Patent Application
20220242117
The present invention relates to an electrostatic deflection continuous ink jet printer, for example an industrial printer suitable for printing onto a succession of objects carried past the printer on a conveyor in an industrial filling, packing or processing line. Typically the objects are products such as manufactured articles or packaged food stuffs and the printer is used to print product and batch information, “use by” dates etc. The present invention also relates to a print head assembly for such a printer.
In the operation of an electrostatic deflection continuous ink jet printer, a continuous jet of ink drops is formed at a print head of the printer. The print head comprises an arrangement of electrodes to trap electric charges on some or all of the ink drops and to create an electrostatic field to deflect the charged drops. 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 a printer body of the printer. Usually the print head is connected to the printer body by a flexible conduit (sometimes called an umbilical) which is typically from 1 m to 6 m long.
The print head of an electrostatic deflection continuous ink jet printer usually has a metal cover that protects it from the environment, encloses the electrodes for reasons of electrical safety and for preventing external interference with the ink jet, and contains the atmosphere inside the print head cover to minimise mixing with the surrounding air. An opening (exit hole) in the cover allows drops of ink to exit for printing. During operation of the printer, very small drops of ink may be formed in the space enclosed by the print head cover in addition to the normal drops in the ink jet. Additionally, further very small drops may be formed by splashback when drops of ink hit the surface being printed onto. These very small drops may settle on nearby surfaces, which may include the print head cover especially at the end of the cover near the gutter and the opening by which ink drops leave. These very small drops may be electrically charged, and the charges will be trapped if they land on a surface that is electrically isolated. If a large trapped electric charge is allowed to build up, it will create an electric field that may interfere with the correct operation of the printer. Therefore the print head cover is normally earthed.
Because the print head cover is earthed it may receive electrostatic discharges. Typically these occur either when a person touches the cover, if the person is carrying an electrostatic electric charge, or when the printer is being used to print onto a plastic web that unwinds from a reel, generating electric charge as it unwinds. The electrostatic discharge will create a large transient current spike in the earth conductor from the print head cover to earth. The earth connection for the print head cover is usually provided by the printer body and the earth conductor usually comprises a metal braid or mesh running the length of the conduit. Metal braid or mesh is used in addition to or in place of a simple conductor wire in order to maximise the conductor surface. This is preferred because the brief transient nature of the discharge current creates high frequency current components which flow mostly along the surface of a conductor rather than through the main bulk of the conductor.
The print head normally contains electronic circuitry which may be damaged by the high voltage present in an electrostatic discharge. Therefore the path from the print head cover to earth is isolated from the electronic circuitry while the path is in the print head and in the conduit, and the conduit carries a separate signal earth conductor between the electronic circuitry and the printer body.
Aspects of the present invention provide an electrostatic deflection ink jet printer in which at least a part of the print head cover is made of a material having an electrical surface resistivity of no more than 1012 ohms per square or an electrical volume resistivity of no more than 109 ohm metres and is electrically connected to a cover earth line.
In one aspect, the print head includes electronic circuits, and a signal earth line for electronic circuits in the print head extends from the electronic circuits into and along the umbilical to its far end. The cover earth line may also extend along the umbilical to its far end or may join the signal earth line. The resistance from the surface of the print head cover to any junction between the cover earth line and the signal earth line is at least 16000 times the resistance of the signal earth line from the junction to the far end of the umbilical.
The resistance from the surface of the print head cover to the near end of the umbilical is at least 16000 times the resistance of any length of the cover earth line inside the umbilical. The print head cover may be moulded from an antistatic or static dissipative material. The limit to the resistivity of the material of the print head cover prevents build-up of electric charge on the cover. The ratio of resistances implies that the total resistance for an electrostatic discharge will be sufficient to avoid large electrostatic discharge currents, avoiding the need for an earthing wire braid in the umbilical. The ratio of resistances also prevents an electrostatic discharge to the cover from disrupting the electronic circuits.
In another aspect, the material of the aforesaid at least a part of the print head cover has an electrical surface resistivity of at least 105 ohms per square or an electrical volume resistivity of at least 100 ohm metres, so that the material is antistatic or static dissipative, and the electrical resistance from the surface of the print head cover to the end of the cover earth line remote from the print head cover is at least 1 kΩ. The print head cover may be moulded from the antistatic or static dissipative material. In this aspect, the print head does not necessarily include electronic circuits, and so the umbilical does not necessarily carry a signal earth line. The limit to the resistivity of the material of the print head cover prevents build-up of electric charge on the cover. The minimum resistance limits the current arising from an electrostatic discharge, avoiding the need for an earthing wire braid in the umbilical and avoiding the need for the cover earth line to carry large currents.
According to an aspect of the present invention there is provided an electrostatic deflection ink jet printer comprising a printer body, a print head and a flexible conduit (often known as an umbilical) extending between the printer body and the print head. The printer body comprises electrical components including electronic circuits of a control system and has an electrical ground for the electrical components. Preferably the printer body has an earth conductor to be connected to an external earth, and the electrical ground is connected to the earth conductor. However, the printer body may not have an earth connector e.g. in the case that it is double insulated, in which case the electrical ground will be floating relative to an external earth. The print head has at least one jet-forming orifice, an arrangement of electrodes for trapping charges on ink drops of the ink jet (or jets) and for providing an electric field for deflecting charged ink drops, and electronic circuits. One or more signal data lines extend between the electronic circuits of the print head and the electronic circuits of the printer body via the flexible conduit. A signal earth line extends between the electronic circuits of the print head and the printer body via the flexible conduit, for providing a signal earth potential to the electronic circuits in the print head. The signal earth line is coupled to the electrical ground of the printer body. The print head has a print head cover that is connected to a cover earth line. The cover earth line may join the signal earth line within the print head or the flexible conduit (preferably within the print head) or even within the printer body, or it may extend via the flexible conduit to be electrically connected to the electrical ground of the printer body independently of the signal earth line. At least a part of the print head cover, which part is exposed to a volume containing a part of the ink jet (or ink jets) when in use, is electrically connected to the cover earth line and. is formed of (i) a material having an electrical surface resistivity of up to 1012 ohms per square (preferably up to 1010 ohms per square) or (ii) a material having an electrical volume resistivity of up to 109 ohm metres (preferably up to 107 ohm metres) All of the exposed exterior surface of the print head cover has an electrical resistance (when dry) to an intermediate point that is at least 16 000 (sixteen thousand) times the resistance from the intermediate point to the electrical ground of the printer body. If the cover earth line joins the signal earth line in the print head (or a short distance, e.g. up to 10 cm, into the flexible conduit), the intermediate point is the junction between the cover earth line and the signal earth line. If the cover earth line joins the signal earth line elsewhere, or does not join it at all, the intermediate point is the place on the cover earth line where it enters the flexible conduit (or a short distance, e.g. up to 10 cm, into the flexible conduit).
Another aspect of the invention provides an electrostatic deflection continuous ink jet printer comprising a printer body, a print head and a flexible conduit extending between the printer body and the print head,
If there is an electrostatic discharge to the print head cover, it will be discharged to the electrical ground of the printer body via a discharge path comprising (i) the part of the print head cover from the point of discharge to the cover earth line together with the cover earth line itself and (ii) any further component, such as the signal earth line, that connects the cover earth line to the electrical ground of the printer body.
If the cover earth line joins the signal earth line (whether in the print head or elsewhere), the signal earth for the electronic circuits in the print head is connected to this discharge path at the junction between the signal earth line and the cover earth line. Therefore the relative resistances of the two parts of the discharge path, as noted above, provides a potential divider that influences the voltage fluctuation experienced by the ground terminals of the electronic circuits in the print head during an electrostatic discharge. By ensuring that the effective resistance of the print head cover together with the cover earth line is sufficiently greater than the resistance of the part of the signal earth line from its junction with the cover earth line to the electrical ground, the voltage during an electrostatic discharge is almost entirely generated across the print head cover and the cover earth line, and the voltage fluctuation that is conducted to the ground terminals of the electronic circuits in the print head is kept to a level that is unlikely to damage or to disrupt the operation of the electronic circuits or corrupt the data sent or received by the electronic circuits in the print head.
Additionally, if the cover earth line extends into the flexible conduit by more than a minimal distance (about 10 cm in practice) it becomes very difficult to avoid a significant capacitive coupling between the cover earth line and the signal earth line (and also between the cover earth line and signal data lines carrying data signals to and from the electronic circuits in the print head). In this case the relative resistances of the two parts of the discharge path, as noted above, provides a potential divider that influences the voltage fluctuation in the part of the cover earth line that is capacitively coupled to the signal earth line and the signal data lines. By ensuring that the effective resistance of the print head cover together with the part of the cover earth line in the print head is sufficiently greater than the resistance of the part of the cover earth line that is in the flexible conduit and is likely to be capacitively coupled to the signal earth and data lines, the voltage during an electrostatic discharge is almost entirely generated across the print head cover and the part of the cover earth line that is in the print head, and the voltage fluctuation that will be capacitively coupled into the signal earth and data lines during an electrostatic discharge event is kept to a level that is unlikely to damage or to disrupt the operation of the electronic circuits or corrupt the data sent or received by the electronic circuits in the print head.
An electrostatic discharge from a person touching the print head cover or from an electrically charged plastic web can be modelled as a discharge from a capacitance of 100 pF charged to 8 kV through an internal resistance of 150Ω. The internal resistance in the electrostatic discharge model adds to the resistance from the electrostatic discharge location on the print head cover to the cover earth line, and therefore further reduces the voltage fluctuation experienced by the ground terminals of the electronic circuits. Therefore it is safe to ignore this internal resistance in the analysis of the effect of the potential divider.
Accordingly, the potential of 8 kV (eight thousand volts) can be regarded as being divided across the potential divider formed by the two parts of the discharge path mentioned above. Because the resistance of the first part of the discharge path is at least 16000 (sixteen thousand) times the resistance of the second part, the voltage fluctuation at any junction between the cover earth line and the signal earth line and the voltage fluctuation coupled into the signal earth and data lines is limited to no more than 0.5 V. This is within the tolerance of some common types of electrical circuitry, so that it is possible to design the electrical circuitry of the print head such that it will not normally be disrupted by such a voltage fluctuation. Additionally, the potential divider implies that the resistance between the external surface of the print head cover and the printer body is sufficiently high that dangerous currents cannot flow between them. Consequently, it is possible to use the signal earth line to connect the print head cover to ground and it is no longer necessary to provide a metal braid or a high-current earth conductor in the flexible conduit in order to earth the print head cover. Even if a separate earth connection is used for the print head cover, i.e. the cover earth line connects to the printer body earth independently of the signal earth line, this connection need only be a simple low-current line such as a thin copper wire, and it is not necessary to provide a metal braid or a high-current earth conductor in the flexible conduit in order to earth the print head cover. This allows the flexible conduit to be manufactured more cheaply and also allows it to be more flexible since the metal braid is comparatively stiff.
Preferably the cover earth line joins the signal earth line within the print head, or less preferably a short distance into the flexible conduit, so that it is not necessary to provide a separate cover earth line along the flexible conduit.
Preferably the minimum electrical resistance from any part on the exposed exterior surface of the print head cover to the junction between the cover earth line and the signal earth line is at least 16 kΩ. This allows the resistance from the junction between the cover earth line and the signal earth line to the electrical ground of the printer body to be up to 1Ω, which should normally be achievable in the design of an electrostatic deflection continuous ink jet printer. The peak current generated by an electrostatic discharge of 8 kV would be 0.5 A in this case.
More preferably, the minimum electrical resistance from any part on the exposed exterior surface of the print head cover to the junction between the cover earth line and the signal earth line is at least 80 kΩ. This allows the resistance from the junction between the cover earth line and the signal earth line to the electrical ground of the printer body to be up to 5Ω. This should be achievable with a normal signal earth wire (e.g. a copper electrical wire of 0.5 mm to 1 mm diameter) in the flexible conduit even if the flexible conductor is 6 m or more long. The peak current generated by an electrostatic discharge of 8 kV would be 0.1 A in this case.
The minimum electrical resistance from any part on the exposed exterior surface of the print head cover to the junction between the cover earth line and the signal earth line may be at least 500 kΩ (0.5 MΩ). This allows this resistance to be much more than 16 000 (sixteen thousand) times the resistance from the said junction to the electrical ground of the printer body. For example, it would be 500 000 (five hundred thousand) times if the resistance from the junction between the cover earth line and the signal earth line to the electrical ground of the printer body is 1Ω. The peak current generated by an electrostatic discharge of 8 kV would be no more than 0.02 A. This means that the voltage fluctuation at the junction between the signal earth line and the cover earth line, and therefore the voltage fluctuation at the ground for the electronic circuits in the print head, would be no more than 0.02 V.
The significance of transient effects can be assessed by considering the time constant of the electrostatic discharge. This time constant will depend on the inductance of the discharge path. This inductance is primarily caused by the inductance of wire used as the signal earth. A typical signal earth wire might have an inductance of about 1 pH per metre. Therefore, a very long 8-metre conduit (umbilical), with an inductance of 1 pH per metre, can be regarded as a worst case example. This would result in an inductance of 8 pH in the discharge path. If the electrostatic discharge path has a resistance of 16 kΩ and an inductance of 8 pH, its time constant (calculated as L/R) will be 0.5 nanoseconds, or 500 picoseconds. This is substantially faster than the typical response time of the electronic circuits in the print head. Therefore even in this worst case, any transient voltage spike at the signal earth for the electronic circuits in the print head, arising from the inductance of the signal earth line, can be ignored as being too brief to disrupt the electronic circuits in the print head. For shorter conduits with less inductance in the signal earth line, and for greater resistances in the discharge path, the time constant will be even shorter.
In practice, it may often be easy to provide a signal earth line with a lower resistance than 1Ω. For example, 2 metres of copper wire having a diameter of 1 mm may have a resistance of about 1/25Ω (0.04Ω). Thus it may be practical to provide a lower electrical resistance from any part on the exposed exterior surface of the print head cover to the junction between the cover earth line and the signal earth line, for example of at least 1 kΩ. This would still be 16000 times the resistance from the junction between the cover earth line and the signal earth line to the electrical ground of the printer body provided that this resistance is no more than 1/16Ω, which can provided reasonably easily. In this case, the time constant of the electrostatic discharge path will typically be 8 nanoseconds or less. However, this is less preferred because the discharge current could be as high as 8 A
If the electronic circuits in the print head have a faster response time than has been assumed in the analysis given above, the time constant of the electrostatic discharge can be shortened appropriately by increasing the resistance from any part on the exposed exterior surface of the print head cover to the junction between the cover earth line and the signal earth line. Assuming a long umbilical with an inductance of 8 pH in the discharge path, a resistance of 80 kΩ would provide a time constant of 100 picoseconds, and a resistance of 500 kΩ would provide a time constant of 16 picoseconds.
The above discussion of resistances, currents and time constants apply in an analogous manner to the case where the cover earth line extends for a significant distance along the flexible conduit and is capacitively coupled to the signal earth and data lines. In this case, the calculations show the currents that will be carried by the cover earth line and the voltages (and time constants thereof) that may be coupled into the signal earth and data lines.
In another aspect of the present invention there is provided an electrostatic deflection continuous ink jet printer comprising a printer body, a print head and a flexible conduit extending between the printer body and the print head,
Preferably the electrical resistance Re is provided by the material of the print head cover.
In this aspect, the print head may not include electronic circuits, in which case it is not necessary to consider the effect of an electrostatic discharge on a signal earth line. However, by making at least a part of the print head cover out of a material having the specified range of resistivity, and ensuring that the resistance from the surface of the print head cover to the electrical reference location is at least a minimum value, it is possible to limit the current of an electrostatic discharge event so that there is no need to provide a metal braid or a high-current earth conductor in the flexible conduit in order to earth the print head cover. In the standard electrostatic discharge model discussed above, the electrostatic charge is considered to be at 8 kV and the electrostatic discharge source is considered to have an internal resistance of 150 ohm. Accordingly, an electrical resistance Re of 100Ω, combined with the electrostatic discharge internal resistance of 150Ω will generate a peak current of up to 32 A (the peak current may be less if there is a significant inductance as well). The discharge current flows only for a short time so that this current can be carried by a normal copper wire without overheating.
The total charge discharged from a human body in an electrostatic discharge event is sufficiently small that any significant current flows very briefly. However, if the resistance of the discharge path is small the brief transient current may be high, and this can generate a significant transient voltage at locations in the printer body that are coupled to the current flow, such as the chassis of the printer body. Such a transient voltage may disrupt the operation of various printer components. The resistance Re limits the magnitude of the transient current and so reduces the degree of disruption to the operation of the printer during an electrostatic discharge event, even if the electrostatic discharge source does not have any significant internal resistance.
Although a value of 100Ω for Re provides some protection, the current limiting effect will be 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.
As discussed above with reference to other aspects of the invention, the upper resistivity limit means that the material of the at least a part of the print head cover is not totally insulating, and allows the dissipation of any charge that is deposited on this part of the print head cover such as charge carried by charged microdrops of ink.
According to another aspect of the present invention there is provided a print head assembly for an electrostatic deflection ink jet printer, the print head assembly comprising a print head and a flexible conduit (often known as an umbilical). The print head has a one or more jet-forming orifices, an arrangement of electrodes for trapping charges on ink drops of the ink jet (or ink jets) and for providing an electric field for deflecting charged ink drops, and electronic circuits. One or more signal data lines extend from the electronic circuits of the print head along the flexible conduit to one or more signal data connectors. A signal earth line, for providing a signal earth potential to the electronic circuits in the print head, extends from the electronic circuits of the print head along the flexible conduit to a signal earth connector. The flexible conduit can be coupled to a printer body so as to extend between the printer body and the print head, so that the signal data lines are coupled to electronic circuits of the printer body via the signal data connectors and the signal earth line is coupled to an electrical ground of the printer body via the signal earth connector.
The print head has a print head cover that is connected to a cover earth line. The cover earth line may join the signal earth line within the print head or the flexible conduit (preferably within the print head) or it may extend via the flexible conduit to a cover earth connector. At least a part of the print head cover that is exposed to a volume containing a part of the ink jet (or jets) when in use is electrically connected to the cover earth line and is formed of (i) a material having an electrical surface resistivity of up to 1012 ohms per square (preferably up to 1010 ohms per square) or (ii) a material having an electrical volume resistivity of up to 109 ohm metres (preferably up to 107 ohm metres). All of the exposed exterior surface of the print head cover has an electrical resistance (when dry) to an intermediate point that is at least 16 000 (sixteen thousand) times the resistance from the intermediate point to the relevant earth connector. If the cover earth line joins the signal earth line in the print head (or a short distance, e.g. up to 10 cm, into the flexible conduit), the intermediate point is the junction between the cover earth line and the signal earth line and the relevant earth connector is the signal earth connector. If the cover earth line joins the signal earth line elsewhere, the intermediate point is the place on the cover earth line where it enters the flexible conduit (or a short distance, e.g. up to 10 cm, into the flexible conduit), and the relevant earth connector is the signal earth connector. If the cover earth line does not join the signal earth line at all, the intermediate point is the place on the cover earth line where it enters the flexible conduit (or a short distance, e.g. up to 10 cm, into the flexible conduit) and the relevant earth connector is the cover earth connector.
A further aspect of the present invention provides a print head assembly for an electrostatic deflection continuous ink jet printer, the print head assembly comprising a print head and a flexible conduit attached to and extending away from the print head,
A further aspect of the present invention provides a print head assembly for an electrostatic deflection continuous ink jet printer, the print head assembly comprising a print head and a flexible conduit attached to and extending away from the print head,
Normally a printer body will provide a very low resistance path from a signal earth connector to an electrical ground, and a very low resistance path from a cover earth connector (if present) to an electrical ground, when a print head assembly is coupled to the printer body. Therefore the resistance from the signal earth connector or the cover earth connector to the electrical ground can be ignored and the discussion and analysis given above, and the preferred and optional values given above, can also be applied to the print head assembly with the resistance to the signal earth connector or the cover earth connector being considered in place of the resistance to the electrical reference location (ground) of the printer body.
The print head cover is typically removable to allow access, e.g. for cleaning. It may extend over substantially all of the print head except where the print head joins the umbilical (flexible conduit) or it may extend over only part of the print head.
In a printing operation of the 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 body of the printer. 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.
The jet-forming orifice on the print head is normally provided in an ink gun. The arrangement of electrodes normally comprises a charge electrode for trapping electric charges on drops of ink and deflection electrodes for creating an electrostatic field for deflecting charged drops of ink. The flexible conduit (umbilical) will normally carry fluid lines, for example for providing pressurised ink to the ink gun and for applying suction to the gutter and transporting ink from the gutter back to the printer body, and electrical lines, for example to provide a drive signal to a piezoelectric crystal or the like for imposing pressure vibrations on the ink jet, to provide electrical connections for the charge electrode and the deflection electrodes, and to provide drive currents for any valves that may be included in the print head.
The print head may be arranged to form a single ink jet, or it be arranged to form two or more ink jets. For example, there may be two or more ink guns. Alternatively, there may be an ink gun that is arranged to provide more than one ink jet.
In the aspects and embodiments of the invention mentioned above, at least a part of the print head cover is formed of (i) a material having a surface resistivity of up to 1012 ohms per square (preferably up to 1010 ohms per square) or (ii) a material having a volume resistivity of up to 109 ohm metres (preferably up to 107 ohm metres) and is electrically connected to the cover earth line. Accordingly the material is not completely electrically insulating. This helps to allow the dissipation of any electric charges that reach the print head cover, such as from small drops of ink that may arise within the volume enclosed by the print head cover or from splashback from printed drops. Preferably this part of the print head cover includes an exit hole that allows ink drops to leave the volume enclosed by the print head cover for printing. This will normally be at the end of the print head nearest the gutter. This is the part of the print head cover that is most likely to receive small charged drops of ink, both as splashback and from within the volume enclosed by the print head cover.
Preferably the part of the print head cover is formed (e.g. moulded) from a material with a surface resistivity of at least 105 ohms per square, or a volume resistivity of at least 100 ohm metres. Such a material can be regarded as anti-static or static dissipative and has a greater resistivity than a conductive material, and therefore it is possible for the material of the print head to provide at least a part of the resistance from the exposed exterior surface of the print head cover to the junction between the cover earth line and the signal earth line. The material may be a mouldable polymer or resin.
Preferably the resistance from the external surface of the print head cover to the junction between the cover earth line and the signal earth line or to the place on the cover earth line that is 10 cm into the flexible conduit, or to the electrical reference location or the cover earth electrical connector, is provided substantially entirely by the material of the print head cover. However, this is not essential and it is possible for example to provide a resistor in the cover earth line. It is possible for part or all of the print head cover to be metal, or another conductive material, in which case a resistor in the cover earth line may be needed in order to provide the minimum resistance from the external surface of the print head cover. It is possible for part of the print head cover to be metal and another part, between the metal part and the cover earth line, to be non-metallic in order to provide the desired resistance, but this is not preferred because it will normally be more complex and expensive to manufacture.
If necessary, there may be an electrically insulating layer on the external surface of the print head cover at least at a region that covers or is adjacent to a connection between the print head cover and the cover earth line. Such an insulating layer can help to ensure that there is the necessary minimum resistance from the external surface of the print head cover to the junction between the cover earth line and the signal earth line or to the place on the cover earth line that is 10 cm into the flexible conduit, or to the electrical reference location or the cover earth electrical connector, by preventing a very short electrical flow path from the external surface of the print head cover to the cover earth line.
Further aspects of the invention and optional features are set out in the accompanying claims.
Surface resistivity may be measured in accordance with IEC 62631-3-2:2015. Volume resistivity may be measured in accordance with IEC 62631-3-1:2016.
The resistance from the exposed exterior surface of the print head to any other location may be measured by covering the exposed exterior surface with a metal foil making good electrical contact with the surface, and then using a conventional ohmmeter to measure the resistance from the metal foil to the other location.
Embodiments of the present invention, given by way of non-limiting example, will be described as reference to the following drawings.
The printer is typically an industrial ink jet printer and is suitable to be used with a conveyor 13 that conveys objects 11 past 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 conveyor 13 that is external to the printer. The object 11 may 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 object 11 from 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. In
Although the ink jet 19 leaves the ink gun 17 as 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 electrode 21, which is positioned so that the ink jet 19 separates into drops while it is in the slot through the charge electrode 21. Other arrangements and other shapes of charge electrode 21 are possible, so long as the ink jet 19 is 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 gun 17 is held at a constant voltage (typically ground). Accordingly, any voltage applied to the charge electrode 21 induces a charge into the part of the ink jet 19 that is subject to the electric field in the slot of the charge electrode 21. As the ink jet 19 separates 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 electrode 21 and different amounts of charge can be trapped on different drops by changing the voltage on the charge electrode 21.
The ink jet 19 then passes between two deflection electrodes 23, 25. A large potential difference (typically several kilovolts) is applied between the deflection electrodes 23, 25 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
Drops of ink that are deflected by the field between the deflection electrodes 23, 25, so as to miss the gutter 27, leave the print head 5 and form printed dots on the surface 9 of the object 11.
The ink gun 17, the charge electrode 21, the deflection electrodes 23, 25 and the gutter 27 are mounted on a baseboard 31. The gutter suction line 29 extends beneath the baseboard 31. It may also be convenient to route the electrical connections for the charge electrode 21 and the deflection electrodes 23, 25 beneath the baseboard 31, as shown in
The ink feed line 15 is also connected to the outlet side of the ink pump 39 and receives pressurised ink. Thus the ink feed line 15 provides an ink feed path to supply pressurised ink from the ink pump 39 to the ink gun 17. An ink feed valve 51 controls the flow of ink along the ink feed line 15. The pump 39 can drive ink continuously through the Venturi 45 and back to the ink feed tank 35, even when the ink feed valve 51 prevents ink from flowing along the ink feed line 15. The flow of ink through the Venturi 45 generates suction and accordingly the Venturi acts as a suction source. The gutter suction line 29 is connected to a suction inlet of the Venturi 45 to receive suction which sucks ink from the gutter 27 through the umbilical 7 back to the printer body 1. The ink from the gutter suction line 29 is sucked into the Venturi 45 and returns to the ink feed tank 35. Fluid flow in the gutter suction line 29 is controlled by a gutter valve 53.
Spare solvent is held in a solvent reservoir 55 which receives suction from the Venturi 45 through a solvent top-up line 57. If solvent needs to be added to the ink in the ink feed tank 35 to dilute the ink and correct its viscosity, a solvent top-up valve 59 in the solvent top-up line 57 is opened briefly. This allows the Venturi 45 to suck a small quantity of solvent from the solvent reservoir 55 into the ink flow through the Venturi 45. The solvent sucked into the Venturi 45 then passes into the ink feed tank 35 to dilute the ink.
Spare ink is held in an ink reservoir 61 which receives suction from the Venturi 45 through an ink top-up line 63. When the level of ink in the ink feed tank 35 becomes low, an ink top-up valve 65 in the ink top-up line 63 is opened. Ink is sucked out of the ink reservoir 61 by the Venturi 45 and is delivered to the ink feed tank 35 in a similar manner to the operation for topping up with solvent from the solvent reservoir 55.
The solvent reservoir 55 and the ink reservoir 61 are supplied from a solvent container 67 and an ink container 69 respectively, and the operator replaces the containers 67, 69 as necessary. In practice, it is not always necessary to provide the solvent reservoir 55 and the ink reservoir 61, and the respective top-up lines 57, 63 may be connected directly to the containers 67, 69.
Fluid lines 75 connect the printer body ink system 71 to the print head 5 through the umbilical 7. These fluid lines will include the ink feed line 15, and the gutter suction line 29 shown in
The printer receives electric power at a power socket 79, which is converted in a voltage converter 81 to the various voltages required internally within the printer. For example, the printer may be designed to receive 24 volt DC at the power socket 79, since power supplies for generating 24 volts DC from an electric mains supply are widely available. The voltage converter 81 uses the received 24 volt supply to generate the voltages required to power the electronics in the control system 73, which may for example be 5 volts. It also supplies power to a component, either in or controlled by the control system 73, to generate the voltages (e.g. up to about 300 V) applied to the charge electrode 21, the EHT voltage (e.g. about 4 kV) applied to the upper deflection electrode 23 and to generate the drive signal for the piezoelectric crystal inside the ink gun 17.
The power socket 79 also provides a connection to an external electrical earth. This is used to earth the external case of the printer body 1. The earth connection is also provided to the voltage converter 81, which uses it to provide an earth to any components that need an earth. The control system 73 uses the earth received from the voltage converter 81 to provide an electrical ground for the electronic circuits in the control system 73 and to provide an electrical ground for connection to the signal earth line in the umbilical 7 so as to provide a signal earth to the electronic circuits in the print head 5.
In the embodiment of
In both
The print head cover 83 is 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 1010 to 1012 ohms per square or an electrical bulk resistivity in the range of 107 to 109 ohm metres and a static dissipative material can be regarded as a material having a surface resistivity in the range of 105 to 1010 ohms per square or a bulk resistivity in the range of 100 to 107 ohm metres. Preferably the material of the print head cover 83 is a plastic or other mouldable material.
In the operation of the printer, the drops of ink in the ink jet 19 either pass into the gutter 27 or pass out of the print head through the hole 85 in order to print dots on the surface 9 of the object 11. Therefore no ink drops should come into contact with the print head cover 83. However, microdrops (which are much smaller than the drops of ink in the ink jet) can also occur while the ink jet 19 is running. Microdrops may be formed as the ink jet 19 breaks into drops at the charge electrode 21 or from the impact of drops on a contact surface inside the gutter 27. They may also be formed outside the print head cover 83 from the impact of drops on the surface 9 that 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 electrodes 23, 25 will 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 cover 83 that miss the deflection electrodes 23, 25 will tend to hit the print head cover 83 in the vicinity of the exit hole 85. Microdrops formed outside the print head cover may also hit the print head cover 83, again in the vicinity of the exit hole 85. Accordingly the print head cover 83 may receive electric charges from the microdrops. If the print head cover 83 was insulated, these electric charges could accumulate on the print head cover 83 and create an electric field that would interfere with the correct deflection of the ink drops. This is avoided because the print head cover 83 is made of an anti-static or static dissipative material as stated above, and is electrically earthed.
When the retaining screw 87 is tightened, it presses the print head cover 83 against the retaining block 91 and so the print head cover makes a good connection to the retaining block 91 both by direct contact and via the retaining screw 87. In this way, any electric charges that arrive at the print head cover 83 will flow slowly through the material of the print head cover 83 or over the surface of the print head cover 83 to reach the retaining screw 87 and the threaded block 91, and will then be earthed via the cover earth line 93 and the signal earth line. Accordingly, electric charges do not accumulate on the print head cover 83.
In this arrangement, the cover earth line 93 is connected to the earthing block 95. As shown in
The print head cover 83 may also receive an electrostatic discharge. This may occur for example if the print head cover 83 is 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 cover 83 results in a sudden large voltage arising at the print head cover 83. Since the print head cover 83 is electrically connected to the signal earth line by the cover earth line 93, 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 cover 83 that receives the electrostatic discharge and the place where the cover earth line 93 joins the signal earth line.
The electric circuit for modelling the effect of an electrostatic discharge is shown in
The print head 5 and the umbilical 7 jointly form a print head assembly that 7 can be disconnected from the printer body 1, e.g. to allow a different print head assembly to be fitted so as to change the type of print head 5 or change the length of the umbilical 7. As shown schematically in
As shown in
The printer body 1 provides a very low resistance connection to earth for the signal earth line 97. Additionally, the length of the signal earth line within the print head 5 is short and provides very little electrical resistance. The electrical resistance between the external earth and the junction 99 (where the cover earth line 93 joins the signal earth line 97) is almost entirely provided by the resistance of the part of the signal earth line 97 that is in the umbilical 7, as this represents almost all of the length of the signal earth line 97. In
In order to avoid disruption of the operation of the electronic circuits 103 in the print head 5 and to avoid corruption of data communicated between the electronic circuits 103 and the control system 73 in the printer body, the voltage on the signal earth line 97 at the electronic circuits 103 (and therefore the voltage at the junction 99) should not fluctuate by more than 0.5 V during an electrostatic discharge event. The voltage fluctuation at the junction 99 is provided by the voltage divider effect of the resistance Rs and the resistance between the junction 99 and the 100 pF capacitor in the human body model of
In practice, the resistance Rs will depend on the length of the umbilical 7 as well as the grade of wire used in the umbilical 7 for the signal earth line 97. 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 circuits 103 in the print head 5 in 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 cover 83, and to provide the cover earth line 93 as a low resistance wire. In the design of
Additionally, if a person touches the print head cover 83 very close to the retaining screw 87 in the design of
Preferably the material of the print head cover has an electrical surface resistivity of at least 107 ohms per square or an electrical volume resistivity of at least 104 ohm 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 wire 93, so that there is no need to provide an insulating layer 117.
Further embodiments are also possible. For example, it may be more convenient to route the cover earth line 93 below the baseboard 31 in the print head 5 rather than to the arrangements shown in
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 line 93 includes 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 line 93 at 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 line 93.
Alternatively an arrangement could be provided such as is shown in
In the embodiment of
However, the design of the print head cover 83 in
The embodiments discussed above enable electrical charge build-up on the print head cover 83 to be avoided and an electrostatic discharge event to be accommodated using the earth connection provided by the signal earth line 97. It is possible with these embodiments to provide sufficient resistance to earth from all points on the print head cover 83 so 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 line 97, and the resistance Rc between the electrostatic discharge event and the signal earth line 97 prevents significant current transients arising in the signal earth line 97. Although the junction 99 between the cover earth line 93 and the signal earth line 97 is preferably in the print head 5, it is possible to place this junction in the umbilical 7 near the end of the umbilical 7 at the print head 5. However it is preferred that the junction 99 should be no further along the umbilical 7 than 10 cm from the end at the print head 5, in order to preserve the benefits provided by joining the cover earth line 93 to the signal earth line 97.
In an alternative embodiment, shown in
In this embodiment, an electrostatic discharge to the print head cover 83 is earthed via the cover earth line 93 and is not connected to the signal earth line 97. Therefore the voltage divider of
In practice it is possible to avoid significant capacitive coupling of the signal earth line 97 and the signal data lines 101 to the first 10 cm of the cover earth line 93 in the umbilical 7, partly because the cover earth line 93 may be held spaced apart from the other lines 97, 101 by the fitting at the end of the umbilical that holds the various lines in the correct positions as they pass into the print head 5, 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 in
As noted above, the voltage on the signal earth line 97 (and on the signal data lines 101) should not fluctuate by more than 0.5 V during an electrostatic discharge event. Therefore the voltage on the part of the cover earth line 93 that is more than 10 cm into the umbilical 7 should 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 line 93 that is 10 cm into the umbilical 7 is 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 to
The various discussions above concerning the values of Rc and Rs in
In this embodiment, the cover earth line 93 provides an extra electrical line in the umbilical 7 compared with the embodiment of
In principle, it would be possible to extend the cover earth line 93 more than 10 cm into the umbilical 7 and then join it to the signal earth line 97, as shown in
If the analysis of
If Re is 100Ω, an electrostatic discharge of 8 kV in accordance with the human body model as shown in
The earth connection for the print head cover 83 is 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 umbilical 7. The cover earth line 93 provides a functional earth, for the purpose of dissipating stray electric charge that might otherwise accumulate on the print head cover 83. However, transient currents carried to the printer body 1 by the cover earth line 93 during an electrostatic discharge event can result in transient potential differences across components in the printer body 1, 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 of
The electrical resistivity of the material used for at least a part of the print head cover 83 makes it easy to design the print head cover 83 so 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 resistor 121 in the cover earth line 93. Because the material of the print head cover 83 around the exit hole 85 is 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 cover 83 to be made more cheaply than a metal cover.
In the embodiments discussed above, the earth lines 93, 97 and the control system 73 are earthed via the voltage converter 81 and the power socket 79 of
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 line 97 and the signal data lines 101) 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 socket 79. 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 in
As will be appreciated by those skilled in the art, the floating electrical reference arrangement of
As discussed above, the print head cover 83 may 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 cover 83 by 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 head 5 may provide two or more ink jets, rather than a single jet as shown in the illustrated embodiments. The ink gun 17 may provide more than one ink jet, or there may be more than one ink gun 17. Normally, each jet will require a separate independent charge electrode 21 so that the drops of different jets can be charged differently. The jets may share a common set of deflection electrodes 23, 25 provided 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 electrodes 23, 25.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1910570.9 | Jul 2019 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2020/051745 | 7/22/2020 | WO | 00 |
