#10 Digital glazing by means of non-piezoelectric systems
- 1. Glazes & application technologies
- 2. A common denominator: the glaze's particle size
- 3. Full field applications
- a) Viscosity & flow limit
- b) Superficial tension
- 4. Relief applications:
- a) Flow limit
- b) Thixotropy
- c) Viscosity
- 5. Possible applications' development & Conclusions
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1. Glazes & application technologies
The ceramic glaze application – both before and after the digital printing process – takes usually place by means of “analogical” systems: by spray / airless application systems. Nevertheless, some new application machines have been recently appeared on the market. They cannot be defined as completely digital, but they cannot even be comparable to traditional analogue techniques. In some way we could talk about digital glazing by means of non-piezoelectric systems, even if the use of the word DIGITAL is probably incorrect.
What does that mean?
Without entering in a very complex field and without categorizing, these new application machines are based on a different kind of injection and deposition of the glaze on the raw ceramic surface: the technology they provide is not totally digital but at the same time it cannot even be defined as analogical. When we talk about digitalization, in this case, we are therefore not referring to the application technology itself but rather to the greater control and to the improved automation that these machines are able to provide.
These machines, even if they are still under implementation and under evaluation by ceramic tiles producers, they have been developed to reach - in the future - some important goals:
- 1. Automated management of the machine and therefore a less operator supervision
- 2.Reduction in material use (tanks to the lack of both suction systems and frequent washing of spray cabins)
- 3. Specific glaze applications with tridimensional relieves (creation of textures during the pressing process)
2. A common denominator: the glaze's particle size
At present, there are on the market several non-piezoelectric machines such as those we are talking about. Each of them is marked by its own features but, however, they all have a common denominator. They can work with same kind of glazes that are used with airless applications: with glazes that have the same particle size. Very similar, if not identical. This is very important, because it allows ceramic producers to be very flexible in terms of purchasing and logistic, since they do not need to buy specific glazes to be only used with these machines.
From a chemical point of view, this also means that each producer can work - or act - on the rheology of the glazes already in use, providing them with the proper parameters according to the different machine set-up. We are implying that the glazes’ rheology must be carefully studied to reach a good performance during the process.
Regardless the specific features of the machine, each of them can work in two directions:
- Full field applications
- Relief applications
Let’s see these two main scenarios in respect of the glaze’s rheology.
3. Full field applications
In case of full field application it is very important to reach a uniform leveling of the glaze on the ceramic support.
VISCOSITY & FLOW LIMIT
To make that happen, the glaze should have a low viscosity values (slightly higher than a glaze for spray application) along with a not too high flow limit.
(The flow limit is the VALUE of the MINIMUM FORCE that must be applied to a fluid in order to make it move).
A low viscosity together with a not too high flow limit allow you to reduce (or even avoid) possible sedimentation phenomena or separation between the liquid and solid phases of the system, that in turn may lead to localized sedimentation problems within the unit circuits of the application machine.
At the same time, it is important to carefully check the surface tension of the glaze to reach a good homogeneitywithout compromising the cohesion. This is very important to provide the glaze with a “compact effect” after application: a compactness both in the middle and at the edges of the tile. Of course, these are general indications because, as we know, all rheology values (and so the features of the glaze) must be regulated according to the machine and the kind of application. This means, for example, that if we replace a glaze during a process (or even we change its density value), we should proceed with a new rheological study to restore the balance (that is very delicate) between all parameters.
4. Relief applications
- A) FLOW LIMIT
In case of relief application, to get high level of printing resolution, the drop of the glaze must remain in its place, where it has been discharged, without collapsing and without expanding on the ceramic support. To make this happen, the glaze should be marked by a high flow limit and it should be able - at the same time - to rapidly restore its structure after its deposition on the raw ceramic surface.
- B) THIXOTROPY
What kind of thixotropic features the glaze should have?
[THIXOTROPY: property of a pseudo-plastic fluid change its viscosity over time when subjected to a shear stress (moving for example from a pasty state (almost solid) to a fluid state)].
The thixotropic effect should not be very high, so that the fluid can keep its original features of viscosity and cohesion,preventing the drops to collapse on the ceramic body and therefore without compromising the definition of the application.
THIXOTROPY OF A FLUID: LA CUISINE AS AN EXAMPLE
To better and more easily explain this common characteristic of a great amount of fluids, let’s give – as usual – an example from the world of cooking. In this area, ketchup is one of the most popular examples of thixotropic fluid. The ketchup, when resting in the bottle it appears as compact, thick, and almost solid but when shaken, instead, it loses or even better it changes its structure, becoming more fluid, able to easily get out of the bottle.
- C) VISCOSITY & COHESION
Given that the glaze should not be marked by a high thixotropic behavior, the system’s viscosity value, unlike full field applications, shouldn’t be too much low so that the drops of the glaze (that of course undergo to the stress of the application machine) do not lose their viscosity and therefore do not collapse on the ceramic body surface. The glaze must not lose cohesion and so the glaze drops must be bond one with the other.
What does that mean?
Suppose you through on a desk simultaneously and violently a little mayonnaise and honey. Mayonnaise, since it’s not very viscous, will lose its cohesive force, splitting into several parts and leading to separate drops all over the surface. Honey, instead, thanks to its high viscosity, will be more compact, remaining still, exactly where it has been thrown (even if – of course - it slowly spreads over time because of its low flow limit value).
In short, all this to say that our glaze should be more like the honey, and so more viscous, to keep its compactness once it has been discharged by the nozzles of the application machine.
5. Possible application development
In case of full field productions, the new machines could be ideally used not only after pressing but also at the end of the production line so to apply a final over-glaze. Top glaze applications may require glazes marked by a lower density so to discharge on the surface a lower solid content, keeping at the same time a good and uniform application.
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