Printing – Processes
Reexamination Certificate
1995-01-27
2001-02-27
Yan, Ren (Department: 2854)
Printing
Processes
C101S232000
Reexamination Certificate
active
06192800
ABSTRACT:
FIELD OF THE INVENTION
This invention concerns method and apparatus for providing improved support for freshly printed sheet material in a printing press.
BACKGROUND OF THE INVENTION
In the operation of a multi-unit rotary offset printing press, freshly printed sheets are transported by transfer devices from one printing unit to another, and then they are delivered to a sheet stacker. Sheet transfer devices are known by various names including transfer cylinders, support rollers, delivery wheels, delivery cylinders, skeleton wheels, transfer drums, support wheels, guide wheels and the like. The ink marking problems inherent in transferring freshly printed sheets have been longstanding. In order to minimize the contact area between the transfer cylinder and the printed sheet, conventional support wheels have been modified in the form of relatively thin disks having a toothed or serrated circumference, referred to as skeleton wheels. However, those types of wheels have not overcome the problems of smearing and marking the printed surface of the printed sheet material due to sliding action between the printed sheet material and the projections or serrations. Moreover, the attempts to minimize the surface support area in contact with the sheet material has also resulted in actual indenting or dimpling of the material itself.
DESCRIPTION OF THE PRIOR ART
Various efforts have been made to overcome the disadvantages of thin disk skeleton wheels. One of the more successful approaches has been completely contrary to the concept of minimizing the surface area of contact. That improvement is disclosed and claimed in my U.S. Pat. No. 3,791,644 wherein I provide for a substantially cylindrical wheel or roller coated with an improved ink repellent surface formed by a layer of polytetrafluoroethylene (PTFE). During the use of the PTFE coated cylinder in high speed commercial printing equipment, the surface of the coated cylinder must be washed relatively frequently with a solvent to remove any ink accumulation.
The limitations on the use of the conventional skeleton wheel and PTFE coated transfer cylinder have been overcome with a transfer cylinder having an ink repellent and supportive flexible jacket covering or the like for handling the freshly printed sheet material. It is now well recognized and accepted in the printing industry world-wide that marking and smearing of freshly printed sheets caused by engagement of the wet printed surface against the supporting surface of a conventional press transfer cylinder is substantially eliminated by using the anti-marking flexible covering system as disclosed and claimed in my U.S. Pat. No. 4,402,267 entitled “Method and Apparatus for Handling Printed Sheet Material”, the disclosure of which is incorporated herein by reference. That system, which is marketed under license by Printing Research, Inc. of Dallas, Tex. under the registered trademark “SUPER BLUE”, includes a movable covering or jacket of flexible material, referred to as a “flexible jacket covering”. The flexible jacket covering provides a yieldable, cushioning support for the freshly printed side of the printed sheet such that any relative movement between the printed sheet and the transfer cylinder surface takes place between the surface of the flexible jacket covering and the support surface of the cylinder so that marking and smearing of the freshly printed surface is substantially reduced.
Although the improved “SUPER BLUE” transfer cylinder has achieved world-wide commercial success, with continuous use such as is common in many printing operations, there is over a period of time a slight accumulation of ink on the surface of the flexible jacket covering. Moreover, some presses do not have sufficient cylinder clearance to accommodate the flexible jacket covering.
Investigation and testing has identified the accumulation of an electrostatic charge on the freshly printed sheets as a factor which tends to impede completely free movement of the printed sheets as they are pulled around the transfer cylinder. The electrostatic charge build-up also appears to cause a faster accumulation of ink so that the support surfaces of the transfer cylinder becomes ink encrusted and needs to be changed more frequently. The build-up of the static electric charge on the freshly printed sheets is caused by “frictional electricity”, which is the transfer of electrons from one material to another when they are pressed or rubbed together.
According to one theory, the transfer of electrostatic charges between two contacting dielectrics, such as the metal press parts and a paper or other substrate sheet, is proportional to the difference between their dielectric constants, with the electrostatic charge moving from the material having the lower dielectric constant to the material having the higher dielectric constant. Since metal has a lower dielectric constant as compared with paper, an electrostatic charge is picked up by the sheets of paper from frictional contact with metal press parts as the sheets travel through the press.
Those transfer cylinders whose transfer surface is covered by a synthetic or natural organic resin, for example, as disclosed in my U.S. Pat. No. 4,402,267, have a low-friction surface but also have electrical insulating, dielectric properties which make them an accumulator of electrostatic charges carried by the printed sheets. That is, the electrical charge which is transferred to the printed sheets is also transferred to the underlying low friction, electrically insulating dielectric covering. As a consequence of such electrostatic charge transfer and accumulation, the freshly printed sheets tend to cling to the underlying cylinder base covering surface and do not move as freely because of the force of electrostatic attraction between the printed sheet material and the electrically insulating cylinder base covering.
SUMMARY OF THE INVENTION
We have discovered that virtually smear-free sheet transfer can be obtained without using a flexible jacket covering as disclosed in U.S. Pat. No. 4,402,267. Smear-free sheet transfer is accomplished by a base covering of electrically semi-conductive material having a frictional coefficient which is less than the frictional coefficient of the transfer cylinder sheet support surface. The detrimental effect of electrostatic charge accumulation on the freshly printed sheets is prevented by interposing a layer or covering of semi-conductive material having a low coefficient of friction which is less than the coefficient of friction of the transfer cylinder surface, whereby electrostatic charges carried by the freshly printed sheet material are discharged through the semi-conductive layer or covering into the grounded transfer or delivery cylinder. Consequently, the build-up or accumulation of electrostatic charges on the semi-conductive covering cannot occur, since such charges are conducted immediately from the printed sheet through the semi-conductive base covering into the transfer cylinder and into the grounded frame of the printing press.
In accordance with one aspect of the present invention, radially projecting surface portions on the semi-conductive base covering define electrostatic precipitation points and reduce the surface area available for frictional engagement. The low friction properties of the semi-conductive base covering permit free movement of the freshly printed sheets relative to the transfer cylinder surface. Electrostatic charges carried by the printed sheet material are discharged into the transfer cylinder through the semi-conductive base covering.
In accordance with another aspect of the present invention, movement of the freshly printed sheets relative to the transfer cylinder is improved by a base covering of a low frictional coefficient material disposed on the sheet support surface of the transfer cylinder. The low frictional coefficient base covering material has a frictional coefficient which is less than the frictional coefficient of the sheet support surface, and has radially projecting surface portions which r
Branson John Andrew
DeMoore Howard Warren
DeMoore Howard W.
Locke Liddell & Sapp LLP
Yan Ren
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