BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:
FIG. 1 is a schematic representation of a printer comprising an image-bearing roller according to the present invention;
FIG. 2 is a schematic representation of an embodiment of a roller according to the present invention;
FIG. 3 is a schematic representation of another embodiment of a roller according to the present invention; and
FIG. 4 is a graph showing the heat transfer from the roller surface to the air flow as a function of the axial position of the roller.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a schematic drawing of an electrophotographic printer 1. The operation of an electrophotographic printer is known in the background art. The heart of the printer 1 is the print engine 3. The print engine 3 includes an image-bearing roller 5. The image-bearing roller 5 includes an outer tube 7 and an inner tube 9. A photoconductive layer (not shown) is applied on the surface of the image-bearing roller 5. The photoconductive layer which is an insulator in the dark and a conductor when exposed to light. Initially, the photoconductive layer is given a total positive charge by the charge corona wire 11, which is a wire with an electrical current running through it. As the drum revolves, the printer shines a tiny laser beam 13 across the surface to discharge certain points, by reflecting the laser beam on a movable polygonal mirror 15. In this way, the laser “draws” the letters and images to be printed as a pattern of electrical charges, i.e. an electrostatic image. After the pattern is set, the toner supply roller 17 coats the photoconductive layer with positively charged toner, i.e. a fine, black powder. Since it has a positive charge, the toner clings to the negative discharged areas of the photoconductive layer, but not to the positively charged “background.” With the powder pattern affixed, the image-bearing roller 5 rolls over a sheet of paper that has been separated from the paper stack 19 and which is moving along below a belt 21. Before the paper rolls under the image-bearing roller, it is given a negative charge by a transfer corona wire (not shown). This charge is stronger than the negative charge of the electrostatic image, so the paper can pull the toner powder away. Since it is moving at the same speed as the image-bearing roller 5, the paper picks up the image pattern exactly. Finally, the printer passes the paper through the fuser 23, i.e. a pair of heated rollers. As the paper passes through these rollers, the loose toner powder melts, fusing with the fibers in the paper. The fuser rolls the paper to the output tray (not shown). The surface temperature of the image-bearing roller 5 tends to increase during operation. The surface temperature may; however, not exceed the melting temperature of the toner powder because then toner powder will fuse with the photoconductive layer on the image-bearing roller, resulting in print artifacts. Therefore, a fan-unit (not shown) blows air through the inner tube 9 and the outer tube 7 in order to cool the surface of the image-bearing member.
FIG. 2 shows a schematic representation of the image-bearing roller 5. The imaging member includes an outer tube 7, two inner tubes 9a and 9b and a central axis (not shown). The tubes define an inter tubular space 11 and an intra tubular space 13. A blocking element 27 is a one-piece disk that fits tightly into the outer tube. The blocking element 27 has two disk-like protrusions 27a and 27b that operatively engage the inner tubes 9a and 9b. Flows of air 35 and 37 are transported through the roller from the left to the right. The inter tubular space 11 and the intra tubular space 13 are divided in a first part upstream of the blocking element and a second part downstream of the blocking element. Ducts 31 in the blocking element guide the flow of air 35 from the upstream part of the inter tubular space 11 to the downstream part of the intra tubular space 13. Ducts 33 guide the air flow 37 from the upstream part of the intra tubular space 13 to the downstream part of the inter tubular space 11. In this manner, air flow 35 cools the first part of the roller surface and the air flow 37 cools the second part of the roller surface. The duct pair 31-33 forms a double helix with a half pitch. The entrances of both ducts are radially aligned, with the entrance of duct 31 lying further away from the central axis than the entrance of duct 33. The exits of both ducts are also radially aligned with the exit of duct 33 lying further away from the central axis than the exit of duct 31.
FIG. 3 is a schematic representation of another embodiment of the image-bearing roller 5. The imaging member includes an outer tube 7, two inner tubes 9a and 9b and the central axis (not shown). The tubes define an inter tubular space 11 and an intra tubular space 13. The blocking member 27 includes two disks 27c and 27d that are operatively engaged with the inner tubes 9a and 9b. The two discs 27c and 27d are connected via a gasket 29. The gasket 29 extends to the wall of the outer tube 7, dividing the inter tubular space 11 in two parts. Flows of air 35 and 37 are transported through the image-bearing roller 5 from the left to the right. The inter tubular space 11 and the intra tubular space 13 are divided in a first part upstream of the blocking element and a second part downstream of the blocking element. The ducts 31 in the blocking element guide the flow of air 35 from the upstream part of the inter tubular space 11 to the downstream part of the intra tubular space 13. The ducts 33 guide the air flow 37 from the upstream part of the intra tubular space 13 to the downstream part of the inter tubular space 11. In this manner, air flow 35 cools the first part of the roller surface and the air flow 37 cools the second part of the roller surface. The duct pair 31-33 forms a distorted double helix.
FIG. 4 shows a graph of the heat flow from the surface of the imaging roller to the air flow that passes though the roller as a function of the axial position of the roller. Trace 40 corresponds to the situation where the roller contains only an outer tube and an inner tube, without a blocking element and without any ducts. The heat flow from the imaging roller surface to the air flow steadily decreases when progressing along the longitudinal axis of the roller. This is due to the fact that the temperature of the air flow increases when passing through the roller. This results in a decrease of the temperature difference between the surface of the imaging roller and the air flow which results in a decreased heat flow. The heat flow in a steady state condition is viz. proportional to the temperature difference between the two bodies. The diminishing heat flow in the course of the roller results in a temperature gradient along the axis of the roller.
Trace 42a, 42b, 42c corresponds to the situation wherein the roller comprises an outer tube and an inner tube with a blocking element with concomitant ducts. The trace 42a, 42b, 42c consists of a first part 42a and a second part 42c which are divided by a dip 42b. The dip 42b is caused by the presence of the blocking element 27. Where the blocking element 27 contacts the inner surface of the outer tube, there is no air flow to carry away the heat from the surface of the roller. Trace 42a shows a decrease of the heat flow when progressing along the axis of the roller. This is due to the increasing temperature of the air flow in the inter tubular space 11 by the uptake of heat from the roller surface. When the air flow reaches the blocking element the air flow goes through the ducts and into the intra tubular space 13 on the other side of the blocking element 27 and then exits the roller. Simultaneously, the air flow that is blown into the first part of the intra tubular space reaches the blocking element 27. This air flow has maintained its initial temperature because it has not exchanged heat with the surface of the roller. This air flow passes through the ducts into the second part of the inter tubular space 11. Now, the air flow comes into heat exchanging contact with the surface of the roller. The heat flow from the surface of the roller to the air flow immediately reaches its maximum value, and then progressively decreases towards the end of the roller. The result is that the absolute differences in heat flow in trace 42 are significantly reduced with respect to trace 40. Dip 42b must be kept as narrow as possible in order to avoid overheating of the corresponding part of the roller surface. This is realized by using the embodiment of FIG. 3.
The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.
Patent Application
20080014003
