The tackiness ( English tack ) referred to in the rheology of the ink the inner train ( cohesion ), opposes an ink layer with which their cleavage. The tack is the second essential rheological variable of a printing ink for printing processes , just after the viscosity and before the elasticity . In contrast to stickiness , in which the wetting of the interface layer ( adhesion ) is also considered, tack is only an internal property of the printing ink itself and is therefore independent of the surface on which the ink is applied.
Knowledge of the speed of a printing ink is necessary in order to be able to predict its behavior in the printing press.
Disadvantages of high speed
Since the rollers have to apply more force in order to split the ink film at high speeds, more energy is converted into heat. A high speed therefore leads to greater heating in the inking unit . The higher force required when splitting the ink layer also leads to stronger plucking forces that are exerted on the printing material . The tendency for fibers or other particles to be torn out of the printing material increases accordingly.
Disadvantages of poor speed
The ink does not split as well through the inking unit. In this context, good cleavage means separation into layers of equal thickness. With the same setting, less ink is transported in the factory with less speed. The poorer splitting also leads to less ink transfer from the rubber blanket to the printing material and is therefore less color-intensive on the latter. The formation of the emulsion with the water is less favorable. Overall, the printed image looks sharper with a swift color. “The quicker the color, the more pointed the dot prints.” For a stable, high-quality printing process, the quickest possible color is advantageous.
In order to prevent a color from splitting the previously applied one, an exact gradation of the speed of the individual colors according to their sequence must be observed in offset printing without water . The color in the first work must therefore always have the highest speed, in the next work the next one, etc. In conventional offset printing machines, color sets with identical speed can be used. The "wet-on-wet" compression results in a sudden increase in tack when the transition from emulsified to pure color. This jump ensures good ink acceptance. Water with its low internal cohesion forms separation points in the emulsion for splitting the color.
In physical terms , the tack refers to a force per area and is therefore measured exactly like a pressure in the SI unit Pascal or Newton per square meter. Since the physically precise measured value is difficult to determine and is not necessary in practical printing applications, methods have been established that have been specially optimized for the needs of the printing industry and that indicate the speed in manufacturer- dependent scales .
In 1959, Amsterdam-based Rudolph Meijer's Drukinktfabriek NV developed the Tack-O-Scope to determine the speed in a practical and process-oriented manner . A certain amount of the paint is distributed on three superimposed rollers. The axis of the top roller is connected to force transducers. Due to the resistance when the paint layer splits, the roller is pulled along by the roller below in its running direction. The force measured is converted by the measuring device into a manufacturer-dependent scale and is also dependent on process variables such as the layer thickness, the temperature and the speed of rotation. For example, the Inko-Tackomat from the company Prüfbau can indicate the speed both in 0-42 Inko and in 0-700 Tacko . In accordance with the two established methods, the Tack-O-Scope is also referred to as the inkometer .
A quick color is colloquially referred to as "long" because it forms long threads during the splitting process. Accordingly, colors with little speed are also referred to as “short” or “buttery”. This relationship is used in the “ finger test” as a simple test for speed. A color sample is pulled apart between two fingers and it is observed when the thread that is forming breaks. A long thread is a sign of good speed. When you squeeze and pull the color several times, a quicker color also creates a louder smacking sound.
Cause and attitude
The tack of a paint is essentially determined by the binder . In contrast to viscosity, which can be viewed as internal friction, the speed is given by the cohesion of the molecules. With suitable binders, these two rheological parameters can be adjusted almost independently of one another. Since the molecular cohesion depends on the size (long chain) of the molecules and polar proportions, binders made from short, non-polar molecules result in only poor tack. For example, if long-chain alkyd resins are used, the color will be quicker.
As Zügigkeitsreduzierer printing products are referred to with which the tack of an ink can be reduced without affecting the oxidative drying unfavorable.
- Dr. Bernd Th. Grande: Rheology for Printing Inks (PDF; 997 kB) Retrieved on September 22, 2012.
- Tack-O-Scope is a registered trademark see trademarken.com ( Memento from February 5, 2013 in the web archive archive.today ).
- Prüfbau Inko-Tackomat Series 200 T and Series 200T / 800 . Archived from the original on February 12, 2013. Retrieved on December 28, 2012.
- Supplement KBA Report 35 2009 - Glossary of printing inks - Effects, applications, interactions, test methods, standards - a collection of terms for offset printing companies (PDF; 1.9 MB) Accessed on December 28, 2012.
- PrintWiki: Tack . Retrieved December 28, 2012.