Preparing art files for flexographic printing.

Main task of preparing art files for flexographic printing is to compensate features of flexographic printing technology and to adapt images to the printing process.

Features of flexographic printing technology:

  • the use of soft polymer printing plates;  

  • printing misregistration;  

  • printing on transparent and metallized materials;  

  • the choice of ruling and the angle of rotation of the raster depends on the equipment.

Result of using polymer printing plates:

  • necessity to distort of art file;

  • dot gain in printing;

  • limitation for minimum size of printed elements;the minimum percentage of the screening- 1-2%.

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Advantages of flexo printing:  

  • high quality of the press at the low cost price of a print;  

  • high speed printing on high-performance flexo printing machines;

  • a wide range of printed materials.

 
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Dot gain - atural part of flexographic printing process, due to the pressure-sensitiveness of the printing form.

 
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You can correct the situation by reducing the size of the raster point - to lighten the image.

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Spreading out, all halftones become much darker.

By measuring the test print with discrete value  scales ​​of halftones, you can get dot gain curve.

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There is a possibility in Adobe® Photoshop®, to make the table of dot gain values and build a corrective curve and profile.

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Corrective curve opposites dot gain curve.

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The prepared image looks much brighter, but the result is likely to be as expected.

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A more accurate result can be obtained by icc-profiling, when testing and profiling creates an accurate color model of the printing  process.

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Minimally stable reproducible elements - each element on the printed form has a tapered base; for small elements (dots, thin lines, or small font), the basis cannot prevent deformation or crushing of the element.

 
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The size of the minimum stably reproducible elements is determined by the thickness of the photopolymer and the manufacturing technology of the printing form.

The reproduction of these elements is determined by the capabilities of the printing equipment.

Raster break - If the image or gradient has areas going to 0%, then at the transition from 1% to 0%, a ragged edge will be formed.

 
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The reason for the break is the impossibility of forming a stable raster dot on the photopolymer form.  Compression of a small raster point during printing leads to the formation of a visible breaking path.
The task of the designer is to control the areas with the filling of 1% and below.

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Trapping - method allowing to compensate for printing misregistration caused by the features and technical condition of the equipment.

 
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Trap values ​​are determined by tests.

White substrate.

Printing on transparent and metalized materials requires the use of white ink as the substrate.

To create a white substrate, use separate layer in the editor or use special plugins are to create the substrate.

 
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Choosing screen angles depends on the used anilox rollers:

 
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For anilox rollers with a cell angle of 60°, “offset angles” are used: 0°, 15°, 45°, 75°, 90°.

When using anilox rollers with an angle of 45°, the angles offset by 7.5°, getting “flexo angles”: 7.5°, 37.5°, 67.5°, 82.5°.

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According to paragraph 4.3.2.3  of  ISO 12647-6 for half-tone dots without a principal axis, the nominal difference between the screen angles for cyan, magenta, and black shall be 30°, with the screen angle of yellow separated at 15° from another color.

The  screen angle of the dominant color should be 45°.

Choosing screening ruling, -  depends on the used anilox rolls.

The recommended ratio between the screening ruling of the printing form and the ruling of anilox roll is 1 to 5.

Having anilox with 500 lpi, you should not increase the print line density above 100 lpi.

For printing areas with less than 5% coverage, the ratio should be changed by 1 to 7

 
 
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1. Screening ruling is chosen correctly, the paint layer is homogeneous.

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2. The same raster dot, but screening ruling of the anilox  is lower, dot rests on the edges of the cell and picks up more ink, which leads to more dot gain.

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3. The ratio is too small and the dot falls through the cells, picking up excess ink - this leads to dirt on printing.

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Screening moiré  -  is actually an artifact caused by either an inappropriate or incorrect halftone screen angle within a CMYK image.

 
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What is most likely to happen is that a screening moiré that is already present is somehow made more visible. For example, the image at right is a blow-up of a screen tint made of Yellow and Cyan. Because the Y and C screen angles are less than 30° apart they create a moiré.

Solutions:

  • change of the regular screening structure - stochastic screening;

  • changing the screening  angle as one of the moire reasons (for example, for yellow - 0°, an angle of black - 45° is assigned);

  • reducing ruling for yellow ink by 15%.