Archive for the ‘issues in press room’ Category

Converting raster images from an RGB colorspace into a print CMYK colorspace has two significant impacts:

1) Typically a compression and alteration of colors as the image is transformed from the original RGB gamut to the different gamut used for CMYK presswork.

2) The on-press printability of the imagery in terms of color stability, press performance/runnability, and ink usage (i.e. cost).

Converting images from one CMYK separation condition into a different CMYK separation condition by reseparating files is primarily intended to enhance the printability of the imagery while maintaining the appearance of the original CMYK
imagery. Put another way, reseparating CMYK files is effectively a way to optimize press forms.

Under Color Removal & Grey component Replacement (UCR & GCR)

The principle of RGB to CMYK separation:
In order to go to press, RGB color images must be converted to their process color counterparts; cyan, magenta, and yellow. An achromatic black channel is added because if the color black in presswork is just made from CMY it can often appear “muddy” or “patchy.” Also, making dark colors from the three chromatic process colors can lead to a higher than desirable volume of ink on the press sheet.

Neutral colors made up of three process colors are also more difficult to maintain consistent on press as solid ink densities normally vary through the run compared with a neutral made primarily of a single black ink. The net effect of introducing black ink in process printing is a reduction of ink usage/costs, stabilization of color (especially gray
tones), and and better printability.
The conversion process is done by taking the 3 channel RGB image, passing it through a 3 channel device independent CIEL*a*b* profile connection color space where the RGB is converted to CMY and the black channel added, and finally
outputting the result as a 4 channel CMYK image.

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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.

Horizontal Stripes

The problem in this case involves disturbances in the printed image, which are manifested as bright or dark stripes running horizontally to the direction of printing.

Horizontal stripes arise for a variety of reasons. Besides adjustment errors, errors in screen vignettes (prepress errors) may also play a role. Additionally, the formation of stripes may also be influenced by contamination from the build up of powder and coating, from damage to the printing blanket or the printing plate, or from the ink/dampening solution feeds, as well as from the quality of the ink. The machines themselves may also be causing the stripes, for example from an incorrect alignment of the ink and dampening units, or from rolling errors (too much pressure). Tone stripes can be caused by the relative lateral movement of the rollers on the printing plate.

remedy for stripe formation conditioned by machines

• When maintaining the rollers, only use appropriate cleansing agents; the weekly application of a 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 becomes glossy and over-smooth. At the same time, as their hardness increases, the edges bulge out in the shape of a trumpet. More pronounced abrasion becomes evident.

• Adjust reciprocation of the ink form rollers

• The dampening distributing cylinder must be kept clean and receptive to water.

• The roll bearings must be in fine working condition (no play, no sticking).

• Lubricate the roll bearings

• By setting the ink form rollers as gently as possible on the printing plate, impacts stemming from the run-on and run-off edges are reduced.

• The bearer ring should be kept clean and grease-free.

• The calibrated under packing should be clean, cut in a format to fit, and correctly inserted.

• Clamping between the plate and the blanket should range between 0.1 mm and a max. 0.13 mm. (0.004 in and 0.051 in).

• Clamping between the blanket and the counter pressure should be set in consideration of the surface of the printing stock; avoid clamping that is set too high.

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

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)