To the average person, both heatset and coldset processes may produce the same product and look exactly the same. The difference lies in the process with which they were produced. The essential difference with heatset is that it has the potential to produce much higher quality. Let’s explore the difference between these two forms of offset printing.
Many newspaper printers nowadays use the coldset form of printing. This basically means that they do not apply any heat to the ink to make it dry. It dries primarily through absorption into the paper and evaporation into the air.
Ever get dirty hands from handling a newspaper? Sometimes ink will rub off on your hands. That’s coldest. The oils in the ink never completely dry – hence the smell and scuff from newspapers.
Many heatset printers also print newspapers. But in addition, they also print flyers, glossy magazines and other high quality work. This is because the process allows for higher quality papers such as glossy and semi-coated papers.
Since ink poorly absorbs into a glossy coated paper, it must dry on the surface. The plus to this is that it can produce a much sharper image. The downside is that you must find another way to dry the ink. Heatset ink is made of ingredients that will quickly dry under extreme heat. So to dry the ink you must add a few components to the press.
Check out this video. It gives you a tour of an extremely fast 100,000 copies per hour heatset Man Roland Lithoman web press.
The Heatset Oven
Heatset ink has solvents in it that, when it reaches what is called a ‘flash point’ it will evaporate. To achieve this, the paper passes through an dryer or oven. The solvents evaporate by means of hot air blowing through air bars against the web of paper.
The a cooling process takes place in the latter part of the oven by exhausting the hot air. Since the paper is so hot, it must be cooled down by means of ‘chill roll’ which set the ink and bring the paper down to more manageable temperature to be folded.
Perhaps what best describes heatset printing is the name itself. It is not called heat-dried printing for a reason, simply because it does not dry. Rather, the solvents evaporate from the ink leaving pigment, resin and wax. Once the paper leaves the oven and touches the chill rollers, it ‘sets’ the ink, solidifying the pigment in the wax and resin. It’s much the same way wax only sets when it has been cooled – drying has no bearing in the process.
Coldset printing is also generally cheaper. Lower quality papers and less expensive machinery to produce the product will naturally translate into fewer dollars required. However as the worlds taste for high quality four color content continues to grow, heatset becomes more in demand.