Dot Gain in Flexography

What is dot gain in flexography ?

All printing processes are subject to the unavoidable occurrence known as dot gain. As dots are transferred from film to plate, they tend to grow in size during light exposure in case an analogue plate process is used. When an ink dot is transferred from the plate to the substrate, it can increase in size once again as the ink spreads during absorption. A dot that began as 50% on film can grow to 51% on the plate, and eventually print on a flexographic press as a 65% or greater dot. The fluid ink and compressible plates used in flexography tend to increase dot gain, but it varies according to the type of press and the substrate.

Smooth non-absorbent films and coated papers will have less dot gain than absorbent and irregular surfaces, such as uncoated papers, newsprint and corrugated liner board.

Dot gain, however, is often consistent and predictable. Image or color separation software can adjust dots based on measurements supplied by the printer. Typically, the printer performs a fingerprinting analysis, which provides dot gain information to the color separator or desktop designer. By printing a target Chart under controlled conditions, dots can be adjusted in the color separation films. In addition, calibration packages built into raster image processors (RIPs), such as Agfa Calibrator, can also make adjustments.

Impression Pressure:

Even though ink is transferred under relatively light pressure in the printing nip, the soft flexographic plate deforms slightly and compresses during image transfer. This causes ink to spread, increasing dot gain.

Plate Durometer:

Because harder plates do not compress as much as softer plates, they produce less dot gain. Softer plates, however, transfer solid images more completely. Dot gain can be minimized by using a thin (0.002″-0.005″)  capped surface layer with a higher durometer than the supporting plate material. Dot gain can also be reduced by mounting the plate with compressible tape or a blanket that absorbs pressure.

Inks:

A higher viscosity ink will not spread as quickly as one with lower viscosity. The spreading, or flow-out, of a low viscosity flexographic ink occurs as it is transferred to the substrate and before it dries, contributing to dot gain. By comparison, lithographic ink is a thicker, paste consistency, and is not prone to excessive flow-out.

Substrates :

The printing surface or finish of a substrate also influences dot gain. When ink is applied to smooth non-absorbent films and coated papers it tends to spread very little, preserving the dot shape. With more absorbent and irregular printing surfaces, such as uncoated paper, newsprint, and corrugated liner board, the paper fibers act as a wick, absorbing the fluid ink and causing it to spread beyond the dot shape and pattern.

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