GB1577236A - Electrostatically-assisted printing - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO ELECTRO STATICALLY - ASSISTED PRINTING (71) We, THORN AUTOMATION LIMITED, a British Company of P.O. Box 4, Rugeley, Staffordshire, WS15 1DR, England, do hereby declare the invention for which we pray that a Patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to electrostaticallyassisted printing systems as used on gravure printing presses.

Electrostatically-assisted printing (E.S.A.) is a technique whereby an improvement of print quality is obtained in gravure printing presses by the use of electrostatic force to assist ink out of the engraved cells on the printing cylinder, this leading also to an increase in running efficiency of the printing press. A number of systems for applying E.S.A. to gravure presses have already been proposed, e.g. in UK patents 1159923, 1169566, and 1349342. However, each of these described systems carries with it certain practical disadvantages. For example, the system in patent No. 1159923 is not universally certifiably safe for operation in the environment for explosive vapours associated with a gravure press, due to the possibility of electrical sparking at the brush and slip ring electrical connections. The system in patent No.

1169566 is certifiably safe but due to the selfgenerated-power principle, it cannot be used on a very slow speed press as is often employed for print proofing. Furthermore, its powerlimiting slipping clutch generates excessive heat on the recently developed type of very fast press; and again, the generator roller cannot be produced short enough to fit very narrow presses due to the inevitable size of the electrical generator pack within the roller. The system in patent No. 1349342 may possibly be constructed to be certifiably safe but by virtue of its corona-discharge-principle the ozone generated can be a health hazard for the operator and furthermore, the rubber rollers are susceptible to deterioration in the presence of ozone. Again the extra high voltage involved tends to attract paper dust necessitating press stoppage for cleaning when some types of paper web are used.

It is therefore an object of the present invention to achieve an E.S.A. system which overcomes the above shortcomings.

According to the invention, a rider roller with an electrically-conducting surface is provided in contact with an impression roller of the press, and a high d.c. voltage is supplied to the rider roller interior and thence to its surface from a flame-proof stationary enclosure located end-to-end with the rider roller and containing a voltage-multiplying network for deriving the high voltage within said enclosure from an external low voltage supply.

An arrangement in accordance with the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a diagram of the arrangements of rollers employed in a press according to the invention, Figure 2 shows the impression roller and rider roller in more detail, and Figure 3 is a diagrammatic longitudinal section through the rider roller and electrical supply arrangements therefore.

In Figure 1 of the drawings, an additional rider 10 is shown placed in contact with and rotating with the impression roller 11 of a gravure press unit. The rider roller 10 and impression roller 11 are each covered with an outer layer of conducting rubber 12 of appropriate electrical resistivity (Figure 2), and each has an inner layer of ebonite 13 to electrically insulate its outer conducting layer 12 from its metal roller body 14.

Referring now to Figure 3, the rider roller metal body 14 is so constructed as to constitute at one end an acceptable rotating flameproof enclosure a. A second and stationary flame-proof enclosure b mechanically supports the rotating roller 10, one end of the roller being mounted on bearings carried on a spigot shaft m projecting from the enclosure b.

The other end of the roller is mounted on a further bearing carried on a second stationary spigot shaft. The shaft m is hollow and contains an insulating rod o through which an electrical lead n passes from the interior of the stationary enclosure b into the rotating enclosure a. The hollow shaft m, the bearings it carries, and associated parts, are adequately dimensioned to satisfy the official safety regulations for operation in explosive atmospheres, using known constructional principles.

The end of the lead n within the rotating enclosure a is connected to a stationary brushj projecting from the end of the shaft m into contact with a brass plug k embedded in and rotating with a cylindrical insulator l secured within the roller 10. The brass plug makes electrical connection with the outer layer of conducting rubber 12 on the roller through a transverse pin x.

In the stationary flame-proof enclosure b, the electrical lead n connects via polarity switch c to the output of voltage multiplier/rectifier d.

This output voltage may be of any value up to 2.5 Kv or as permitted by the safety authority.

Low voltage a.c. input leads to the multiplier/ rectifier d pass through a flame-barrier e into a connection chamberf. A small portion of the d.c. high voltage output of the multiplier/ rectifier d also passes out through'the flamebarrier e from the junction of resistors g for external voltage indication purposes. A further lead brings a low d.c. voltage (say 5v) into the flame-proof enclosure b through the flamebarrier e, for energising a low-speed cut-out arrangement h and relay i. The cut-out h is a conventional proximity detector, and produces an electrical pulse at every roller revolution due to the passage past it of a metallic projection!i on the adjacent end of the roller 10. Instead of the relay i, an electronic circuit can be employed which inhibits the low voltage supply feeding the multiplier/rectifier d when the pulse rate falls below a predetermined level.

The polarity switch c may be manually operated via a screw c" projecting through the wall of the enclosure b as shown, or via a shaft projecting through the barrier e, or remotely operated from a control panel via an electrical solenoid adjacent to the switch.

Leads from the connection chamberf pass out through officially approved trumpet cable entry e.The external cable a connects to a control panel containing an adjustable low a.c.

voltage (e.g. max. 20 volts at 100 KHz) power supply in its own flame-proof housing b The adjustable low a.c. voltage is produced by an oscillator and adjustable power amplifier. Alternatively, this oscillator and amplifier may be situated in the flame-proof enclosure b.

The voltage multiplier/rectifier d may consist of a step-up transformer followed by a conventional half or full-wave voltage rectifier circuit. The output resistance of'the multiplier in conjunction with a resistor c' and the resistivity of the external rubber layer 12 of the roller 10, is arranged to be of such a value as to limit any short circuit current (caused by an external short circuit on the roller surface) to a safe value related to the hazardous operational atmosphere. The output capacitance is able to be small and yet give adequate smoothing, due to the high frequency of 100 KHz of the input supply. This limits any momentary short circuit current and the energy in any possible spark discharge.

The low speed cut-out arrangement h, i disconnects the 100 KHz supply voltage from the mulllplieqrectiher d when the roller speed drops below the rotational speed corresponding to the lowest press speed at which printing can take place. This provides operator safety, i.e.

reduces risk of electric shock when the press is stationary or almost stationary. The switch may be mechanical or an electronic pulse-counting system operating in association with a rotating magnet and an electronic switching device.

Alternatively, but not preferably, this cut-out at the low-printing speed can be arranged apart from the roller, e.g. a magnetic pulse generator on the press-drive operating an electronic switch in the supply in the control unit.

The electrical capacitance of the outer conductive rubber layer on the roller metal body may be suitably increased by adding a material of high permittivity, e.g. titanium dioxide, to the ebonite intermediate layer.

The provision of a stationary high voltage generator as described has the following ad vantage: i) The electronics are more reliable due to less vibration; ii) the generator high-voltage output does not need to return to press frame earth via any rotating assembly; iii)it is a simple matter to provide a cable con nection to give an indication of the high voltage at the control unit; iv) the high-voltage polarity can be changed fron the control unit without stopping the press; v) a slipping clutch assembly is not needed.

Further advantages of the arrangement described are that no alternator is employed, making the high voltage generator more compact, the roller bearings are enclosed and protected from the ink environment, the generator can be adjusted to give maximum voltage even on a very slow printing press, and the high frequency supply means that the capacitors in the voltage-multiplying network can be quite small so making possible a smaller pack and reducing the energy available in the case of a short circuit discharge.

WHAT WE CLAIM IS: 1. An electrostatically-assisted printing system in a gravure printing press, wherein a rider roller with an electrically-conducting surface is provided in contact with an im

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. being mounted on bearings carried on a spigot shaft m projecting from the enclosure b. The other end of the roller is mounted on a further bearing carried on a second stationary spigot shaft. The shaft m is hollow and contains an insulating rod o through which an electrical lead n passes from the interior of the stationary enclosure b into the rotating enclosure a. The hollow shaft m, the bearings it carries, and associated parts, are adequately dimensioned to satisfy the official safety regulations for operation in explosive atmospheres, using known constructional principles. The end of the lead n within the rotating enclosure a is connected to a stationary brushj projecting from the end of the shaft m into contact with a brass plug k embedded in and rotating with a cylindrical insulator l secured within the roller 10. The brass plug makes electrical connection with the outer layer of conducting rubber 12 on the roller through a transverse pin x. In the stationary flame-proof enclosure b, the electrical lead n connects via polarity switch c to the output of voltage multiplier/rectifier d. This output voltage may be of any value up to 2.5 Kv or as permitted by the safety authority. Low voltage a.c. input leads to the multiplier/ rectifier d pass through a flame-barrier e into a connection chamberf. A small portion of the d.c. high voltage output of the multiplier/ rectifier d also passes out through'the flamebarrier e from the junction of resistors g for external voltage indication purposes. A further lead brings a low d.c. voltage (say 5v) into the flame-proof enclosure b through the flamebarrier e, for energising a low-speed cut-out arrangement h and relay i. The cut-out h is a conventional proximity detector, and produces an electrical pulse at every roller revolution due to the passage past it of a metallic projection!i on the adjacent end of the roller 10. Instead of the relay i, an electronic circuit can be employed which inhibits the low voltage supply feeding the multiplier/rectifier d when the pulse rate falls below a predetermined level. The polarity switch c may be manually operated via a screw c" projecting through the wall of the enclosure b as shown, or via a shaft projecting through the barrier e, or remotely operated from a control panel via an electrical solenoid adjacent to the switch. Leads from the connection chamberf pass out through officially approved trumpet cable entry e.The external cable a connects to a control panel containing an adjustable low a.c. voltage (e.g. max. 20 volts at 100 KHz) power supply in its own flame-proof housing b The adjustable low a.c. voltage is produced by an oscillator and adjustable power amplifier. Alternatively, this oscillator and amplifier may be situated in the flame-proof enclosure b. The voltage multiplier/rectifier d may consist of a step-up transformer followed by a conventional half or full-wave voltage rectifier circuit. The output resistance of'the multiplier in conjunction with a resistor c' and the resistivity of the external rubber layer 12 of the roller 10, is arranged to be of such a value as to limit any short circuit current (caused by an external short circuit on the roller surface) to a safe value related to the hazardous operational atmosphere. The output capacitance is able to be small and yet give adequate smoothing, due to the high frequency of 100 KHz of the input supply. This limits any momentary short circuit current and the energy in any possible spark discharge. The low speed cut-out arrangement h, i disconnects the 100 KHz supply voltage from the mulllplieqrectiher d when the roller speed drops below the rotational speed corresponding to the lowest press speed at which printing can take place. This provides operator safety, i.e. reduces risk of electric shock when the press is stationary or almost stationary. The switch may be mechanical or an electronic pulse-counting system operating in association with a rotating magnet and an electronic switching device. Alternatively, but not preferably, this cut-out at the low-printing speed can be arranged apart from the roller, e.g. a magnetic pulse generator on the press-drive operating an electronic switch in the supply in the control unit. The electrical capacitance of the outer conductive rubber layer on the roller metal body may be suitably increased by adding a material of high permittivity, e.g. titanium dioxide, to the ebonite intermediate layer. The provision of a stationary high voltage generator as described has the following ad vantage: i) The electronics are more reliable due to less vibration; ii) the generator high-voltage output does not need to return to press frame earth via any rotating assembly; iii)it is a simple matter to provide a cable con nection to give an indication of the high voltage at the control unit; iv) the high-voltage polarity can be changed fron the control unit without stopping the press; v) a slipping clutch assembly is not needed. Further advantages of the arrangement described are that no alternator is employed, making the high voltage generator more compact, the roller bearings are enclosed and protected from the ink environment, the generator can be adjusted to give maximum voltage even on a very slow printing press, and the high frequency supply means that the capacitors in the voltage-multiplying network can be quite small so making possible a smaller pack and reducing the energy available in the case of a short circuit discharge. WHAT WE CLAIM IS:

1. An electrostatically-assisted printing system in a gravure printing press, wherein a rider roller with an electrically-conducting surface is provided in contact with an im

pression roller of the press, and a high d.c.

voltage is supplied to the rider roller interior and thence to its surface from a flame-proof stationary enclosure located end-to-end with the rider roller and containing a voltagemultiplying network for deriving the high voltage within said enclosure from an external low voltage supply.

2. A system according to claim 1, wherein the stationary enclosure has a hollow spigot projecting from one end constituting a stationary shaft for supporting one end of the rider roller, and a high voltage lead passes through an insulating rod within said spigot.

3. A system according to claim 1 or claim 2, including a low-speed cut-out responding to the rate of rotation of the rider roller and operable to interrupt the low voltage supply to the voltage-multiplying network when the rate of rotation falls to a predetermined level.

4. A system according to any one of the preceding claims, wherein the low voltage supply is given a high frequency, e.g. 100KHz.

5. A system according to claim 4, wherein the stationary enclosure also contains an oscillator and amplifier for generating the high frequency of the low voltage supply.

6. A system according to any one of the preceding claims, wherein a polarity switch is provided within the stationary enclosure, operable from the exterior of said enclosure to switch the polarity of the high voltage supply fed by the voltage-multiplying network to the rider roller.

7. A system according to any one of the preceding claims, wherein the overall resistance in the output circuit of the voltage-multiplying network is such as to limit any short circuit current at the roller surface to a safe value.

8. An electrostatically-assisted printing system in a gravure printing press, substantially as described with reference to the accompanying drawings.