Screens are Screaming for Attention

Innovative screening technologies are providing package printers with new tools to improve graphic quality.

CREATIVE PROBLEM SOLVING is a beautiful thing. When technical process limitations are overcome resulting in tangible quality and productivity improvements, it truly is rewarding for both the developers of the solutions and the people that use them.

In the case of packaging screening technologies, suppliers have been hard at work improving and developing innovative techniques and technologies that provide real improvements for package printers and their customers. These solutions have made positive impacts in both offset and flexographic printing applications. And for the most part, the solutions are applicable to both computer-to-film and computer-to-plate systems.

A brief overview

Four primary variables are manipulated in the screening technologies that are used to produce the dot patterns formed on offset and flexo printing plates. These are dot size, dot frequency (number of dots in a given area), dot formation, and most recently, dot shape.

Conventional AM (amplitude modulated) screening changes the dot size through the entire tonal range, while maintaining a fixed center spacing on the dots, denoting the lineature. A smaller overall dot size can provide a higher line per inch (lpi) screen, while lower lpi screens use larger dot sizes. The choice of which to use is determined by several printing and substrate factors. AM screening generally provides good reproduction in the mid-tones, but runs into problems in the highlights (below 10 percent) and shadows (above 90 percent), especially in flexo.

FM (frequency modulated) screening (also called random or stochastic) uses a small dot that remains constant in size. Color density is achieved by randomly varying the spacing of the dots to achieve a different number of dots in a given area. This method can improve highlights and shadows, and eliminates moires and rosettes that can result from the set line-pattern of AM screening. In the mid-tone areas, however, reproductions can appear grainy.