WO1990015714A1 - A method and a scanning device for preparation of large working surfaces, particularly printing patterns on serigraphic printing frames - Google Patents

Abstract

For producing high quality patterns on large surfaces such as serigraphical printing frames having a light coating it is advantageous to produce the pattern by laser scanning, line by line, though by this technique the working width is rather limited. According to the invention, however, a large effective width is obtained by scanning with normal width a number of neighbouring zones (a-b, b-c) of the working surface. Extreme care should be taken for ensuring the side edges of the zones to coincide, but a high accuracy is achievable almost automatically with the use to a surface coating of a type that changes colour locally where it is exposed to the laser beam. Thereby a sensor (4) associated with the laser gun (2) can successively detect the location of the side of the neighbouring zone line by line, and the sensor signal can be used to control the new line scanning such that the new line will stop exactly at the end of the corresponding line in the neighbouring zone. Hereby even local irregularities in the zone border will be taken into account.

Description

A method and a Scanning Device for Preparation of Large Working Surface , Particularly Printing Patterns on Serigraphic Printing Frames.

The present invention relates to a method of expos¬ ing large film surfaces, primarily for preparing graphic printing elements of the serigraphic frame type, where a photographic emulsion on the surface of a serigraphic frame is irradiated in accordance with a master pattern so as to later on form a printing plate with subject determined subareas being respectively permeable and non-permeable for printing ink.

According to a widely used technique the permeable fabric layer of a serigraphic frame is coated by a pho¬ tographic emulsion and covered by a photographic film that represents the desired printing pattern at full size, whereafter the unit is irradiated by a strong ultraviolet light for exposing the emulsion on the fabric by a contact print, such that it is possible to thereafter treat the emulsion to make it selectively ink permeable according to the desired pattern.

This, however, is a troublesome and expensive pro¬ cess, i.a. because it can be expensive to produce the required films in the relatively very large sizes that may be actual, e.g. with edge lengths well more than one meter in both longitudinal and transverse direction.

The invention is based on the knowledge that it is possible to affect the photographic emulsion in a more direct manner, viz. by irradiating it with a concen¬ trated light beam, which is controlled to describe the desired printing pattern on the printing plate. Typical¬ ly the beam can be a laser beam which, from a transmit¬ ter head, is brought to sweep over the plate by a scan¬ ning, which in a computer controlled manner produces just the desired printing pattern, without the latter having to be represented by a full size film. It is well known, however, that by such a scanning it is very difficult to produce an irradiation zone with a width of more than 50-60 cm, which is far too little for many relevant printing works, so for broader pat¬ terns it has been required to make use of the said con¬ tact prints anyway. The said limitation of the width is due to some basic limitations for a modulated scanning, and with the present invention it has been found pos¬ sible to make some change in that.

On this background, according to the invention, it is proposed to prepare a wide, emulsion coated printing plate area by successively irradiating the area from a scanner head in parallel, mutually adjacent zones, each of a restricted width, the operation being controlled such that the zones are coherent as far as the printing pattern is concerned.

For obtaining a reasonably good result, i.e. with¬ out really visible distortions of the printing pattern in the border line areas between the various scanning zones, it is not sufficient just to effect an accurate mechanical guiding of a scanner unit relative to the printing element, since in connection with the scanning there are several problems in providing a side delimita¬ tion of the scanning zone with an accuracy high enough to satisfy the demand as to an exact continuity with the adjacent zone or zones. For a single zone edge displace¬ ments of the magnitude 1 mm will normally be insignifi¬ cant, but such an inaccuracy will be entirely unac¬ ceptable e.g. in a vertical separation line inside a printing pattern; it should be endavoured that possible displacements should not exceed 0.1 mm, as otherwise, even in coarse printing patterns, separation lines may be formed which can be sensed visually even when they are difficult to point out locally.

It is an integrated part of the invention, there¬ fore, that the applied scanner or scanners are con- trolled in such a manner that an extremely accurate coincidence of the side limits of the scanning zones is achieved, and a solution to this effect has already been suggested, confer WO 89/11116, which is concentrated on the provision partly of a scanning zone with a very small tolerance on the zone width and partly on an accurate parallel guidance of the scanner, e.g. by a guiding along parallel laser beams. In connection with the pre¬ sent invention it has been recognized that with an ac¬ curacy in the micrometer range it is relevant to control the function based on another criterion, which will actually reduce the tolerance requirements with respect to both the width of the scanning zones and the degree of rectilinearity of the guiding of the scanner, whether or not it is chosen to weaken these tolerances.

It has been found that certain photographic emul¬ sions have the property of changing colour when being exposed by irradiation, this being observable in a non-exposing light; this is particularly easy to notice with the use of a powerful ultraviolet laser beam for the exposure, as the emulsion surface may then be viewed in ordinary roomlight or daylight. The change of colour takes place practically momentarily or at least so fast that an exposed scanning zone will have shifted its colour on the exposed partial areas thereof when it becomes actual to add the next scanning zone.. With the use of a well focused laser beam the change of colour will be abrupt at the edge of the irradiation zone or line, and by means of a suitable sensor it will be pos¬ sible to detect the colour change with a high accuracy.

By means of such a sensor mounted on the scanner it will thus be possible to detect the location of the adjoining side limit of the previously produced, adja¬ cent scanning zone or rather the exposed partial areas thereof located at the side borders of each zone. If in the border area between the two zones only non-exposed sub areas occur, then it will not be possible to detect any side limit, but in that case it will be entirely unimportant whether the next zone is added spaced a few micrometers more or less from the foregoing zone, where¬ by it is fully sufficient to operate based on the guid¬ ing of the scanner by means of a superior, coarser con¬ trol system. In principle the same will be true where the adjoining zone edge areas are fully exposed, but here, due to the full exposure, it may happen that the zone edges are not brought fully together, whereby there may be a tendency of a formation of a more or less pro¬ nounced, non-exposed line of partition between the two zones in the midst of the exposed area. Here the said sensor can be operative in detecting that at the end of the scanning line there is not produced the colour change which should be produced by an exposure inwardly against an already exposed area, and as mentioned below the sensor may then effect a rapid correction of the location of the scanner or of the swing of the scanner beam determining the side limit of the new scanning zone. A tendency of a separation line being formed in exposed areas may thus be compensated for immediately or so fast and effectively that a real line will not ac¬ tually be created.

For that sake it would be acceptable should a de¬ tectable line be left, if it is only so thin, i.e. a few micrometers, that it will hardly permit a penetration of printing ink, or simply 'invisibly thin'. As the sensor can well detect such very small line widths it will be fully in order to adapt the control system to operate based on the attainment of an 'invisible' separation line between the exposed areas.

The more precarious separation areas will be those in which oblique transitions occur between exposed and non-exposed areas in the border area between two neigh- boring zones, because here even minor displacements between the zones will be visible in the final picture. This situation is shown in the drawing, in which:Fig. 1 is a plan view of two neighboring scanning zones,

Fig. 2 is a more detailed view of the transition area between these zones, and

Fig. 3 is a view illustrating an applied sensor.

In Fig. 1 is shown a scanner 2, which is moved downwardly in the Figure and is about to produce a scan¬ ning line x between indicated scanning zone border lines b and c. Previously the scanner (or optionally another scanner) has been moved downwardly along a first scan¬ ning zone between the lines a and b, having exposed a pattern area M which is to be continued obliquely down¬ wardly towards the left, across the line b. Above and outside the sides of the area M the laser beam of the scanner is switched off, whereby it is irrelevant whether the potential swing of the beam in the area b-c is sufficient to make the beam reach or perhaps overlap the left hand end of the neighboring scanning line in the area a-b.

The same, however, will be very important when the scanning line gets down to the crossing area between the line b and the left hand border of the area M:

As shown in Fig. 2 the left hand ends of the scan¬ ning lines in the zone a-b will not be located very accurately vertically below each other, even if the differences amount to few micrometers only, and when corresponding inaccuracies occur at the right hand ends of the scanning lines in the neighboring zone b-c the result may be a noticeable disturbance in the oblique transition between the exposed areas M in the to areas a-b and b-c. An additional and highly noticeable lateral jump will occur if the scanner 2 when moved along the zone b-c is not guided with extreme precision relative to the location of the left hand edge of the exposure zone a-b.

According to the invention, however, these diffi¬ culties are remedied by the scanner 2 being provided with a sensor 4, which in Fig. 1 is shown separated from the scanner, but in practice is mounted on the scanner such that this sensor may keep under surveillance the transition area at the line b in the level of the line x or rather somewhat below this line.

As shown in Fig. 3 the sensor 4 may operate in the manner that a pencil of rays L radiated against the working surface is reflected therefrom backwards against a row of very narrow light sensitive cells 6 through a light filter, which will attenuate the intensity of the light reflected from non-exposed surface portions, while attenuating to a smaller degree the light reflected from the exposed areas, this light having a changed colour. If in the surveilled area a transition occurs between a non-exposed and an exposed condition of the working surface, as shown at 0-^ in Fig. 2, it will thus be pos¬ sible for the sensor 4 to localize the transition very accurately, even with a degree of resolution of a magni¬ tude as low as 1 micrometer.

When the transition O-^ has thus been localized it will be possible, with the aid of the associated com¬ puter equipment, to ascertain geometrically where the next transition 0₂ by the subsequent scanning in the zone b-c should be located. It is a possibility that the transition 0^ is located "wrongly", namely if the exten¬ sion of the left end portion of the associated working line xl in the scanning zone a-b is somewhat larger or smaller than the desired value, by which the left hand end is located exactly in the line b; if, regardless of that, the scanning along the zone b-c was effected with reference solely to the right hand line b, then the result could well be erroneous, visually noticeable jumps between the successive transition areas. By the exact localization of the transition O-i there will be time to correct the adjacent working line x in the zone b-c in such a manner that this line - irrespectively of its own inaccuracy with respect to the location of the ends of the working line - will nevertheless be com¬ pressed or expanded such that its right hand end is adjusted into an at least well approximated correct location of the transition 0₂ relative to the transition 0_{l t} also when the latter is not fully correctly located. Thus, any uncontrolled jump in the total transition will be avoided, and the same will be achievable by the sub¬ sequent transition between the exposed area M and the non-exposed area further down along the line b. Here the sensor 4 will detect the 'inverted' transition between the respective areas just before the scanner is moved down to the level at which the exposure area M leaves the zone a-b.

The controlling of the scanner beam for achieving a balanced displacement of the transition area 0₂ can be effected either by an adjustment of the moment of the switching on or off of the light beam, as a delayed switching on in the situation shown in Fig. 2 (in which the ray swing is supposed to be directed towards the line b) will result in a displacement of the transition 0₂ towards the right, or by effecting via the control of the optical scanning system an increase or a decrease of the ray swing itself, this being possible e.g. by means of a piezoelectrically moved mirror as proposed for another purpose in the said WO 89/11116, published by 16.11.89.

The sensor 4 or another similar sensor will also be able to detect the ordinate positions of the left hand ends of the lines x in the zone a-b, and the scanner or the scanner beam may, as a function of such a detection, be controlled in such a desirable manner that at least the ends of the scanning lines in the zone b-c will be brought to be located just adjacent to the neighboring line ends in the zone a-b, this of course being impor¬ tant for the production of fully clean transitions be¬ tween the lines when the said corrections are effected. By way of example, with the use of a piezoelectrically moved mirror it will be possible, based on such a detec¬ tion, to provide for a minor, but sufficient displace¬ ment of the working line of the light beam in order to achieve the desired coincidence, and it is hereby less important whether the working line is displaced trans¬ versely or is just tilted to some degree. The degree of displacement or tilting relative to the normal run of the line may change during the movement of the scanner along the zone b-c, viz. if the moving velocity of the scanner happens to deviate from the correct velocity as determined by the successive addition of new scanning lines.

As mentioned, for the particular technical effect of the invention it is decisive that a sensor connected with the scanner be operable to detect the colour change effected by the exposure of the photographic emulsion, and it can be extra desirable, therefore, to develop emulsions, the colour changing abilities of which are particularly pronounced for the purpose of the present invention. It will be appreciated, therefore, that the invention will thus also comprise such specialized emul¬ sions.

Another possibility is that the surface of the emulsion layer can be coated by a special material layer, which is easily permeable for the scanner light, but yet is affected by that light so as to change colour or character in a detectable manner by the penetrating scanner light; in stead of a colour change, which may hereby even be extra marked, the change can be a change of state, by which the surface of the transparent layer changes between a rough and a smooth appearance, such that the material at the opposite sides of the transi¬ tion o will be reflecting and non-reflecting, respec¬ tively, or otherwise be respectively more and less re¬ flecting for the radiation or energy field to which the sensor 4 responds.

It is a still further possibility that the sensor 4 be placed at the opposite side of the irradiated plate element, exactly opposite to or relative to the scanner in such a manner that the sensor will be affected di¬ rectly by the radiation which acts exposing on the emul¬ sion while partly penetrating the same, whereby the sensor will then not at all be affected as long as the scanner beam is switched off.

In the foregoing reference has been made specifi¬ cally to serigraphic printing frames as an example, but it will be appreciated that the invention will be rele¬ vant also in other connections, as the principle of the invention, per se, is not depending of what type of workpiece is to be treated or in which manner the work¬ ing reveals itself. What is important for the invention is the very controlling of the working medium.

Claims

C L A I M :

A method of preparing material surfaces, e.g. gra¬ phical printing plates, which in accordance with a master pattern are subjected to a selective irradiation of different area portions, viz. by irradiation and non-irradiation, respectively, by e.g. laser light, which is brought to successively irradiate the surface by a scanning with a modulated beam, the surface being prepared with a surface layer which reacts in a selec¬ tive manner on the selective irradiation, characterized in that a material surface having a width of more than 20 cm and an arbitrary length is irradiated for the formation of a coherent irradiation picture by scanning along parallel zones with respective, exactly coinciding side limits, the work being controlled such that the irradiation picture is reproduced in a coherent manner across the side limit or limits, that the material surface is coated with a sub¬ stance which in response to local irradiation changes colour or character in a detectable manner without pre¬ venting the selective reaction in the said surface layer, that during the scanning of a new zone a successive detection of the location of the edge of the adjacent edge area of the neighboring zone is effected, in so far the latter area is of a detectable type, and that this detection is used for control purposes for producing a location of the edge of the next scanned line exactly coinciding with or overlapping the edge of the relevant area of the neighboring zone.