Ceramic Pressing: How the Internal Quality of a Tile Is Built Before Firing

Ceramic pressing: the stage where tile quality begins

In ceramic tile manufacturing, there is one fundamental principle:

The final quality of a tile starts to be defined long before the product enters the kiln.

When a tile breaks after firing, shows dimensional deformation or develops problems during rectification, the defect usually becomes visible in the final stages of the process.

However, many of these deviations originate at an earlier stage: ceramic pressing.

Pressing is not simply about applying pressure to atomized powder to achieve a specific shape.

At this stage, the initial structure of the tile is formed:

• the distribution of material within the mould;
• the compaction of the granules;
• the evacuation of the air trapped between the granules;
• the mass distribution;
• the initial mechanical strength of the green tile.

For this reason, pressing should be understood as a critical stage in which internal conditions are established that can significantly influence the subsequent behaviour of the tile.

A correctly pressed tile will be better prepared to undergo the subsequent transformations of the process:

• drying;
• glazing;
• firing;
• grinding;
• cutting;
• sorting.

The key is not only to obtain a tile with the correct shape, but also to achieve a homogeneous internal structure.

From atomized powder to the finished tile: a continuous transformation

Before becoming a finished ceramic tile, the material passes through different states:

Atomized powder → Mould filling → Pressing and compaction → Green tile → Drying → Glazing and decoration → Firing → Finished product

Each stage modifies the properties of the material.

However, there is one key principle:

Firing transforms the tile, but it does not completely eliminate the conditions with which it enters the kiln.

A tile with internal differences in compaction may react differently during:

• water removal during drying;
• thermal transformations;
• sintering;
• cooling.

For this reason, analysing only the final defect does not always reveal the origin of the problem.

Ceramic manufacturing must be understood as a chain of transformations in which each stage affects the next.

What actually happens during ceramic pressing?

Ceramic pressing is a dry forming process in which atomized powder is introduced into a mould and subjected to high pressure to obtain a compacted tile.

During this operation, several physical phenomena occur simultaneously:

• particle rearrangement;
• granule deformation;
• reduction of internal voids;
• increase in density;
• generation of internal stresses.

Particle rearrangement during mould filling

Before pressure is applied, the atomized powder must be correctly distributed within the mould.

This stage is essential because it determines the amount of material present in each area of the tile.

The behaviour of the powder depends on factors such as:

Variable	Influence
Moisture	Modifies flowability and deformability
Particle size distribution	Affects particle packing
Granule shape	Affects rearrangement
Particle size distribution	Influences compaction

Irregular feeding can cause local mass differences that subsequently translate into density variations.

The preparation of atomized powder is therefore a stage directly related to the final quality of the tile. You can explore this relationship further in our article on how atomized powder moisture affects ceramic tile density.

Compaction and reduction of internal porosity

When pressure begins to be applied, the granules start to rearrange and deform.

The objective of pressing is to reduce the volume of internal voids and achieve a sufficiently compact structure.

Apparent density can be expressed using a basic relationship:

ρ = m / V

where:

• ρ represents density.
• m represents mass.
• V represents the occupied volume.

In ceramic manufacturing, increasing density is not the only objective.

The main challenge is to ensure that density is homogeneous throughout the entire tile.

Two tiles can have similar average values and yet behave differently during subsequent stages.

The green tile: the stage where the tile's future behaviour begins

The green tile is the result of pressing before firing.

Although it does not yet have the visual characteristics of the finished product, key properties have already been defined at this stage.

Green tile property	Subsequent influence
Density	Affects behaviour during firing
Mass distribution	Affects dimensional stability
Mechanical strength	Allows handling without breakage
Internal homogeneity	Reduces deviations during the process

A tile may appear correct externally while containing internal differences that only become apparent when the product is subjected to more demanding conditions.

This is one of the main challenges of traditional quality control: many deviations only become apparent after the tile has already absorbed a significant share of the process's added value.

Density is not just a value: it is how that density is distributed within the tile

One of the most important aspects of ceramic pressing is understanding that density should not be analysed solely as an average value.

Apparent density provides information, but an overall measurement can conceal internal differences.

For example:

Tile	Average density	Internal distribution
Tile A	2.05 g/cm³	Homogeneous compaction
Tile B	2.05 g/cm³	Areas with different levels of compaction

Both tiles have the same average value.

However, their behaviour during manufacturing may be different.

Why?

Because areas with different levels of compaction may experience:

• different shrinkage;
• different levels of pore removal;
• different mechanical responses.

For this reason, knowing the internal distribution of the tile provides much more complete information than a single average value.

Pressing variables that determine the internal structure of the tile

The result of ceramic pressing depends on the interaction between different process variables.

There is no single parameter that determines the final quality of a tile on its own. Product stability depends on how the characteristics of the powder, the pressing conditions and the distribution of the material within the mould interact.

The main influencing variables are:

Process variable	Influence during pressing	Possible consequence
Moisture content of the atomised powder	Modifies the flowability and deformability of the granules	Compaction variations
Particle size distribution	Affects particle packing	Internal density differences
Applied pressure	Determines the degree of compaction achieved	Changes in mechanical strength
Pressing speed	Influences the evacuation of trapped air	Possible structural defects
Powder distribution in the mould	Determines the amount of material in each area	Dimensional differences
Condition of the mould and punches	Affects pressure transmission	Compaction gradients

Atomized powder moisture and its influence on compaction

The moisture content of atomized powder plays a decisive role during pressing.

Moisture content influences:

• the strength of the granules;
• their ability to deform;
• the ease of rearrangement;
• the degree of compaction achieved.

A powder with unsuitable behaviour can make it difficult to achieve a homogeneous structure throughout the tile.

The relationship between moisture, compaction and density is especially important because small variations in powder preparation can lead to differences in the tile's subsequent behaviour.

To explore this relationship in greater depth, you can consult our article on how atomized powder moisture affects ceramic tile density.

Pressing pressure: more pressure does not always mean a better tile

The pressure applied during pressing is one of the most important process variables.

Increasing the pressure can increase material compaction, but the industrial objective is not to achieve the highest possible pressure.

The objective is to achieve adequate and uniform compaction.

Incorrectly adjusted pressure can cause:

• density differences within the tile;
• internal stresses;
• dimensional variations after firing.

Therefore, pressure should be analysed together with other process variables, such as powder moisture, pressing speed or the distribution of material within the mould.

How a deviation during pressing can become a problem during firing

One of the most complex aspects of the ceramic process is that the moment when a defect appears does not always coincide with the moment when it is generated.

A compaction difference created during pressing can remain invisible for several stages and only become apparent later during firing.

The reason is related to the thermal evolution of the tile.

During firing, the tile undergoes transformations such as:

• removal of residual water;
• mineralogical changes;
• densification of the ceramic body;
• shrinkage of the tile.

When different areas of the same tile have different internal structures, their evolution may not be uniform.

Differential shrinkage during firing

During firing, the tile undergoes a reduction in its dimensions as a result, among other phenomena, of the sintering process.

When the tile has a homogeneous structure, its dimensional evolution also tends to be more uniform. However, if there are areas with different degrees of compaction or different initial porosity, they may evolve differently during firing.

As a result, different areas of the same tile may undergo different levels of shrinkage. These differential shrinkages can generate internal stresses and contribute to the appearance of:

• deformation;
• loss of flatness;
• dimensional deviations;
• subsequent breakage.