Technical Field

The field of the invention is impression rollers for an electrostatically assisted printing machine.

Background Art

Printing machines have been developed in which the transfer of ink to a web of material, such as paper, is not only provided by direct transfer of ink from an engraved cylinder, but also by electrostatic attraction of ink to the web. This is accomplished by passing he web through a nip region where the underside of the web is contacted by the engraved cylinder as it is being pressed down against the cylinder by an impression roller, which in turn is urged downward by hydraulic means.

Transfer of the ink is assisted by applying an electrical voltage in the nip region in a pattern determined by the engraved cylinder. In machines where a backup roller contacts the impression roller in an area halfway around its circumference from the nip region, a voltage may be conveniently applied to the impression roller at that point and conducted through it to the engraved cylinder which is at ground potential.

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Impression rollers of the type suitable for use in such a machine have typically been formed around a steel core that is grounded and insulated from the semiconduc¬ tive layer that is formed on the exterior of the roller to conduct current to the nip region. The material used in the outer layer of the impression roller exhibits the resistance of a semiconductor, and is also resilient, which allows for flattening of the outer surface of the impression roller in the nip region.

Disclosure of the Invention

The invention is embodied in an impression roller of adjustable resistance and is practiced in a method of making such a roller. Given a semiconductive resilient material that exhibits a variation in resistivity over its mechanical life, the invention teaches that a plural¬ ity of conductive elements can be added to such a roller and adapted for interconnection to adjust the resistance of the roller in several steps during its mechanical life to maintain the resistance of the roller within an accept- able range despite a change in the resistivity of the material in the outer layer.

More particularly, the invention is demonstrated in an impression roller for conducting current from a first region of a relatively higher electrical potential to a second region of relatively lower electrical potential that is displaced around a portion of the circumference of the roller from the first region. The roller has a body that includes an insulated cylindrical core and a layer of semiconductive material around the insulated core. A set of conductive elements are disposed around the outside of the insulated core and in contact with the semiconductive material. Each conductive element has an

electrical resistance of at least two orders of magnitude less than the layer of semiconductive material. The conductive elements have termination portions and means are provided for connecting the termination portions of selected conductive elements so that current bypasses portions of the layer of semiconductive material as such current is conducted from the first region to the second region.

The invention provides an impression roller of adjustable resistance to allow utilization of the roller throughout its mechanical life.

The invention also provides a principle of roller construction that will be useful in various applications where it would be desirable to control the electrical properties of the covering, for example, electrostatic printing and copying.

The invention allows the adjustment of the resistance of an impression roller within an operating range in view of other parameters that may be adjustable or present in a particular printing machine.

Two constructions of a roller embodying the invention are illustrated by way of example in the accompanying drawings.

Brief Description of the Drawings

Fig. 1 is a perspective view of a printing machine roller of the present invention;

Fig. 2 is a sectional view of the roller of Fig. 1; Fig. 3 is a first schematic view of a section of the printing machine roller of Figs. 1 and 2 in its operating environment;

Fig. 4 is a second schematic- view of the same section of the printing machine roller after it has been rotated 22.5 degrees from its position in Fig. 3;

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Fig- 5 is a third schematic view of the same section of the printing machine roller as Figs. 3 and 4, with certain of its conductive elements being left unconnected; Fig. 6 is a schematic view of a longitudinal section of the printing machine roller taken in plane indicated by line 6-6 in Fig. 1; and .

Fig. 7 is an end view of a second embodiment of the invention in which resistors are interconnected between the leads extending from the roller of Fig. 1.

Best Mode of Carrying Out the Invention

Referring to Fig. 1, the present invention is em¬ bodied in an impression roller 10 for use in a printing machine (not shown) . The roller 10 has an elongated, cylindrical body 11 that is closed on its opposite ends by hubs 12. A pair of journal shafts 13 extend from the hubs 12 on opposite ends to be received in bearings in the printing machine.

A printing machine of the type in which such a roller 10 may be employed is described in Adamson et al, U.S. Patent No. 3,477,369, issued November 11, 1969. As described there and as represented schematically in Figs. 3-5, the impression roller 10 is journaled in bearings for rotation between an engraved printing cylinder 14 and a backup roller 15 that is also journaled in respective bearings for rotation. Printed matter is transferred to a web 16 of a material such as paper, paperboard, fabric, plastic film or a laminate, by feeding the web 16 through a nip region 10b where the impression roller 10 and the engraved cylinder 14 bear upon each other through the web 16. The printed matter is transferred both by directly impressing and by electrostatically attracting ink from the cylinder 14 to the web 16 in a pattern determined by

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the engraved pattern on the cylinder 14. The present invention is related to characteristics of an impression roller 10 which affect the electrostatic attraction of ink to the web 16. Referring to Fig. 2 and 6, the impression roller 10 has a tubular core 17 that is preferably made of metal. The core 17 has an outside diameter of 5-1/2 inches, while the outside diameter of the roller is 7 inches. The core 17 is insulated by covering it with an inner layer 18 of insulating material of 1/4 inch thickness to separate the core 17 from an outer layer 19 of semicon¬ ductive material that is 1/2 inch in thickness. The insulating material is preferably either natural or synthetic rubber or a mixture of these, but other known insulating materials can also be used. The preferred material in the outer layer 19 is resilient and has an

3 electrical resistivity in a range from 10 ohm-centimeters o to 10 ohm-centimeters and a relative hardness in the range from 60-95 according to the Shore A scale. A chlorinated synthetic elastomer such as epichlorohydrin is suitable for use in forming the semiconductive layer 19. In addition, other semiconductive materials, including natural or synthetic materials, that exhibit the above described electrical characteristics, can also be employed. Furthermore, while resilient materials are preferred, in some applications non-resilient materials can also be used.

Still referring to Eigs. 1, 2 and 6, the roller 10 includes a set of conductive elements 20a-20h extending longitudinally from one end to the other, an as seen in Fig. 2, the elements 20a-20h are spaced radially from the central longitudinal axis of the roller 10. This radial distance is slightly greater than the radius of the interior boundary between the inner layer 18 of insulating

material and the outer layer 19 of semiconductive material, The conductive elements 20a-20h are embedded in the outer layer 19 and equally spaced around the boundary between the two layers 18 and 19 to define a circle concentric therewith. Referring to Fig. 1, the conductive elements 20a-20h in this embodiment are metal wires having ends that extend out one end of the roller 10 and are con¬ nected by jumpers 21 and electrical connectors 22 in any of several arrangements to be described. Referring to Fig. 3 in a typical arrangement with a backup roller 15 and an engraved printing cylinder 16 as described above, a d.c. voltage of 1500 volts is supplied through the backup roller 15. This voltage is supplied to a first region 10a of the outer surface of the semi- conductive layer 19 and conducted to a second region 10b on the impression roller 10 that defines the nip region. The engraved cylinder 16 is grounded as is the frame of the printing machine so that a current will be transmit¬ ted through the impression roller 10 and through the portion of the web 16 passing through the nip region 10b. A suitable range for current in the nip region 10b is from 0.5 to 3 milliamperes. A new impression roller, without the conductive elements described above, typic¬ ally has a resistance between 500 kilo-ohms and 1500 kilo-ohms, thereby providing an operating current in the desired range. As the roller 10 is used over a period of time, however, the outer layer 19 of semiconductive material experiences deterioration in its electrical characteristics to a relatively greater degree than it experiences mechanical wear. The result is that after several months of usage, the resistance of the semiconduc¬ tive outer layer 19 begins to approach 3000 kilo-ohms, thereby requiring a much higher voltage through the backup roller 15 to maintain a current that is suitable

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for electrostatically assisted printing. It will be seen that the present invention provides a roller with an adjustable resistance, so that as the resistivity of the semiconductive outer layer increases with use, the resist- ance of the roller 10 can be decreased in steps by con¬ necting the conductive elements 20a-20h. Where use of the roller 10 results in a decrease in resistivity, selected conductive elements can be connected before the roller is used and then disconnected as required to increase the overall resistance of the roller 10 in steps.

Referring to Fig. 3, it will be seen that with all eight conductive elements 20a-20f connected at one end of the roller 10, and with conductive element 20a positioned directly beneath the first region where the backup roller 15 and the impression roller 10 make contact, and with conductor 20e positioned 180 degrees around an arc from element 20a, the operating current (represented by dotted line 23) will be conducted through the semiconductive outer layer 19 for a distance equal to twice its thickness. After the current passes from the backup roller 15 through a first thickness, it is conducted through element 20a and the jumpers 21 to element 20e and then passes through a second thickness of the outer layer 19. Other conductors 20b-20d and 20f-20h provide a similar result as they are rotated into the positions of conductors 20a and 20e, respectively. Without the conductive elements 20a-20h, current would flow approximately half the mean arcuate length of the semiconductive layer as indicated by the dotted line 24 in Fig. 3.

Although the resistivity of the preferred conductive elements 20 is less than 1 ohm per ten feet of wire which in ohm centimeters is approximately six orders of magnitude less than the resistivity of the preferred semiconductive

material, the present invention is applicable to embodi¬ ments where the conductive elements 20a-20h provide a negligible resistance when compared with the resistance through twice the thickness of the semiconductive layer 19. For purposes of this description wherever the resist¬ ance through a conductive element 20 is less than 1% of the resistance through the semiconductive layer 19 it will be considered to be negligible.

Referring to Fig. 4, it can be seen that some varia- tion in the effective resistance of the impression roller 10 will be experienced when conductors 20a and 20e are not aligned along a diameter from the source voltage region 10a to the nip region 10b. This variation is tolerable, however, provided that the effective resistance of the roller 10 and the operating current are kept within a suitable range. This variation is also limited as the speed of rotation of the roller 10 is increased, and as the number of conductive elements is increased. In other embodiments of the invention, an odd number of conductive elements 20 might be employed to reduce this variation.

Referring to Fig. 6, it is also necessary to consider the path of current from points near and remote to the end of the roller 10 at which the jumpers 21 connect the conductive elements 20a-20f. With conductive elements 20a-20h of negligible resistance as defined above, the added resistance to current traveling from point A to point F via B and Ξ will also be negligible in comparison with current traveling from point C to point H via D and G. Any resistance provided by the conductors 20a-20h can be further reduced by connecting the conductive elements 20 at both ends of the roller 10.

Referring to Fig. 5, it can be seen that when conduc-= tive elements 20d and 20h are not connected by jumper

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wires 21 and when the conductive elements 20a-20h of the impression roller 10 are positioned the same as in Fig. 3, the overall resistance of the roller 10 will be greater because the operating current will first travel along an arcuate portion of a circumferential path through the semiconductive layer 19 before bypassing a second arcuate portion and being conducted through the jumper wires 21 connecting conductors 20a-20c and 20e-20g respectively. The invention thus teaches that the impression roller can be installed in a printing machine without connections between the conductive elements 20a-20h, or with connections between conductive elements to provide a resistance between a source of d.c. voltage and a grounded, engraved cylinder 14, and as the electrical property of resistivity of the semiconductive material 19 in the roller 10 changes over its mechanical life, connections between the conductive elements 20a-20h can be made or interrupted to adjust the resistance of the roller 10 in steps to maintain its resistance and the operating current within an acceptable operating range for that printing machine.

In making a roller 10 of the present invention, the core 17 is coated with a metal bonding agent and the insulating layer 18 is formed by applying one or more laminations of sheet stock that are steam vulcanized under pressure to form a unitary layer. The semiconduc¬ tive layer 19 is also formed by applying sheet stock in successive laminations which are vulcanized individually or as a group or together with the insulating layer 18 to form a unitary body 11 for the roller 10. ^' The conductive elements 20a-20h are positioned around the outside of the insulated core 17 in slots made in the innermost lamination of the semiconductive layer 19, and after this layer 18 has been vulcanized the conductive elements 20a-20h are

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embedded in the locations described earlier herein. The termination portions of the conductive elements 20a-20h are allowed to extend out of the outer layer 19 to be connected in a pattern that will allow current to flow between the higher potential region 10a and the nip region 10b while bypassing portions of the semiconductive layer 19 as seen in Fig. 5.

An additional, optional step is to enclose the connected termination portions, which limits access by the user. With this additional step, calibration of the roller is made at the manufacturing site by making the necessary connections. When, after use in the field, a roller does not provide the necessary current control, the roller can be returned to the manufacturer for adjust- ent through the making of additional connections or interruption of original 'connections, to bring the roller back within the specified range of performance. Switches can be installed between the termination portions of the conductive elements so that the connections need only be made and interrupted "electrically" without mechanically altering the roller. When the connections between the termination portions of the conductive elements 20a-20h are to be enclosed or sealed, the semiconductive layer 18 is formed to be shorter in length than the insulated core 17 so that portions of the roller that enclose the connec¬ tions will not extend beyond the ends of the roller body 11. The roller body 11 typically extends a short distance beyond the end of the back-up roller 15 and the engraved cylinder 14 to provide non-contacting areas in which the connections between the conductive elements 20a-20h can be conveniently made and enclosed for their protection. Referring to Fig. 7, additional steps in adjusting the resistance of the roller 10 may be provided by connecting resistors 25 of various values between the

electrical conductive elements instead of jumper wires 21 of negligible resistance.

Thus, the character of the roller 10 is changed from that of a single conductive object to a plurality of conductive segments, with the preferred embodiment showing one way of dividing the roller into such segments. In Figs. 3-5, the connection of any two adjacent conductive elements bypasses one of these segments. Non-adjacent conductive elements can also be connected to bypass multiple segments. It should be apparent from the above description that the roller may be divided into segments in different ways by positioning the conductive elements 20a-20h differently in other embodiments. Similarly, it will also be apparent that the number and type of conduc- tive elements may vary. For example, the conductive elements can be conductive rubber strips, or can be metalized elements formed on the outer surface of the insulating layer 18 using circuit board manu acturing techniques. The invention is also applicable to three layer rollers wherein a conductive layer is disposed between the semiconductive layer and the insulating layer, providing that some non-negligible resistance is provided by the conductive layer as current is conducted from region 10a to region 10b. The invention could be used to adjust resistance in such a roller although within a more limited range than in the two layer roller seen in Figs. 1-7.

Accordingly, the scope of the invention has been reserved for the following claims.