How different are heatset and coldset offset?

As you get your morning cup of coffee along with a newspaper – it is a sheer sense of early morning fulfilment. Yet, you have never bothered to know how his favourite newspaper is created, produced and folded from a huge web roll of paper and delivered right at his doorstep.

24 Aug 2016 | By Som Nath Sapru

As you sip your coffee and start turning the pages, just to go through major news headlines, you start cursing the newspaper delivery boy, as you observe blackened fingertips and experience a foul smell emanating from the newspaper.

The newspaper delivery boy has nothing to do with both of your complaints. Well, the smell happens to be of the ink which has neither been dried nor been absorbed by the print area of the newspaper and this is the real reason why your fingertips are blackened.

As we know, the newspaper industry uses offset process of printing for the production of newspapers and other allied publications like weekly magazines, etc. There are two processes in offset web printing –heatset and coldest. To an average person, both heatset and coldest processes may produce the same product and look the same. The difference lies in the process. The essential difference with heatset is that it has the potential to produce superior quality print.

Most newspaper printers use coldset printing. This means they do not apply heat to the ink to make it dry. It dries primarily through absorption into the paper and evaporation into the air. This is the reason why hands get dirty while handling a newspaper – when the ink rubs off in one’s hands, it is a clear indication that the newspaper was printed with coldest process. Since the oils in the ink never completely dries, this also results in the smell and scuff.

These days newspapers with colour pages are produced with heatset process. These printers utilise more than 60% of printing time for the printing of other publications like supplements, magazines, flyers, and commercial print jobs, which demand superior quality work and coated and glossy paper stock. The heatset process prints beautifully on both coated and glossy substrates of less and heavy grammages, besides newsprints.

We should understand that ink absorbs poorly in glossy, coated or even semi-coated papers – so it has to dry on the surface. The additional advantage of this process is that it can produce much higher and sharper images. The downside is that one 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 one has to add a few components to the press.

Heatset ink has solvents in it –when it reaches what is called ‘flashpoint’, it will evaporate. To achieve this, the paper passes through an oven dryer. The solvents evaporate by means of hot air blowing through air bars against the web of paper – then a cooling process takes place in the latter part of the oven by exhausting the hot air.

Since the paper is hot, it has to be cooled by means of ‘chill roll’, which set the ink and brings the paper down to manageable temperature so that it can be folded.

Why then the process is called heatset printing whereas it should have been named heat-dried printing. Solvents evaporate from the ink leaving behind pigments, resin and wax on the printed area and this is the reason it is called heatset. Once the paper leaves the oven and touches the chill rollers – it sets the ink, solidifying the pigments along with the wax and resin – it is much the same way wax only sets when it has been cooled – drying has no bearing in the process.

Fluting happens in heatset web offset printing. Fluting is produced in the printing process and defined as a series of waves or corrugations which run in paper-grain and web-travel direction. The problem is exclusive to web heatset printing with undesirable intensities of fluting being most evident on lightweight papers. Although varying flute levels may relax and partially flatten-out after the printed signatures are trimmed and re-acclimated to ambient environment, the condition is considered to be permanent.

Fluting intensity can vary from run to run in both amplitude (wave height) and wavelength (distance between waves). The primary and variable factors that contribute to fluting are heat intensity, ink coverage, layout, and web tension.

Undoubtedly, coldest printing is cheaper. With coldest, one can use lower quality papers and less expensive printing units. However, in the recent years, print-media and publishing industry are in upgrade mode. As print consumers’ taste for high quality four-colour content continues to grow – heatset produced publications are in demand.

Now, let’s understand in more simpler terms that heatset and coldest web offset printing both refer to a process used to dry ink after it has already been applied to the paper. In both coldest and heatset web offset printing, the paper is run through the printing press in the same way – the only difference comes when it is time for the ink to dry.

For publications, mostly printed on low-grammage uncoated paper stocks like newsprint, etc, printers prefer to use coldest offset printing. This is because coldest is one of the most economical forms of printing that allows multiple webs to run concurrently while using less expensive paper in a short space of time and power consumption and need.

As said earlier, coldset web presses use far less energy and produce far fewer volatile organic compounds (VOCs) than their heatset counterparts produce. Heatset presses use a powerful heating source to bake the ink into the paper. Heatset printing’s energy inefficiency is made worse by the fact the process uses toxic inks that are very high in VOCs. This needs high-energy equipment for the chill stands required to cool the paper. Coldest process of printing is used to produce textbooks, workbooks and even exercise books besides standard small-town newspapers.

Recent models of coldest web offset presses produce quality reproductions, which are more than comparable to the publications produced on heatset web offset printing presses. In fact, new screen technology enable coldest printing presses produce and print 175 lines screen very successfully.

To recap, in coldest, the ink dries gradually through evaporation and absorption into the paper and this printing process is most commonly used on uncoated paper stocks. Coldset machines consume less power and the process proves to be economical while successfully using cost-effective paper stocks with equally sharp and qualitative reproductions. Heatset printing, on the other hand, is a process by which ink is dried by running the printed paper through an heat chamber immediately after ink is applied by the printing units.

Right from the inception of heatset and coldest printing machines – there has been a constant debate as to which one gives a better and economical reproduction quality of the published materials – not only the quality of printing, but also minimal initial paper wastage at set-up time and of course, ink and power consumption. Recent models of heatset web offset machines consume less energy by using laminar airflow to drive out the oxygen and putting a nitrogen or inert atmosphere next to the substrate, which creates optimal curing conditions with reduced thermal stress on the substrate of any grammage.

For high-speed coldest web offset machine, a recent innovative system has been introduced in place of experimental UV curing. The system is Eltex Innocure. A serious and comprehensive technical innovation, it has attempted to overcome many of the shortcomings of earlier attempts at UV curing on earlier models of large and high-speed coldest presses. 

Newspaper publishers have always tried to publish pictures/ visuals or illustrations in a better print quality, by whichever process they may adapt to print – but the overall cost of production of the newspaper is also an essential factor to be kept in mind.

Initially, the letterpress process used on rotary printing machines was the only printing process available, and as we know, it offered limited possibilities of reproduction quality.

About a century ago, gravure printing entered the scene. The publishers of German newspaper Frieburger Zeitung used gravure printing process around 1912-13. They linked it along with their existing letterpress rotary printing machine and had a very different quality of their print reproduction. What they did – they printed all the pictures and illustrations by gravure process and text areas by normal letterpress rotary and this marked the birth of hybrid printing press almost a century back – but this hybrid system of printing was too time consuming and economically not viable at all.

As we know, gravure is much costlier than letterpress rotary and time consuming as well due to its lengthy imagedrum etching process, etc. Moreover, it was experienced that hybrid printing units proved more expensive and time consuming.

Another negative aspect was that the heatset section caused a relatively high level of setup waste, which the cold section had to follow due to the linking-up arrangement, even though it could manage with less waste. Another major problem happened with web tension, which culminated in major register deviations that could not always be compensated quickly.

Over a period of time, printers found technologically upgraded and improvised versions of web offset machines which had answer for the problems faced by using heatset/ coldest machines, besides printing neat and clean high quality reproduction and being cost-effective and great time saver due high speeds.

Both coldest and heatset web offset printing have a prominent place in newspaper and publishing industry. The choice of process depends on what type of publication is being printed. Number of copies and kind of paper stock used is also considered, beside the shelf life of the publication and the target audience.