Patent ID: 12233646

DETAILED DESCRIPTION OF EMBODIMENTS

FIG.1shows an electrostatic deflection type continuous ink jet 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 printer body1. An operator communicates with the printer via a touchscreen display3. The ink jet is formed within a print head5, which also includes the electrode arrangement for charging and deflecting the ink drops, and the print head5is connected to the printer body1by a flexible connection7known as a conduit or an umbilical. Drops of ink, deflected as necessary to create the desired pattern, travel from the print head5and strike the surface9of an object11conveyed past the print head5, in order to print the desired pattern on the surface9of the object11. The print head5and the umbilical7form a print head assembly that may be disconnectable from the printer body1.

The printer is typically an industrial ink jet printer and is suitable to be used with a conveyor13that conveys objects11past the print head to be printed onto. This is in contrast to a document printer that prints onto flat sheets, and which normally conveys the sheets itself rather than being used with a conveyor13that is external to the printer. The object11may be a manufactured product item, such as a bottle or can of drink, a jar of jam, a ready meal, or a carton containing multiple individual items. The desired pattern may comprise product information such a batch number or a “use by” date. The printer may print onto the object11from the side so that the ink jet travels in a direction generally across the conveyor, or from above so that the ink jet travels in a direction generally towards the conveyor, or from any other angle. For example, bottles are normally printed onto from the side whereas ready meals are normally printed onto from above. InFIG.1the printer is set up to print from the side and partially above.

FIG.2is a schematic top view andFIG.3is a schematic side view of the main components of the print head5in the region of the ink jet. The terms “top view” and “side view” represent conventional directions from which to view the print head on the assumption that the printer will print onto an object11from the side, and do not necessarily correspond to the orientation of the print head when in use. Pressurised ink, delivered from the printer body1through the umbilical7, is provided via an ink feed line15to an ink gun (or nozzle)17. The pressure of the ink drives it out of the ink gun17through a small jet-forming orifice to form an ink jet19. Provided that pressurised ink is received by the ink gun17and any valves in the ink gun17are in the appropriate state, the ink jet19is 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 jet19leaves the ink gun17as a continuous unbroken stream of ink, it rapidly breaks into separate drops. The path of the ink jet passes through a slot in a charge electrode21, which is positioned so that the ink jet19separates into drops while it is in the slot through the charge electrode21. Other arrangements and other shapes of charge electrode21are possible, so long as the ink jet19is subject to the electric field of the charge electrode at the position where it separates into drops. The ink is electrically conductive and the ink gun17is held at a constant voltage (typically ground). Accordingly, any voltage applied to the charge electrode21induces a charge into the part of the ink jet19that is subject to the electric field in the slot of the charge electrode21. As the ink jet19separates into drops, any such charge is trapped on the drops. Accordingly, the amount of charge trapped on each drop can be controlled by the voltage on the charge electrode21and different amounts of charge can be trapped on different drops by changing the voltage on the charge electrode21.

The ink jet19then passes between two deflection electrodes23,25. A large potential difference (typically several kilovolts) is applied between the deflection electrodes23,25to 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 inFIG.2, uncharged ink drops, which pass through the electric field without deflection, travel to a gutter27where they are caught. Suction is applied to the inside of the gutter27by a gutter suction line29, and so the ink received by the gutter27is sucked away and returned through the umbilical7to the printer body1, for re-use.

Drops of ink that are deflected by the field between the deflection electrodes23,25, so as to miss the gutter27, leave the print head5and form printed dots on the surface9of the object11.

The ink gun17, the charge electrode21, the deflection electrodes23,25and the gutter27are mounted on a baseboard31. The gutter suction line29extends beneath the baseboard31. It may also be convenient to route the electrical connections for the charge electrode21and the deflection electrodes23,25beneath the baseboard31, as shown inFIG.3. The print head5also contains electronic circuits (not shown inFIGS.2and3), which may be positioned beneath the baseboard31. The deflection electrodes23,25may be mounted so that they each extend perpendicular to the plane of the baseboard31. Alternatively they may extend parallel to the plane of the baseboard, as shown inFIGS.2and3, with one deflection electrode23lying on the baseboard31and the other deflection electrode25being spaced above the baseboard31and supported by one or more electrode supports33. Normally the deflection electrode23lying on the baseboard will be connected to ground and the deflection electrode25spaced above the baseboard31will be connected to a high voltage supply to create the deflection field. The electrical connection for the deflection electrode25spaced above the baseboard31may be carried in one of the electrode supports33.

FIG.4is a simplified schematic diagram of a fluid system for the ink jet printer ofFIG.1. Ink is held in an ink feed tank35in the printer body1. The ink feed tank35is the main ink tank of the printer. The interior of the ink feed tank35is held at atmospheric pressure by a vent37. Ink is sucked out of the ink feed tank35by a pump39, via a filter41and an ink supply line43. The ink, pressurised by the pump39, flows through a Venturi45and back to the ink feed tank35via an ink return line47. A pressure transducer (pressure sensor)49is used to sense the ink pressure on the outlet side of the ink pump39.

The ink feed line15is also connected to the outlet side of the ink pump39and receives pressurised ink. Thus the ink feed line15provides an ink feed path to supply pressurised ink from the ink pump39to the ink gun17. An ink feed valve51controls the flow of ink along the ink feed line15. The pump39can drive ink continuously through the Venturi45and back to the ink feed tank35, even when the ink feed valve51prevents ink from flowing along the ink feed line15. The flow of ink through the Venturi45generates suction and accordingly the Venturi acts as a suction source. The gutter suction line29is connected to a suction inlet of the Venturi45to receive suction which sucks ink from the gutter27through the umbilical7back to the printer body1. The ink from the gutter suction line29is sucked into the Venturi45and returns to the ink feed tank35. Fluid flow in the gutter suction line29is controlled by a gutter valve53.

Spare solvent is held in a solvent reservoir55which receives suction from the Venturi45through a solvent top-up line57. If solvent needs to be added to the ink in the ink feed tank35to dilute the ink and correct its viscosity, a solvent top-up valve59in the solvent top-up line57is opened briefly. This allows the Venturi45to suck a small quantity of solvent from the solvent reservoir55into the ink flow through the Venturi45. The solvent sucked into the Venturi45then passes into the ink feed tank35to dilute the ink.

Spare ink is held in an ink reservoir61which receives suction from the Venturi45through an ink top-up line63. When the level of ink in the ink feed tank35becomes low, an ink top-up valve65in the ink top-up line63is opened. Ink is sucked out of the ink reservoir61by the Venturi45and is delivered to the ink feed tank35in a similar manner to the operation for topping up with solvent from the solvent reservoir55.

The solvent reservoir55and the ink reservoir61are supplied from a solvent container67and an ink container69respectively, and the operator replaces the containers67,69as necessary. In practice, it is not always necessary to provide the solvent reservoir55and the ink reservoir61, and the respective top-up lines57,63may be connected directly to the containers67,69.

FIG.5shows schematically some of the components inside the printer body1of the printer. The printer has a printer body ink system71, which includes the components inFIG.4that are shown inside the printer body1. The printer body ink system71and other parts of the printer operate under the control of a control system73that comprises electronic circuits. The control system73, for example, sends drive currents to the ink pump39and to the various valves,51,53,59,65of the printer body ink system71. The control system73receives outputs from the pressure sensor49and also from level sensors in the ink feed tank35, the solvent reservoir55and the ink reservoir61. The electronics in the control system73communicates with the electronics in the print head5via the umbilical7. The control system73also provides outputs to, and receives inputs from, the touchscreen display3. Typically, the control system will include a processor such as a microprocessor and other electronic components as is well known in the art.

Fluid lines75connect the printer body ink system71to the print head5through the umbilical7. These fluid lines will include the ink feed line15, and the gutter suction line29shown inFIG.4. Electrical lines77connect the control system73to the print head5via the umbilical7. These electrical lines include signal lines for communication between the electronics of the control system73and the electronics in the print head5and also lines for applying the appropriate voltages to the charge electrode21and the deflection of electrodes23,25, and for applying a drive signal to a piezoelectric crystal inside the ink gun17that applies a vibration to the ink that forms the ink jet19in order to control the manner in which it breaks into drops.

The printer receives electric power at a power socket79, which is converted in a voltage converter81to the various voltages required internally within the printer. For example, the printer may be designed to receive 24 volt DC at the power socket79, since power supplies for generating 24 volts DC from an electric mains supply are widely available. The voltage converter81uses the received 24 volt supply to generate the voltages required to power the electronics in the control system73, which may for example be 5 volts. It also supplies power to a component, either in or controlled by the control system73, to generate the voltages (e.g. up to about 300 V) applied to the charge electrode21, the EHT voltage (e.g. about 4 kV) applied to the upper deflection electrode23and to generate the drive signal for the piezoelectric crystal inside the ink gun17.

The power socket79also provides a connection to an external electrical earth. This is used to earth the external case of the printer body1. The earth connection is also provided to the voltage converter81, which uses it to provide an earth to any components that need an earth. The control system73uses the earth received from the voltage converter81to provide an electrical ground for the electronic circuits in the control system73and to provide an electrical ground for connection to the signal earth line in the umbilical7so as to provide a signal earth to the electronic circuits in the print head5.

FIGS.6and7are side views of the part of the print head5where the ink jet19is present. A removable print head cover83is shown in section. The cover83ensures that the full length of the ink jet19from the ink gun17to the gutter27is enclosed while the printer is in use, but removal of the cover allows access to the space where the ink jet19is formed in order to enable inspection or cleaning.

In the embodiment ofFIG.6the cover83is generally cylindrical and fully encloses the corresponding part of the print head5. In the embodiment ofFIG.7the cover83is generally semi-cylindrical, and covers the upper half of the corresponding part of the print head. InFIG.7the external surface of the print head5is exposed at the lower half of the print head. Many other designs of cover83are possible. In bothFIG.6andFIG.7the downstream (with respect to the direction of travel of the ink jet19) end of the cover83is closed but has an exit hole85to allow ink drops used for printing to exit from inside the cover.

In bothFIG.6andFIG.7, the cover83is secured to the rest of the print head by a cover retaining screw87. The retaining screw87is metal, and its exposed end is covered by an insulating handle89. The handle allows an operator to turn the screw, to release or fasten the cover83, by hand. Other fastening arrangements are possible. However, the retaining screw87is convenient because it also ensures that the cover83makes a good contact with an electrical earth connection, as discussed below with reference toFIGS.8and9.

The print head cover83is made of an anti-static or static dissipative material. An anti-static material can be regarded as a material having an electrical surface resistivity in the range of 1010to 1012ohms per square or an electrical bulk resistivity in the range of 107to 109ohm metres and a static dissipative material can be regarded as a material having a surface resistivity in the range of 105to 1010ohms per square or a bulk resistivity in the range of 100 to 107ohm metres. Preferably the material of the print head cover83is a plastic or other mouldable material.

In the operation of the printer, the drops of ink in the ink jet19either pass into the gutter27or pass out of the print head through the hole85in order to print dots on the surface9of the object11. Therefore no ink drops should come into contact with the print head cover83. However, microdrops (which are much smaller than the drops of ink in the ink jet) can also occur while the ink jet19is running. Microdrops may be formed as the ink jet19breaks into drops at the charge electrode21or from the impact of drops on a contact surface inside the gutter27. They may also be formed outside the print head cover83from the impact of drops on the surface9that is being printed onto.

It is likely that some of the microdrops will carry an electric charge. Any charged microdrops that hit one of the deflection electrodes23,25will discharge their charge to the electrode, and the charge will be dissipated by the electrical connection to the electrode. Any microdrops in the space enclosed by the print head cover83that miss the deflection electrodes23,25will tend to hit the print head cover83in the vicinity of the exit hole85. Microdrops formed outside the print head cover may also hit the print head cover83, again in the vicinity of the exit hole85. Accordingly the print head cover83may receive electric charges from the microdrops. If the print head cover83was insulated, these electric charges could accumulate on the print head cover83and create an electric field that would interfere with the correct deflection of the ink drops. This is avoided because the print head cover83is made of an anti-static or static dissipative material as stated above, and is electrically earthed.

FIG.8shows an arrangement for earthing the print head cover83. The print head cover83is fastened to the body of the print head5by the retaining screw87, which passes though the print head cover83and engages with a threaded block91mounted in the body of the print head5. The retaining screw87and the threaded block91are both metal, and therefore electrically conductive, and the threaded block91is connected to a cover earth line93for earthing the print head cover83. As mentioned above, there are electronic circuits in the print head5and a signal earth line extends from the electronic circuits in the print head along the umbilical7so as to provide a signal earth via the printer body1. The cover earth line93is connected to the signal earth line within the print head5and thereby provides an earth connection for the print head cover83via the signal earth line.

When the retaining screw87is tightened, it presses the print head cover83against the retaining block91and so the print head cover makes a good connection to the retaining block91both by direct contact and via the retaining screw87. In this way, any electric charges that arrive at the print head cover83will flow slowly through the material of the print head cover83or over the surface of the print head cover83to reach the retaining screw87and the threaded block91, and will then be earthed via the cover earth line93and the signal earth line. Accordingly, electric charges do not accumulate on the print head cover83.

FIG.9shows an alternative arrangement in which the print head cover is not earthed via the cover retaining screw87and the threaded block91. Instead, the print head cover83contacts a separate metal earthing block95that is fitted into the body of the print head5.

In this arrangement, the cover earth line93is connected to the earthing block95. As shown inFIG.9, the earthing block extends outwards slightly further than the adjacent surface of the body of the print head5, to ensure that the print head cover83makes a good contact with the earthing block95.

The print head cover83may also receive an electrostatic discharge. This may occur for example if the print head cover83is touched by a nearby person who carries an electrostatic charge. It may also occur if the printer is being used to print onto a continuous plastic web that may become charged as it unwinds from a reel. An electrostatic discharge to the print head cover83results in a sudden large voltage arising at the print head cover83. Since the print head cover83is electrically connected to the signal earth line by the cover earth line93, there is a possibility that the operation of the electronic circuits in the print head may be disrupted, or the circuits themselves may even be damaged, by a sudden large voltage appearing on the signal earth line. This is avoided by ensuring that there is adequate electrical resistance between the place on the print head cover83that receives the electrostatic discharge and the place where the cover earth line93joins the signal earth line.

The electric circuit for modelling the effect of an electrostatic discharge is shown inFIG.10. The source of the electrostatic discharge is represented by a human body model for electrostatic discharge. This is based on JEDEC standard JS-001. InFIG.10, the human body is modelled as a 100 pF capacitor charged to 8 kV and connected for discharge through a 150Ω resistance. As shown inFIG.10, the cover earth line93is connected inside the print head5to the signal earth line97at a junction99. The signal earth line97, together with multiple print head signal data lines101, extends from the electronic circuits103in the print head5along the umbilical7to the printer body1.

The print head5and the umbilical7jointly form a print head assembly that7can be disconnected from the printer body1, e.g. to allow a different print head assembly to be fitted so as to change the type of print head5or change the length of the umbilical7. As shown schematically inFIG.11, where the umbilical7meets the printer body1an umbilical fluid line connector105mates with a printer body fluid line connector107, an umbilical electrical connector109mates with a printer body electrical connector111and an umbilical HT connector113mates with a printer body HT connector115. The fluid line connectors105,107make connections between the umbilical7and the printer body1for fluid lines75such as the ink feed line15and the gutter suction line29. The electrical connectors109,111make connections between the umbilical7and the printer body1for electrical lines77. The electrical lines77include the signal earth line97, the print head signal data lines101and lines carrying other signals such as the drive signal to a piezoelectric transducer in the ink gun17that imposes pressure vibrations on the ink as it forms the ink jet and the drive signal for the charge electrode21. Accordingly, the umbilical electrical connector109comprises a signal earth line umbilical connector and signal data line umbilical connectors, and the printer body electrical connector111comprises a signal earth line printer body connector and signal data line printer body connectors, amongst other connectors. The electrical line carrying the high voltage to be applied to the deflection electrode25is connected using the separate HT connectors113,115.

As shown inFIG.10, the signal earth line97and the print head signal data lines101are connected in the printer body1to the control system73. Electronic circuits in the control system73communicate with the electronic circuits103in the print head5via the print head signal data lines101. The control system73provides an earth connection for the signal earth line97via the voltage converter81to the power socket79. The power connection to the power socket79provides a connection to an external earth.

The printer body1provides a very low resistance connection to earth for the signal earth line97. Additionally, the length of the signal earth line within the print head5is short and provides very little electrical resistance. The electrical resistance between the external earth and the junction99(where the cover earth line93joins the signal earth line97) is almost entirely provided by the resistance of the part of the signal earth line97that is in the umbilical7, as this represents almost all of the length of the signal earth line97. InFIG.10, this resistance is represented by resistance Rs. Resistance Rc inFIG.10represents the resistance between the place on the print head cover83where the electrostatic discharge occurs and the junction99where the cover earth line93joins the signal earth line97.

In order to avoid disruption of the operation of the electronic circuits103in the print head5and to avoid corruption of data communicated between the electronic circuits103and the control system73in the printer body, the voltage on the signal earth line97at the electronic circuits103(and therefore the voltage at the junction99) should not fluctuate by more than 0.5 V during an electrostatic discharge event. The voltage fluctuation at the junction99is provided by the voltage divider effect of the resistance Rs and the resistance between the junction99and the 100 pF capacitor in the human body model ofFIG.10. InFIG.10, the electrostatic discharge is modelled as providing an electric potential of 8 kV. Therefore the resistance between the junction99and the 100 pF capacitor (i.e. Rc plus 150Ω) must be 16,000 times Rs. If the resistance Rs is 1Ω, the resistance between the junction99and the 100 pF capacitor must be at least 16 kΩ. This is much greater than the 150Ω resistance in the human body model, and so in effect this requires Rc to be at least 16 kΩ.

In practice, the resistance Rs will depend on the length of the umbilical7as well as the grade of wire used in the umbilical7for the signal earth line97. In practice, if the signal earth line is provided by a copper wire having a diameter of 1 mm and the umbilical is only 0.5 m long, the resistance Rs may be about 0.01Ω and so Rc need only be 160Ω. If the signal earth line is provided by a copper wire having a diameter of 0.5 mm and the umbilical is 8 m long, the resistance Rs may be about 0.6Ω so that resistance Rc should be at least 9,600Ω. Therefore if the resistance Rc is at least 16,000Ω this should be adequate to avoid an undesirable spike in the voltage at the earth connection for the electronic circuits103in the print head5in all printer designs and all umbilical lengths that are likely to be used under normal circumstances.

It is preferred to provide the resistance Rc by the resistance of the material of the print head cover83, and to provide the cover earth line93as a low resistance wire. In the design ofFIG.8the cover retaining screw87is metal and is in electrical contact with the cover earth line via the threaded block91. Therefore the screw handle89must be electrically insulating or alternatively have sufficient electrical resistance so that at least the minimum desired value for Rc is provided between retaining screw87and the hand of an operator touching the screw handle89.

Additionally, if a person touches the print head cover83very close to the retaining screw87in the design ofFIG.8or very close to the position of the earthing block95inFIG.9, the path from the person's hand to the retaining screw87or the threaded block91inFIG.8or the earthing block95inFIG.9may be only a few millimetres. Depending on the material used for the print head cover83, this distance may be insufficient to provide the desired minimum value for Rc. In this case, a layer117of insulating material as shown inFIGS.8and9can be provided on the outer surface of the print head cover83in the vicinity of the retaining screw87or the earthing block95in order to ensure that the desired minimum value for Rc is maintained under these circumstances.

Preferably the material of the print head cover has an electrical surface resistivity of at least 107ohms per square or an electrical volume resistivity of at least 104ohm metres. This will usually be adequate to provide the desired minimum value for the resistance Rc even if the print head cover is touched as close as possible to the electrical connection to the cover earth wire93, so that there is no need to provide an insulating layer117.

Further embodiments are also possible. For example, it may be more convenient to route the cover earth line93below the baseboard31in the print head5rather than to the arrangements shown inFIGS.8and9. Accordingly,FIG.12shows a schematic sectional view of the end part of the print head5and the print head cover83. In this embodiment the earthing block95is provided in the end surface of the body of the print head5, immediately below the baseboard31. The end surface of the print head cover83extends far enough below the level of the baseboard31to make contact with earthing block95. This position for the connection of the print head cover83to the cover earth line93also provides a shorter (and therefore lower resistance) path to the cover earth line for charges that arrive on the print head cover83in the vicinity of the ink jet exit hole85.

Although it is preferred to make the print head cover from an anti-static or static dissipative material, it is also possible to make all or part of it from an electrically conductive material provided that the path from the conductive material to the cover earth line93includes something to provide the required resistance Rc. For example, it would be possible to make part of the print head cover from a conductive material and part from an anti-static or static dissipative material, and to provide the connection to the cover earth line93at the part made from an anti-static or static dissipative material. The anti-static or static dissipative part would still provide the necessary resistance Rc between the electrically conductive part and the cover earth line93.

Alternatively an arrangement could be provided such as is shown inFIG.13. In this case the main part of the print head cover83is made of an anti-static or static dissipative material, but an end plate119of the print head cover, surrounding the ink jet exit hole85, is metal and electrically conductive. The connection to the cover earth line93is made by an earthing block95below the baseboard31in the same way as inFIG.12. Therefore the earthing block95contacts the metal end plate119. There is negligible electrical resistance between the earthing block and all places on the end plate119. Therefore the required resistance Rc is provided by a resistor121in the cover earth line93.

In the embodiment ofFIG.13the metal end plate119is at the part of the print head cover83that receives almost all of the charged microdrops that reach the print head cover83. Therefore it is possible in this embodiment to make the remainder of the print head cover83from an electrically insulating material while still avoiding a substantial build-up of electrical charge on the print head cover83.

However, the design of the print head cover83inFIG.13is less preferred than the designs of the print head cover83inFIGS.8,9and12because it is more complex to manufacture.

The embodiments discussed above enable electrical charge build-up on the print head cover83to be avoided and an electrostatic discharge event to be accommodated using the earth connection provided by the signal earth line97. It is possible with these embodiments to provide sufficient resistance to earth from all points on the print head cover83so that a safety earth connection is not required. By comparison, it is known to provide a metal print head cover for an electrostatic deflection continuous ink jet printer, which has a very low resistance safety earth connection to the printer body via the umbilical. Charges from microdrops that strike the print head cover and electrostatic discharge events will also be earthed by the safety earth connection. An electrostatic discharge event will cause high frequency current transients in the safety earth connection, and these will tend to flow over the surface of the earth conductor and not through its bulk. Therefore a wire braid earth connection is usually provided along the length of the umbilical in addition to the safety earth connection, in order to provide a large surface area to carry these current transients. This adds to the cost of the umbilical, makes it more awkward to assemble, and also makes it stiffer and more awkward to handle. In the embodiments discussed above, it is not necessary to use a safety earth or this wire braid because the earth connection is made via the signal earth line97, and the resistance Rc between the electrostatic discharge event and the signal earth line97prevents significant current transients arising in the signal earth line97. Although the junction99between the cover earth line93and the signal earth line97is preferably in the print head5, it is possible to place this junction in the umbilical7near the end of the umbilical7at the print head5. However it is preferred that the junction99should be no further along the umbilical7than 10 cm from the end at the print head5, in order to preserve the benefits provided by joining the cover earth line93to the signal earth line97.

In an alternative embodiment, shown inFIG.14, the cover earth line93does not join the signal earth line97. Instead, the cover earth line93extends along the entire length of the umbilical7to the control system73, and the control system73provides an earth connection for the cover earth line93via the voltage converter81to the power socket79. The signal earth line97is earthed separately via the electronic circuits of the control system73. In this case, the electrical lines77ofFIG.5include the cover earth line and inFIG.11the umbilical electrical connector109and the printer body electrical connector111comprise respective connectors for the cover earth line93as well as connectors for the signal earth line97.

In this embodiment, an electrostatic discharge to the print head cover83is earthed via the cover earth line93and is not connected to the signal earth line97. Therefore the voltage divider ofFIG.10does not exist in this embodiment. However, the cover earth line93extends adjacent to the signal earth line97and the signal data lines101along the length of the umbilical7, and there will almost inevitably be a significant capacitive coupling between at least some of the lines. Consequently, if an electrostatic discharge event creates a voltage pulse on the part of the cover earth line93in the umbilical7, this voltage pulse will be capacitively coupled into the signal earth line97and/or some of the signal data lines101. Consequently there remains a possibility that the electronic circuits103may be damaged or disrupted or the data on the signal data lines101may be corrupted.

In practice it is possible to avoid significant capacitive coupling of the signal earth line97and the signal data lines101to the first 10 cm of the cover earth line93in the umbilical7, partly because the cover earth line93may be held spaced apart from the other lines97,101by the fitting at the end of the umbilical that holds the various lines in the correct positions as they pass into the print head5, and partly because the degree of capacitive coupling depends on the length of line involved and so the degree of coupling from the first 10 cm is low. Resistance Rp inFIG.14represents the electrical resistance between the place on the print head cover83where the electrostatic discharge occurs and the place on the cover earth line93that is 10 cm into the umbilical7. The electrical resistance between the external earth and the place on the cover earth line93that is 10 cm into the umbilical7is almost entirely provided by the resistance from that place on the cover earth line93to the end of the umbilical7at the printer body1. InFIG.14, this resistance is represented by resistance Ru.

As noted above, the voltage on the signal earth line97(and on the signal data lines101) should not fluctuate by more than 0.5 V during an electrostatic discharge event. Therefore the voltage on the part of the cover earth line93that is more than 10 cm into the umbilical7should not fluctuate by more than 0.5 V. The electrostatic discharge is modelled as providing an electric potential of 8 kV.

The voltage fluctuation at the place on the cover earth line93that is 10 cm into the umbilical7is provided by the voltage divider effect of the resistance Ru and the resistance between this place and the 100 pF capacitor in the human body model (i.e. Rp plus 150Ω). As discussed with reference toFIG.10, the contribution of the 150Ω resistance in the human body model can be ignored. Therefore the voltage coupled into the signal earth line97and the signal data lines101can be limited to no more than 0.5 V if resistance Rp is at least 16,000 times Ru. If the resistance Ru is 1Ω, this requires Rp to be at least 16 kΩ.

The various discussions above concerning the values of Rc and Rs inFIG.10, and the ratio of their values, can be applied in an analogous manner to Rp and Ru inFIG.14.

In this embodiment, the cover earth line93provides an extra electrical line in the umbilical7compared with the embodiment ofFIG.10. However, the value of Rp will be such that the current carried by this line will be low, even during an electrostatic discharge event, and so it can be provided as a simple small-diameter copper wire and it is still unnecessary to provide a metal braid to carry high-frequency current transients or a high-current safety earth.

In principle, it would be possible to extend the cover earth line93more than 10 cm into the umbilical7and then join it to the signal earth line97, as shown inFIG.15. In this case, the electrical resistance between the place on the print head cover83where the electrostatic discharge occurs and the place on the cover earth line93that is 10 cm into the umbilical7is resistance Rp as inFIG.14, and the electrical resistance between the junction99(where the cover earth line93joins the signal earth line97) and the end of the umbilical7is resistance Rs as inFIG.10. The resistance from the place on the cover earth line93that is 10 cm into the umbilical7to the junction99is shown as resistance Rx inFIG.15. If the analysis ofFIG.14is applied toFIG.15, Ru ofFIG.14is provided by Rx plus Rs inFIG.15. Therefore Rp should be at least 16000 times Rx+Rs.

If the analysis ofFIG.10is applied toFIG.15, Rc ofFIG.10is provided by Rp plus Rx. Since Rp is at least 16000 times Rx+Rs, Rp is more than 16000 times Rs. Therefore Rp plus Rx must be more than 16000 times Rs. Consequently, inFIG.15if Rp is at least 16000 times Ru as required byFIG.14, it is inevitable that Rc is at least 16000 times Rs as required byFIG.10.

FIG.16shows a further embodiment, in which the print head5is simpler and does not include any electronic circuits103. Therefore there is no signal earth line97and no print head signal data lines101for the electronic circuits, although there may be other electrical lines for other electrical components such as valves, electrodes and an ink pressure vibration source. In this embodiment, at least the part of the print head cover83around the exit hole85, and preferably the entire print head cover83, is made from a material having an electrical surface resistivity of at least 105ohms per square and no more than 1012ohms per square or an electrical volume resistivity of at least 100 ohm metres and no more than 109ohm metres. The material is preferably a mouldable polymeric material. The print head cover83may have any of the designs discussed above other than the use of a metal end plate119as shown inFIG.13. The cover earth line93extends along the entire length of the umbilical7to the control system73, and the control system73provides an earth connection for the cover earth line93via the voltage converter81to the power socket79, in the same way as inFIG.14. The resistance between the place on the print head cover83where the electrostatic discharge occurs and the external earth is represented by resistance Re inFIG.16. Resistance Re is at least 100Ω, in order to limit the current that flows in the cover earth line93if there is an electrostatic discharge to the print head cover83.

If Re is 100Ω, an electrostatic discharge of 8 kV in accordance with the human body model as shown inFIG.16will flow through a total of about 250Ω including the resistance in the human body model. This will result in a peak current of about 32 A (or less if there is a significant impedance). Since the current flow is brief, there will be no sustained heating of the cover earth line93and so this current can be carried by a 1 mm copper wire without problems.

The earth connection for the print head cover83is not a safety earth and the current-limiting effect of Re means that there is no need to provide a stiff metal earth braid or a high-current earth line in the umbilical7. The cover earth line93provides a functional earth, for the purpose of dissipating stray electric charge that might otherwise accumulate on the print head cover83. However, transient currents carried to the printer body1by the cover earth line93during an electrostatic discharge event can result in transient potential differences across components in the printer body1, and these may disturb the correct operation of the system. The resistance Re limits these currents and so limits the degree of electrical disturbance to the printer operation during an electrostatic discharge event.

The minimum practical value for Re is 100Ω. This ensures that there is some effective current limitation, even if there is a discharge in circumstances where the internal resistance of the discharge source is lower than that of the human body model ofFIG.16. However, the current limiting effect is greater, creating less disturbance to the printer operation and making its performance more predictable, if the value of Re is greater and therefore a value of at least 1 kΩ is preferred. This would limit the peak current from an electrostatic discharge of 8 kV to 8 A. Still higher values of Re provide better protection. For example, a resistance Re of at least 8 kΩ would limit the peak current to no more than 1 A. If for example this current flows to earth through the printer chassis, and the connection through the chassis has a resistance of 1Ω, this will result in a voltage change at the chassis of 1 V. It is reasonably straightforward to protect other components from the influence of a voltage fluctuation of this magnitude. Preferably the resistance Re is at least 80 kΩ, so that the peak current is no more than 0.1 A. More preferably the resistance Re is at least 800 kΩ, so that the peak current is no more than 10 mA. This ensures that any voltage fluctuation at the printer body will be very small and would be unlikely to result in any noticeable disruption to the operation of any components in the printer.

The electrical resistivity of the material used for at least a part of the print head cover83makes it easy to design the print head cover83so that the minimum value for the resistance Re is provided by the material of the print head cover and there is no need to provide a separate resistor121in the cover earth line93. Because the material of the print head cover83around the exit hole85is not completely insulating, any electrical charges reaching this part of the print head cover are dissipated and do not build up. The use of a mouldable polymeric material enables the print head cover83to be made more cheaply than a metal cover.

In the embodiments discussed above, the earth lines93,97and the control system73are earthed via the voltage converter81and the power socket79ofFIG.5. However, it is also possible (for example if the printer body is double insulated) that the components inside the printer are not earthed and instead the earth lines93,97and the control system73are connected to an electrical reference which provides a common reference potential for electrical components. An example of this is shown inFIG.15, where the external casing of the printer body acts as an electrical reference location.

During an electrostatic discharge event, the high frequency components of the discharge will tend to be earthed by capacitive coupling between the printer and other nearby objects. The dc component of the discharge will charge the entire printer, so that its electrical potential relative to earth will change. This will not disrupt the electronic circuits or other electrical components, nor corrupt data, because the potential of all parts of the printer (including both the signal earth line97and the signal data lines101) will be affected equally. Over time, the common electrical reference potential of the printer will slowly return to earth potential by leakage, for example between the secondary and the primary circuits of a power supply plugged into the power socket79. Preferably, this earth leakage is assisted by a high resistance connection to earth (e.g. in the range of 100 kΩ to 1 MΩ) shown as Rg inFIG.15. Provided that this high resistance connection is provided in an appropriate manner it need not compromise the double-insulated characteristic of the printer.

As will be appreciated by those skilled in the art, the floating electrical reference arrangement ofFIG.15may also be applied toFIGS.10,14and16, and the earthed arrangement ofFIGS.10,14and16may also be applied toFIG.15.

As discussed above, the print head cover83may be made from an anti-static or static dissipative material. Such materials are often mouldable plastics (typically thermoplastic polymer materials, which may be inherently dissipative polymers or may be other polymers mixed with inherently dissipative polymers and/or non-polymeric conductive materials). Consequently it may be possible to manufacture the print head cover83by moulding, allowing it to be made more cheaply than a metal print head cover.

The embodiments described above and illustrated in the drawings are provided by way of non-limiting example and further embodiments are possible. For example, the print head5may provide two or more ink jets, rather than a single jet as shown in the illustrated embodiments. The ink gun17may provide more than one ink jet, or there may be more than one ink gun17. Normally, each jet will require a separate independent charge electrode21so that the drops of different jets can be charged differently. The jets may share a common set of deflection electrodes23,25provided that the geometry of the print head allows a strong enough deflection field to be provided for each jet, or there may be more than one set of deflection electrodes23,25.