GB2386344A

GB2386344A - Computer to plate imaging

Info

Publication number: GB2386344A
Application number: GB0205661A
Authority: GB
Inventors: Graham Daking; Paul Sinclair Drysdale
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-03-11
Filing date: 2002-03-11
Publication date: 2003-09-17
Also published as: GB0205661D0

Abstract

A method for enabling a film image setter to function as a plate image setter is described, using a number of novel techniques including the use of a specially designed plate carrier and appropriate means of achieving accurate registration of the printing plate modifications are made to the internal drum and the film cassette slots of the film image setter to add the functionality of a plate image setter.

Description

Developments in computer to print imaging Background

Historically, high volume printing has depended upon the offset lithographic printing process, whereby text and images are transferred from a printing plate via a 'blanket' to the paper. This process must be repeated several times (usually four times), in order to provide 'full' colour printing.

Print quality depends upon a number of features, including both the quality of the printing plate(s) and, in the case of full colour printing, the alignment, or registration', of the printing plates carrying the individual colours, which are effectively printed one on top of the other. Consequently much attention is given to the preparation of the printing plates.

In recent years, much of the text and images has been created using computers, and a significant industry has grown up providing equipment to prepare printing plates from computer generated text and images (data). Initially computer data was transferred to film using a film imagesetter, and the printing plate then produced from the film by photographic means. Latterly it has been possible to transfer the computer data direct to plate using a plate imagesetter. This technique is referred to as CtP (Computer direct to Plate), and offers users greater speed and lower production costs.

There are however still a large number of film image setters in regular use due both to the equipment's longevity and the high cost of replacing them with the new CtP equipment. Introduction

The objective of this invention is to describe means for modifying an existing film image setter such that it may function as a plate image setter, using a number of novel techniques. Description

A conventional film image setter is shown simplistically in figure 1, and comprises of a stationary drum (1) and a moveable scanning carriage (2). Drum (1) is a segment (approximately one half) of an open ended cylinder, upon the internal surface of which a photographic film (3) is mounted, usually being held in place by vacuum.

Scanning carriage (2) carries an optical projection device (4) which is capable of projecting a narrowly focussed beam of light onto film (3). Scanning carriage (2) is capable of traversing along the central axis of drum ( 1).

Optical projection device (4) is capable of spinning within scanning carriage (2), and is so positioned that whilst traversing along drum (1), remains a constant distance from the internal surface of drum (1). The aforementioned beam of light thus illuminates a chord of the internal surface of drum ( 1) as it spins. Since photographic film (3) is secured to the surface of drum (1), the light beam exposes a line on the film perpendicular to the axis of cylinder (1). The mechanics of the entire unit are so \

designed that the beam remains in focus over the entire surface of photographic film (3). Furthermore, the beam of light may be modulated by an electronic signal, usually derived from a computer. The exact position of the beam of light with respect to drum (1) (and hence film (3)) is accurately known at all times through electronic sensors (not shown) so that the computer data can modulate the light beam in synchronism with the position signals and hence expose the film to light where required. As optical projection device (4) spins continuously, the beam of light is turned on or off according to whether it is directed onto film (3) or not.

In use, an unexposed photographic film (3) is mounted on the internal surface of drum (1) and scanning carriage (2) with the attached spinning optical projection device (4) positioned at one end of drum (1). Exposure of the complete surface of photographic film (3) can then be accomplished by allowing scanning carriage (2) to traverse the full length of drum (1).

A machine such as described above is designed only to carry film or polyester plates in roll form mounted on drum (1).

According to the invention, for which we pray a patent may be granted, the basic film image setter as described above can be modified to carry printing plates in place of the photographic film.

The modifications include changes to the original film image setter and the addition of a number of new components: this package of modifications is described in detail in a preferred embodiment of the invention.

Drum (1) is shown in more detail in figure 2, whereby it can be seen that the internal surface of drum (1) contains a series of tracks (6) which are connected to a vacuum pump (not shown) to hold the film in position. Drum (1) also contains film register pins (7) which are used to locate the film on. The only modifications to drum (1) consist of adding guides (5) at each end using existing threaded holes in the drum.

Guides (5) are required to enable drum (1) to carry a newly designed plate carrier (described below).

Figure 3 shows the new plate carrier (8) made typically of thin semirigid sheet material such as spring steel. Rollers (9) are positioned at both corners of the leading edge (upper edge in figure 3) of plate carrier (8), and further small rollers (10) fitted as shown. A series of holes (11) are punched in plate carrier (8) and are so positioned as to align with the existing vacuum tracks (6) of drum (1) when carrier (8) is mounted in drum (1). Plate carrier (8) also carries location strips (12), one close to the leading edge and one towards the opposite end as shown. Although location strips (12) are securely attached to plate carrier (8), their facing edges are raised slightly from the surface of carrier (8) such that a printing plate may be pushed underneath the facing edges of location strips (12) a short distance. Location slots (13) are positioned along the leading edge of plate carrier (8) to mate up with the existing film register pins (7) of drum (1). Registration pins (14) are positioned as shown close to the leading edge of carrier (8) in order to engage with corresponding pre-punched holes in the printing plate once it is inserted under the upper location strip (12). Holes (15) and hand-apertures (16) are positioned as shown in figure 3 close to the trailing edge of plate carrier (8). Holes (15) aid in securing plate carrier (8) inside the film image

setter, and hand-apertures (16) are used to manually hold plate carrier (8) when withdrawing it.

When it is desired to use the film image setter as a plate image setter, an unexposed printing plate is loaded into plate carrier (8) by 'trapping' its edges underneath location strips (12) and engaging its pre- punched holes onto register pins (14). Plate carrier (8), complete with the unexposed printing plate, is loaded into drum (1) by engaging rollers (9) into guides (5) and pushing plate carrier (8) all the way along until register pins (7) of drum (1) enter location slots (13) and so 'centralize' plate carrier (8) in drum (1). During this loading operation, plate carrier (8) assumes the curved profile of drum (1), as a consequence of which the printing plate 'creeps' further under the lower location strip (12), ensuring also that it is held firmly inside carrier (8). The subsequent application of vacuum through tracks (6) and holes (11) ensures that this alignment is maintained.

Film image setters conventionally carry a film cassette mounted on top of the setter to aid in the loading and unloading of film. The film cassette is a self contained unit which simply slots into position, and needs to be replaced by a newly designed plate loading cassette when the film image setter is to be used to prepare printing plates.

Figure 4 shows diagrammatically such a plate loading cassette (17), containing two legs (18) which serve both to locate cassette (17) into corresponding slots in the top of the film image setter, and to guide plate carrier (8) into drum (1). Although the film image setter already contains slots for the film cassette, these require widening in order to accomodate the slightly wider legs of the plate loading cassette. Plate loading cassette (17) also contains two sprung catches (19), one at each end of the cassette (for clarity, only one is shown).

Catches (19) are shown in greater detail whilst engaged with plate carrier (8) in figure 5. Catches (19) consist of a tension spring and over-centre toggle, the overall function of which is to engage in holes (15) of plate carrier (8) once it is fully inserted into the film image setter and so exert a constant downward force on the plate carrier to hold it in good contact with the internal surface of drum ( 1).

In use, plate loading cassette (17), is slotted into position on top of the film image setter, and plate carrier (8), loaded with an unexposed printing plate, is fed through the slots in legs (18) into the film image setter. Plate carrier (8) will wrap itself within drum (1) until register pins (7) of drum (1) enter location slots (13) as previously described. Catches (19) are now engaged with holes (15) and locked into position so that plate carrier (8) remains secure within the film image setter. Exposure of the printing plate starts, at the conclusion of which, catches (19) are released from

engagement with holes (15) and plate carrier (8) withdrawn by pulling it by hand-

apertures (16).

Claims

1. A system comprising a specially designed plate carrier, plate cassette and modifications to the internal drum and film cassette slots of a film image setter, so as to add the functionality of a plate image setter.

2. A system as described in claim 1 whereby the aforementioned plate carrier comprises a flat semi-rigid sheet.

3. A plate carrier as described in claim 2 containing associated registration pins (14) generally as shown in figure 3.

4. A plate carrier as described in claim 2 containing associated location slots (13) generally as shown in figure 3.

5. A plate carrier as described in claim 2 that has roller wheels (9) at each end of the leading edge as shown in figure 3.

6. A plate carrier as described in claim 2 that has roller wheels (10) attached to the leading edge as shown in figure 3.

7. A plate carrier as described in claim that has rows of holes as generally shown in figure 3 to allow a vacuum to hold down the printing plate.

8. A plate carrier as described in claim 2 fitted with location strips generally as shown in figure 3 that trap the edges of the printing plate and are so positioned as to accurately locate the aforementioned printing plate once the plate carrier is loaded into the film image setter allowing the printing plate to 'creep' within the aforementioned plate carrier to achieve this.

9. A plate carrier as described in claim 2 with means (shown in figure 3 as holes (15)) to enable it to be positively secured inside the film image setter.

10. A plate carrier as described in claim 2 containing one or more hand apertures as generally shown as slots (16) in figure 3 to assist loading and unloading.

11. A system as described in claim 1 whereby the plate cassette is so designed as to be capable of replacing the normal film cassette with modifications onlt to the existing slots in the film image setter.

12. A system as described in claim 1 whereby the plate cassette is fitted with a system of springs and catches as generally shown in figures 4 and 5 capable of engaging with appropriately located and designed points of engagement on the plate carrier to provide a positive locking force whilst the plate carrier is within the film image setter.

13. A system as described in claim 1 whereby the modifications to the internal drum of the film image setter consist of the addition of two guides (shown as

(5) in figure 2) to enable the plate carrier to be fed into the drum of the film image setter.

14. A system as described in claim 1 whereby a 'swing-up' table may be attached to the rear of the film image setter to aid loading.