5 Root Causes of Spiral & S-Cracks in Clay Bricks
Clay brick production faces persistent quality defects: spiral cracks and S-cracks on brick surfaces. These common brick defects greatly reduce mechanical strength, damage drying and sintering uniformity, and decrease the commercial value of finished clay bricks. For global brick manufacturers, identifying root causes and practical elimination methods is critical for daily quality control, production yield improvement, and manufacturing cost reduction in the heavy clay industry.
1. Uneven Extrusion Speed in Vacuum Extruders
The primary cause of spiral lines and S-shaped cracks is differential clay flow velocity inside the vacuum extruder. When raw clay is pushed forward by spiral blades, the central clay moves faster than the peripheral materials. This uneven extrusion creates layered shear planes, trapping tiny air bubbles and residual water inside the clay strip. After drying and sintering procedures, invisible shear layers expand and form obvious spiral cracks and S-cracks on brick bodies.
2. Excessively Fine Clay Particles & High Plasticity
High-plasticity raw clay with ultra-fine particles easily causes layered separation during extrusion. Fine clay particles below 0.05mm increase clay viscosity and internal sliding force. Without coarse aggregate addition, pure clay materials cannot form stable internal friction. In contrast, shale bricks and coal gangue bricks with coarse particles effectively avoid spiral layering defects.
3. Improper Raw Material Moisture Content
Excessive moisture increases drying shrinkage stress of green bricks. Extra pore water accumulates on shear layers, expanding during the dehydration process. If the green brick moisture exceeds the standard range, rapid water loss will generate tensile stress, triggering S-type cracks and expanding tiny spiral lines into visible brick defects.
4. Worn Extruder Components & Unreasonable Clearance
Long-term operation causes spiral blade abrasion, cylinder liner wear, and enlarged extruder head gaps. When the blade clearance exceeds 3mm, clay flow becomes disordered and unstable. Misaligned blades generate uneven shear force, resulting in local indentation and irregular S-cracks on brick surfaces.
5. Unbalanced Drying & Sintering Procedures
Unreasonable heating curves are key thermal-induced crack reasons. Rapid heating speed over 30°C/h hardens the green brick surface prematurely, while internal moisture cannot escape smoothly. Sealed steam pressure breaks the brick structure, aggravating spiral and S-crack propagation in the kiln.
Spiral cracks and S-cracks in clay bricks mainly derive from five factors: uneven extrusion, poor raw material gradation, abnormal moisture, worn mechanical parts, and unoptimized thermal processing. By optimizing clay formula, adjusting extruder parameters, controlling moisture content, and standardizing kiln operation, brick factories completely eliminate these typical defects to produce high-quality solid clay bricks and hollow bricks.
