Ask PNEAC Archive: Re: Heatset Litho vs Gravure

Re: Heatset Litho vs Gravure
Mon, 23 Aug 1999 18:14:54 EDT

Date: Mon, 23 Aug 1999 18:14:54 EDT
Subject: Re: Heatset Litho vs Gravure

Dear PNEAC_customer:

There are several dramatic differences between gravure printing and heatset
lithographic printing and there are literally books written on each of the
major printing processes. I will try and provide a brief explanation of the
differences. The first place to start would be a review of the major print
processes. There are five major printing processes and they are offset
lithography, flexography, rotogravure, screen printing, and letterpress.
While they are all the same in that the goal of the process is to deliver ink
to a substrate, they all differ in how it is actually accomplished.

In gravure printing, the process is distinguished by the fact that the image
area is recessed relative to the nonimage area and the image is transferred
directly tot he substrate from a cylinder. The cylinder is typically copper
based and the image is engraved into the copper. Since copper is a soft
metal, the cylinder is plated with chrome prior to being used to print an
actual product.

Since the engraved image is made of tiny cells, the inks used to print need
to be able to fill the cells and as a result, the inks are very liquid or
fluid in nature. They can either be solvent-based or water-based. Gravure is
used to produce a variety of products ranging from long-run publication work
to packaging as well as other products such as wall and floor coverings.
Typically dryers are used to evaporate the ink solvent or water.

The substrate is typically fed as a continuos roll or web. Occasionally,
individual sheets are fed into the press.

Heatset lithography is a subdivision of lithography which includes nonheatset
web and sheetfed. Lithography is distinguished from the other print processes
by the fact that the image area and nonimage area are in the same geographic
plane and the distinctions between the two are maintained chemically.
Lithography works on the principal that oil and water do not mix together.
Lithographic ink is very thick and pasty, very much like peanut butter and is
oil based. It is applied to the image area of the plate which is made oil
receptive. Fountain solution, which is principally water with other additives
such as gum arabic, buffers, surfactants, and wetting additives, is applied
to the nonimage area that has been made water receptive.

Since the image on the plate is very thin, it can't transfer the image
directly to the substrate or it would quickly be worn away. Therefore, the
inked image is transferred to an intermediate carrier called a blanket which
in turn transfers the image to the substrate. The blanket is made of rubber
and like the plate is attached to a cylinder on the press.

As the name implies, heatset inks dry by evaporation and dryers are used at
the end of the press line to dry the ink. The substrate, which is usually
paper, is almost always fed from a continuous roll feeding system and is
referred to as web fed.

I hope this answers your question. Please email me back if you need more

Gary Jones
Graphic Arts Technical Foundation



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