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Description

When a job is ripped with screening, it can often happen that very small or broken dots sometimes called 'scum dots' can appear in highlight areas. These dots can be smaller than the minimum dot size that can be held on a flexo plate, which can cause ‘dragging’ and other unwanted effects when the job is printed.

Simply removing all dots below a particular size is not a good solution, since this can result in support dots and micro-surface screening effects being removed as well.

To prevent problems caused by too-small dots, Esko has developed a feature for Imaging Engine called Dot Clean-Up. When enabled, Dot Clean-Up will look inside the output files generated by the Image to Screened Separations ticket and identify and remove those dots that are too small to be printed.

In the above screenshot, the unprocessed output with unwanted small dots is on the left with the output after Dot Clean-Up in the center. The right-hand pane highlights the differences between the two files in blue.

The In-Rip dot removal preserves special features created in Custom screens or HDFlexo screens, like micro surface screening and supporting dots.

Procedure

Dot Clean-Up can be found in the Post-RIP tab of the Image to Screened Separations ticket if the required license is available.

If the feature is enabled, the screened output from the rip will be passed through the Dot Clean-Up function and small dots will be removed according to the parameters set for the feature. Note that this can add some time to the process of ripping a screened job, so it should only be applied if needed.

The feature can be enabled by enabling the Clean up dots in highlights check box. This can also be enabled by means of a SmartName.

The feature uses two parameters namely Ruling and Minimum Dot Size.

Ruling

This should be set to the ruling in the areas of the job that are to be affected by Dot Clean-Up. When there are several screen rulings used in the job, this should be set to the ruling of those areas where the Dot Clean-up function is particularly required, or to the most widely-used ruling in the job.

The Ruling value allows the Dot Clean-Up function to determine whether a dot is ‘isolated’ (i.e. not close to another dot or object, in which case it can be removed, if it is smaller than the defined minimum dot size (see below)) or ‘non-isolated’ (i.e. close to another dot or object, in which case it can be retained). Whether a dot is considered to be isolated is determined by testing its proximity to other objects.

When the Dot Clean-Up function is launched, an 'isolation distance', marked as 'i' in the diagrams below, is calculated as the currently selected rip resolution divided by the ruling as defined in the Dot Clean-Up Ruling parameter.

In the below diagrams, 'i' is the isolation distance, and 'd' is the minimum dot size as defined in the Dot Clean-Up parameters.

Here, the dot is below the minimum dot size, and the distance from the dot to the closest object is greater than the isolation distance. The dot will not be supported by nearby objects, and so will be unstable.

In this case, the dot will be removed. 

Similarly, here the closest object to the target dot is another dot. It lies outside of the isolation distance, and will not help to support the target dot.

In this case, the dot will be removed.

Here, the dot is below the minimum dot size, and the distance from the dot to the closest object is less than the isolation distance. The dot will be supported by the nearby object, and so will be stable.

In this case, the dot will not be removed.

Similarly, in this case, the dot has another dot within the isolation distance. The two dots will support one another, and so will be stable.

In this case, the dot will not be removed.

Here, the dot is above the minimum dot size, and the distance from the dot to the closest object is less than the isolation distance.

In this case, the dot will not be removed.

Similarly, in this case, even though the nearest dot to the target dot falls outside the isolation distance, the target dot itself is above the minimum dot size, and so does not require additional support.

In this case, the dot will not be removed.

In this case, the target dot is not a full dot. Even though it would print well if it were a full dot, since it now falls below the size specified for the minimum dot size, and also falls outside of the isolation distance, it will be removed.

Minimum Dot Size

This should be set to the smallest allowable screen dot size. It can be defined in terms of pixels in the output file, the percentage value of the smallest allowable dot, or as a real size in microns. If a value is entered for one of the units, the values for the other units will be calculated automatically, based on the Ruling value set in the Dot Clean-Up parameters and the ripping resolution that is defined in the ticket. For example, for a screen ruling of 175lpi and a ripping resolution of 4000dpi, a minimum dot size of 3% will also be calculated to be 28.367 microns or 16 pixels.

Note that the size in microns refers to the diameter of the dot, while the size in pixels refers to the total number of pixels that comprise the dot, rather than the diameter of the dot.

The diagram below illustrates how the dot size and the number of pixels relate to one another.

Screen dots smaller than the defined Minimum Dot Size will be removed, while small dots that are used for support or as part of a screen’s surface effects will be retained.

For HDFlexo or Full HDFlexo screens which use Highlights or a minimum dot size, the selected Minimum Dot Size in the Dot Clean-Up tool must be the minimum dot size for that screen. For example, for an HDFlexo screen whose name contains 'C16', the Minimum Dot Size should be set to 16 pixels.

HD Flexo Screens

A problem with the current implementation means that it is possible that HDFlexo screens which use FM in highlights or otherwise use a minimum dot size can occasionally generate dots that are one pixel smaller than the stated minimum size, which will result in these dots being removed by the Dot Clean-Up tool.

To prevent this, users should regenerate any HDFlexo or Full HDFlexo screens that have a minimum dot size (in the latest version of HD Flexo Screens where the problem is fixed), if they intend to use them with the Dot Clean-Up tool.

Dot Clean-Up requires the purchase of an additional license.

Article information
Applies to

Imaging Engine

Created06-Apr-17
Last revised 
AuthorJACH
Case NumberN/A
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