Posts Tagged ‘Print control’

A host of factors affect the quality of your printed products: in addition to the machine itself, there are the printing stock and inks, as well as external factors such as atmospheric humidity, and room temperature. Often, when problems arise in the pressroom, the root cause is not immediately apparent. Frequently, it takes time to filter out the underlying cause from among the many possible variables.

This series of blog with Professional Tips offers advice on tips in the most common problem cases. This handy guide should not be missing from any pressroom.

Picking

Figure 1: Picking

Picking occurs when small particles are torn out of the surface of the paper, or printing stock rips during the printing process. This can cause damage to the printing blanket. It becomes necessary to wash more frequently, and print quality is adversely affected. Picking becomes a problem when surfaces are not sufficiently resilient.

This indicates that the tractive force of the printing ink is greater than the fiber adhesion in the paper structure, or the bonding of the pigment used in the coating. Inadequate gluing, or the poor anchoring of the coating material on the paper’s surface

may cause picking.

Remedy

• Employ a soft series of inks

• Reduce the ink’s tractive force (add printing oil)

• Allow the machine to run on standard working condition

• Raise the rasp temperature (temper the inking system)

• Reduce the speed of the machine; this lessens the traction on the paper

• Use a quick release blanket

• Employ paper of better quality

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A host of factors affect the quality of your printed products: in addition to the machine itself, there are the printing stock and inks, as well as external factors such as atmospheric humidity, and room temperature. Often, when problems arise in the pressroom, the root cause is not immediately apparent. Frequently, it takes time to filter out the underlying cause from among the many possible variables.

This series of blog with Professional Tips offers advice on tips in the most common problem cases. This handy guide should not be missing from any pressroom.

Ghosting:

By ghosting, one understands a circumstance where printing form elements other than the desired positive or negative ones duplicate themselves onto the printed surface. These “stencils” or “ghost images” emerge from repeated passes of the ink form roller over the plate cylinder, and from a reduction or accumulation of ink. Used (hard) or poorly adjusted ink-rollers facilitate ghosting, and the tendency toward ghosting increases with weak pigment inks. An optimal balance between ink and dampening solution helps prevent ghosting, since too much water negatively influences printing outcomes.

remedy

• When maintaining rollers, only use the appropriate cleansing agents; a weekly application of wash paste removes lime deposits, and will regenerate the rollers.

• Adjust the rollers in accordance with the instruction manual; inspect the adjustment regularly.

• Change used rollers: the rubber surface of older rollers will become glossy and over-smooth. At the same time, as their hardness increases, the edges bulge out in a trumpet shape. More pronounced abrasion becomes evident.

• Adjust for maximum lateral distribution

• Adjust reciprocation of the ink form rollers

• Shift the engaging point of the lateral distribution

• Reduce the quantity of dampening solution being used (smearing limit)

• Employ high pigment inks

• Turn on the vario

• Turn the printing form (prepress)

Version 4 of the ICC profile specification was published in 2001. The v4 profile format has introduced a number of changes when compared to the previous v2 specification. These changes provide a number of advantages, the most significant of which follow from the removal of ambiguities from the specification and a more precise definition of the PCS. More information about the advantages of the V4 specification can be found here.

These lead to an improved predictability of performance of a profile in use which will lead to a reduction of major differences of interpretation. Therefore, when pairs of profiles are used they should always produce the same result – regardless of which CMM is used. A summary of reasons to use the v4 specificationis available.

ICC – International Color Consortium

As color management requirements evolve, the profile specification undergoes a process of continuous review by members of the ICC. The current version is ICC.1:2004-10 (Version 4.2.0.0) and is available on the ICC Specification page, together with Amendments approved by ICC members.

The specification has been approved as an ISO standard (ISO 15076) and the version on the ICC Specification page is compatible with the ISO publication.

Most platforms and color management applications have now migrated to ICC V4, or are in the process of doing so. You can test whether your system is V4-compatible here

In color management, an ICC profile is a set of data that characterizes a color input or output device, or a color space, according to standards promulgated by the International Color Consortium (ICC). Profiles describe the color attributes of a particular device or viewing requirement by defining a mapping between the device source or target color space and a profile connection space (PCS). This PCS is either CIELAB (L*a*b*) or CIEXYZ. Mappings may be specified using tables, to which interpolation is applied, or through a series of parameters for transformations.

Every device that captures or displays color can be profiled. Some manufacturers provide profiles for their products, and there are several products  that allow an end user to generate his or her own color profiles, typically through the use of a tristimulus colorimeter or preferably a spectrophotometer.

The ICC defines the format precisely but does not define algorithms or processing details. This means there is room for variation between different applications and systems that work with ICC profiles. As of 2009, the current version of the specification is 4.2,  but most devices support only version 2.

ICC – International Color Consortium

 Background

A large number of companies and individuals, from a variety of industries, participated in the development of the ICC specification which is designed to provide developers and other interested parties with a clear description of the profile format. A nominal understanding of color science is assumed, such as familiarity with the CIELAB color space, general knowledge of device characterizations, and familiarity of at least one operating system level color management system.

Device profiles provide color management systems with the information necessary to convert color data between native device color spaces and device independent color spaces. The specification divides color devices into three broad classifications: input devices, display devices and output devices. For each device class, a series of base algorithmic models are described which perform the transformation between color spaces. These models provide a range of color quality and performance results which provide different trade-offs in memory footprint, performance and image quality.

The device profiles obtain their openness by using a well-defined reference colour space and by being capable of being interpreted by any ICC operating system or application that is compliant with the specification. In combination with profiles for other devices colour transformations may be determined that enable colours captured on one device to be reproduced satisfactorily on many others. The information required in the profile is adequate to ensure the level of color fidelity selected by the user and for the design of a default color management module (CMM) to transform color information between native device color spaces. Such CMMs are found in many operating systems and applications

In addition to providing a cross-platform standard for the actual profile format, the specification also describes the convention for embedding these profiles within graphics documents and images. Embedded profiles allow users to transparently move color data between different computers, networks and even operating systems without having to worry if the necessary profiles are present on the destination systems. The intention of embedded profiles is to allow the interpretation of the associated color data.

The International Color Consortium Profile Format supports a variety of device-dependent and device-independent color spaces divided into three basic families: 1) CIEXYZ based, 2) RGB based, and 3) CMY based (including CMYK). A subset of the CIEXYZ based spaces are also defined as connection spaces.

More to follow…. Next we will discuss about Rendering intents and Profile connection space

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)

 

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.