Printing blankets put the dot onto the paper

by Jan Jungmann, Head of Sales

What effect does the sinking of the printing blanket have on the dot gain value and delta E value measurement? How is the correlation between the compressibility of a printing blanket and the required nip forces? How do the various face compound polarities and surfaces affect printing quality? What are the feeding characteristics of a printing blanket and what is their significance for sheet-fed and web offset printing? In short: What is the significance of the printing blanket for the standardization of offset printing processes?

The significance of the printing blanket as a quality factor in the printing process is often underestimated, although it transfers the printed dot and thus the quality to the paper. The printing blanket transfers not only the ink from the offset printing plate to the material being printed, it also absorbs vibrations from the printing machine and compensates for different paper grades and qualities.
In the printing process, the blanket affects a series of key variables and parameters which reflect the dot gain value of the printing machine, which in turn is the basis for the ICC profile of the machine and therefore complete digital workflow. These gains should be stable throughout the life of a printing blanket in order for the desired printing quality to be reproduced.

As the printing quality is determined to a large extent by the printing blanket, it is an important product for the process standard for offset printing as to ISO 12467-2 for sheet-fed and heatset offset printing and ISO 12467-3 for coldset newspaper printing.

With its CONTI-AIR® brand, ContiTech Elastomer Coatings is one of the world's leading manufacturers of printing blankets. As the sole German manufacturer of printing blankets, the high-tech company, which belongs to Continental AG, has the know-how to have a major impact on and improve process stability in offset printing.

Standardization

Nowadays, if the chain of various output devices in a printing plant are to be standardized, the printing blanket should be taken in consideration and not be ignored. Certain printing blanket characteristics such as blanket sinking, nip speeds, face compound polarities and grinding technologies, different compressibility and face compound manufacturing process as well as various fabric types are all quality features that are specifically controlled by the printing blanket manufacturer ContiAir, as these features have a direct impact on the printed dot and thus the dot gain value.

The printing blanket

The 3-ply (1.69 mm) or 4-ply (1.95 mm) thick printing blanket is a composite consisting of a solid rubber, a compressible rubber and various fabric plies and grades, all of which are used to produce printing blankets for various applications fields in various process stages such as calendering, vulcanization and grinding. The ContiAir fabrics, are woven exclusively in Germany, determine the stretch resistance which is vital for tensioning the blanket onto the printing blanket cylinder.

In part, the printing blanket affects the dot gain value significantly as a result of various factors. The elasticity of the face rubber compound makes it possible to adapt to the paper surfaces well. Rubber has another characteristic that is important for the print result: The compressible layer ensures uniform pressing force in the nip to compensate for irregularities in the paper surface.

ContiTech is the first printing blanket maker to produce the blanket face compound and compressible layer using solvent-free and environmentally-friendly calendering technology for blankets in the sheet-fed, heatset and coldset sector which Mr. Jungmann added.

Pressure forces in the nip zone

The compressibility of the printing blanket affects the pressure between the plate cylinder / printing blanket cylinder as well as the printing blanket cylinder / impression cylinder in what is referred to as the printing nip. For optimum ink transfer, the industry strives for a nip pressure of from 70 to 100 N/cm2. This is the optimum nip pressure to transfer the printed ink dot from the printing plate to the printing blanket and from there onto the paper. The compressibility is determined by the design of the printing blanket carcass and the product process used for the compressible layer based upon the microsphere or blowing agent process.

For the some 120 different printing blanket carcasses available on the market from various manufacturers, little attention is paid to the compressibility of a printing blanket when determining the underpacking. Just as an example if this would be the case in the automotive industry, all shock absorbers available on the market would have the same "stiffness" and "maximum load". For instance, there would be no choice between soft or stiff when tuning a chassis.

Sinking characteristic

To ensure a stable printing process, the height loss of a printing blanket should be as little as possible to ensure as stable dot gain value during the lifetime of the printing blanket. If this value fluctuates, the process reliability is instable and makes it more difficult to measure the delta E value in accordance with
ISO 12647-2.

Since, depending on manufacturer, the printing blanket can drop from 0.02 to 0.14 mm when in use, the nip pressure to the plate and to the paper is insufficient. Some printers compensate this blanket sinking by over packing the press with calibrated board or mylar foil, but this should be questioned for quality reasons seeing that the dot gain will vary till the blanket has reached it’s final height.  

Feeding characteristics

The speed status in the printing nip is described by the feeding characteristics of the printing blanket. Here, the printing blanket construction with the respective material composition plays a major role, since, depending on printing blanket type, it determines whether the speed in the nip is the same, faster or slower than the speed of the machine or paper. Original it was thought that the feeding characteristics of printing blankets would be an important factor only for web offset printing machines. Practice has shown, however, that this is to be taken seriously for sheet-fed printing machines as well.

Sheetfed presses:
Above all when focusing on sheetfed printing presses, the actual cylinder diameters of the plate, blanket and impression cylinders of the respective machine types are responsible for the surface speeds which should be taken into account. If different shear forces or variations of nip speeds arise between the cylinder contact zones, it is possible that in the long term the sheetfed printing blanket will not withstand these high forces and elongation issues are possible.
Field tests have shown that certain sheetfed machine types with positive feeding characteristics blankets achieve better printing results than when the blanket has comparatively negative feeding characteristics. And some sheetfed machine types achieve better results if the printing blanket has negative feeding characteristics than if it has positive ones due to these above mentioned issues.

Web presses:
The feeding characteristics on web printing machines depend on the individual motor drive technology and the printing blanket cylinder width in relation to the diameter. Long and narrow cylinders are subject to deflection. The older printing blankets generations reacted more sensitive to the various pressing loads convey the paper differently across the width of the cylinder, which can lead to register problems.

To make the printing process more stable, ContiTech has developed a generation of printing blankets that demonstrate consistent feeding characteristics despite various impressions. The new CONTI-AIR® printing blanket brochures provide information about whether a printing blanket has neutral, slightly positive, positive, slightly negative, or negative feeding characteristics so Mr. Jungmann added.

Face compound polarity

The polarity of the face compound reflects the ink- or water-friendly characteristics of the cover stock. In offset printing, a distinction is made between polar and non-polar printing blanket face compounds. Conventional offset printing inks and cleaning agents are non-polar media, whereas UV-inks and UV-cleaning agents are categorized as polar. The face compounds of a printing blanket consist either of a synthetic polar NBR rubber type, which is used with conventional offset printing inks, or the non-polar EPDM rubber for UV offset inks. In keeping with the chemistry maxim "similar dissolves similar", printing blanket manufacturers use opposing polarities to prevent swelling of the face compound.

Face compound characteristics

Depending on the application field, the printing blanket must have various surface characteristics in order to accommodate a wide range of paper grades and qualities.

In addition to the polarity of the face compound, the grinding technology and the respective surface roughness (RZ and RA) of the face compound are also important for the quick release characteristics of a printing blanket.

If a printing blanket has poor quick release, the sheet or web will adhere to it longer. Here, the paper is peeled off the blanket in a shorter time interval, which in turn can deform the paper.

Summary

The printing blanket is responsible for transferring the ink dot from the printing plate to the paper. The shape and uniform structure of the printed dot should be the same on the paper, and, to the greatest extent possible, be independent of the paper grade used. This complex task places high demands on modern printing blankets, which as a key link in the offset printing process standards are often underestimated. Key parameters are sink characteristics, feeding characteristics, polarities, compressibility and face compound technologies. These parameters can be adjusted by the printing blanket manufacturer ContiTech to ensure a constant dot gain value over the life of a blanket. This constant dot gain value is, in turn, the basic requirement for determining the ICC profile of the machine and for certification in accordance with ISO 12647.

About the author:

Jan Jungmann is the Head of Sales manager for CONTI-AIR® printing blankets at ContiTech Elastomer Coatings in Northeim. The trained printer and print engineer presented this topic to an audience of more than 150 at the Printing Forum in Stuttgart.