The digital universe of paper and printing knowledge

07/05/14 ● www.sappi.com/houston

CAF in heat-set web offset printing

CaF stands for Cracking at the Fold. The term refers to the phenomenon of coated paper

breaking under the pressure of a folding device.

These breaks vary in severity, depending on a number of product and process characteristics, like paper

grammage, paper surface, coating quality, ink coverage, printing system, folding pattern, etc.

The problem, traditionally associated mainly with multi-coated boards and high grammage papers, has been

known for many years and is extensively documented. There are numerous studies on the subject and even

more recommendations on how to prevent or reduce the effect, generally centred around the influence of

fibre direction and the importance of pre-fold creasing.

Recently, we have been seeing a new version of this old problem; this time in the field of heat-set web offset

printing, where cracking at the fold is not uncommon even with very low grammage papers. What's more, the

breaks, when they occur, are not limited to the coating layer, but penetrate into the actual fold as if the paper

had been cut with a knife.

Since this is a general trend across the board of all common paper types from all major paper producers, we

can not look to particular manufacturers for a solution. The problem is not caused by specific paper

characteristics that can be modified in the production process. It is the result of general paper properties that,

in the actual process of converting, determine its flexibility and, as a result, its suitability for folding. As

opposed to its rigidity and, as a result, proneness to breaking. It is these paper properties that we need to

focus on, that we need to understand properly in order to reverse the trend.

One key parameter to take into account is humidity. When the question is "to break or not to break", water

content will definitely make the difference.

Water content is measured in percentage of relative humidity, with absolute humidity being expressed in

grammes of water per kilogramme of dry air or content of water as a component percentage of the paper

mass. The same paper, in other words, can be described as having a RH of 50%, 10 g H2O/kg dry air or 5%

mass in component value.

Handling these concepts correctly and understanding the exact meaning of each expression is important to

get a proper idea of what we are dealing with and what can be done to arrive at the optimal values. Sappi’s

technical brochure on climate and paper is an excellent guide to better insight in the subject.

The normal water content in paper for sheet-fed presses is 50 % RH or 10 g H2O/kg dry air, compared to 40

% RH or 8 g H2O/kg dry air in paper for heat-set web offset presses (HSWO). The lower quantity of water in

the case of HSWO is to prevent blistering. In the drying section of a printing press, the paper is submitted to

extremely high temperatures that vary according to dryer type and press speed and size, but are always in

excess of 100oC. At this temperature, water evaporizes and, depending on surface finishing and ink

coverage, the paper needs to have sufficient porosity to allow this vapour to escape. If not, it produces

blisters.

When faced with the issue of cracking associated with low humidity, we understandably tend to focus on the

water content of paper as it is being fed into the press. In truth, however, the initial humidity does not really

matter all that much, because prior to folding, the paper passes through the press's drying section, where

nearly every last particle of water is removed anyway.

The digital universe of paper and printing knowledge

07/05/14 ● www.sappi.com/houston

At this point, when the paper leaves the drying section, its humidity is practically zero (0.5 to 1% RH). Any

effort to convert paper in this condition is an exercise in futility. It will simply break into pieces. The obvious

answer, then, is re-humidifying.

Practical tests show that paper must have a minimal humidity of 30 % RH or more than 7 g H2O/kg dry air to

be converted without problems. This humidity, if the paper has been folded, must be measured at the

stacker. If sheeted, at the delivery.

There are different ways of re-humidifying. One method is the use of chill rollers, which, apart from reducing

the paper's temperature, produce a crystallizing effect in the ink, thus increasing humidity. The cooler the

rollers, the more re-humidification.

Another opportunity for increasing water content is at the silicon applicator, where a layer of chemical

material is put onto the paper in order to improve foldability. The same process can be used to increase

water content. Reducing the concentration of silicon and adding more water is a good way to partly restore

humidity.

A third option is the installation of humidification chambers, sprayers, fog fans, mist pumps or other devices

that add to restoration of water content.

Last but not least, there is the important matter of precise oven temperature control. Every single degree of

reduced temperature that does not negatively impact effective drying, is a point gained in preventing breaks

in the fold, with the additional benefit of saving in energy consumption. So, reducing oven temperatures is

one key target for press operators. While drying at high temperatures does not really lower the risk of

scuffing, it has disastrous effects on the quality of the fold - not to mention the overall operational costs of

printing.