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Contents

1. 1. Grit suspensions
2. 2. The production ine
3. 3. The machine set-up
4. 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

The suspension, before being applied by means of spray system technology, usually rests inside the tanks for a period that can range depending on the situation and where is constantly under stirring so to avoid possible sedimentation phenomena.

Among the several ingredients that made up the suspension the big three are grit powder, water, and binders.

The percentage ratio of each component can of course range according to the production parameters but, in any case, the formula must be developed to reach STABLE SUSPENSION.

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 the tank the suspension is added with water and leveling agents (or more generally functionalized agents) to reach the right parameters in terms of density and viscosity and therefore to get a proper application. Even these rheological values cannot be standard since the production parameters of each ceramic company are different from each other. By implication, this means that all grit suspensions must be previously studied inside the lab to avoid critical issues during production.

3. Machine set-up 

The proper set-up of the spray cabin is the precondition of the grit correct application. Among all aspects that must be considered we list here the most important:

1. 1. NUMBER AND TYPE OF NOZZLES
2. 2. DISTANCE BETWEEN ONE NOZZLES AND THE OTHER
3. 3. DISTANCE BETWEEN NOZZLES AND CERAMIC TILE
4. 4. PRESSURE OF THE SPRAY APPLICATION (PRESSURE WITH WHICH THE GRIT SUSPENION IS DISCHARGED ON THE TILE)
5. 5. SPEED OF THE CONVEYOR BELT 

All nozzles are provided with a hole that can 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 must be removed, cleaned, and reassembled, the blockage of the nozzles significantly impacts on the production, often leading to production stop.

What are the causes and origins of the obstruction?
The origins of the problem can have several reasons. Let’s list the most representative.

A) IMPROPER MACHINE SET-UP

Even before changing the suspension parameters, the first recommended action is to check the machine set-up, modifying or - when necessary – changing the nozzles as well as the pressure with which the suspension is applied. This first action in several cases, could solve the problem without any intervention on the suspension in terms of grit and/or chemicals. 

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 best 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 (and the most effective) is to replace the grit with a proper batch. 

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, when the production is already in progress.

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. This kind of 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 leading to their homogeneous dispersion within the fluid, therefore producing a de-agglomeration of the solid elements.
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 wettability, the lack of flow limit (the ability to suspend the grit) together with viscosity’s low values, can lead to rapid grit sedimentation phenomena.

In these cases, two different steps are required:

1. 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. 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 by analyzing and rectifying all the parameters