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#22 Ceramic glazes, dusting phenomena & digital printing decoration

Contents

  1. 1. Definition
  2. 2. A little history
  3. 3. Advent of digital printing systems
  4. 4. Problem's solutions   

 


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1. Definition

What do we mean with “dusting phenomena”?
What are the features of a glaze marked by this characteristic?

In general, we are talking about a glaze that shows a low cohesion after application by means of traditional airless spray systems. This lack of cohesion occurs not only between the glaze’s particles but also between the glaze and the raw ceramic support. This kind of glaze, that we could also define as fragile, is easily removable from the ceramic surface, especially when it undergoes to the mechanical solicitations that take place along the production line. The problem is particularly clear after the evaporation and draining process, when the glaze has definitely lost its solvent part (the liquid component, that is water).

2. A little history

Dusting phenomena are a serious problem that must be properly managed and it was even worst in the past, when the digital technology had not replaced yet the outdated decoration techniques: flat and rotary screen printing or silicon roller decoration systems. Why?
The physical contact between the application machine and the tile used to directly compress the surface, also promoting the removal of a superficial layer of glaze’s particles that usually adhered to the screen mesh or to the roller.

To avoid the problem, ceramic producers used to apply a FIXER (watery solution of polyvinyl alcohol) on the surface of the glaze already discharged on the raw tile. This was a preventive action that, thanks to product’s cohesive power, inhibited the detachment and displacement of the glaze, also bypassing possible alterations of the screen printing pastes or superficial defects after the decoration process.

 

Chemically speaking, FIXERS are polymers in water that develop a high binding power after the water’s evaporation and draining process (the solvent of the solution). Unlike all other binders for glazes and engobes, polyvinyl alcohol – with the same active material – are marked by a strong cross-linking ability towards the particles, promoted by their chemism. However, despite this important feature, they cannot be applied within the matrix of the suspension: these are chemicals, in fact, that can be handled only by means of airless spray systems and discharged on the glaze already placed on the support, because of their strong incompatibility with the glaze (or engobe) suspensions.

3. Advent of digital printing systems

With the digital printing decoration technology that does not involve and physical contact between the machine and the raw ceramic tile, things have gradually changed, until the complete replacement of fixers and similar. Why?
At the beginning, the idea was that these kinds of products were not interesting or useful anymore: thanks to the lack of physical contact, the superficial glaze’s removal was a solved problem. Actually, the presence of a low cohesive glaze (and so marked by a dusting phenomenon) is still a problem, even with the new digital technology that have replaced the old decoration systems.  Among all the several problems, the most important is about the accumulation of powder under the bars of the ceramic printing machine.

 

What does that mean?

 

A low cohesive glaze can be removed by the “violence” with which the ink lands or reaches the surface (the speed of discharge ranges from six to seven meters per second), leading to a glaze’s particles displacement, and therefore to their dispersion in the atmosphere, in the tiny space between the raw tile and the bars of the printing machine. In general, in fact, the distance between the tiles and the bars of the printer is usually about 2 or 3 millimeters: this very small distance, together with the temperature of the tile that has previously been inside the dryers, produces an amount of water vapor that in turns causes turbulence and air movements that affect the glaze’s particles. The latter, after detaching from the surface, accumulates on the bars and on the printheads of the digital printer, leading over time to printing defects and compromising the nozzles efficiency.

And so, the subject of glaze’s dusting phenomena must be carefully addressed to avoid problems and to not compromise the digital printing process.

 

4. Problem's solutions

And here a fair question: why don’t we add the fixers already used in the past?
Unfortunately, this is impossible due to the nature of these products and to the kind of action they promote: their very high cross-linking power “closes and seals” the glaze, making it practically impenetrable and therefore reducing (or even stopping) the absorption of the ink. The latter, since it cannot penetrate the glaze, expands on the surface, producing an optical effect that turns into a loss of printing definition. It may seem quite obvious but it is important to remember that the glaze’s surface must receive and partially absorb the ink’s drops. Even better: the ink drops must be integrated within the matrix of the glaze to promote a good resolution and a proper color development during the firing stage. Adding to this, the use of these products can easily slow down the drying times, negatively impacting on the entire process. Finally, fixers cannot be used both inside or over the glaze, since they cannot be matched with production targets.

 

The solution of the problem, as usual, starts from the R&D labs where it takes place the study and development of specific binders for glazes (engobes and smaltobbi) able to bind the glaze’s particles, decrease or avoid dusting phenomena by producing a draining cross-link that does not compromise the proper ink’s discharge.

From a chemical point of view, these binders are basically made of organic molecules characterized by not too long molecular chains. This morphology is essential to produce between the particles a sort of “bridge” instead of a film within the glaze’s matrix that would negatively affect the ink’s absorption.
The reduction of dusting phenomena by means of this chemicals’ category is able to solve not only technical and aesthetic problems, but it also ensures a longer life of the printheads, since they are not “attacked” by the dust of the glaze, therefore reducing times and costs of the printing machines’ maintenance.



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