Apparatus for processing printing plates precoated on both sides

Abstract

An apparatus for simultaneously processing two sides of an offset printing plate, including a developer solution dispenser for dispensing developer solution onto two sides of the plate, a developer section, a washing section, a gumming section and transport rollers for propelling the plates through the apparatus. The improvement lies primarily in the developer section which includes two free-floating pads of a soft material mounted in a manner to receive a plate between them and reciprocatable transversely to the direction of plate travel.

Description

This invention relates to an apparatus for simultaneously processing two sides of a printing plate, for example a planographic printing plate as is used in the lithographic printing process. It has long been known in the lithographic industry that a subtractive presensitized offset printing plate may be developed by what is known as the "hand developing process." In this process, the carrier, which is usually a thin sheet of aluminum, is first coated by the manufacturer with a light-sensitive coating usually containing, among other things, one or more resins and the plate is then sold in the presensitized condition to the customer.

Some printing plates are available with such a subtractive coating on two sides thereof. In this case, the customer exposes one side at a time or, in suitable equipment, both sides simultaneously through a master to a source of strong light such as a carbon arc, for example. The light-sensitive coating when subjected to such a light is converted in such a manner that, after development, the image areas are retained on the plate surface and the non-image areas are removed. After development, for example by rubbing a developing agent into the surface of the plate by means of a sponge, the plate may be fixed and/or washed and finally a coating of gum arabic or similar material is applied to the surface of the plate to protect the image-free areas thereof. The plate is then ready for the printing press.

Where this hand developing process is employed to produce a printing plate, the following procedure is generally followed: From a typewritten set-up or make-up equivalent to the material to be printed and provided in any desired manner including typed articles, pictures of various kinds of art work of different sizes and the like, all assembled onto a suitable cardboard or other support, a master is prepared in the conventional manner. The image of the master is then transferred onto the sensitized printing member, such as is described above, by a suitable exposure means.

The printing member thus prepared is then subjected to a developing operation using the "chemistry" prescribed by the manufacturer of the printing member. This "chemistry" is worked into the exposed coating by hand rubbing, for example with a sponge, and the non-image areas are subtracted or removed from the carrier leaving an exact replica of the image on the plate carrier. The printing member is then fixed and/or washed and a solution of gum arabic, or a similar solution, is applied to the plate surface, resulting in a printing plate which is ready for the press.

As described above, some plates have a light-sensitive coating on two sides of the carrier or support, thus resulting in advantages to the printer who is then able to print two different jobs from a single printing member. This means that each side of the printing member must be exposed, as described above, and each side must be hand developed and processed in a sink, as described above.

The disadvantages of hand development of offset printing plates are numerous. The process is slow and expensive. Uniformity of pressure in applying the developing solution to remove the undesired coating is almost impossible to attain and exposure to the developing solution is uneven. Thus, defective printing often results from an inadequate development or under-development of an area or from applying varying pressure by hand, which may adversely affect the desired printing image. Drying of the developer on certain portions of the plate before it can be rubbed in to remove the undesired coating also may leave a residue on the plate. A further difficulty with the hand method is in the application of the developing solution. It is presently the practice of the craftman to pour a quantity of developer onto a developing sponge or pad and onto the center of the plate, which quantity is supposedly sufficient to process the plate, and the craftman then works his sponge from the "reservior." This procedure may lead to a high degree of contamination of the processing fluids by the removed photosensitive coating as well as a change in the chemistry of the fluid because of evaporation, which will, in turn, either reduce the efficiency of the chemistry resulting in incomplete removal of the undesired coating in highly critical areas of halftones, or increase its potency resulting in image attack.

Further, the development of a two-sided plate by developing one side at a time in a sink and turning the plate over can result in damage to the first developed side by pieces of contaminated material in the sink being forced back into the image or non-image areas of the first developed side when the second side is developed. These defects may not become apparent until the plate is clamped into a printing press and the expense of developing the plate has been incurred. All of these problems become more critical with increasing plate sizes.

The present invention overcomes the difficulties associated with the conventional hand developing of a two-sided plate by providing an enclosed developer apparatus which includes a receiving station for receiving a printing plate, a continuous transport system, a dispensing station in which developer is applied onto two sides of the plate simultaneously and evenly over the complete surface of the two sides, a rubbing or scrubbing station wherein two slightly curved, free floating velour type cloth coated pieces of rubber are mounted one above and one below the plate in a manner such as to facilitate easy entrance of the plate between the two scrubbing devices, the two scrubbing devices being mounted for reciprocation transversely across the surface of the plate in directions opposite to each other. When no plate is in the developing system, the two scrubbing devices rub against each other.

Prior to entering the scrubbing unit, developer is applied onto both sides of the plate. Developer is also introduced into the opening or nip between the scrubbing pads. By reciprocating the slightly curved, floating pads in directions opposite to each other and by introducing developer into the nip of the scrubbing pads, the removed coating which would eventually foul the scrubbing pads is effectively and continuously scrubbed and washed away. Nip rolls carry the plate into the developing apparatus and nip rolls removed excess developer from the plate and carry it into a washing station which washes the top and bottom of the plate effectively with or without mechanical scrubbing action. Another pair of nip rolls removes excess water from the plate and passes the plate into a gumming station where a gum solution is applied to both surfaces of the plate and which, in turn, is metered by a pair of soft rubber squeegee rolls in intimate contact with each other to minimize the amount of protective material applied, consistent with the requirement to protect the plate. The plate then passes into a drying station where warm air is blown onto the top and bottom of the plate before it exits from the apparatus.

The apparatus according to the present invention provides a uniform and even processing on the top and bottom of the plate without scratching or otherwise damaging the image. Also, by including means for recirculating the developer, it is continuously filtered and used to exhaustion, resulting in additional savings to the user.

The invention will be further illustrated by reference to the accompanying drawings in which:

FIG. 1 is a schematic view of the apparatus of the present invention, and

FIG. 2 is a detail view of the scrubbing or rubbing apparatus.

Referring to FIG. 1, a planographic printing plate sensitized on both sides and imaged on both sides by exposure to a desired master, is inserted into the apparatus at the point of the arrow 2. Alternatively two plates, each sensitized and exposed only on one side thereof may be inserted back to back into the apparatus. The inserted plate passes over the trigger mechanism 4 and thus actuates the entire apparatus. The plate then passes through a pair of feed rolls 6, then through a first pair of shielded developer dispensers 8, and through a second pair of developer dispensers 10, which introduces developer into the free-floating reciprocating scrubber pads 14 and 16 respectively. The upper pad 14 is mounted on the upper reciprocator shaft 18 and the lower scrubber pad is mounted on the lower reciprocator shaft 20. The upper scrubber pad is guided at its lower end in its transverse reciprocating movement by the guide 22 and the lower scrubber pad is guided in its transverse reciprocating movement by the guide 24. Both of the guides 22 and 24 cooperate with fixed guide means on the frame of the apparatus, not shown. After passing through the scrubber pads, the plate passes between a third pair of developer dispensers 26 which remove any residual coating which may still adhere to the surface. The plate then passes between the developer transport-squeegee rolls 28 which remove excess developer and propel the plate or plate assembly between the water wash tubes 30 to the rinse transport-squeegee rolls 32. This pair of rolls now assumes the propulsion of the plate or plate assembly and passes it between the gum dispensing tubes 34 to the gum applicator transport-squeegee rolls 36. These rolls, in turn, pass the plate between a pair of air-drying tubes 38 to the exit rolls 40, which guide the now developed, rinsed, gummed, and dried planographic printing plate or plate assembly out of the apparatus in condition for mounting on a press.

Referring to FIG. 2, a plate 42 is shown as it passes between the upper and lower scrubber pads 14 and 16 respectively. As noted above and as shown in FIG. 2, the pads 14 and 16 are free-floating, being secured to the upper and lower shafts 18 and 20, respectively, and travel freely at their unsecured ends by means of the guides 22 and 24. These pads may be made of any desired soft material which does not harm the surface of the plate and, as noted above, velour-covered rubber has been found to give good results.

As will be seen from FIG. 2, the scrubbing pads 14 and 16 are curved away from each other at the leading edges 44 and 46, respectively, the leading edges being those edges through which the plate 42 first passes as it travels from left to right, as shown in FIG. 2. The upper and lower reciprocator developer sprays 10 introduce sprays into the entrance nip of the scrubber pads 14 and 16 so that the removed coating is effectively and continuously washed away. The upper and lower developer dispensers 26 remove any residual coating which may still adhere to the plate surface prior to the entry of the plate into the developer transport-squeegee rolls.

The scrubbing pads 14 and 16 are reciprocated, by means not shown in the drawings, in directions opposite to each other.

The entire apparatus is enclosed in a housing, not shown.

The operating conditions of the scrubber or rubbing means may be widely varied depending, for example, upon the plate size, plate travel speed, and the like. However, the pad reciprocation rate may be in the range of 300 to 400 strokes per minute, with 350 strokes per minute providing excellent results. The pad stroke may be in the range of 1 to 3 inches and a stroke or travel of 1.25 inches has been found to give excellent results. The plate travel speed may be in the range of 24 to 60 inches per minute, with a speed of 36 inches per minute providing excellent results.

The total pad contact area per plate side is approximately 90 square inches for a processor accommodating 32-inch plates, and 100 square inches per plate side for a processor accommodating 48-inch plates, but the pad to plate contact area is, of course, relative to the plate size.

The length of the pad contact with the plate may be in the range of 2 to 4 inches or more, and a length of about 2.5 inches has been found to give excellent results. The pressure of the pad contact with the plate may be in the range of about 0.08 to 0.3 psi or more and a pressure of about 0.12 psi has been found to give excellent results. This pressure results in a total pressure of 10.8 pounds per plate side in an apparatus for processing 32-inch plates and 15.6 pounds per plate side in an apparatus for processing 48-inch plates.

It will be obvious to those skilled in the art that many modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.

Claims

What is claimed is:

1. In an apparatus for simultaneously processing two sides of an offset printing plate, including means for dispensing developer solution onto two sides of the plate, developer means, washing means, gumming means and means for propelling the plate through the apparatus, the improvement comprising a developer means which includes a pair of opposed pad means, of a soft material, mounted in a manner to receive a plate between them, the upper one of said pad means being free-floating, whereby the pressure applied to a plate passing between said pad means is essentially the weight of said upper pad means assembly, and said pad means being reciprocatable transversely to the direction of plate travel.

2. In an apparatus for simultaneously processing two sides of an offset printing plate, including means for dispensing developer solution onto two sides of the plate, developer means, washing means, gumming means and means for propelling the plate through the apparatus, the improvement comprising a developer means which includes a pair of opposed pad means, of a soft material, mounted in a manner to receive a plate between them and curved away from each other on the side where the plate enters between them, the upper one of said pad means being free-floating, whereby the pressure applied to a plate passing between said pad means is essentially the weight of said upper pad means assembly, and said pad means being reciprocatable transversely to the direction of plate travel.

3. An apparatus according to claim 2 including means for introducing a spray of developer between the curved portions of the pad means.

4. An apparatus according to claim 2 in which the pad means are fabricated of velour type cloth covered rubber.

5. An apparatus according to claim 2 in which the two pad means reciprocate in directions opposite to each other.

6. An apparatus according to claim 2 in which the plate is a subtractive presensitized plate.