ink tank, a reversible ink drive arrangement, an ink flow path having a plurality of inlets inside the ink tank and extending from the inlets to an inlet of the reversible ink drive arrangement, and an ink return path extending from an outlet of the reversible ink drive arrangement to the interior of the ink tank, whereby the reversible ink drive arrangement is operable in a forward mode to withdraw ink from the ink tank via the inlet of the ink flow path and cause it to flow along the ink flow path, through the reversible ink drive arrangement and along the ink return path back to the ink tank and whereby the reversible ink drive arrangement is operable in a reverse mode to withdraw fluid from the interior of the ink tank and cause any ink already in the ink return path or the ink flow path to flow into the ink tank via the inlet of the ink flow path.

The fluid withdrawn from the ink tank in the reverse mode is not necessarily ink, and may be air, even if ink is present at the inlet of the ink flow path, since the ink return path preferably opens into the ink tank at a position higher than the inlet of the ink flow path.

Preferably the reversible ink drive arrangement comprises a reversible pump. However, other arrangements are possible. For example, a switchover valve can be used to connect the ink flow path to the inlet of a pump and the ink return path to the outlet of the pump in the forward mode, and to connect the ink flow path to the outlet of the pump and the ink return path to the inlet of the pump in the reverse mode, so that the pump itself does not have to operate in reverse.

An embodiment of the present invention provides a continuous inkjet printer having an ink gun, an ink circuit including an ink tank for holding a supply of ink to be driven out of the ink gun and an ink pump for circulating ink from the ink tank around the ink circuit and back to the ink tank, and a gutter for collecting ink that has been driven out of the ink gun and has not been used for printing and returning the ink to the ink tank, the ink circuit comprising an ink supply path for supplying ink from the ink tank to the ink pump and the ink supply path starting at a plurality of ink entrance openings at the bottom of the ink tank, the ink entrance openings facing at least partially towards the floor of the ink tank or facing parallel to the floor of the ink tank, and the ink pump being operable to drive fluid around the ink circuit in a reverse direction such that fluid flows from the ink pump along the ink supply path and out through the ink entrance openings. Preferably the ink supply path has a portion that extends from the one or more ink entrance openings within the ink tank and reaches a point higher than the one or more ink entrance openings. The ink supply path may exit the ink tank through the top or a side of the ink tank.

There are a plurality of ink entrance openings, more preferably at least four ink entrance openings, spaced from each other across the bottom of the ink tank. Preferably the ink entrance openings face at least partially along the floor of the ink tank.

In the case that the ink supply path includes an ink filter inside the ink tank, the ink entrance openings may be provided in a wall that surrounds the ink filter.

Preferably the linear flow rate of ink through at least some of the ink entrance openings is at least <NUM>-<NUM> during operation in the reverse mode or direction and/or during an operation to drive ink in through the inlet or ink entrance opening and forward round the ink circuit without forming an ink jet.

If there is an operation to drive ink in through the inlet or ink entrance opening and forward round the ink circuit without forming an ink jet, it is desired that the ink pump or drive arrangement is arranged or controlled to drive the ink during this operation at a volume flow rate that is at least one and a quarter times the volume flow rate of the ink pump or drive arrangement while the ink jet is formed.

It is anticipated that the total cross-sectional area of all of the ink entrance openings will normally not exceed <NUM><NUM>. It is also anticipated that the ink entrance openings will be no more than <NUM> above the floor of the ink tank. In principle, a larger total cross-sectional area and/or a greater distance above the floor may be possible, but in such cases the fluid flow velocity out of the ink entrance openings during reverse flow may have to be inconveniently high in order to be effective to disturb pigment that has settled on the floor of the ink tank.

Preferably the ink tank has a flat floor over substantially all of its entire width and breadth, in contrast to an inclined tapered (e.g. conical) floor as is known in some designs of printer for use with pigmented ink. The use of a flat floor can enable a better use of space within the printer, and may also allow the ink tank to be cheaper to manufacture.

Preferably the ink pump is reversible. However, other arrangements may enable the ink pump to be operable to drive fluid around the ink circuit in the reverse direction. For example, there may be an arrangement of one or more valves to redirect the flow of fluid from the ink pump.

An embodiment of the present invention provides a method of operating a continuous ink jet printer, the printer having an ink gun for forming an inkjet, and an ink circuit comprising an ink tank for holding ink to be supplied to the ink gun, and an ink pump arrangement for pumping ink from the ink tank around the ink circuit and back to the ink tank, an ink supply path extending from a plurality of inlet openings inside the ink tank to an inlet of the ink pump arrangement, and an ink return path extending from an outlet of the ink pump arrangement to the interior of the ink tank, and the printer further having an ink feed path extending from the ink return path to the ink gun to supply ink, that has been pressurised by the ink pump arrangement, to the ink gun, the method comprising (a) a jet forming operation in which the ink pumping arrangement operates in a forward direction to drive ink from the ink tank along the ink supply path, through the ink pump arrangement and along the ink return path back to the ink tank and in which ink pressurised by the ink pump flows from the ink return path along the ink feed path to the ink gun and (b) an ink mixing operation that comprises a reverse flow step of operating the ink pump arrangement in a reverse direction to drive fluid from the ink tank along the ink return path to the ink pump arrangement and then along the ink supply path from the ink pump arrangement to the ink tank and out through the plurality of inlet openings. If the ink circuit contains ink at the start of the reverse flow step, the ink will tend to be forced out through the plurality of inlet openings. This can create a movement of ink inside the ink tank that tends to disturb ink pigment that may have settled out of the ink in the ink tank.

Preferably the ink mixing operation also comprises a forward flow step of operating the ink pump arrangement in a forward direction to drive ink from the ink tank along the ink supply path, through the ink pump arrangement and along the ink return path back to the ink tank, preferably without any ink flow along the ink feed line to the ink gun, the forward flow step being performed before the reverse flow step. Preferably the forward flow step is performed for long enough to fill the ink supply path and the ink return path with ink before the reverse flow step is carried out.

The fluid that is driven out of the ink tank in the reverse flow step may be a gas, usually air, that is in the ink tank above the ink. Preferably, in cases where the fluid is a gas and the forward flow step is carried out before the reverse flow step, the reverse flow step is carried out for long enough to drive all of the ink in the ink return path and the ink supply path out through the plurality of inlet openings and is continued so as to drive gas out through the plurality of inlet openings.

Alternatively, the fluid that is driven out of the ink tank in the reverse flow step may be ink. the ink return path may have an opening below the normal ink surface level in the ink tank.

Additionally, if the forward flow step is carried out, the ink may exit the ink return path through an opening in the ink tank that is below the normal ink surface level, and is preferably at the bottom of the ink tank, as this can further help to disturb pigment that has settled on the floor of the ink tank.

Preferably, ink from the ink jet that is not used for printing is returned to the ink tank in the jet forming operation. In this case, air may also be delivered to the ink tank together with the ink that is returned from the ink jet. Preferably such ink and air are delivered to a location in the ink tank above the normal ink surface level, as this minimises the tendency of air bubbles to be retained in the ink during jet forming operation. In some designs of printer, this ink and air may be delivered to the ink tank along a part of the ink return line, in which case the ink return line preferably has an opening in the ink tank that is above the normal ink surface level. If the ink return line also has an opening in the ink tank below the normal ink surface level, as described above, the ink return line may be branched and there may be one or more valves to select which opening of the ink return line is used. Preferably the ink mixing operation comprises one or more mix cycles, each mix cycle comprising a forward flow step followed by a reverse flow step. The ink mixing operation may be carried out after a period when the printer has been idle (wherein "idle" means that the ink pump arrangement has not been operated) and before a jet forming operation. In this case, the number of mix cycles carried out before the jet forming operation may be determined, at least in part, by the length of time for which the printer has been idle. Preferably the linear flow rate of ink through at least some of the inlet openings is at least <NUM>-<NUM> during the reverse flow step and/or during the forward flow step of the ink mixing operation (if present). It is preferred that the linear flow rate is at least <NUM>-<NUM> and it is more preferably at least <NUM>-<NUM>. The linear flow rate may be up to <NUM>-<NUM> or may even be more than <NUM>-<NUM>.

If the ink mixing operation comprises a forward flow step, it is desired that the volume flow rate of ink through the ink pump arrangement during at least a part of the forward flow step is at least one and a quarter times the maximum volume flow rate of the ink pump arrangement while the ink jet is formed.

Both the flow of ink in the forward flow step and the flow of fluid in the reverse flow step tend to disturb settled pigment on the floor of the ink tank in the region of each inlet opening. There are a plurality of inlet openings, spaced apart from each other across the ink tank (spaced in one or more directions parallel to the floor of the ink tank). This will tend to increase the total area of the floor over which settled pigment is disturbed.

Preferably there are at least four inlet openings. This increases the amount of settled pigment that is disturbed, thereby increasing the opacity of the ink.

The reverse flow of fluid through the ink inlets is useful to disturb pigment that has settled on the floor of the ink tank. However, there may be circumstances in which it is not convenient to use reverse flow, either because it is difficult or expensive to provide a pump arrangement for reverse flow or because of some effect that reverse flow has in the ink tank. In these cases, it may be possible to disturb the pigment using forward flow alone.

In some designs of ink tank it may be the case that four ink inlet openings will not be adequate to disturb settled pigment over the majority of the area of the floor of the ink tank, and for this reason there are preferably at least eight ink inlet openings. Greater numbers of ink inlet openings will provide a better coverage of the floor of the ink tank and therefore there are preferably from twelve to twenty (e.g. sixteen) ink inlet openings. Although the coverage over the floor of the ink tank is improved as the number of ink inlet openings is increased, it becomes difficult to maintain an adequate flow rate through each individual opening if there are too many of them. For this reason, it is preferred that there are no more than twenty four ink inlet openings.

In an embodiment, the entrance to the ink path from the interior of the ink tank to the ink pump is made up of a plurality of small inlet openings, which may be provided by nozzles formed in a shroud that fits around an ink filter in the ink tank. The openings are provided at the bottom of the ink tank, close to the floor, and face parallel to the floor or at least partially towards it. If the ink pump is driven in reverse, any ink in the ink path is driven at speed out through the inlet openings into the interior of the ink tank, followed by air. This tends to disperse pigment that may have settled to the bottom on the ink tank.

In a printing operation of the continuous ink jet printer, a continuous jet of ink drops is formed. Usually, the drops are deflected in flight so that only some drops are used for printing. Drops of ink that are not required for printing are caught by a gutter and are normally returned to an ink tank within the main body of the printer. Preferably the continuous inkjet printer is an electrostatic deflection inkjet printer in which the deflection of ink drops is performed by trapping electric charges on some or all of the drops of ink, and using an electrostatic field to deflect the charged drops. Typically the ink includes a solvent which is normally highly volatile so that the drops of ink dry quickly after printing.

The solvent also tends to evaporate from the ink that is caught in the gutter and returned to the ink tank, so that the ink used by the printer loses solvent over time. In order to maintain the correct ink viscosity, additional solvent may be added from time to time. Additionally, the ink is slowly used up as the printer prints and therefore the ink in the ink tank may also be replenished from time to time.

Embodiments of the invention, given by way of non-limiting example, will now be described with reference to the accompanying drawings, in which:.

In the illustrated embodiments of the present invention, the continuous ink jet printer is an electrostatic deflection continuous inkjet printer.

<FIG> shows an electrostatic deflection type continuous inkjet printer. The printer forms a continuous jet of ink and has an arrangement of electrodes for charging drops of ink and deflecting the drops electrostatically in order to print a desired pattern. The main fluid and electrical components are housed within a main printer body <NUM>. An operator communicates with the printer via a touchscreen display <NUM>. The ink jet is formed within a print head <NUM>, which also includes the electrode arrangement for charging and deflecting the ink drops, and the print head <NUM> is connected to the main printer body <NUM> by a flexible connection <NUM> known as a conduit or an umbilical. Drops of ink, deflected as necessary to create the desired pattern, travel from the print head <NUM> and strike the surface <NUM> of an object conveyed past the print head <NUM>, in order to print the desired pattern on the surface <NUM> of the object. The print head <NUM> and the umbilical <NUM> may be disconnectable from the main printer body <NUM>.

<FIG> is a schematic top view and <FIG> is a schematic side view of the main components of the print head <NUM>. The terms "top view" and "side view" represent conventional directions from which to view the print head and do not necessarily correspond to the orientation of the print head when in use. Pressurised ink, delivered from the main printer body <NUM> through the umbilical <NUM>, is provided via an ink feed line <NUM> to an ink gun <NUM>. The pressurised ink leaves the ink gun <NUM> through a small jet-forming orifice to form an ink jet <NUM>. Provided that pressurised ink is received by the ink gun <NUM> and any valves in the ink gun <NUM> are in the appropriate state, the ink jet <NUM> is formed continuously. Accordingly, this type of ink jet printer is known as a continuous ink jet printer, by contrast with a drop-on-demand printer in which a drop of ink is ejected only when a dot is to be printed.

Although the ink jet <NUM> leaves the ink gun <NUM> as a continuous unbroken stream of ink, it rapidly breaks into separate drops. The path of the inkjet passes through a slot in a charge electrode <NUM>, which is positioned so that the ink jet <NUM> separates into drops while it is in the slot through the charge electrode <NUM>. The ink is electrically conductive and the ink gun <NUM> is held at a constant voltage (typically ground). Accordingly, any voltage applied to the charge electrode <NUM> induces a charge into the part of the ink jet <NUM> that is in the slot of the charge electrode <NUM>. As the ink jet <NUM> separates into drops, any such charge is trapped on the drops. Accordingly, the amount of charge trapped on each drop can be controlled by changing the voltage on the charge electrode <NUM>.

The ink jet <NUM> then passes between two deflection electrodes <NUM>, <NUM>. A large potential difference (typically several kilovolts) is applied between those electrodes <NUM>, <NUM> to provide a strong electric field between them. Accordingly, the drops of ink are deflected by the electric field and the amount of deflection depends on the amount of charge trapped on each drop. In this way, each ink drop can be steered into a selected path. As shown in <FIG>, uncharged ink drops, which pass through the electric field without deflection, travel to a gutter <NUM> where they are caught. Suction is applied to the inside of the gutter <NUM> by a suction line <NUM>, and so the ink received by the gutter <NUM> is sucked away and returned through the umbilical <NUM> to the main printer body <NUM>, for re-use.

Drops of ink that are deflected by the field between the deflection electrodes <NUM>, <NUM>, so as to miss the gutter <NUM>, leave the print head <NUM> and form printed dots on the surface <NUM> of the object.

<FIG> is a simplified schematic diagram of a fluid system for the ink jet printer of <FIG>. Ink is held in an ink feed tank <NUM> in the main printer body <NUM>. The ink feed tank <NUM> is the main ink tank of the printer. The interior of the ink feed tank <NUM> is held at atmospheric pressure by a vent <NUM>. Ink is sucked out of the ink feed tank <NUM> by a pump <NUM>, via a filter <NUM> and an ink supply line <NUM>. The ink, pressurised by the pump <NUM>, flows through a Venturi <NUM> and back to the ink feed tank <NUM> via an ink return line <NUM>. Ink will flow continuously around this loop as long as the pump <NUM> is running. Thus the loop comprising the ink feed tank <NUM>, the ink supply line <NUM>, the ink pump <NUM>, the line from the pump <NUM> to the Venturi <NUM>, the Venturi <NUM> itself, and the ink return line <NUM> provides an ink circuit. The flow of ink through the Venturi <NUM> generates suction and accordingly the Venturi acts as a suction source. A pressure transducer (pressure sensor) <NUM> is used to sense the ink pressure on the outlet side of the ink pump <NUM>.

The ink feed line <NUM> is also connected to the outlet side of the ink pump <NUM> and receives pressurised ink. Thus the ink feed line <NUM> provides an ink feed path to supply pressurised ink from the ink circuit to the ink gun <NUM>. An ink feed valve <NUM> controls the flow of ink through the ink feed line <NUM>. The gutter suction line <NUM> returns ink from the gutter <NUM> through the umbilical <NUM> to the main printer body <NUM>, and receives suction from the Venturi <NUM>. Fluid flow in the gutter suction line <NUM> is controlled by a gutter valve <NUM>.

During operation of the printer, the solvent in the ink used to form the ink jet <NUM> tends to evaporate, causing a change in the viscosity of the ink. In order to restore the ink to the correct viscosity, it is necessary to add further solvent from time to time. Spare solvent is held in a solvent reservoir <NUM> which receives suction from the Venturi <NUM> through a solvent top-up line <NUM>. In order to add solvent to the ink, a solvent top-up valve <NUM> in a solvent top-up line <NUM> is opened briefly, allowing the Venturi <NUM> to suck a small quantity of solvent from the solvent reservoir <NUM>. Solvent sucked in by the Venturi <NUM> joins the ink flow through the Venturi and therefore passes into the ink feed tank <NUM>, so as to dilute the ink in the ink feed tank.

As the ink jet printer prints, it will slowly use up ink from the ink feed tank <NUM>. When the ink level becomes too low, the ink feed tank <NUM> is topped up from an ink reservoir <NUM>. Ink is sucked out of the ink reservoir <NUM> by the Venturi <NUM> via an ink top-up line <NUM>, controlled by an ink top-up valve <NUM>, in a similar manner to the operation for topping up with solvent from the solvent reservoir <NUM>.

The solvent reservoir <NUM> and the ink reservoir <NUM> are supplied from a solvent container <NUM> and an ink container <NUM> respectively, and the operator replaces the containers <NUM>, <NUM> as necessary. In practice, it is not always necessary to provide the solvent reservoir <NUM> and the ink reservoir <NUM>, and the respective top-up lines <NUM>, <NUM> may be connected directly to the containers <NUM>, <NUM>.

<FIG> shows schematically some of the components inside the main body <NUM> of the printer. The printer has a main body ink system <NUM>, which includes the components in <FIG> that are shown inside the main printer body <NUM>. The main body ink system <NUM> and other parts of the printer operate under the control of a control system <NUM>. The control system <NUM>, for example, sends drive currents to the ink pump <NUM> and to the various valves, <NUM>, <NUM>, <NUM>, <NUM> of the main body ink system <NUM>. The control system <NUM> receives outputs from the pressure sensor <NUM> and also from level sensors in the ink feed tank <NUM>, the solvent reservoir <NUM> and the ink reservoir <NUM>. The control system <NUM> also provides outputs to, and receives inputs from, the touchscreen display <NUM>. Typically, the control system will include a processor such as a microprocessor and other electronic components as is well known in the art.

Fluid lines <NUM> connect the main body ink system <NUM> to the print head <NUM> through the umbilical <NUM>. These fluid lines will include the ink feed line <NUM>, the gutter suction line <NUM> and the purge line <NUM> shown in <FIG>. Electrical lines <NUM> connect the control system <NUM> to the print head <NUM> via the umbilical <NUM>. These electrical lines will include lines for applying the appropriate voltages to the charge electrode <NUM> and the deflection of electrodes <NUM>, <NUM>, and a drive signal to a piezoelectric crystal inside the ink gun <NUM> that applies a vibration to the ink that forms the inkjet <NUM> in order to control the manner in which it breaks into drops.

<FIG> is vertical section through the centre of the ink feed tank <NUM> showing the main components contained within it. In this embodiment, the ink feed tank <NUM>, together with the components contained within it, forms a removable and replaceable service module that can be inserted into the main body <NUM> of the printer, and removed from it, by the operator. Inside the ink feed tank <NUM> there is a filter assembly made up of the ink filter <NUM>, a filter base plate <NUM> and a filter top plate <NUM>. The filter <NUM> is a pleated cylindrical filter, and is shown in section in <FIG> so that the inner pleated cylindrical surface is visible and the outer pleated cylindrical surface is not visible. A shroud is fitted to the filter assembly, surrounding the filter <NUM>, but for ease of illustration the shroud is not shown in <FIG>. Above the filter assembly there is an arrangement for promoting the separation of ink and air, made up of four flat plates <NUM> that provide a serpentine path for ink and air that enter the ink tank from the Venturi <NUM>, and a sloping conical plate <NUM>.

As can be seen in <FIG>, the floor of the ink feed tank <NUM> is flat over substantially the entire area (width and breadth) of the ink feed tank <NUM>, with a radius where it joins the side walls. The ink filter <NUM> sits at the bottom of the ink feed tank <NUM>, with the filter assembly base plate <NUM> resting on the floor of the ink feed tank <NUM>. The filter assembly top plate <NUM> has an upwardly extending central cylindrical extension, and the top of this supports the sloping conical plate <NUM> of the ink/air separator arrangement. This sloping conical plate <NUM> in turn supports the stack of four flat plates <NUM>.

Fluid connections to the ink feed tank <NUM> are made at a port <NUM> at the top of a side wall. Three fluid connections are provided, corresponding to the three connections shown for the ink feed tank <NUM> in <FIG>. One connection (not shown in <FIG>) receives the ink and air mixture output from the Venturi suction device <NUM> via the ink return line <NUM>, and delivers this ink/air mixture onto the topmost of the flat plates <NUM>. The second connection (also not show in <FIG>) allows air to flow from inside the ink feed tank <NUM> to the vent <NUM>. The third connection (shown in <FIG>) provides an ink outlet from the ink feed tank <NUM> to the ink pump <NUM> via the ink supply line <NUM>. Ink that has passed through the filter <NUM> is carried by an ink outflow tube <NUM> from the space in the centre of the filter <NUM> up to the top of the ink feed tank <NUM> and then to the port <NUM> for connection to the ink supply line <NUM>.

Accordingly, while the ink pump <NUM> is operating, ink in the main volume of the ink feed tank <NUM> is sucked through the filter <NUM> and the ink outflow tube <NUM> out of the ink feed tank <NUM> and along the ink supply line <NUM> to the inlet of the ink pump <NUM>. From the ink pump <NUM> the ink is driven through the Venturi <NUM> and along the ink return line <NUM> and re-enters the ink feed tank <NUM>. While the ink jet <NUM> is running, unused ink from the jet is captured by the gutter <NUM>, and suction developed by the Venturi <NUM> sucks the ink along the gutter suction line <NUM> so that it enters the Venturi <NUM> and joins the ink flow along the ink return line <NUM> to the ink feed tank <NUM>. Suction at the gutter <NUM> also draws in air, which is also sucked down the gutter suction line <NUM> to the Venturi <NUM> and joins the ink flow along the ink return line <NUM> to the ink feed tank <NUM>. Therefore when the ink jet <NUM> is running the ink re-entering the ink feed tank <NUM> from the ink return line <NUM> is mixed with air. The ink/air mixture from the ink return line <NUM> is delivered onto the topmost of the flat plates <NUM>. The ink then flows over the flat plates <NUM> and the sloping conical plate <NUM> while air separates out from it, and re-joins the ink in the main volume of the ink feed tank <NUM>. The amount of ink in the ink feed tank <NUM> is maintained so that the surface of the ink is slightly above the bottom of the sloping conical plate <NUM>, as shown by the broken line A-A in <FIG>. The arrangement of flat plates <NUM> and the sloping conical plate <NUM> allow the ink filter <NUM> to receive ink that is free of air bubbles. This is advantageous since the presence of air bubbles in the ink at the ink gun <NUM> would tend to disrupt the formation of the ink jet <NUM>.

<FIG> show the filter assembly in the ink feed tank <NUM>, with the shroud <NUM> in place. <FIG> shows an external view of the shroud <NUM>. In <FIG>, the shroud <NUM> is in section but (in contrast to <FIG>) the ink filter <NUM>, the filter assembly base plate <NUM> and the filter assembly top plate <NUM> are not in section. The shroud <NUM> extends from the filter assembly top plate <NUM> to the filter assembly base plate <NUM>, closing off the ink filter <NUM> from the ink in the ink feed tank <NUM>, except where nozzles <NUM> enable ink to flow from the ink feed tank <NUM> to the filter <NUM>. Preferably, there are a plurality of nozzles <NUM> spaced around the circumference of the shroud <NUM>. For clarity of illustration, <FIG> show an arrangement in which the shroud <NUM> has eight nozzles. In practice, more nozzles might be provided, for example there could be twelve or sixteen nozzles <NUM>. In the embodiment of <FIG> each nozzle is formed by a semi-cylindrical bulge in the shroud <NUM>, and provides a semi-cylindrical flow path from a nozzle opening at the bottom of each nozzle <NUM> to the volume enclosed by the shroud <NUM>. As shown in <FIG>, the filter assembly base plate <NUM> and the filter assembly top plate <NUM> have a slightly larger diameter than the filter <NUM>. Since the shroud <NUM> fits round the outside of the filter assembly base plate <NUM> and the filter assembly top plate <NUM>, it encloses a circumferential space <NUM> around the outside of the filter <NUM>.

During normal operation of the printer, the ink pump <NUM> sucks ink from the ink feed tank <NUM>. The ink inside the ink feed tank <NUM> flows in through the opening at the bottom of each nozzle <NUM> and then via the nozzles <NUM> into the space <NUM> around the filter <NUM>. From the space <NUM> the ink flows through the filter <NUM>, the ink outflow tube <NUM> and the ink supply line <NUM> to the ink pump <NUM>. The pressurised ink from the ink pump then flows through the Venturi <NUM> and the ink return line <NUM> back to the ink feed tank <NUM>. If the inkjet <NUM> is running the pressurised ink also flows through the ink feed line <NUM> to the ink gun <NUM> to form the ink jet <NUM>, and unused ink is caught by the gutter <NUM> and returns to the Venturi <NUM> along the gutter suction line <NUM>. Thus the openings at the bottoms of the nozzles can be regarded as inlets or inlet openings or ink entrance openings to an ink flow path or ink supply path that comprises the nozzles <NUM>, the space <NUM>, the ink outflow tube <NUM> and the ink supply line <NUM>. An ink return path is provided by the line from the ink pump <NUM> to the Venturi <NUM>, the Venturi <NUM> itself, and the ink return line <NUM>.

Because the total cross section of the ink flow paths through all the nozzles <NUM> is relatively small, the ink velocity through the nozzles <NUM> is relatively high. However, in order to minimise the pressure drop across the ink filter <NUM> it is desirable to provide a large filter area for the ink filter <NUM> and enable a low ink velocity through the filter <NUM>. Therefore the ink filter <NUM> is pleated, and the space <NUM> inside the shroud <NUM> enables the ink that has entered through the nozzles <NUM> to circulate around the ink filter <NUM> and reach the entire area of the ink filter <NUM>.

This arrangement is suitable for printing both with inks containing a pigment and with inks not containing a pigment. Alternatively, the shroud <NUM> may be provided if the printer is to be used with pigmented ink and may be omitted if the printer is to be used with non-pigmented ink. When a pigment-containing ink is used, the continual flow of ink from the ink feed tank <NUM> through the ink tank <NUM> and back into the ink feed tank <NUM> tends to keep the pigment dispersed within the ink. The relatively high ink velocity through the nozzles <NUM>, and the consequent relatively high velocity of the ink flow in the ink feed tank <NUM> immediately adjacent the openings of the nozzles <NUM>, also helps to keep the pigment dispersed.

However, if the printer is left unused for an extended period with a pigment-containing ink in the ink feed tank <NUM>, the pigment may tend to settle out of the ink and accumulate on the floor of the ink feed tank <NUM>. Accordingly, the printer can perform a mixing operation when it is started up, in order to re-disperse pigment that may have settled out. In the mixing operation, the ink pump <NUM> is used to drive a fluid (ink and/or air) in the reverse direction along the ink supply line <NUM> and into the ink feed tank <NUM> via the nozzles <NUM>.

The relatively small total cross-sectional area of all the flow paths through the nozzles <NUM> results in a relatively high fluid flow velocity through them. This relatively high fluid flow velocity, combined with the downward direction of the reverse fluid flow out of the nozzles <NUM> into the ink feed tank <NUM>, tends to agitate and disperse pigment that has settled onto the floor of the ink feed tank <NUM>. The dispersed pigment is once again suspended in the ink. Subsequent normal operation of the ink pump <NUM> then sucks the ink and the suspended pigment in through the nozzles <NUM> to flow along the ink supply line <NUM> to the ink pump <NUM>, and back through the Venturi <NUM> and the ink return line <NUM> to the ink feed tank <NUM>. This rapidly provides an even mixing of the suspended pigment in the ink. Additionally, the agitation caused by this flow tends to break up any lumps where pigment particles have joined together. Optionally, the ink pump <NUM> may be a gear pump, which tends to break down any accumulated lumps of pigment that pass through it.

The reverse flow along the ink supply line <NUM> may also be useful to dislodge any accumulations of pigment in the path from the interior of the ink feed tank <NUM> to the ink pump <NUM>, especially at the nozzles <NUM> or on the surface of the ink filter <NUM>.

Because there are several nozzles <NUM> around the circumference of the shroud <NUM>, the flow of fluid out through the nozzles <NUM> for disturbing settled pigment is directed to several different places on the floor of the ink feed tank <NUM>. This increases the overall proportion of the ink tank floor where pigment is disturbed. Additionally, as shown in <FIG> the lower ends of the nozzles <NUM> are cut at an angle (chamfered) so that the openings at the lower ends of the nozzles <NUM> face outwards as well as downwards. This tends to promote an outward component in the direction of flow of ink from the nozzles <NUM> to the floor of the ink tank <NUM>, with result that the flow of ink tends to extend across the floor of the ink tank <NUM> away from the shroud <NUM>, increasing the overall area of the floor of the ink tank <NUM> that receives the flow of fluid for dispersing settled pigment.

An example of a mixing operation will now be described with reference to the flow diagram of <FIG>.

When the printer is started up after a period of being idle, it performs a start-up routine. At a suitable point during the start-up routine the control system <NUM> may check whether the ink currently in the printer contains pigment, and if so it controls the printer to perform the ink mixing operation. Alternatively, the printer may perform the ink mixing operation in every case regardless of the type of ink. At the beginning of the ink mixing operation, the control system <NUM> determines how long the printer has been idle, i.e. how much time has elapsed since the printer was last active. As the length of time increases, the amount of pigment that will have settled out of the ink will increase. Therefore a greater amount of ink mixing will be required. Consequently, in step <NUM> the control system <NUM> sets the number of mixing cycles to be performed depending on how long the printer has been idle.

Assuming that the control system <NUM> determines that at least one mixing cycle is required, the operation moves onto step <NUM> in which control system <NUM> operates the ink pump <NUM> and brings it up to its normal full operating speed. Normally, whenever the ink pump <NUM> is turned on after being idle, its speed it increased gradually so as to increase the output ink pressure over a period rather than driving the pump <NUM> from stationary to full speed as quickly as possible. Accordingly, step <NUM> may take several seconds to complete. Once the pump has reached the desired operating speed, it is maintained in that state for a first predetermined period in step <NUM>. The purpose of steps <NUM> and <NUM> is to pump ink from the ink feed tank <NUM> around the ink circuit (comprising the ink supply line <NUM>, the ink pump <NUM>, the Venturi <NUM> and the ink return line <NUM>) in the same way as during normal operation of the printer, in order to ensure that the ink circuit is fully charged with ink in preparation for next phase of the mix cycle. In step <NUM> the first period is selected so as to be long enough to ensure that ink has circulated fully around the ink circuit and is returning back into the ink feed tank <NUM> via the ink return line <NUM>.

Following the end of the first period in step <NUM>, control system <NUM> returns the ink pump <NUM> to the idle state briefly in step <NUM> and then the reverse flow phase of the mix cycle begins with control system <NUM> controlling the pump <NUM> to operate at maximum speed in reverse in step <NUM>. Normally, the pump <NUM> will be brought to its maximum reverse speed in step <NUM> as quickly as possible, in contrast to the more gradual increase in pump speed in step <NUM>, in order to minimise the amount of ink driven through the pump <NUM> before it has reached its maximum reverse speed. Once the pump <NUM> has reached its maximum reverse speed, it is maintained in that condition in step <NUM> for a predetermined second period. Preferably, the predetermined second period of step <NUM> is longer than is required for all of the ink in the ink circuit to be discharged.

Therefore, during step <NUM>, all of the ink in the ink circuit is driven out through the nozzles <NUM> into the ink feed tank <NUM> with the maximum velocity that the ink pump <NUM> is able to provide. This generates a strong liquid flow within the ink tank <NUM>, directed at the floor of the ink tank by the nozzles <NUM>, so as to dislodge pigment that has settled on the floor of the ink tank <NUM>. While the ink is being discharged from the nozzles <NUM>, air from the top of the ink feed tank <NUM> is sucked into the ink return line <NUM>. Once all of the ink in the ink circuit has been discharged through the nozzles <NUM>, and the ink circuit is filled with air, the continued operation of the ink pump <NUM> in reverse drives air out of the nozzles <NUM> into the ink tank <NUM> for the remainder of the second period in step <NUM>. This air flow into the bottom of the ink feed tank <NUM> continues to agitate the ink and tends to mix the dislodged sediment thoroughly into the ink in the ink tank <NUM>.

At the end of the second period, control system <NUM> returns the ink pump <NUM> to the idle state in step <NUM>, and the mixing cycle is complete. Then, in step <NUM> the control system <NUM> determines whether the printer has yet carried out the required number of mix cycles as decided in step <NUM>. If the required number of mix cycles have not yet been performed, the operation returns to step <NUM> in order to perform another mix cycle. Once it is determined in step <NUM> that the required number of mix cycles have been performed, the mixing operation is completed and in step <NUM> the printer moves on to the next operation in the start-up procedure.

One specific example of a printer, which has been tested, used a shroud <NUM> generally as shown in <FIG> but having sixteen nozzles <NUM>. This was fitted around a filter assembly having a diameter of <NUM> in the manner shown in <FIG>. As shown in <FIG>, each nozzle <NUM> had the shape of a semi-cylinder, and provided a semi-cylindrical recess in the inner side of the shroud <NUM> so as to provide the nozzle flow path. Each semi-cylindrical recess had a radius of about <NUM>, so that the total cross sectional area of all the nozzle paths combined was about <NUM><NUM>. By way of comparison, the pipework used for the ink supply lines <NUM>, the connection from the pump <NUM> to the Venturi <NUM> and the ink return line <NUM> had an internal radius of <NUM> and an internal cross section of about 12½ mm<NUM>. During normal full speed forward operation of the pump <NUM>, ink flows around the ink circuit with a volume flow rate of about <NUM><NUM> per second, resulting in an ink velocity in the ink supply line <NUM> and the ink return line <NUM> of about <NUM> millimetres per second (<NUM>/s), and a velocity through the nozzles <NUM> of about <NUM> millimetres per second (<NUM>/s).

This design was tested with a pigmented ink, and by way of comparison a test was also done using the same design of printer but omitting the shroud <NUM> so that the arrangement of parts inside the ink tank <NUM> resembled that shown in <FIG>. The arrangement without the shroud is known to work satisfactorily with inks having a dye but no pigment.

The absence of the shroud <NUM> means that ink passing from the bottom of the ink feed tank <NUM> through the filter <NUM> and into the ink outflow tube <NUM> can approach the filter assembly around its entire circumference and over the full height of the space between the filter assembly base plate <NUM> and the filter assembly top plate <NUM>. Therefore, assuming that the volume flow rate provided by the ink pump <NUM> remains the same, the absence of the shroud <NUM> results in a much lower ink velocity at the bottom of the ink feed tank <NUM>.

It was found that if the printer was operated continuously, the arrangement with the shroud <NUM> enabled the pigment to remain suspended in the ink sufficiently that no loss of opacity in the printer output of the printer was detected. However, in the absence of the shroud <NUM> there was eventually a detectable loss of opacity in the print out, indicating that a significant amount of pigment had settled out of the ink. It is assumed that the increased ink velocity at the nozzles <NUM> enabled the design having the shroud <NUM> to be more effective at keeping the pigment in suspension.

With the detailed example described above, the ink pump <NUM> was operated at maximum reverse flow to provide a volume of flow rate of about <NUM><NUM> per second, resulting in an ink velocity in the ink supply line <NUM> and the ink return line <NUM> of about <NUM> millimetres per second (<NUM>-<NUM>) and an ink velocity out through the nozzles <NUM> into the ink feed tank <NUM> of about <NUM> millimetres per second (<NUM>-<NUM>).

With this design, the mix cycle as described with reference to <FIG> was conducted so that in step <NUM> it took <NUM> seconds to increase the pump speed steadily from idle to its normal full forward speed. The first period of step <NUM>, for which the pump was maintained at its normal full forward speed, was <NUM> seconds, and in step <NUM> it was returned to the idle state for <NUM> seconds. In step <NUM>, the pump was brought to the maximum reverse flow operation in <NUM> seconds, and in step <NUM> the second period, in which the pump was maintained at its maximum reverse speed, was <NUM> seconds. Then in step <NUM> the pump was returned to the idle state for <NUM> seconds. The ink circuit contained approximately <NUM> millilitres of fluid. Accordingly, while the ink circuit was charged with ink in steps <NUM> and <NUM>, there was a significant flow of ink around the ink circuit and back into the feed tank <NUM>, with approximately <NUM> of ink being driven around the ink circuit. In the reverse flow phase, all the ink in the ink circuit was driven out through the nozzles <NUM> during the first <NUM> seconds of step <NUM>, and during the remaining <NUM> seconds of step <NUM> air was driven out through the nozzles <NUM>.

If the printer is left idle for an extended period, the pigment tends to settle out of the ink and a mixing operation can be performed to disturb the settled pigment and mix it back into the ink. This was tested using the printer used in the example discussed above. The printer was allowed to sit idle for approximately <NUM> weeks. At the end of this period, sufficient pigment had settled out of the ink so that when the printer was started up the opacity of the print out was noticeably reduced. When the printer was used without a shroud <NUM>, a mixing cycle as discussed above was not sufficient to re-disperse enough pigment to provide good opacity to the ink. However, opacity was provided when the mixing cycle was performed with the arrangement using the shroud <NUM>.

It is proposed that the number of mixing cycles performed during start-up of the printer should depend on the length of time that the printer has stood idle. With the specific example discussed above, it is proposed that the printer can restart without any mixing operation if it has been idle for less than <NUM> hours. The mixing operation should include two mixing cycles if the printer has been idle for a period between four hours and two days. The mixing operation should include <NUM> mixing cycles if the printer has been idle for a period between two days and one week. There should be five mixing cycles if the printer has been idle for a period between one week and two weeks. If the printer has been idle for more than two weeks, it is proposed that the mixing operation should include ten mixing cycles.

The embodiment described above provides a simple, cheap and compact arrangement for enabling a continuous inkjet printer to work with pigmented ink. Compared with the design used for inks containing dye but no pigments, the only design change is a provision of the shroud <NUM> so as to create nozzles <NUM>. The design enables pigmented ink to be used in a printer with an ink tank <NUM> having a flat floor for substantially the entire area of the ink tank, and therefore it avoids the wasted space associated with the use of a conical-floored ink tank. There is no need to provide an extra component beneath the ink tank, as there is in magnetic stirrer arrangements, thereby avoiding the cost and loss of space associated with such components, and equally there is no need to provide additional components such as special pipe work to introduce ink flows at the floor level of the ink, which can take up space around the tank.

In the mixing operation described with reference to <FIG>, both ink and air are driven out of the nozzles <NUM> during the reverse flow period. As an alternative, it is possible to provide only air during the reverse flow period, by omitting steps <NUM>, <NUM> and <NUM> so that the ink circuit is not charged with ink before the pump is operated for reverse flow in step <NUM>. However, this is less preferred. First, the period of forward flow ensures that ink flows through the pump <NUM>, thereby wetting the pump and ensuring that it can operate efficiently. Additionally, it is desirable to obtain a good flow of ink across the floor of the ink tank <NUM> in order to disturb pigment that has settled. As movement is transmitted from the fluid driven out through the nozzle <NUM> to the ink in the ink tank <NUM>, momentum must be conserved. Since the ink has a greater density than air, a flow of ink through the nozzles <NUM> will have a greater momentum than a flow of air with the same velocity, and therefore it is more effective in creating a strong ink flow across the floor of the ink feed tank <NUM>.

It is also possible for the reverse flow period to provide only a reverse flow of ink, and no reverse flow of air. This can be done, for example, by ending step <NUM> no later than the supply of ink in the ink circuit is used up. However, unless the ink circuit is designed so it contains an unusually large volume of ink, or a design modification is provided to enable ink to be sucked out of the ink tank <NUM> during the reverse flow phase of the mix cycle, the period of reverse flow during each mix cycle will inevitably be brief (about <NUM> seconds in the example discussed above). Therefore it is normally preferable to extend the period of reverse flow and include some reverse flow of air. Additionally, the air bubbles rise through the ink tank <NUM> owing to the buoyancy of the bubbles and this tends to promote vertical mixing within the ink, helping to disperse the pigment that has been dislodged by the initial reverse flow of ink.

In the example described above, the ink pump <NUM> was operated in the forward flow step at its normal full speed for forward operation. This is the maximum pump speed that is permitted during normal printing operation, when the ink jet <NUM> is running. However, it is also possible to operate the ink pump <NUM> at a higher forward speed during the forward flow step of an ink mixing cycle, since the ink jet will usually not be running at this time. The higher forward speed will increase the ink flow rate in through the entrance openings of the nozzles <NUM>, and this will enhance the tendency of the ink flow to disturb the pigment during the forward flow step. For example, the pump could operate at the same speed in the forward flow step as in the reverse flow step. In the example given above, this results in a flow rate that is about 1¼ times the normal full speed for forward operation while the inkjet <NUM> is running. The pump speeds in both the forward and the reverse flow steps may be increased further if the pump is capable of this and the remainder of the ink circuit can withstand the higher flow rates and higher ink pressures. For example, the pump speeds could be 2½ time the maximum pump speed that is permitted during normal printing operation. These higher speeds will enhance the disturbance of pigment during the mixing cycle.

In the embodiment of <FIG>, the nozzles <NUM> provide vertical semi cylindrical flow paths, but a chamfered edge around the bottom of the shroud <NUM> means that the openings at the bottom of each nozzle <NUM> are angled, with the result that the ink flow into and out of the nozzle openings tends to have a sideways component. This sideways component of the flow direction increases the spread of the flow plume during the reverse flow and increases the region of localised disturbance caused by ink flow into the nozzle during normal forward flow. However, other nozzle designs are also possible.

In general, the nozzles should either face towards the floor of the ink feed tank <NUM> or should be situated at the floor of the ink feed tank and face across it. This enables the flow of ink out of the nozzles <NUM> during reverse flow to interact with sediment that has accumulated on the floor of the ink tank. It also enables the localised ink disturbance caused by ink flow into the nozzles during normal operation to agitate pigment that might be falling towards the floor, and thereby to tend to mix it back into the ink.

An alternative nozzle design is shown in <FIG>, which is a view similar to <FIG> of an embodiment in which the shroud <NUM> is modified. In <FIG>, each nozzle <NUM> provides an angled ink flow path through it, rather than a vertical flow path as in <FIG>. This design can be effective to ensure that the direction of ink flow into and out of the nozzle includes a horizontal component, for the purpose of increasing the area on the floor of the ink feed tank <NUM> that is subjected to disturbance by ink flow through the nozzle <NUM>.

A further embodiment is shown in <FIG>, in which the sides of the path through the nozzle <NUM> converged towards the nozzle opening. This tends to increase the flow velocity through the nozzle opening. It will also tend to alter the geometry of the flow plume created by the nozzle during reverse flow, and will therefore alter the way in which the reverse flow affects pigment that has settled on the floor of the ink feed tank <NUM>. It is also possible to angle the sides of the flow path through the nozzle so that they diverge towards the nozzle opening, which will reduce the nozzle velocity but increase the width of the flow plume.

Other designs are possible. For example, as shown more schematically in <FIG>, the nozzles <NUM> can provide vertical flow paths around the shroud <NUM>, with no chamfered edge, so that the nozzle openings are horizontal and flow from and to the ink tank <NUM> into and out of the nozzles <NUM> is essentially vertical.

As shown in <FIG>, the nozzles <NUM> can spiral round the shroud <NUM>, so that the ink flow into and out of the nozzle openings is in a direction that has a component which is circumferential of the shroud <NUM>.

The various possibilities for varying the positioning and internal geometry of the nozzles <NUM>, as illustrated in the embodiments discussed above, provide the designer with options for adjusting the design of the nozzles <NUM> to suit the particular geometry of the ink tank in which the system will be used.

It is also possible to replace the nozzles <NUM> by a single nozzle that extends around the entire circumference of the shroud <NUM>. This is shown in <FIG>, which respectively show the shroud <NUM> from the outside and show it in section, in a similar way to <FIG>. In this case, the nozzle opening extends around the entire circumference of the filter assembly, and in order to provide the required cross sectional area for the nozzle opening, the space between the bottom of the shroud <NUM> and the filter assembly base plate <NUM> must be very small. For example, in the detailed construction described above the filter assembly is <NUM> in diameter and therefore <NUM> in circumference. The total nozzle area is about <NUM><NUM>. Consequently, the gap between the shroud <NUM> and the filter assembly base plate <NUM> needs to be less than <NUM> to provide the correct total cross sectional area. Therefore this design is less preferred than designs using multiple nozzles, because the necessary manufacturing tolerances tend to make it expensive.

In the embodiments described above, the nozzles <NUM> have been provided by a shroud <NUM> fitted around the filter assembly. However, the nozzles <NUM> can be provided in other ways. For example, as shown in <FIG>, the ink filter <NUM> may not be provided inside the ink feed tank <NUM> at all, but may be provided in the ink supply line <NUM> between the ink feed tank <NUM> and the ink pump <NUM>. In this case, the ink outflow tubes <NUM> in the ink feed tank <NUM> extends further down towards the bottom of the ink feed tank <NUM> than is shown in <FIG>, and may have an enlarged diameter fitting <NUM> at the end, as shown in <FIG>, to provide the nozzles <NUM>. The fitting <NUM> has an enlarged diameter in order to accommodate a sufficient number of nozzles <NUM> and also in order to spread the nozzles <NUM> out over a larger area of the floor of the ink feed tank <NUM> and thereby increase the area of the floor of the ink tank <NUM> where settled pigment is disturbed by the fluid flow out of nozzles <NUM>.

In <FIG> the fitting <NUM> is suspended slightly above the floor of the ink tank <NUM> and the nozzles <NUM> face downwardly and outwardly. Accordingly, when the ink pump <NUM> operates in reverse the fluid flow out through the nozzles <NUM> have a direction that is partially downwards and particularly outwards, as shown by the arrows in <FIG>. In the alternative construction in <FIG>, the nozzles <NUM> face outwardly, and the fluid flow out through the nozzles is horizontal, as shown by the arrows. In this case, the nozzle <NUM> are very close to the floor of the ink feed tank <NUM> so that the horizontal flow dislodges the pigment that has settled on the floor, and in this case the fitting <NUM> can rest on the floor of the ink feed tank <NUM> as shown in <FIG>.

Preferably the ink pump <NUM> is reversible, in order to provide the reverse flow of fluid out through the nozzle <NUM>. In this way, the ink pump <NUM> is an example of a reversible ink drive arrangement or an ink pump arrangement that is operable in both a forward direction and a reverse direction. However, a non-reversible pump can be used provided that there is an arrangement to enable fluid to be driven in the reverse direction along the ink supply lines <NUM> and the ink return line <NUM>. For example, a valve arrangement may be provided to switch the route of fluid flow around the ink circuit in order to enable both forward and reverse flow. An example of such an arrangement is shown in <FIG>, where the ink pump <NUM> is not reversible and a spool valve <NUM> can switch the inlet and outlet connections of the pump <NUM> between the ink supply lines <NUM> and the line to the Venturi <NUM>. In <FIG>, if the spool valve <NUM> connects the lines together as shown in the unbroken lines through the valve <NUM>, the pump <NUM> drives fluid around the ink circuit in the forward direction. If the spool valve <NUM> is operated so as to make the connections shown in broken lines and not the connections shown in unbroken lines, the effect of the pump <NUM> will be to pump fluid around the ink circuit in reverse direction, even though the direction of fluid flow through the pump <NUM> does not change. Thus in <FIG> the pump <NUM> is operable to drive fluid around the ink circuit in both the forward direction and the reverse direction even though the pump is not itself reversible. The pump <NUM> and the valve <NUM> together provide a reversible ink drive arrangement or an ink pumping arrangement operates in a forward direction and in a reverse direction.

In the embodiments described above, the ink return line <NUM> opens into the interior of the ink feed tank <NUM> at a location above the normal level of the ink surface (shown by the broken line A-A in <FIG>). <FIG> shows an alternative arrangement, in which the ink return line <NUM> branches. A first branch 39a opens into the interior of the ink feed tank <NUM> above the ink surface level and a second branch 39b opens in to the interior of the ink feed tank <NUM> below the ink surface level, as shown in <FIG>. A valve <NUM> in the ink return line <NUM> selects which branch is used.

During normal operation of the printer, when the ink jet <NUM> is running, the first branch 39a is used so that the ink and air in the ink return line <NUM> are delivered into the space above the ink in the ink feed tank <NUM>. However, during the ink mixing operation the second branch 39b may be used for all or part of a reverse flow step or for all or part of a forward flow step or for all or part of both.

If the second branch 39b is used during a reverse flow step, ink will enter the ink return line <NUM> instead of air, and a longer period of ink flow out through the nozzles <NUM> is possible. This can help to disturb the settled pigment on the floor of the ink tank <NUM>.

If the second branch 39b is used during a forward flow step, the ink returning to the ink tank <NUM> will flow directly into the ink already in the ink tank <NUM>, and this will increase the movement of ink within the ink feed tank <NUM> and will this help to disturb the settled pigment. If the second branch 39b of the ink return line <NUM> opens into the interior of the ink feed tank at or near the bottom of the tank, as shown in <FIG>, and the opening faces parallel or at least partially towards the floor of the ink tank <NUM>, the flow of ink into the ink tank from the ink return line during the forward flow step will be effective to disturb pigment that has settled on the floor of the ink tank <NUM>. In this case, the second branch 39b of the ink return line <NUM> may have multiple openings into the interior of the ink feed tank <NUM>, e.g. using a fitting similar to the fitting <NUM> described with reference to <FIG>, in order to enhance the ability of the ink flow from the ink return line <NUM> to disturb the pigment on the floor of the ink feed tank <NUM>.

Claim 1:
A continuous ink jet printer comprising an ink circuit, the ink circuit comprising an ink tank (<NUM>), an ink pump arrangement (<NUM>), an ink supply path (<NUM>) from a plurality of ink inlet openings inside the ink tank (<NUM>) to the ink pump arrangement (<NUM>) and an ink return path (<NUM>) from the ink pump arrangement (<NUM>) to the interior of the ink tank (<NUM>), the plurality of ink inlet openings being spaced in one or more directions parallel to the floor of the ink tank,
the printer further comprising a control system (<NUM>) arranged (a) to control the ink pump arrangement (<NUM>) to pump ink around the ink circuit in a forward direction, in which the ink flows from the interior of the ink tank (<NUM>) through the plurality of ink inlet openings and along the ink supply path (<NUM>) to the ink pump arrangement (<NUM>), and from the ink pump arrangement (<NUM>) along the ink return path (<NUM>) back to the interior of the ink tank (<NUM>),
characterised in that
the control system (<NUM>) is also arranged (b) to control the ink pump arrangement (<NUM>) to pump fluid around the ink circuit in a reverse direction, in which the fluid flows from the interior of the ink tank (<NUM>) along the ink return path (<NUM>) to the ink pump arrangement (<NUM>), and from the ink pump arrangement (<NUM>) along the ink supply path (<NUM>) and through the plurality of ink inlet openings back to the interior of the ink tank (<NUM>).