Posts Tagged ‘UGRA’

UV drying of printing inks (and varnishes) is based on radical polymerizable vehicles. UV inks with appropriate UV dryers are suitable for sheet-fed printing presses and web presses. Drying between the print units – interunit drying  can be used to prevent a reversal of the ink splitting in the following inking unit. In flexographic and gravure printing, drying has to be carried out after each inking unit (e.g., straight (recto) printing and perfecting) because of the ink properties (ink trapping, etc.). Very often an overall drying becomes necessary after the last inking unit, possibly at a higher output rate.

UV inter-unit dryers for sheet offset printing press after printing unit and coating unit; UV dryer (blue) can be replaced by IR dryer (red),(IST Strahlungstechnik metz)

In case of UV drying, the ink film polymerizes and dries completely as soon as radiation occurs. Polymerization takes fractions of a second. The UV drying method, however, requires special inks  containing completely different binders (vehicles) and additional photoinitiators.  The color black prevents UV radiation from penetrating in the ink layer and the curing effect is less than with chromatic colors or varnishes. 

Conventional UV dryers work with one or several mercury vapor lamps.  The wave length range lies between 100 and 380 nm. The system is enclosed by a reflector housing. Optimum cooling and extraction of generated ozone is necessary for the complete system.The units are designed in such a way that the permissible threshold limit value of 0.ppm (parts per million = one millionth of the volume of the substance in question, e.g., air) is not exceeded and damage (e.g., irritation to the mucous membrane) to one’s health is prevented.

UV drying system.
a) Ranges of the UV spectrum and their effect;
b) UV radiator reflector system (Dr. Hönle)

 

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Color reflection densitometers such as Ihara R730 are equipped with  color filters to deliver density values for the specific process colors Cyan, Magenta, Yellow and Black. For each of the colors Cyan, Magenta and Yellow a specific filter is used which reflects the spectral characteristics of the respective ink.

Fig. 1 shows the remission curves of the solid tones of Cyan, Magenta, Yellow and for Paper White. Remission is the term for the degree of reflectance at a certain wavelength. The remission curve describes the reflectance among the entire visible spectrum and is a unique description for the color.

Fig. 1: The remission curves of the process colors Cyan,Magenta and Yellow and for Paper White.

The solid (optical) density is linear to the amount of ink applied and therefore a valid method for controlling the quality of printwork. Depending on the solid density, the shape of the remission curve changes. As seen in an example in Fig. 2 for three different densities in Cyan the changes are the largest at remission values beyond 590 nm (red).

This is why densities for Cyan are measured with a red filter with a spectral transmittance peak at 600 nm. The Cyan sensor (which consists of a photoelement with a red-sensitive filter in front) just detects the red portion of light and will be susceptible to small changes in Cyan density.

Fig. 2: Example for the remission curves for different densities for Cyan. The density measurement is carried out with a red filter, where changes in remission are susceptible.

Respectively, Yellow is measured with a blue filter at 430 nm and the process color Magenta is measured with a green filter at 530 nm wavelength.  Each process color is detected with its complementary filter:

• Cyan with a red filter

• Magena with a green filter

• Yellow with a blue filter.

The black color could be measured with any (or with no) filter, because its remission is even along the entire spectrum. To achieve a standardized density value, a filter called V is used which reflects the visual gray sensitivity.

 About Ihara R730:

 The Ihara R730 Colour Reflection Densitometer is a sophisticated QC tool. It has all of the measuring functions possible with a densitometer for full analysis of process work.

For a sophisticated densitometer with every possible function for measuring process work, then you need the Ihara R730 – you won’t find better value or a more accurate instrument! Simple operation means it is still fine for basic press control, but the extra features will help you detect and sort out just about anything that can possibly go wrong!

Not only does the R730 allows you to measure tints, and enable you to achieve a consistent ink densities on press, but the wide range of measuring functions available is ideal for QC departments, as well as sorting out printing problems.

  • Top Performance & Reliability  Made in Japan by Ihara Electronic Industries, the R730 has proven reliability and accuracy.
  • Menu Driven Commands  Simply select the required functions from the on-screen menu – you won’t ever need to read a manual. Clear and precise instructions for every measurement are displayed on the large LCD screen.
  • Help Key  Displays explanations for all measuring functions, along with answers to FAQs.
  • Quick Calibration  Recalibrate the unit in seconds.
  • Auto Function  Automatically measures density or dot gain.
  • Printer & Computer Interface  Configurable RS 232 serial interface provided for connecting to a PC or printer.
  • Fully Portable  Rechargeable batteries provide up to 4,000 readings before recharging.
  • Plate Reading  Optimized function for checking control strip tints on plates – available at small extra cost.
  • Filter/Aperture Options  Standard models supplied with Status T filters, 3 mm aperture and polarized. Manufacturer’s options available for Status E filters, 1.7 mm aperture and no polarization.

    Ihara Densitometer R730 – The best in class

Example of Dithering

Dither is an intentionally applied form of noise used to randomize quantization error, preventing large-scale patterns such as color banding in images. Dithering has mainly been used for laser and inkjet printers. The individual laser dots are distributed as finely as possible in an orderly pattern, as you can see in the following example. Today, error diffusion is usually used

You will notice that these images become considerably darker when they are copied and are not really suited for further processing. The laser dots are not distributed well enough for this purpose, with a border line that is much too long appearing between the black and white elements.
 
Errors occur mainly at the borders of screen dots when film is copied to the printing plate and as a result of dot gain in print.

For that reason, screen dots should be placed as compactly as possible to minimize the size of the border line as much as possible.

As an alternative to dot gain the relative print contrast Krel.(%) is often determined, particularly to check the screen in the three-quarter tone.

A print should have a contrast as high as possible. This means that the solids should have a high ink density, but the screen should still print open (optimum halftone value difference). When the inking is increased and the ink density of the dots rises, the contrast is increased.

However, the increase in ink feed is only practicable up to a certain
limit. Above that limit the dots tend to exhibit gain and, especially in three-quarter tone, to fill in. This reduces the portion of paper white, and the contrast decreases again.

If there is no measuring device available with a direct contrast display, the relative print contrast can be calculated or determined on the basis of the FOGRA PMS.

If the contrast value deteriorates during a production run in spite of constant ink value in solid DV, this may be a sign that the blankets need washing.

If the solid density is correct, the contrast value can be used to assess various factors which influence the print result such as rolling and printing pressure, blankets and underlays, dampening, printing inks and additives.

Since the contrast value, unlike the dot gain, depends to a large extent on the solid density it is not suitable as a variable for standardisation. This is why in the recent past its importance has decreased significantly.

ISO 12647-2 LAB values for different paper types

Like all complex systems, digital plate setters, also known as computer to plate systems, require suitable control aids in order to monitor output quality during daily production and to be able to guarantee produc tion standards. The complexity of computer to plate systems is apparent in the large number of components involved in the process.

The combina tion of digital data from various different application programs, with various different RIP and output device parameters, different types of plate and development conditions for plates as well as printing requirements for the transfer of tonal values place high demands on the control of the work as it progresses.

After extensive test exposures and practical investigations a plate control strip that is suitable for use in produc tion was developed. It is a further development of the Ugra/FOGRA-PostScript Control Strip. This is the standard tool for PostScript capable output devices and continues to be important as a control aid for fi lm exposure and for digital work flow.

The Ugra/FOGRA Digital Plate Control

Wedge is arranged as a total of 6

functional groups or control panels:

• Information panel

• Resolution panel

• Geometrical diagnostics panels

• Chequered panels

• Visual reference steps (VRS)

• Progress wedge

The 11 visual reference steps are a new feature. In this group a chequered panel is surrounded by a reference panel, and the dot percentage increases in 5 % steps from 35 % to 85 %. Under theoretically ideal conditions and if the transfer characteristics are linear then when the two panels have a dot percentage of 50 % they should blend with each other, i.e. the impression of brightness and the measurable tonal value should give a dot percentage of 50 % in both areas.

Due to the plate type, exposure calibration, developer and transfer characteristics this is hardly ever actually achieved under operational conditions, and shifts upwards or downwards occur. The VRS panels that are important for day to day production are those that enable the optimum setting to be chosen and output results to be achieved. Deviations from the appearance of the VRS panels that is ideal for the production process can be identifi ed by a visual check. Other panels provide information about resolution as well as the progress wedge, with which the tonal value transfer can be checked.

In order to eliminate production dependent differences in plate materials, zero points are located between the rows of halftone panels. Consequently the position for the densitometric measurement of zero (substrate only) and the dot percentage lie beside each other.

The resolution panel contains two semicircular panels. In the first panel positive lines radiate out from a point and in the second the lines are negative.

The thickness of the rays corresponds to the theoretical resolution of the output device. The geometrical diagnostics panels also contain lines that are oriented to the particular resolution setting of the output device. The chequered panels are below the geometrical diagnostics panel.

Labels are positioned over each of the panels and the sides of the squares are one, two and four units long.

Ihara LIthocam Plate reader – world class equipment