Patent Grant
6905266
The present invention is directed to a method and apparatus for applying a uniform film of fluid to a flat surface being conveyed through a workstation and specifically for applying a uniform film of developer to lithographic printing plates in a developing station. The invention is particularly directed to the use of a wire-wound rod for uniformly distributing the fluid over the workpiece and to the technique for metering and delivering the fluid to the wire-wound rod. The present invention is applicable to various planar workpieces and various types of workstations but is particularly applicable to lithographic printing plates which have been imaged and require the application of a developer to remove the areas of the coating on the plate which have been rendered soluble by the imaging process. Although the invention has a broader application, it will be described with particular reference to lithographic printing plate development.
One method for the development of the imaged plates entails the application of a thin film or layer of developing solution to the imaged plate surface of each imaged plate to be developed. This thin film of developer solution is allowed to dwell on the plate for a time sufficient to complete the development and then rinsed from the plate. Because only a thin film of developer solution is applied to each plate, any variation of any part of the surface of the plate from being substantially flat and horizontal and any variation in the thickness of the film of developer and any variation in the dwell time of the developer on different areas of the plate can result in the improper development of the coating.
The use of a wire-wound rod as a coating means is well suited to the continuous coating of web materials with a fluid, and well known in the art. Typically, a wire-wound coating rod is used in a coating method where some volume of fluid is continuously applied to the web surface prior to the rod, and surface. However, the coating of individual, discrete plates requires the ability to precisely initiate the coating process and precisely terminate the coating process on individual plates delivered at irregular intervals. In the case of lithographic printing plates, the developer fluid must be applied in the correct amount uniformly distributed across the width and length of the plate, with minimal waste.
The use of a wire-wound rod in metering the developer in a lithographic plate processor is known in the art. U.S. Pat. No. 4,737,810 teaches the application of excess developer with the wire-wound rod serving as the means to meter off the excess into some recovery means. The rod thus serves as the means to control the volume of fluid consumed in the development process. The developer fluid is applied to the plate ahead of the wire-wound rod and it is indicated in this patent that the path between the delivery of the fluid and the metering at the wire-wound rod is sufficiently short that development does not commence within this area. The excess developer removed in this area is intended to be reused.
Typical imaging methods include exposure to radiation and writing by ink jet. As is well known in the art, the imaging process renders the coating soluble in the imaged areas of a positive-working plate and renders the coating insoluble in the imaged areas of a negative-working plate. In either case, it is the coating which has been rendered soluble or the coating which has remained soluble that is removed. The particular compositions of the developer solutions for these different types of printing plates are well known. For example, many of the printing plates currently in use are positive-working plates and have coatings that contain alkali-soluble resins, specifically phenolic or acrylic resins. These coatings usually contain dissolution inhibitors that render them insoluble in the alkaline developers. The imaging process reverses this dissolution inhibition and the coating then becomes soluble in the areas subjected to the imaging radiation.
Excess developer that has been applied to the surface, metered off by a wire-wound rod, and subsequently recovered has been exposed to the atmosphere, and as such is subject to degradation. It is well documented in the art that atmospheric carbon dioxide rapidly reduces the alkalinity of aqueous alkaline developers of the type very commonly used in the processing of positive-working plates. Thus the recovered developer that is being reused will not have the same alkalinity as fresh, new developer. This recovered developer is in fact therefore reused in a way that with each application of developer to the plate, some fraction of the developer metered off by the wire-wound coating rod will have been removed in a previous cycle of development. Thus the repeated exposure to carbon dioxide and resultant degradation will further alter the effective alkalinity of the developer. Further, if the developer is applied some distance ahead of a wire-wound rod, some degree of development is certain to take place and it may very well be uneven.
With positive-working plates, the difference in the solubility of the imaged and non-imaged areas of the coating is generally less than the difference in solubility for negative-working plates. For that reason, the development process is more critical for positive-working plates. Also, the development mechanism for positive-working plates is a percolation process and a quiescent film of developer solution is critical. Any relative movement between the developer and the surface of the plate must be minimized or eliminated. Furthermore, the film of developer must be uniform with no bubbles. For these reasons, it is critical how the developer is applied to the plate.
The present invention is directed to an apparatus and a method for applying a uniform film of a fluid to the surface of a flat workpiece as it is being conveyed through a workstation. The invention is specifically directed to the development of lithographic printing plates and comprises a novel system and method for applying the thin film of developer solution to each plate in a controlled manner using a wire-wound rod coating device and a unique manner of metering and feeding fresh fluid to the wire-wound rod. Specifically, the fluid is gently fed onto the wire-wound rod by simple volumetric displacement and overflow and the flow is controlled by sensing the beginning and end of each workpiece or plate. The developer is allowed to dwell on the plate as it is conveyed across a platen or other support structure for a sufficient time to allow for percolation into and/or dissolution of the soluble areas of the coating. The spent developer is rinsed and sent to waste. According to the invention, only fresh developer solution is applied at a uniform film thickness, thereby achieving uniform image development.
In the present invention, a novel design of a wire-wound rod coating system including means for metering and feeding the developer solution to the rod is used to control the thickness and assure the uniformity of the developer solution on the plate. In contrast to the known techniques where an excess of fluid is applied and subsequently metered off by the wire-wound rod, the present invention delivers precisely the required volume of developer at precisely the rate required to obtain a uniform film of developer on the plate in the amount needed to process the plate with very little excess. The preferred means for controlling the volume of developer delivered and the rate at which it is delivered is a peristaltic pump. The delivery of the developer is commenced at the beginning of the plate and continues at the appropriate rate of flow until the end of the plate where it is stopped. The wire-wound rod in the present invention is merely a means for ensuring the uniform distribution of the developer across the surface of the plate since there is little if any excess developer to be removed. Thus all problems with developer degradation that arise from the recirculation of excess developer are eliminated.
In the context of the present invention, the term “wire-wound” includes what are termed “formed rods”. These formed rods are manufactured by machining a rod or tube to produce a rolled thread-like profile that duplicates the pitch and radius of the rods formed by winding wire and are to be understood to be the equivalent of rods formed by winding wire onto a core. In one embodiment, a wire-wound coating rod is fabricated using a hollow tube as the core on which the wire is wound. The developer is delivered to the interior of the tube. There are penetrations through the wall of the tube allowing the developer to flow out from the interior. There are two main alternatives for this rod embodiment. In one alternative, the wire is tightly wound around the circumference of the rod, i.e., there are no gaps between adjacent winds. In this alternative, it is necessary to provide some means for the fluid to flow through the wire. This can be accomplished by drilling holes at a series of locations between adjacent wraps of the wire. The drilling may be any means of providing holes including direct mechanical drills or the use of laser beams to remove material and form a hole. In this embodiment, the hollow tube that serves as the core of the rod may be provided with slots or holes prior to being wound with wire. The holes between the wraps of the wire are subsequently drilled at locations corresponding to the positions of the holes or slots in the tube. Alternatively, a hollow tube with no slots or holes may be wound with wire, and the subsequent drilling of the holes between the wire wraps may be done such the penetration through the hollow tube is made during this drilling process.
In the second alternative, the wire is loosely wound around the hollow tube, i.e., the helical pitch of the winding exceeds the diameter of the wire, resulting in a gap between adjacent winds. For example, if a wire with a 0.010″ (10 mils) diameter is wound on a helical pitch of 0.011″, there will be a gap of 0.001″ between adjacent wraps. The gaps allow for the liquid to flow out between the wires. The hollow tube is preferably provided with slots or holes prior to the winding, but the drilling of holes in the tube in the gaps between the wire after winding is possible.
The holes or slots provided are distributed along the length of the tube to enable the developer to be spread uniformly across the width of the plate. The tube is filled with the fluid to the level of the holes or slots. The small openings in the tube inhibit the exchange of air into the interior of the tube. This is an important advantage when using alkaline developers that are subject to degradation by atmospheric carbon dioxide.
In a preferred embodiment, a peristaltic pump is used as the means for delivering the developer to the interior of the hollow tube. The pump commences operation when the leading edge of the plate is detected by a sensor. The volume of developer delivered by the peristaltic pump causes an equal volume of developer to overflow through the slots or holes out onto the wire, where it is distributed across the plate width. The pump rate is matched to the plate speed and developer quantity requirement to maintain a uniform coverage along the length of the plate. The pump stops in conjunction with the sensing of the trailing edge of the plate. In
Another embodiment of the wire-wound rod of the present invention is shown in FIG. 3. The tube 44 still has the slots 48 but the spirally wound wire 46 is now loosely wound with gaps 52 between adjacent winds. These gaps permit the fluid to flow out from the slots 48 between the wires. The relative sizes of the wire and gap are distorted in this
A further embodiment of the invention is shown in
The printing plate which has been coated with the developer solution continues to travel across the platen. The length and speed of travel is selected such that the developer solution will have completed the development process by the time the printing plate reaches the discharge end of the platen. A typical development time is 20 to 60 seconds. At this point, rinse water from the supply 65 is sprayed onto the plate through the spray nozzles 66 and 68. Located below the conveyor structure is a collection pan 70 which collects all of the liquid run off from the printing plate including the spent developer solution and rinse water now containing the portion of the coating which has been dissolved away. The developer solution which is rinsed from the plate is collected at 72 and sent to waste. It can be seen that there is always only fresh developer solution being applied to the plates and that there is only a small quantity of developer solution applied to each plate. It has been discovered that the consumption of developer solution can be reduced by as much as 50% when compared to a conventional printing plate development processor.
In order to properly develop an imaged plate in accordance with the present invention, it is essential that the thin film of developer solution be substantially uniformly distributed over the entire upper, imaged surface of the plate as it is being conveyed across the platen. This requires that the plate on the conveyor be substantially flat and substantially horizontal or level and begins with having a substantially flat, horizontal support structure and, therefore, a substantially flat horizontal conveyor belt. Since the printing plates are very thin and flexible, surface tension is used to hold the plate firmly in position and flat on the conveyor belt. For example, this can be done by providing a film of water between the plate and the conveyor belt.
The present invention uses simple volumetric displacement and overflow as the means for controlling the rate and volume of fluid applied to the plate. Referring back to
The following examples compare the plates which are developed according to the prior art and plates which are developed according to the present invention. These examples clearly show a significant and unanticipated benefit of the invention.
An 830 plate commercially available from Anocoil Corporation, Rockville, Conn., which is a plate thermally imaged by infrared radiation, was imaged on a Creo/Scitex Trendsetter Imager, commercially available from Creo/Scitex, Vancouver, British Columbia, Canada. The plate was imaged at an exposure of 200 mj/cm2. The image comprised halftone target areas at a 175 line per inch ruling.
The imaged plate was developed in a Glunz and Jensen Model 135 Plate Processor, commercially available from Glunz and Jensen, Elkwood, Va. The developer used was T-4 Developer commercially available from Anocoil Corporation, Rockville, Conn., which is an aqueous sodium metasilicate solution. This processor immerses the plate in a sump of developer that is recirculated during use and replenished at a rate based on usage.
A processor was constructed as depicted in
The wire-wound coating rod was constructed according to the depiction shown in FIG. 3. The core was 1¼″ stainless steel tube. Slots ½″ in length and {fraction (1/16)}″ in height were cut along the length of the tube at 1″ intervals. The tube was subsequently wound with 0.020″ stainless steel wire. The wire was wound at a helical pitch of 0.021″, giving a 0.001″ gap between adjacent winds on the tube. The tube was placed in the processor on the continuous conveyor belt so that the axis of the tube was perpendicular to the direction of travel of the belt and the orientation of the slots was toward the plate entry end of the processor. One end of the tube was capped and the other end was fitted with a flexible tubing connection. The wire-wound tube was connected to a variable flow peristaltic pump available from VWR International of Bridgeport, N.J.
An 830 T plate was imaged in the same manner as in the Example 1.
The imaged plate was processed in the processor of the present invention using a developer comprising an aqueous sodium metasilicate solution.
A summary of the measured halftone dot values is given in Table 1. The area of interest on the plates was a series of vertical targets of different halftone values. These target values going from left to right across the plate were 50%, 30%, 10%, 70%, 10%, 30% and 50%. It is clear that the Example 2 plate of the present invention has dot values that are much closer to the nominal target values. More significantly, the halftone dot values for the Example 2 plate are more consistent on the left and right sides of the plate than for Example 1. The 50% target values are 45% left and 46% right for the plate processed according to the method of present invention. By contrast, Example 1 shows a very large variation in the 50% target values; 39% left and 19% right. It is clear that the method of the present invention yields a more uniformly processed plate than the prior art method which is typically used commercially.
This is a continuation of U.S. application Ser. No. 10/1446,357 filed May 28, 2003 now U.S. Pat. No. 6,793,418, the benefit of which is claimed under 35 U.S.C. Sec. 120.
| Number | Name | Date | Kind |
|---|---|---|---|
| 4737810 | Kobayashi et al. | Apr 1988 | A |
| 5210005 | Takekoshi et al. | May 1993 | A |
| 5984539 | Patton et al. | Nov 1999 | A |
| 6423138 | Piccinino et al. | Jul 2002 | B1 |
| Number | Date | Country |
|---|---|---|
| 2 163 151 | May 1973 | DE |
| Number | Date | Country | |
|---|---|---|---|
| 20050013608 A1 | Jan 2005 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10446357 | May 2003 | US |
| Child | 10916153 | US |
