IE72104B1 - A method for manufacturing rotary letterpress printing plates - Google Patents

Abstract

Artwork is separated into a number of associated plate images each corresponding to a particular colour of the design (20). Sets of registration marks are arranged (24) on opposite side margins of each plate image. Each set of registration marks comprises a central mark (6) and associated graduated lines (7, 8) extending outwardly from the central mark at each end of the plate image (3). The plate images are then distorted (26) to correspond to the shape of the cylinder used to mount the final printing plate and a photographic negative (28) of each plate image prepared. Each negative is mounted on an uncured photopolymer plate and exposed (30) onto the plate using UV light. Non-image areas are then chemically removed (32) to leave the finished printing plate which is mounted on a cylinder.

Description

A METHOD FOR MANOOCTORING ROTART-LETTERPRBSS PRINTING PLATES* This invention relates to a method for manufacturing a set of rotary-letterpress printing plates, in particular photopolymer printing plates.

Photopolymer rotary-letterpress printing plates are widely used in the printing industry and comprise a plate having a print design in relief thereon. The plate is releasablv mounted on a support cylinder of a printing machine for printing, typically being adhesively secured to the support cylinder. Usually a set of associated printing plates is used, each plate corresponding to a particular colour used in the design which is to be printed. The material on which the design is to be printed is passed through a number of printing stations in which each colour of the design is successively laid on the material to build up the complete design.

When a new set of printing plates is delivered to a printer typically, they are oversized and the printer cuts the plate to size for his printing machine support rollers. This is a relatively skilled and time-consuming exercise as the plates must be accurately measured and cut to size ensuring that all of the plates In the plate set correspond so that when the plates are mounted on the support rollers of the printing machine they can be brought into register.

Many printing plates carry a repeat pattern with gaps between adjacent relief images. During printing the inking roller which engages the printing plate tends to move down Into a gap and then up over the next .relief image. As it hits the leading edge of a relief image„ because of the initial impact in which the roller surface - 2 hits and then rides up over the leading edge, more ink tends to be deposited at the leading edge which can result in smudging or indistinct printing of that portion of the design when printed.

The present invention is directed towards overcoming these problems .

According to the invention there is provided a method for manufacturing a set of rotary-letterpress printing plates for printing a design, comprising the steps; (a) colour separating artwork into a number of associated plate images each corresponding to a particular colour, including arranging the spreads and chokes of the plate images, (b) putting corresponding sets of associated registration marks on opposite side margins of each plate image, each set of marks comprising on each side margin a central mark and associated graduated lines extending outwardly from the central mark adjacent each end of the plate image, (c) distorting each plate image to allow for subsequent distortion of the associated printing plate when it is mounted on a support cylinder for printing, (d) preparing a specially treated photographic negative for each distorted plate image, (e) mounting each specially treated photographic • negative on an uncured photopolymer plate and exposing the negative onto the plate using a UV light source and, (£) chemically etching away non-image areas of each plate to leave a relief image on the plate.

In a particularly preferred embodiment of the invention the graduations of the registration mark graduated lines correspond to the support cylinders on which they are mounted for printing.

In another embodiment the method includes the step of marking an associated scale adjacent the graduations.

In a further embodiment, the method includes the step of providing additional registration marks between each central mark and Its associated graduated lines.

Conveniently also, the method may include the step of providing indicia with information relating to the printing plate intermediate the registration marks on the side margins. In some cases, bearers may also be provided extending along the side margins.

In a particularly preferred embodiment the printing plates are prepared by the steps of; (a) loading artwork into a camera and producing a photographic negative image of the artwork, (fo) colour separating the artwork into a number of associated plate images, using masks and pins, including arranging the spreads and chokes, (c) putting the registration marks onto side marginal edges of each plate image, (d) distorting each plate image and producing a photographic positive plate image, (e) producing a specially treated photographic negative plate image from each positive image, mounting the negative image on an uncured photopolymer plate and exposing the negative onto the plate using a UV light source, and (f) chemically etching away non-image areas of each plate to leave a relief image on the plate.

In a further embodiment the process includes the steps of; (a) transferring the artwork on to a computer screen and manipulating the artwork image on the screen to suit a desired plate size, (b) separating the artwork image Into a number of associated plate images, optionally including tints and vignettes, and arranging the spreads and chokes, (c) adding registration marks to side marginal edges of each plate image on the screen, (d) distorting each plate image to allow for distortion of the printing plate in use, and outputting a high resolution film negative for each printing plate, (e) mounting each photographic negative on an uncured photopolymer plate and exposing the negative onto the plate using a UV light source, and (f) chemically etching away non-image areas of each plate to leave a relief image on the plate.

The invention will be more clearly understood from the following description of some embodiments thereof given by way of example only, with reference to the accompanying drawings, in which? Fig. 1 is a flow chart illustrating a method for manufacturing a set ox rotary-letterpress printing plates according to the invention.

Figs. 2 and 3 are flow charts illustrating alternative ways of carrying out the method shown In Fig 1, Fig. 4 is a plan view of a rotary-letterpress printing plate formed according to the method, and Fig. 5 is an end view of the plate shown in Fig. 4.

Referring to the drawings, a method for manufacturing a set of rotary-letterpress printing plates according to the invention will be described. Referring briefly first to Figs. 4 and 5 there is illustrated a photopolymer rotaryletterpress printing plats according to the invention indicated generally by the reference to the numeral l. The plate 1 has a base portion 2 with upstanding thereon images 3 of a print design. Associated registration marks are provided on opposite side margins 4,5 of the plate 1. Each set of registration marks comprises a central mark 6 and associated graduated lines 7,8 extending outwardly from the central mark 6 adjacent each end of the plate 1. The distance between the graduations, correspond to the support cylinder on which the plate is to be mounted for printing. It will be noted also that a scale corresponding to the repeat length of the plate 1 is marked along each set of graduations. This advantageously facilitates the printer when the plate 1 is to be cut to a desired repeat length, the printer simply trimming the ends of the plate 1 to the required repeat length marking on the graduations. Also provided on the side margins 4,5 are information blocks 10 carrying indicia with information relating to the printing plate 1.

It will be noted that the registration marks 6,7,8 and the blocks 10 all form side bearers for the plate 1. Optionally, additional bearing strips may also be provided on the side margins 4,5 ox the printing plate 1.

Referring in particular to Fig. l the method for manufacturing the printing plate 1 according to the invention will be described. Artwork corresponding to the design which is to be printed is first colour separated (20) into a number of associated plat® images, each corresponding to a particular colour for the design. Also at this stage the spreads and chokes of the plats images are arranged. Each plate image is then passed through a step and repeat machine 22 in which the desired step and repeat pattern for each plat© is produced. Next, the registration marks are arranged (24) along each side margin of a plate image. The plat® image is then distorted (26) to allow for subsequent distortion of the associated printing plate when mounted os a support cylinder for printing. A photographic negative for each distorted plate image is produced (28). This negative is then mounted on an uncured photopolvmer plate and at an exposure station 30 the negative is exposed onto the plate using a W light source. Non-image areas of the plate are subsequently chemically etched away (32) to leave the relief image on the plate.

Referring in particular to Fig. 2 a manual process for deriving a plate negative for exposure onto the plate is shown. Artwork is mounted in a camera (40) and a photographic negative image of the artwork is produced. Next the negative image is colour separated (42) into a number of associated plate images each corresponding to a particular colour using masks and pins, at the same time arranging the spreads and chokes. Each prepared negative plate image is delivered to a step and repeat machine 44 to produce & plats image with the required step and repeat pattern of the design. Then the registration marks 6,7,8 and blocks are arranged along the side margins of the plate image such that they correspond for all of the set of plate images. Each plate image is delivered to an image distortion machine 48 and a positive plate image produced- From the positive plate image a negative plate image is produced (50) for exposure onto an uncured photopolymer plate as previously described to produce a printing plate 1.

Referring now to Fig. 3 an alternative method for production of the set of plate negatives is shown. Artwork 60 is transferred onto a computer screen 62 either directly if the artwork is on tape or disc or if necessary first digitizing the information on a CCD camera for transfer to the computer screen 62. On the computer screen 62 the artwork is manipulated (S4) to suit a desired plate size. The artwork is then colour separated (66) into a number of associated plate images each corresponding to a particular colour, optionally including tints and vignettes , and arranging the spreads and chokes . Next the required step and repeat pattern (68) is arranged on the screen 62 for each plate image. Then the registration marks 5,7,8 and information blocks 10 are arranged (70) in the marginal portions of a plate image on the computer screen 62 and then repeated for all the other plate images in the set of plate images such that they all correspond. Each plate image is then distorted (72) and high resolution photographic negatives are outputted for each plate image. Each photographic negative can then be used as previously described to expose an uncured photopolymer plate to produce the rotary-letterpress printed plate 1.

It will be appreciated that the particular registration mark arrangement on the printing plates advantageously provides an accurate alignment facility to aid a printer in the cutting and mounting of plates. Further, the nature of the registration marks enables a printer to easily check registration between the various plates in the set of printing plates during operation of the printing machine. This facilitates the setting up of a printing press and also the maintenance of registration throughout a print run.

Advantageously also the information blocks can contain information such as colour reference and order number to facilitate the printer. For colour work, the information blocks can include control strips to take densitometer readings.

It will also bs appreciated that the arrangement of the registration marks and the information blocks is such that they fox» bearers engageable with the print roller of the print machine to maintain the print roller at an even distance above the plat© so that ink is evenly deposited on the relief images on the plate.

The invention is not limited to the embodiments hereinbefore described, but may be varied in construction and detail ae defined by cfee eteims.

Claims (3)

1. CLAIMS IS A method for manufacturing a set of rotaryletterpress printing plates for printing a design, comprising the steps; (a) colour separating artwork into a number of associated plate images each corresponding to a particular colour, including arranging the spreads and chokes of the plate images, (b) putting corresponding sets of associated registration marks on opposite side margins of each plate image, each set of marks comprising on each side margin a central mark and associated graduated lines extending outwardly from the central mark at each end of the plat® image, (c) distorting each plate image to allow for subsequent distortion of the associated printing plate when it is mounted on a support cylinder for printing, (d) preparing a photographic negative of each distorted plate image, (e) mounting each specially treated photographic negative on an uncured photopolyiaer plate and exposing the negative onto the plate using a uv light source, and (f) chemically etching away non-image areas of each plate to leave a relief image on the plate. 5 3. 4. 5. 6. A method as claimed in claim 1 wherein the graduations of the registration mark graduated lines correspond to the support cylinders on which they are mounted and used for printing. A method as claimed in claim 2 including the step of marking an associated scale adjacent to the graduations. A method as claimed in any preceding claim including the step of providing additional registration marks between each central mark and its associated graduated lines. A method as claimed in any preceding claim including the step of providing indicia with information relating to the printing plate intermediate the registration marks on the side margins. A method as claimed in any preceding claim wherein the printing plates ar© prepared by the steps of? (a) loading artwork into a camera and producing a photographic negative image of the artwork, (b) colour separating the artwork into a number of associated plate images using masks and pins, including arranging the spreads and chokes, (c) putting the registration marks on side marginal edges of each plate image, (d) distorting each plate image and producing a photographic positive plate image, (e) producing a specially treated photographic negative plate image from each positive
2. 2. 5 image, mounting the negative image on an uncured photopolymer plate, and exposing the negative onto the plate using a UV light source, and (f) chemically etching away non-image areas of 10 each plate to leave a relief image on the plate.
3. 3. 7. A method as claimed in any of claims 1-5 including the steps; (a) transferring artwork onto a computer screen 15 and manipulating the artwork image on the screen to suit a desired plate size, (b) colour separating the artwork image into a number of associated plate images, optionally including tints and vignettes, and arranging 20 the spreads and chokes, (c) adding registration marks to side marginal edges of each plate image on the screen, (d) distorting each plate image to allow for distortion of the printing plate in use, and 25 outputting a high resolution film negative for each printing plate, (e) mounting each photographic negative on an uncured photopolymer plate and exposing the 13 negative onto the plate using a uV light source, and (£) chemically etching away non-image areas of each plate to leave a relief image on the plate. A set of printing plates whenever produced hv the method as claimed in any preceding claims.