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Don’t Overthink It

Flexo platemaking is complex only if you let it become so.

March 2009 by John Anderson
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Flexo platemaking, in its most basic concept, is very simple. You start with a sheet of solid (or liquid) material, selectively alter it chemically using UV light, then remove the areas you don’t want, leaving the areas you do want as the image. More than 30 years ago flexo plates were made by hand, carved out by skilled plate makers, but they really did equate in many ways to sophisticated “potato printing” and gained flexo its low print quality reputation. But, today it is very different, increasingly scientific, and significantly better. So, why do people find it so complex? This article will explain the key points, and clarify common confusions.

Flexo plate terminology

To explain the process of platemaking, it is important to understand the plate and its terminology. When manufacturing a plate, the raised image area is a distance above the polymer base, which is called the floor. The height from the floor to the image area surface is the plate relief. The back of the plate is supported with a film layer to provide dimensional stability.

The thickness of the plate depends on the press and its final application. Plate thicknesses generally vary from 0.045˝ (1.14 mm) to 0.250˝ (6.35 mm), with the average thin plate in North America being 0.067˝ (1.70 mm), used for labels, folding cartons, flexible packaging, etc. Thick plates range from 0.125˝ to 0.250˝ and are used more for corrugated post-print and multiwall bags, etc.

Basic plate and platemaking details

There are two fundamental starting photopolymer formats—liquid or sheet—that are used to make most flexo plates. Photopolymer plates are made of monomers and UV initiators that, when exposed to UV light, experience a chemical reaction to make the monomers join together to become a solid mass of polymers. Whatever the plate format—liquid, analog, or digital—the same basic chemical reaction in the platemaking process exists.

Normally the first step is to expose the back of the plate to UV light, curing it evenly to build and raise the plate floor to its required level. This also activates the monomers and photoinitiators throughout the rest of the photopolymer mass. 

The top of the plate is then selectively exposed to UV light, through some form of mask, to build the image areas, while the non-image areas remain un-polymerized monomer. The finer the details, the longer this takes.

 

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