Maximizing Gradients for Flexography

Gradients and vignettes can be a particular challenge for Flexography. So, it is important to understand the print variables when creating or modifying a file with gradients to minimize the appearance of banding or hard lines. The best way to determine print capabilities is to run a fingerprint or characterization on the press that includes a complete tonal scale. The scale should include patches for 1-2-3-4-5-10-20-30-40-50-60-70-80-90-95-100. Since minimum dot capabilities are particularly difficult in Flexography, you need to make sure that the minimum dot you use in your gradient is not less than what can be consistently held throughout the Flexo process.

It is important to understand that the minimum dot capability is not entirely based on the printer's consistency and capabilities. Minimum dots are also limited by the consistency and capabilities of the film (for conventional processes) and plate processing as well as the screen ruling. For example, the plate-making department may find that for a 120 screen ruling the minimum film dot percentage they can consistently hold on the plate is a 1% dot. When evaluating the film consistency it may be determined that a 1% dot in the digital file may come out on the film as anywhere from a 0.7% to 1.3% film dot, 120 lpi. That is a tolerance of ±0.3%. The 2% dot may also vary ±0.3%. With this information, the best decision would be to hold the minimum dot in your gradients to a 2% for 120 lpi. In most cases, when printing a halftone screen, it is safest to make sure that no gradient ends before the edge of the area in which it is located. Meaning, do not take the gradient down to 0%, especially before the end of the panel in which the gradient is located or you will see a hard break.

Another quick tip for Photoshop: If you have a gradient blend that does not print smoothly, try adding about 2% noise using the noise filter. Some RIPs will RIP a blend from a raster application and add noise.

Here are some general rules:

• Raster blends are generally better than vector blends.
• The longer the blend, the more likely it is to show banding. So it is best to minimize the physical length if possible.
• The shorter the range, the more likely it is to show banding. For example, a 25% to 50% gradient (that is a range of 25%) is more likely to show banding than a 15% to 65% gradient (that is a range of 50%).
• The fewer steps used, the more likely it is to show banding.
• Understandably, banding is also more visible with darker colors. This is important to keep in mind as you are designing a job.
• Higher screen rulings (lpi) are more likely to show banding.
• Lower resolutions are more likely to show or cause banding. This is a tricky point as monitors and some printers are low-resolution devices. Keep this in mind, as you do not want to waste time trying to fix a banding problem on your monitor or printer that may not be an issue when you go to a high-resolution output device such as an imagesetter or platesetter.
• When blending one spot color into another it is best to provide specific instructions for how the blend is to be handled. Unless you are blending process colors, there is no easy way to correctly show how a spot-to-spot blend will appear.

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