- 1. Grit suspensions
- 2. The production ine
- 3. The machine set-up
- 4. Critical issues and corrective actions
A) Improper machine set-up
B) Abnormal grit grain size
C) Low wettability of the grit
D) Grit localized sedimentation
1. Grit suspensions
Grit suspensions applied by means of spray systems applications are usually prepared in the glazes’ grinding department. The three main ingredients of the suspension are poured inside tanks provided with stirrers.
The proportion of each component has to meet a predetermined recipe (that varies according to production) in order to reach a STABLE SUSPENSION, without sedimentation and gelling phenomena.
2. The production line
Once it has been prepared and mixed, the suspension is brought to the glazing line and put in smaller tanks connected to the spray cabin. In this phase, water and leveling agents (or more generally functionalizing agents) are added in order to reach proper parameters in terms of density and viscosity and therefore in order to get a correct application.
Even these rheological values cannot be standard, changing according to all process’ parameters of each ceramic company. This therefore means that all grit suspensions must previously studied inside the lab in order to avoid critical issues during production.
3. Machine set-up
The proper machine set-up is the precondition of the grit correct application. Only as an example we cite a few:
- 1. NUMBER AND TYPE OF NOZZLES
- 2. DISTANCE BETWEEN NOZZLES
- 3. DISTANCE BETWEEN NOZZLES AND CERAMIC TILE
- 4. PRESSURE OF THE SPRAY APPLICATION
- 5. CONVEYOR BELT SPEED
All nozzles are provided with an orifice that may vary both in terms of diameter and of opening angle. The hole dimension can range from 0,02 to 0,07 inch (0,05 / 0,17 cm) while the opening angle can swing from 65 to 130 degrees.
4. The problem
During grit application many critical issues may occur.
One of the most popular and frequent is the nozzles obstruction. Since they have to be removed, cleaned and reassembled, their blockage significantly impacts on the production yield, frequently stopping the process along the glazing line.
What are the causes of the obstruction?
The origins of the problem can have several reasons. Let’s list the most representative.
A) INCORRECT MACHINE SET-UP
Even before you analyze the suspension parameters, the first recommended action is to check the machine set-up, modifying or change - if necessary - the kind of nozzles as well as the applied pressure. This first intervention could, in several cases, solve the problem without having to act on the suspension in terms of grit and/or additives.
B) ABNORMAL GRIT GRAIN SIZE
Even if grits producers are extremely careful about the quality of the raw material, grit batches can sometimes contain non-standard grain size. These grains, exceeding the requested dimensions, cannot pass through the nozzles’ holes therefore clogging the application machine and stopping the production.
This scenario is actually very extreme and uncommon and it is very difficult to find a remedy during application. In the happiest cases it is possible to use a sifter inside the tank in order to retain and remove the biggest particles. More frequently, the only way forward is to replace the grit with a proper batch. In this case this is most effective action.
C) LOW WETTABILITY OF THE GRIT
Sometimes, the grit inside the solution can be not perfectly and homogeneously dispersed in water. This means that grit behaves independently and separately from the fluid in which is contained (that is mainly composed by water and additives).
The low wettability behavior makes the suspension a heterogeneous system, producing phenomena that are very similar to the sand behavior inside a glass of water: during application, it is possible to find aggregations of matter right at the nozzles.
In these cases it is strictly recommended the use of wetting agents that can be previously added during the suspension’s preparation or directly along the glazing line, during application.
How does a wetting agent act?
Wetting agents are partially or slightly water-soluble medium composed of organic molecules able to decrease the water’s high surface tension. Such decrease helps the water penetration between the suspended solid particles that are holding air between one and another.
The water, thanks to this process, hydrates every single particle facilitating their homogeneous dispersion within the fluid, therefore producing a de-agglomeration phenomena.
Once they have been wet, grit’s particle can freely move inside the suspension and slip on each other, avoiding possible clogging phenomena at the nozzles.
It is important to point out that both dosage and type of wetting agent always change according to the solution as well as the water’s properties, the kind of grit and the application parameters. Sometimes, however, the wetting agent does not reach the target.
In this case it is necessary to study the suspension inside the labs in order to identify the proper and more adequate mediums that can match the grit in use.
D) GRIT LOCALISED SEDIMENTATION
Sometimes, even if the system is homogeneous and provided with a good wettabilty, the absence of flow limit (the ability to suspend the grit) together with viscosity’s low values, can lead to rapid grit sedimentation phenomena that may interfere with the application steps.
In these cases, two different steps are required:
- 1. Take immediately action along the production line by using proper rheological modifiers, able to restore the proper values in terms of viscosity, density and flow limit (making the system more cohesive
- 2. Once the problem has been solved along the glazing line, it is a good practice to proceed inside the lab with a new study of the suspension’s sample analyzing and rectifying all the parameters
Back to How To