Cracks, deformation or dimensional failures found after firing do not prove that the kiln caused them. Slip rheology, wall-thickness differences, demoulding loads, support methods and drying moisture gradients can leave microcracks or residual stress in the green body. Firing may only make these weaknesses visible. The 25% figure is a plant-diagnostic scenario, not an industry-average scrap rate.
The calculation boundary comes first. Is the denominator formed pieces, kiln input or fired inspection quantity? Are repaired, downgraded and refired products counted? How is one piece with multiple defects classified? Different definitions can produce different rates from the same production flow.
Stratify the number by product model, defect type, location, shift and detection stage. Then connect each group with its slip batch, mould, forming recipe, dryer position and kiln location. A total percentage without this structure cannot identify a cause.
Path | Process abnormality | Possible result |
Slip and forming | Rheology change, unequal drainage, wall-thickness or density variation | Unequal shrinkage, dimensional variation and deformation |
Demoulding and handling | Low strength, incorrect load points, impact or twisting | Hidden-crack growth and profile shift |
Finishing and support | Local moisture differences or mismatched supports | Joint cracks, warpage and local deformation |
Drying | Fast surface water loss, uneven airflow or poor loading | Drying cracks, twisting and firing breakage |
Research on industrial-scale sanitary ware drying explains that temperature and moisture gradients create internal stress and dimensional change. Separate sanitary ware slip research shows that preparation conditions affect rheology, casting rate, green density and strength, with effects that can continue into fired properties.
These findings do not let an inspector assign a cause from appearance alone. They establish a traceable sequence: confirm the material and formed body, inspect demoulding and support, evaluate drying conditions, and then examine firing.
The kiln deserves priority when defects repeatedly cluster on a kiln car, loading level or temperature zone and correspond with actual temperature, pressure, combustion, cooling or loading changes. Compare the same upstream batch across different kiln positions before changing the firing curve.
The kiln can create a defect directly or amplify one that already exists. Useful checks include the actual curve versus the recipe, measurement reliability, loading supports, product spacing, air leakage, burner or fan exceptions, and cooling-stage records. Without links among batch, position and actual operation, adjusting the kiln from crack appearance remains guesswork.
1. Define scrap, repair, downgrade and first-pass acceptance consistently. 2. Separate products and defect types instead of combining every loss. 3. Record defect position, direction and form, retaining representative images. 4. Trace the affected batch through slip, mould, demoulding, support, drying and kiln position. 5. Change one principal variable in a controlled trial and retain a comparison group. 6. Confirm the result over consecutive batches and check for defect transfer. |  |
The KPI framework in ISO 22400-1 can support common definitions and data relationships across operations. A useful scrap record does more than say rejected after firing. It connects the final decision with the process conditions capable of explaining it.
Post-kiln detection does not prove a kiln origin. A defect may begin with slip and forming, grow during demoulding or support, develop stress during nonuniform drying, and become visible during firing. When forming, drying and stress records share one traceability chain, the factory can replace repeated firing-curve adjustments with evidence-based root-cause improvement.