The occurrence of various defects significantly impacts product yield and manufacturing costs in sanitaryware production. These defects typically originate from three primary sources: raw material quality, process parameter control, and production handling. Below is a systematic classification and cause analysis of twenty-five common defects.
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Structural Defects in Body and Glaze
Body Cracking: Microscopic or macroscopic cracks in the body, caused by insufficient green strength or uneven drying leading to internal stresses, manifesting before or during firing.
Glaze Crazing: A network of fine cracks on the glaze surface resulting from a mismatch in the thermal expansion coefficients between the glaze and the body, releasing stress during cooling.
Dunting: Cracks specifically formed within the body during the quartz inversion stage (around 573°C) due to overly rapid cooling, often filled subsequently by the molten glaze.
Glaze Peeling: Partial or extensive detachment of the glaze layer from the body surface due to poor adhesion.
Glaze Pinholes: Micron-scale pores on the glaze surface formed by gases trapped and unable to escape during glaze melting.
Glaze Blisters: Sharp-edged bursts or raised bubbles on the surface caused by gases expanding within the glaze layer at high temperatures.
Glaze Pitting: Localized depressions in the fired glaze caused by its rejection of contaminants or impurities on the body surface.
Body Bloating: Localized swelling of the body occurs when internally generated gases cannot escape before the glaze surface seals during firing.
Ware Warpage: Bending or distortion of the product resulting from uneven drying, excessive firing temperature, or inadequate structural design for load-bearing.
Surface Appearance and Decorative Defects
Orange Peel Glaze: An uneven glaze texture resembling orange peel, caused by inappropriate glaze slurry viscosity or improper spray gun pressure or distance.
Glaze Crawling: Retraction of the glaze during firing, exposing the underlying body, due to oily or dusty body surfaces or issues with the glaze formulation itself.
Glaze Color Variation: Inconsistent coloration or deviation from the standard shade caused by fluctuations in kiln atmosphere (oxidizing/reducing), batch-to-batch glaze differences, or uneven glaze thickness.
Low Gloss / Underfiring: A matte glaze surface failing to achieve the intended gloss, resulting from insufficient firing temperature or incomplete glaze maturation.
Sulphuring: A surface scumming on the glaze surface formed by sulfur compounds from fuel or materials under insufficient oxidizing conditions during firing.
Iron Specks: Prominent brown or black spots on the glaze surface caused by iron contaminants introduced from raw materials or equipment.
Defective Logo: Issues such as blurring, misplacement, or poor adhesion arising during the transfer, screen printing, or inkjet marking process.
Poor Finishing: Burrs or unevenness on edges or joints after firing, stemming from inadequate trimming or smoothing at the green stage.
Process Handling and Performance Defects
Light Glaze / Insufficient Coverage: Inadequate glaze application or incorrect specific gravity of the glaze slurry resulting in incomplete coverage, allowing the body color to show through.
Heavy Glaze / Excessive Coverage: An overly thick glaze layer leading to glaze running, or even cracking after firing.
Knocked Glaze: Physical chipping of the glaze layer during handling between glazing and firing stages.
Stuck Ware: Sticking or between a product and kiln furniture during firing due to insufficient spacing.
Chipping: Small fragments broken from the edges of the ware after removal from the kiln or before packaging, due to external impact.
Leakage: Failure in water retention tests caused by hidden body micro-cracks or defects from high-pressure casting.
Poor Repair Marks: Visible differences in color or texture between touched-up areas (on green or bisque ware) and the surrounding surface after firing.
Blown Out: Blowout on the surface caused by the combustion and escape of combustible impurities embedded within the body during firing.
The systematic identification and understanding of the root causes behind these defects are fundamental to establishing an effective quality control system and continuously improving the first-pass yield.