GB2248801A

GB2248801A - Temperature dependent method of controlling the supply of damping fluid to the printing plate of an offset press.

Info

Publication number: GB2248801A
Application number: GB9119646A
Authority: GB
Inventors: Georg Hartung
Original assignee: MAN Roland Druckmaschinen AG
Current assignee: Manroland AG
Priority date: 1990-09-14
Filing date: 1991-09-13
Publication date: 1992-04-22
Anticipated expiration: 2011-09-13
Also published as: JPH0720694B2; GB2248801B; SE9102626L; GB9119646D0; FR2666765B1; FR2666765A1; JPH04234648A; SE9102626D0

Abstract

- The present invention relates to a method for controlling the feed of moistening agent to the printing plate (D) of an offset printing press, the feed being carried out in various quantities transversely to the direction of printing. - According to the invention, the variation in temperature which the printing plate (D) exhibits transversely to the direction of printing is detected, and the supply of moistening agent is carried out depending on the variation in temperature detected, so that a deficit in moistening agent, linked with the evaporation of the moistening agent transversely to the direction of printing, is compensated. <IMAGE>

Description

METHOD OF CONTROLLING THE SUPPLY OF DAMPER TO THE PRINTING PLATE OF AN OFFSET PRESS The invention relates to a method of controlling the supply of damper to the printing plate of an offset press.

To achieve optimum adjustment of damper supply in offset printing and to maintain such adjustment for the duration of printing - i.e., neither to overdamp nor underdamp in all its non-printing zones a printing plate clamped to a plate cylinder of an offset press - a method is known from German Patent Specification 3 636 507 Al wherein a measuring head provides quantitative detection of the quantity of damper present on the printing plate, is disposed for movement transversely to the printing direction and automatically makes search movements. The function thereof is to discover what is known as the critical point - i.e., that place transversely of the printing direction where there is the smallest quantity of damper on the printing plate. Damper feed is adjusted in accordance with the value measured at such place until a set-value is produced.As a rule, the critical point is near the largest printing proportion of the surface.

As described in detail in the specification just referred to - so that there is no need to go further into it here - the distribution of damper quantity transversely to the printing direction varies when speed is raised from the printing speed of the adjusting or setting-up operation (adjustment in and transversely to the printing direction of the quantity of damper necessary for optimal printing quality) to run-on speed. Guiding of the damper therefore needs to be readjusted for the run-on speed.

Alterations must also be made in damper supply during run-on to ensure that optimal printing quality is maintained constantly throughout the run. The reasons for the latter adjustment are, for example, ink layering in the damper unit and evaporation of damper due to heating of the press, plate cylinder and printing plate.

The latter effect might be obviated by a plate cylinder controlled to a constant temperature by water cooling and/or by similarly cooled damping unit rollers, but this would greatly complicate press construction.

Variations transversely to the printing direction of the quantity of damper on the printing plate can be detected by the search runs of the measurement head in accordance with the specification previously mentioned, but creeping changes in damper quantity as a result of slow heating are detected only if the search movements are performed continuously, in which event it is impossible to provide a permanent measurement at the critical point.

According to the present invention, there is provided a method of controlling the supply of damper to the printing plate of an offset press, the supply being effected in a differing quantity transversely to the printing direction, wherein the temperature pattern of the printing plate transversely to the printing direction is sensed and damper is so supplied in dependence upon the temperature pattern thus found as to compensate for any damper shortfall transversely of the printing direction due to evaporation of the damper.

Control of the feed of damper to the printing plate of an offset press in this way enables slow changes of the quantity of damper on the printing plate to be counteracted.

When carrying out the method of the invention, the temperature pattern on the printing plate is sensed transversely to the printing direction and the damper is guided in accordance with the pattern thus found, so that creeping changes in the quantity of damper on the plate are obviated.

The method according to the invention will be described in greater detail hereinafter with reference to the accompanying drawings wherein: Figure 1 shows temperature patterns on the printing plate; Figure 2 shows the control of damper supply according to the invention, and Figure 3 shows a development for controlling the supply of damper.

When the press is running, the printing plate clamped to the plate cylinder of an offset press heats up. The temperature pattern is not constant transversely to the printing direction, measurements having shown that the temperature increases from the centre of the plate cylinder outwards to the warmer side walls (frame walls) of the press. Also, the temperature on the drive side of the press may be higher than on the servicing side.

Figure 1 shows the pattern of the temperature T transversely to the printing direction - i.e., in the direction of plate cylinder axis AR - at three different times a), b) and c). The reference FD denotes the format of a printing plate D while the reference A denotes the input side, B the servicing side and M the centre of the plate D. It will be assumed that the press and, therefore, the plate cylinder and the printing plate D, are still "cold" at the time a) - i.e., they are at the ambient temperature (taking of the press into operation).

At a later time b) after, for example, the press has been adjusted for run-on and run-on has started, the temperature pattern alters. The printing plate D has warmed up as a whole and the temperature T rises from the centre M of the plate D towards the sides A and B. At a still later time c) (during run-on), yet another temperature pattern arises. The printing plate D has heated up still more and the rise of the temperature T towards the printing plate edges has increased since the press side walls have become hotter during the run-on time and evolve correspondingly more heat.

If there is no alteration in damper supply, the quantity of damper on the printing plate decreases because of increased evaporation at the elevated printing plate temperatures. The result in the edge zones of the printing plate may therefore be an uptake of ink in nonprinting image zones due to lack of damper if, as the tendency always is, it is required to print with minimal damper feed.

If, as indicated by chain lines in Figure 1, the printing plate temperature is measured at the centre M, on the drive side A and on the servicing side B throughout printing, these three temperatures represent the trend of the true temperature pattern. As temperature sensors TF (Figures 2 and 3), there can be used, for example, thermistors (cold or hot conductors) or other pickups which change their electrical properties in dependence upon temperature and which are in thermal contact with the printing plate D. The measurement signals of the temperature sensors are transmitted from the plate cylinder by means of a rotating transformer. Referring to Figures 2 and 3, the temperature sensors TF can be disposed on the frame and the printing plate temperature can be detected contactlessly (without contact being made with the printing plate D), in which event, for example, three infrared radiation thermometers would be required.

Figure 2 diagrammatically illustrates the method according to the invention of controlling the supply of damper. Temperature sensors TF sense the heating of the printing plate D on the drive side A, servicing side B and at the centre M. The temperature pattern of the printing plate D transversely to the printing direction corresponds to the pattern of the evaporation rate i.e., to the shortfall of damper. The necessary adjustments of the damper unit can be determined from a relationship, determined empirically or on a model basis, between damper consumption and temperature (damper consumption proportional to temperature). These adjustments can then be translated into actuating instructions to produce the required corrections of the damper unit, the same having corresponding actuating drives.The corrections include, for example: Altering the speed of the damper ductor; Altering the contact pressure of damper unit rollers in their axial separation parallel to one another (e.g. ductor and dispensing roller); Altering the axial directions of damper unit rollers relatively to one another (adjustment of roller inclination).

The effect of the latter correction of the damping unit is that either the printing plate D receives more damper on one side than on the other side or it receives less damper in the centre than in its edge zones (damper unit rollers skewed relatively to one another). The dispensing of the damper transversely to the printing direction in the sense that more damper is supplied to the edge zones can be achieved by corresponding adjustment of a spherical dispensing roller (a roller having a barrel-shaped cross-section in the plane of its rotational axis).

When, in accordance with Figure 1, the temperature pattern at the time b) changes to the temperature pattern at the time c), then, for example, the skew orientation of the rollers is increased automatically to compensate automatically for damper evaporation towards the edges of the printing plate D. Overall, more damper is added too, for example, by varying the speed of the damper ductor and/or the contact pressure of damper unit rollers since the heating increases to the overall damper consumption of the printing plate D. The method according to the invention precludes the occurrence of a shortfall of damper. Damper feed adjustments corresponding to the temperature measurement can be performed, for example, fully automatically by control electronics.Also, the measured temperatures (temperature pattern) and/or the correspondingly necessary adjustments for damper supply can be displayed and can be remotely controlled by an operator (display of recommended adjustments).

Figure 3 shows the combination of the damper control method according to the invention with the conventional prior art damper control method. The quantity of water on the printing plate D is measured by means of a water monitor WKG. The necessary adjustments for the damper unit are then derived in combination from the measured values of the water monitor WKG (damper control, control of damper supply in accordance with a set value) and of the damper control previously described on the basis of detection of the temperature pattern of the printing plate D.

A damper feed control of the kind according to the invention therefore represents a temperature-compensated damper feed. Instead of relatively elaborate means being used to obviate temperature changes on the printing plate or in the damper unit, the damper feed is adapted to temperature conditions. Consequently, despite variations in temperature and temperature pattern on the printing plate, the extent of damping remains the same everywhere and the printing plate is never underdamped.

Claims

1. A method of controlling the supply of damper to the printing plate of an offset press, the supply being effected in a differing quantity transversely to the printing direction, wherein the temperature pattern of the printing plate transversely to the printing direction is sensed and damper is so supplied in dependence upon the temperature pattern thus found as to compensate for any damper shortfall transversely of the printing direction due to evaporation of the damper.

2. A method according to Claim 1 wherein the printing plate temperature pattern transversely of the printing direction is determined by a temperature measurement in three zones of the printing plate, one zone being disposed at the centre thereof and the other two zones being disposed one at each printing plate edge.

3. A method according to Claim 1 or 2 wherein the extra supply of damper necessary in response to a temperature increase is provided in accordance with a relationship which is determined empirically or on the basis of a model and which reflects the relationship between damper consumption and printing plate temperature.

4. A method according to any one of the preceding Claims wherein the quantity of damper present at a particular place of the printing plate is also measured and is adjusted to a predetermined set value.

5. A method for controlling the supply of damper to a printing plate in an offset press substantially as hereinbefore described with reference to the accompanying drawings.