In the production of tiles made of porcelain tile and large formats, quality control has traditionally been reactive. When the cracks in the grinding or ceramic cutting process, the usual response on the production floor tends to be to look for issues with the cutting process, the diamond grinding wheels, vibrations in the spindles or faults in the cooling system. These breakages can either completely destroy the workpiece, breaking it into two or more pieces, or they can manifest as visible cracks along the edge of the workpiece, which is both an aesthetic defect and a potential trigger for a subsequent total fracture.
However, ceramic manufacturing is a continuous process characterized by deep interdependence. Grinding or cutting is rarely the primary source of the problem; it is almost always the trigger for a latent defect. A variation of just 0.5 to 1% in the distribution of apparent density of a part during pressing creates a stress imbalance that is released in an unstable manner during abrasive machining, significantly increasing material loss.
The dust cycle: the origin of variability
The uniformity of the tile begins with the preparation of the atomized powder, where its particle size, morphology, and moisture content determine how it fills the mold.
- Accelerated drying: If heat transfer in the atomizer is too rapid, an impermeable outer crust forms on the granule. The trapped steam raises the internal pressure and causes the particle to burst, resulting in hollow granules or an excess of fines.
- Fluency issues: An excess of fine particles affects the consistency of the dough, causing it to feed unevenly into the press chamber.
- Inadequate rest: The atomized powder must be allowed to settle in the silos. If it reaches the press without sufficient settling time or with uneven moisture content, the water (which acts as a plasticizer) will not be distributed evenly. Areas that are drier or contain more fine particles will offer greater resistance to compaction, limiting the density achieved in those specific areas.
Mass defects and their impact on density
| Anomaly in the dust | Physical mechanism | Filling behavior | Effect on bulk density |
| Accelerated drying: | Thick crust and steam buildup. | Hollow pellets and an excess of fines. | Lower packing density and greater friction. |
| Insufficient rest | Uneven distribution of water. | Agglomeration and poor powder flow. | Fluctuations in compressibility. |
| Segregation | Separation of coarse and fine materials in silos. | Uneven flow at the ends of the cart. | Gradients between the center and the edges. |
The physics of uniaxial compaction and the 1% gradient
Forming is performed by dry uniaxial pressing at pressures ranging from 30 to 50 MPa. However, this force is not transmitted uniformly throughout the entire volume of the part due to internal friction forces (between granules) and external friction forces (against the mold walls).
This lateral friction causes an exponential drop in effective pressure as depth or distance from the punch increases. As a result, the areas adjacent to the active punch achieve maximum densification, while the corners and the lower center remain less compacted.
In addition to this, there are two critical operational factors:
- Trapped air andspringback: If the pressing speed is too high, the air cannot escape through the gaps in the die. When the punch is withdrawn, instantaneous elastic decompression (post-pressing expansion or springback) causes the compressed air to expand, opening microscopic planes of weakness or internal laminations.
- Asymmetric mold filling:Wear on the carriage guides or mold blades, accumulated dust or dirt in the feeding system, or improper adjustment of the filling speed can alter the local volume distribution of powder in the mold. As the punch descends to a fixed depth, areas with less material are subjected to lower specific pressure, resulting in a variation of approximately 1% in the distribution of bulk density.
The oven as a voltage amplifier
During sintering in the roller kiln (1150°C – 1250°C), the fluxing components generate a liquid phase that fills the pores by capillary action. This process causes linear shrinkage of between 5% and 8%.
Cooking shrinkage (Sc) is inversely proportional to dry bulk density (ρd) according to the following equation:
Sc = −k ⋅ ρd + C
Because of this law of physics, regions with a density that is 1% lower will experience a substantially greater contraction. Since the tile is a continuous, monolithic body, adjacent areas restrict each other’s movement. The less dense areas (which tend to shrink more) are subjected to intense mechanical tensile stresses, while the more compact areas are subjected to compressive stresses.
[Variación 1% Densidad en Prensa]
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[Contracción Diferencial en Horno]
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[Asymmetric Stress Profile] ──► Tensile displacement at the surface
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[Machining: Grinding / Cutting] ───► Sudden release and crack propagation
This stress distribution worsens during cooling. As the temperature drops below the glass transition temperature (Tg), the material can no longer relieve stresses through viscoelastic deformation. Furthermore, at 573°C, free quartz undergoes the α→β polymorphic transition, contracting abruptly by 1%. If the density and quartz are not distributed homogeneously, the inversion occurs prematurely, inducing severe thermal gradients.
Under normal conditions, residual stresses keep the surfaces under protective compression and the core under tension. However, a 1% deflection during pressing completely disrupts this parabolic profile, shifting the maximum tensile forces toward the surface and leaving the tile in a state of unsustainable elastic vulnerability.
The tribology of grinding and ceramic cutting: the trigger
The ceramic grinding It uses diamond abrasive heads to finish edges with surface roughness (Ra) of less than 0.1 μm. This is an extremely aggressive process that involves three primary physical interactions:
- Cut: The diamonds penetrate the edge, overcome the toughness of the stoneware, and tear off micro-chips.
- Grooved: Abrasives plastically deform the material without removing it, causing localized deformation.
- Friction: Direct contact generates significant friction and raises the local temperature at the point of contact.
If cooling is inadequate or the grinding wheel is dulled by accumulated dust, the heat causes violent thermal expansion at the edge, generating tensile stresses as it cools rapidly when exposed to water.
The main reason why grinding causes 12% of breakages is the removal of the tile’s outer layer. When the edge is mechanically removed, the surface layers that contained the protective compressive stresses are eliminated. A similar mechanism occurs during cutting processes due to abrasion by the cutting tool.
To compensate for the loss of symmetry, internal stresses must be redistributed instantly. If the part retains the press’s 1% gradient, this redistribution is concentrated violently at the density transition interfaces (where the substrate is more porous and weaker). The microcracks introduced by the grinding wheel or disc act as stress concentrators; the crack finds a path of very low energy and propagates catastrophically, destroying the part.
Strength parameters based on compaction uniformity
| Operating Parameter | With a 1% Change in the Press | Homogeneous Compaction ( |
| Local bending strength | Reduced by up to 12% due to porosity. | High and uniform (>35 N/mm²). |
| Differential contraction | When raised, it distorts the flatness. | Negligible, stable caliber. |
| Residual stresses | Asymmetrical and concentrated in weak areas. | Symmetrical, parabolic, and balanced. |
| Effect of the grinding wheel / disc | Exceed the tensile strength limit and break the part. | Elastic absorption of abrasive impact. |
| Breakage rate in finishing | It has skyrocketed to 12%. | Minimal, kept below 1%. |
The shift toward preventive X-Ray screening
Attempting to mitigate these losses by adjusting the grinding process (reducing the line speed or changing grinding wheels or discs) merely masks the symptom, thereby reducing productivity. Traditional press control systems also fail to solve the problem: they are blind to internal volume, and because they are destructive, they measure only part of the parts, failing to detect the potential stress distribution throughout the entire mass.
In response to this, Tekinn has developed systems based on the physics of the X-ray inspection and laser telemetry. The equipment measures the attenuation of a high-energy beam as it passes through the green tile, applying physical laws to calculate the exact density at each point.
The system scans formats up to 130 x 130 cm in just 6 minutes, analyzing more than 2 million points with an accuracy of ±4 kg/m³. This generates a two-dimensional false-color map that instantly reveals uneven distributions of density, thickness, or mass in the compacted part.
Comparison of density control systems
| Technical aspect | Mercury immersion test | Digital preventive inspection (Tekinn) |
| Material consumption | Destructive; generates green waste. | Non-destructive; |
| Response time | 30 to 60 minutes per sample. | A full scan in less than 8 minutes. |
| Analysis results | Average discrete values per sample | A complete map with more than 2 million points. |
The major economic benefit is immediate: by detecting variations quickly and with high precision on the press, parts at risk of breaking are identified before they enter the kiln. This allows for quick and efficient corrections on the press to compensate for differences in compaction, thereby reducing potential breakage issues during cutting or grinding.
Technical recommendations for reducing waste at the plant
To systematically eliminate breaks in the finishing line, technical directors should implement the following operational guidelines, among others:
- Implement X-ray inspection of crude oil: Continuously monitor the density at the press outlet to prevent the spread of defects before the thermal phase.
- Set strict tolerances: Set up alarms so that the local deviation in bulk density within the part does not exceed the control limits, and implement corrective measures on the press if this occurs.
- Adaptive press adjustment:Use the information obtained from the X-ray inspection to immediately adjust the dosing of the feed car and compensate for any misalignment of the mold.
- Optimizing tribology in grinding: Ensure a plentiful and precise flow of coolant at the point of contact between the grinding wheel and the tile. Follow strict protocols for diamond dressing of the grinding wheels at frequent intervals to prevent dulling and reduce mechanical vibrations.
- Stabilize the rheology of the spray: Monitor the holding times in the silos to keep moisture fluctuations in the powder below ±0.2%, ensuring optimal flowability and eliminating the risk of agglomeration caused by trapped air.