Impression roller for limiting charge distribution

An impression roller for electrostatically assisted printing distributes a charge across a web of packaging material. The roller has considerably more resistance in the longitudinal direction than in the circumferential direction to restrict the distribution of charge to portions of the roller extending beyond the web. The lower resistance in the circumferential direction is provided by a set of looped conductors--either conductive rubberized strips or wire loops--which are embedded in an outer layer of semiconductive material of the roller to allow electrical current traveling in the circumferential direction to bypass portions of the semiconductive material. The electrical current encounters considerably more resistance in the longitudinal direction, because the conductors are spaced longitudinally along the rollers at intervals and any current traveling from one looped conductor to another travels through the semiconductive material.

BACKGROUND OF THE INVENTION 
The invention relates to an impression roller for use in electrostatically 
assisted printing. 
In this method of printing, the transfer of ink from a printing design 
cylinder to a web of material such as paper, paperboard, fabric or plastic 
film, is assisted by the electrostatic attraction of ink to the web. This 
is accomplished by passing the web through a nip region, where the web is 
contacted on its topside by the impression roller and on its underside by 
the printing design roller. A voltage is applied to the impression roller 
and current is conducted through it to the area of the nip region. There, 
a voltage is generated between the impression roller and the printing 
design roller which is at ground potential, and an electrical charge is 
distributed across the area of the web contacting the two rollers. 
Impression rollers have generally been of two types: a two-layer roller and 
a three-layer roller. In the two-layer roller, an insulated core is 
covered with an outer layer of semiconductive material. A voltage is 
applied to the outer layer of semiconductive material at the top of the 
roller, so that current travels through the semiconductive layer to reach 
the nip region at the bottom of the roller. In the three-layer roller, a 
layer of material considerably more conductive than the semiconductive 
material is disposed around the insulated core, but beneath the outer 
surface and the semiconductive layer. When a voltage is applied across the 
top of the impression roller, current travels through the semiconductive 
material to the relatively more conductive layer. It then bypasses a great 
portion of the semiconductive layer as it travels around the circumference 
of the roller to the nip region. This results in the three-layer roller 
having considerably less resistance than a two-layer roller with a 
comparable semiconductive layer. 
Printing applications are numerous and varied, and the owner of an 
electrostatically assisted printing machine may desire to print images on 
webs of varying widths. In some instances the webs may be considerably 
narrower in width than the length of the rollers over which it travels. In 
those instances there is a great deal of excess electrical energy 
dissipated in areas of the impression roller that extend beyond the web. 
To remedy this, a voltage has been applied through a group of stainless 
steel sliding blades. The blades outside the width of web are lifted to a 
disengaging position to shorten the effective width over which voltage is 
applied to the impression roller. This system, however, is not apparently 
suitable for use with a three-layer roller, because substantial current 
would travel to the ends of the roller as well as around its 
circumference, due to the charge-distributing function of the conductive 
layer. 
SUMMARY OF THE INVENTION 
The invention is embodied in a machine roller which provides greater 
electrical resistance in the longitudinal direction than in the 
circumferential direction to limit the distribution of current in regions 
of the roller extending longitudinally beyond the region where the 
potential is applied. 
Such a roller has an elongated body with an insulated cylindrical core and 
a layer of semiconductive material extending around the insulated core to 
allow current to flow between a first region of higher voltage and a 
second region of lower voltage. A set of conductive elements are spaced 
along the length of the roller, with each conductive element encircling 
the insulated core and contacting the semiconductive layer beneath its 
outer surface. The conductive elements each have an electrical resistance 
of at least two orders of magnitude less than the electrical resistance 
through twice the thickness of the layer of semiconductive material. This 
provides a roller with substantially greater resistance in the 
longitudinal direction than in the circumferential direction where the 
conductive elements act to reduce the resistance. 
The general object of the invention is to limit charge distribution outside 
the nip area to reduce power and energy requirements for printing on webs 
that are narrower than the widest web that can be handled by the printing 
machine. 
Another object of the invention is to provide an impression roller with a 
resistance characteristic of a three-layer roller in the circumferential 
direction and a resistance characteristic of a two-layer roller in the 
longitudinal direction.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, a first embodiment of an impression roller 10 that 
incorporates the present invention is a type used 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 of the 
printing machine. 
A printing machine of the general type in which such a roller 10 may be 
employed is described and illustrated in Adamson et al, U.S. Pat. No. 
3,477,369, issued Nov. 11, 1969. As described there, the impression roller 
10 is journaled in bearings for rotation between a printing design 
cylinder (engraved) and a back-up roller for applying a voltage to the 
impression roller. The rollers are all journaled in respective bearings 
for rotation. 
As seen schematically in FIG. 3, a voltage application roller 14 is located 
above the impression roller 10, while the printing design roller 15 is 
located below. Printed matter is transferred to a web 16 of a material, 
such as paper, paperboard, fabric, plastic film, a laminate, or types of 
packaging material, by feeding the web 16 through a nip region 17 where 
the impression roller 10 and the printing design cylinder 15 bear against 
each other through the web 16. The printed matter is transferred both by 
directly impressing and by electrostatically attracting ink to the web 16 
in a pattern determined by the design on the printing design cylinder 15. 
The present invention is related to the characteristics of an impression 
roller 10 which affect the electrostatic attraction of ink to the web 16. 
As seen in FIGS. 1-3, the impression roller 10 has a tubular core 18 that 
is preferably made of metal. The core 18 is insulated by covering it with 
an inner layer 19 of insulating material to separate the core 18 from an 
outer layer 20 of semiconductive material. The insulating material is 
preferably a 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 20 is resilient and has an electrical resistivity in a range 
from 10.sup.6 ohm--centimeters to 10.sup.9 ohm--centimeters and a relative 
hardness in the range from 60 to 95 according to the Shore A scale. A 
chlorinated synthetic elastomer such as epichlorohydrin is suitable for 
use in the semiconductive layer 20. 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 FIGS. 1-3, the roller 10 includes a set of conductive 
bands 21. The bands are formed by conductive strips of synthetic rubber, 
each of which encircles the insulated core. These bands are spaced along 
the length of the roller, i.e. longitudinally relative to the roller, at 
regular intervals. As seen in FIGS. 2 and 3 the conductive bands 21 lie 
close to the boundary between the outer semiconductive layer 20 and the 
inner, insulating layer 19. The bands 21 are completely covered by the 
semiconductive layer 20. 
The volume resistivity of the material of the conductive bands 21 is 
preferably at least two orders of magnitude less than the volume 
resistivity of the material in outer layer 20. The invention is applicable 
wherever it is desired to provide conductive bands 21 of negligible 
resistance when compared with the resistance through twice the thickness 
of the semiconductive layer 20. For purposes of this description, where 
the resistance through one half of the mean circumferential length of a 
conductive band is equal to or less than 1% of the resistance through 
twice the thickness of the semiconductive layer, it is considered to be 
negligible. 
Referring to FIG. 3, in a typical arrangement, a d.c. voltage of 1500 volts 
is supplied through the voltage application roller 14. This voltage is 
applied to a first region 22 of the outer surface of the semiconductive 
layer 20 and is conducted to a second region of the roller 10, which in 
this example is the nip region 17. The printing design roller 15 is 
electrically grounded, as is the frame of the printing machine, so that a 
current will be transmitted through the impression roller 10 to the nip 
region 17. A suitable range for current in the nip region 17 is from 0.5 
to 3 milliamperes. 
As seen in FIG. 3, application of the electrical voltage in the first 
region of the roller will cause operating current (represented by dashed 
line 23) to be conducted radially through a first thickness of the 
semiconductive outer layer 20, then around the roller in the 
circumferential direction through the conductive bands 21, and then 
radially through a second thickness of the semiconductive outer layer 20. 
The roller 14 for applying the electrical voltage in FIG. 3 is shorter than 
the width of the web 16. As seen in FIG. 2, this roller 14 does not extend 
all the way to the end of the impression roller 10, and is not therefore 
disposed over all of the conductive bands 21. For electrical current to 
reach the outlying conductive bands 21 it must travel longitudinally along 
the roller, as well as radially inward (illustrated by dashed line 24). 
This requires that the current travel through the semiconductive layer 20, 
but due to the much higher resistance through that layer than through the 
conductive bands 21 very little current will travel in the longitudinal 
direction. Instead, the bulk of the current will be conducted through 
conductive bands 21 in the circumferential direction. 
In this embodiment the conductive bands 21 are each one-half inch in width 
and the space between adjacent bands is one inch. Spacing of one-half inch 
is suitable as is spacing of two inches or four inches between conductive 
bands 21. 
FIGS. 4-6 show a second embodiment of the invention in which the basic 
roller 10' is similar to the first embodiment. The relationship of the 
parts of the second embodiment to the parts of the first embodiment has 
been indicated by assigning numbers with the prime notation. An exception 
to this similarity is that the conductive elements are formed by brass 
wire loops 25. As seen best in FIG. 5 these loops 25 are spaced along the 
length of the roller 10' at intervals of about one-half inch and are 
embedded in the outer layer 20' of semiconductive material. The resistance 
of the brass wire is less than 1 ohm per ten feet. The semiconductive 
material has the same electrical and mechanical characteristics as in the 
first embodiment so that the brass wire loops 25 provide conductive 
elements of negligible resistance. 
Like the first example, the voltage application roller 14 in FIG. 5 does 
not extend to the end of the impression roller 20', and is shorter than 
the width of the web 16. This results in current being conducted primarily 
in the circumferential direction and limited to that portion of the roller 
20' to which the voltage is applied. 
While two embodiments of the impression roller of the invention have been 
disclosed it should now be apparent that other types of conductive 
elements could be used to reduce resistance in the circumferential 
direction, and that these types of conductive elements are deemed to be 
within the scope of the invention. Therefore, to fairly define the scope 
of the invention, the following claims are made.