WO2020245417A1

WO2020245417A1 - Double sided register controlled printing unit

Info

Publication number: WO2020245417A1
Application number: PCT/EP2020/065713
Authority: WO
Inventors: Renzo Melotti; Davide ROSSELLO
Original assignee: Bobst Italia S.P.A.
Priority date: 2019-06-06
Filing date: 2020-06-05
Publication date: 2020-12-10
Also published as: CN114025962B; EP3980270A1; CN114025962A

Abstract

This invention relates to a web (2) printing unit (5) and a register method to align the different colour planes in a printing press. Despite the use of inexpensive equipment, the invention is able to compensate for diagonal misalignments and lateral shrinking of the web (2). It uses a photocell (40) positioned on each side of the web (2) and can be fully automated. It also performs a fast correction, thereby reducing the waste caused by misalignments.

Description

DOUBLE SIDED REGISTER CONTROLLED PRINTING UNIT

FIELD OF THE INVENTION

The invention is in the field of register control for printing machines. In particular, the invention is about a method to register the print medium in a printing unit and about the related printing unit.

TECHNICAL BACKGROUND

An industrial printing press is composed of several printing units where each unit is configured to print a single colour, called a colour plane. The superposition of colours printed by each unit results in the final colour print. The different colour planes must be adequately aligned to produce a print of good quality. The distance between printing units is in the order of several meters of web, while the accuracy requirements between the colour planes are in the order of the 10^th of a millimetre. Thus, a mere change of temperature or a small change in the ink amount dropped on the web might cause a shift larger than 10^th of a millimetre. Thus, a printing press requires a real-time adjustment mechanism, which is called the register control. A modern press uses seven colour units and some additional ad-hoc ones to handle most of the printing jobs without having to use custom inks (called Pantones). Thus, compared to an older press using Pantone inks, a modern press gains in job changeover time and ink waste. The downside of this evolution is a tighter requirement in the alignment specifications between the colour planes, which is now below 50 microns (at two sigmas, assuming a Gaussian error distribution). On older machines, the register is controlled by reading ad-hoc marks on one side of the print using a photocell, thereby compensating for the register along the longitudinal and lateral direction. In practice, the web register might differ depending on the side of the web that we consider. This phenomenon may be caused by a web that gets shrinked more on one of its side or by a misalignment of the engraving on the printing cylinder. Also, the web might undergo a shrink along the lateral direction whose amplitude depends on a multitude of parameters like the temperature, the amount of ink on the web or whether the ink is wet or dry. The lateral shrink may be particularly severe for recyclable or biodegradable media. These last phenomena cannot be measured using a single-sided measurement system.

SUMMARY OF THE INVENTION

The invention is about a printing subsystem comprising a printing unit and a heater and about a method that measures the register on both sides of the web for each printing unit. It computes or corrects the lateral register and the lateral shrink of the web. The lateral shrink being a variation in the width of the web.

The method is adapted for a web, which is printed with register marks on both of its sides. The marks may be printed by a printing unit or cut by a cutting unit. A photocell measures the mark on each side of the web. The web travels under the photocells along a longitudinal direction. The mark shape is chosen so that the length of the mark measured by the photocell depends on the lateral position of the mark. We use, for example, triangular-shaped marks.

The lateral shrink may be controlled by setting the temperature of the web at the input of the printing cylinder. The temperature may be controlled upstream from the first printing unit by a preconditioning hood that preheats the web. By preheating the web, we avoid too much shrinking in the printing units due to a temperature change. For the printing units following the first unit, the temperature may be controlled by the dryer associated to the preceding unit in the machine (or alternatively by preconditioning hoods placed in-between printing units).

The tension of the web at the input of the printing unit may also be used to control the lateral shrink. For example, the tension can be set with a tensioning roller. Some materials tend to shrink laterally when tension is applied longitudinally. Thus, the tensioning roller may influence the lateral shrink and the longitudinal stretch of the web. The amount of stretching and shrinking is material dependent. The longitudinal stretch being a variation in the length of the web.

Advantageously, the lateral shrink may be controlled by setting both the tension of the web and the temperature.

The lateral register may be compensated by controlling the lateral position of the printing/cutting cylinder.

Preferably, two physical quantities may be measured on each mark: the longitudinal position and the length of the mark. By comparing the longitudinal positions stemming from the marks on each side of the web, we can compute the longitudinal register for each side of the web or, equivalently, the longitudinal register and the diagonal register. By combining all the measurements stemming from the marks on each side of the web, we can compute the longitudinal and diagonal register in addition to the lateral register and the lateral shrink.

The diagonal register can be compensated by changing the orientation of a skewing roller, a roller which is located upstream from the printing roller, (and downstream from the previous printing unit). The orientation is modified by controlling the position of at least one side of the roller, for example, by attaching one side of the skewing roller to a (linear) guide. The guide may be controlled by a motor, or manually with a knob. Thus by changing the position differently on the right side and on the left side of the skewing roller, one may influence its orientation. A printing unit according to the invention comprises two photocells, each mounted on a manual or motorised linear guide. The linear guides are able to displace the photocell laterally for handling webs of different width in a printing machine. The photocells are placed on the path of the register marks printed on the medium by a printing cylinder, downstream from the printing cylinder. The signal recorded by the photocell is analysed by a processor, which computes for each photocell a length of the register marks crossing the field of view of the photocell. Then, the processor computes a lateral register and a lateral shrink parameter. The printing unit comprises a heater that may be used to control the lateral shrink by heating the incoming web.

A printing machine is designed to keep the web tension approximately constant throughout all the printing units. It often comprises an infeed tension control group upstream from the first printing unit (or within the first printing unit but upstream from its printing roller) to set the web tension. The infeed tension control group may also be used to set the web shrinking in addition to or in combination with the heaters in the printing machine.

Advantageously, the printing cylinder in the printing unit may comprise a lateral adjustment mechanism to adjust the lateral register The printing unit may comprise a skewing roller positioned upstream from the printing cylinder, which is used to influence the diagonal register.

Advantageously, the processor computes for each photocell a longitudinal position. The processor may then compute a longitudinal and a diagonal register of the medium in addition to the lateral register and the lateral shrink. The printing unit may comprise a skewing roller positioned upstream from the printing cylinder, which is used to influence the diagonal register.

Please note that the current invention can be equivalently applied to a rotary cutting unit instead of a printing unit, where a rotary cutting cylinder replaces the printing cylinder and cuts out the register marks. Also, when mentioning a register system, we refer to the subsystem of the printing unit made of the two photocells, their linear guides and the related computing capabilities to compute the register values. A BRIEF DESCRIPTION OF THE FIGURES

Embodiments of the present invention are illustrated by way of example in the accompanying drawings in which reference numbers indicate the same or similar elements and in which; Figure 1 shows a web with lateral marks printed on each side of the web;

Figure 2 shows the detail of a printed mark and the effect of the lateral register on the measurement of the photocell;

Figure 3 shows a principled setup of a printing unit according to the invention;

Figure 4 is a perspective view of the unit in Figure 3;

Figure 5 shows several examples of marks that are suitable for the measurement system according to the invention;

Figure 6 illustrates an example of a printing machine with a preconditioning hood at the entrance.

A DETAILED DESCRIPTION OF THE INVENTION AND OF SOME OF ITS EMBODIMENTS

Figure 1 shows an example of a web 2 printed with marks adapted for the printing units according to the invention. The web 2 is running according to a longitudinal direction 3. Marks are printed on the left side (10,11) and on the right side (20,21) of the web. In this example, mark 10 and 20 are printed by the former printing unit in the press while mark 11 and 21 are printed by the current printing units. Thus, distance 15 along the longitudinal direction between mark 10 and mark 11 can be used to measure the longitudinal register of the current printing unit by comparison with the specifications. The difference in lateral register 16 between said two marks (10,11) can be measured by measuring a lateral register value for each mark and comparing them. The lateral register is measured using the size 31 of the mark along the longitudinal measurement line 19. The lateral register can be computed from the size by using the knowledge of the shape of the mark.

The marks that are adapted for measuring the lateral register and/or the lateral shrink parameter exhibit a monotonic variation of the size along the (longitudinal) measurement line as a function of the lateral position (when the line is within the measurement boundaries). Figure 2 shows an example of a traditional mark 10 in two different lateral positions 30 and 34. The photocell of the register system measures the mark along line 19. In position 30, the photocell measures a size 31, corresponding to a lateral register 32, while in position 34, the photocell measures a size 35 corresponding to a lateral register 36. It suffices that the shape of the mark is known and that the relation between the size (along line 19) and the lateral position is monotonic to compute the lateral registers 32, 36 from the size values 31, 35. Preferably, one of the sides of the mark has an edge perpendicular to the measurement line, to make its longitudinal position independent from the lateral position for measuring the longitudinal register directly (this feature is useful but not mandatory). Figure 5 shows some examples of possible mark shapes. Any shape of figure 5 can be mirrored along the longitudinal axis to constitute another compatible shape.

Figure 3 shows a principled example of a printing unit 5 according to the invention. A web 2 is printed (or cut) using cylinder 41 and a counter cylinder. The path length between the printing unit 5 and the printing unit located upstream is controlled by skewing roller 42, which can move along direction 43. The axle of the skewing roller is attached to a linear guide. The guide can also be non-linear. Fully automated embodiments use a motorised guide. By moving one side of the skewing roller along direction 43, while keeping the other side fixed modifies the diagonal register, as shown in Figure 4. The longitudinal register may be controlled with the phase of the printing cylinder.

The lateral register may be controlled by displacing the printing (or cutting) cylinder 41 along its rotation axis.

The lateral shrink is computed by comparing the lateral register of the mark printed on the left and on the right or the web and stemming from the same printing unit and comparing it to the specification. The shrink parameter is influenced by modifying the temperature or the tension of the web (or both) coming into the printing unit. The temperature can be influenced either with a preconditioning hood 51 upstream from the printing unit or by changing the power delivered by the dryer 50 associated with the former printing unit (or by using both). The tension can be influenced using an infeed tension control group. With some materials, especially the biodegradable, the thermosensitive or the extensible ones, adding tension reduces the width of the web. A dryer 50 is used to dry the ink in-between printing units and uses warm air, infrared light or UV light, or a combination thereof, depending on the type of ink and support used for the printing job.

Figure 6 shows a typical arrangement used in a printing machine. It shows an unwinder 53, followed by a preconditioning hood 51 that heads the web to a predetermined temperature, followed by a number of printing units (here only two of them are shown). The web runs according to direction 52 throughout the printing machine, from upstream to downstream.

This invention is adapted for printing units printing on the web, as well as for converting unit configured to cut shapes out of the web. A printed mark may be a cut mark with the same shape. Indeed, a photocell is able to detect a printed mark as well as a mark cut out of the web. Thus, the invention may be used in printing machines where each printing unit has a register system (except sometimes the first one) and where the last unit may be cutting unit with the same register system. In some situations, the first printing unit does not need a register system since it may print on a blank sheet because the register is a relative measurement between a reference (printed by the first unit or any preceding unit) and the printing unit under control. The first printing units needs a register system if the printing machine requires the capability to print on a pre-cut or a pre-printed medium.

The longitudinal direction is defined by the travel direction of the print medium, locally. The lateral direction is defined as the perpendicular to the longitudinal direction within the plane of the print medium. The longitudinal register is measured along the longitudinal direction. The lateral register and the lateral shrink are measured along the lateral direction.

Web or print medium or medium is used interchangeably in this document and refers to the same concept.

In this document, when using expressions like former or preceding or next or last about the printing unit, we refer to the order defined by the path of the print medium. Thus, the medium reaches first the former (or preceding) printing unit, then the current printing unit, then the next printing unit. The medium travels from upstream to downstream.

Claims

1. Method for measuring the register of a print medium (2) in a printing unit where the medium travels according to a longitudinal direction (3) from upstream to downstream, the method comprising

- Measuring a first length along the longitudinal direction of a first register mark (10,11) on a first side of the medium using a first photocell;

- Measuring a second length along the longitudinal direction of a second register mark (20,21) on a second side of the medium using a second photocell;

- Computing a lateral register and a lateral shrink parameter of the medium from the first length and from the second length.

2. The method according to claim 1, for registering a print medium, further comprising the adjustments of the lateral shrink parameter by a control of the temperature of the print medium (2) entering the printing unit;

- said control of the temperature is performed using a preconditioning hood (51) or a dryer (50), positioned upstream from the printing unit.

3. The method according to claim 1, for registering a print medium, further comprising the adjustments of the lateral shrink parameter by a control of the print medium tension entering the printing unit.

4. The method according to claim 2 and claim 3.

5. The method according to any claim 1-4, further comprising

- Setting a lateral position of the printing cylinder in the printing unit to adjust the lateral register.

6. The method according to any claim 1-5, further comprising

- Measuring a first longitudinal position of the first register mark (10,11) on the first side of the medium using the first photocell;

- Measuring a second longitudinal position of the second register mark (20,21) on the second side of the medium using the second photocell;

- Computing a longitudinal register and a diagonal register of the print medium from the first position and the second position;

- Adjusting an orientation of a skewing roller (42) for setting the diagonal register to a predefined value.

7. The method according to claim 6, wherein the longitudinal register is computed from the average value between the first longitudinal position and the second longitudinal position.

8. The method according to any claim 6-7, further comprising an adjustment of a phase of the printing cylinder to set the longitudinal register to a predefined value.

9. A printing subsystem, for printing on a print medium that travels from upstream to downstream, comprising

- A heater (50,51) for heating the print medium;

- A printing unit positioned downstream from the heater (50,51), the printing unit comprising

- A printing cylinder (41);

- A skewing roller (42) positioned upstream from the printing cylinder (41);

- Two photocells (40) positioned downstream from the printing cylinder, each mounted on a linear guide oriented along the printing cylinder rotation axis;

- A processor;

Wherein

- The photocells (40) are arranged on each side of the print medium (2) for recording register marks (10,11,20,21) carried by the medium;

- The processor is configured to record a signal from each photocell (40) and to compute a lateral shrink parameter;

- The lateral shrink parameter is controlled by adjusting the temperature of the printing medium with the heater

10. The printing subsystem according to claim 9, wherein

- The processor is configured to compute a diagonal register of the medium;

- The axle of the skewing roller is attached to a guide at one of its sides for controlling the diagonal register of the medium.

11. A printing machine, comprising

- a first printing subsystem according to claim 9 or claim 10; and

- several following printing subsystems according to claim 9 or claim 10, positioned downstream from the first printing subsystem;

- a dryer (50) for each printing subsystem, located downstream of the printing unit of said printing subsystem;

- wherein the heater of the first printing subsystem is a preconditioning hood (51); and - wherein the dryer (50) is the heater (50,51) of the printing subsystem located next on the print medium path.

12. The printing machine according to claim 11 further comprising an infeed tension control group located upstream from the printing unit of the first printing subsystem.