Fig 1 : Register mark reader on the control console
of a sheet-fed press and measuring marks
At the same time as measuring and controlling the ink supply/the color, the color register is measured and controlled: that is, the color separations are put into register in the individual printing units.
Once again, systems with different levels of automation are available. As detailed in Automation in Print Media (we will discuss it later), Production, this may be anything from simple visual aids (optical magnifying glasses) to handheld instruments that check the register reliably, quickly, and accurately from register marks aided by opto-electric components.
Figure 1 and 2 shows a register measurement and control device, which is also shown installed in the control console in figure 4, in more detail in terms of its constituent components and how it works.
Fig 2 : Electronic/optical components of the
measuring instrument and a measuring mark “in the sight”
Fig 3 : “Video magnifying glass” for color register measurement and control.
a. Video magnifying glass with color monitor on the control console
of a sheet-fed offset press;
b. Video magnifying glass with lighting unit and display of the
register mark positions on the monitor (CCI, MAN Roland)
Fig 4 : Remote control console for multicolor sheet-fed offset press
with lighting unit for printed sheet inspection and handheld
measuring instrument for color register adjustment (CPC1/CPC41
Register mark reader, Heidelberg)
The cutaway view (fig. 2 left side) shows how the unit uses two high-resolution CCD line sensors (accurate to approximately 5 μm) to measure the distance between lines and register marks: one CCD line senors for the register lines for measuring the circumferential register, and one CCD sensor for the register lines in print direction to check the lateral register.
This measuring system is designed so that any variations in the register marks regarding reference values are passed to the computer in the control console via infrared light, from where the plate cylinders are adjusted.
Another register measurement system is shown in figure 3. The video magnifying glass can be used to display register marks in the image area on the monitor. By checking the register marks by computer and using appropriate image processing algorithms, any register adjustments required can be detected and quantified. The operator then adjusts the press accordingly.
Thanks to today’s register systems used in the platemaking process, plates can be mounted with such a high degree of register accuracy that only minor corrections in accordance with the image are still necessary. These can then be carried out quickly and easily by the operator. Setting the ink supply correctly is the greatest challenge and also where the greatest potential for reducing make-ready times lies. The register is generally set up at the same time as the ink supply; in fact, the correct color register is reached more quickly than the correct ink supply.