Introduction
The production process of sanitaryware is intricate and demanding. From raw material preparation to high-temperature firing, any minor oversight in each step may result in product defects or even total scrap. These defects arise from complex causes and manifest in diverse forms: some become apparent immediately after forming, others remain latent until the firing stage, while others are directly triggered by improper firing process control. Major cosmetic defects in sanitaryware are categorized into over a dozen types, including deformation, cracks, brown spots, and stains. To enhance product yield rates, it is essential to understand defect origins at their source. This paper provides an in-depth analysis of the mechanisms behind several common defects occurring during the forming process and explores corresponding prevention and control methods, aiming to offer practical guidance for production (as production conditions vary across kilns, it is challenging to develop a comprehensive, universally applicable solution).
Various defects may occur during ceramic forming. Below are some common defects and their preventive measures:
Cracking Causes: Insufficient plasticity of raw materials, excessively fine slurry particle size, uneven moisture content in plaster molds, inconsistent thickness of greenware, interruption during slip casting. Prevention: Maintain slurry temperature between 25-35°C; ensure adequate weathering and aging periods for raw materials; adjust greenware formulation to achieve optimal plasticity clay content; inspect plaster molds for uniform moisture distribution prior to use; avoid interruptions during slip casting. |  | |
Bubbling High carbonate/sulfate content in body or glaze; incomplete removal of deliquescent materials; improper firing temperature or atmosphere control. Prevention: Optimize body/glaze formulations to reduce impurities; control firing ramp rate and maintain reducing atmosphere to ensure thorough oxidation and decomposition. |  | |
Pinholes Causes: High organic matter or iron oxide content in body material; rapid heating causing volatiles to escape. Prevention: Select raw materials to reduce organic content; adjust firing curve to avoid rapid heating during high-temperature stages; optimize glaze formulation to enhance flowability. | ||
Orange Glaze Causes: Excessive temperature rise during glaze melting, uneven glaze thickness, or poor high-temperature flow. Prevention: Control firing temperature and heating rate to ensure uniform glaze melting; optimize glaze grinding and application processes. |  | |
Glaze Cracking Causes:Mismatched expansion coefficients between body and glaze, excessive firing temperature, or improper cooling regimen. Prevention: Adjust body and glaze formulations to match expansion coefficients; optimize firing curves and control cooling rates. |  | |
Deformation Causes: Unsuitable body structure design, improper kiln loading methods, inadequate control of firing temperature or holding time. Prevention: Optimize product design to avoid excessive thickness variations; standardize kiln loading procedures to ensure flatness of saggers and spacers; strictly control firing temperature and holding time. |  | |
Yellowing Causes: Excessively rapid heating, insufficient reducing atmosphere, excessive TiO₂ content in raw materials. Prevention: Adjust firing curves to extend reduction phase duration; control TiO₂ content in raw materials, adding masking agents when necessary. |  | |
Smoke Stains Incomplete oxidation of greenware, excessive calcium in glaze, poor kiln ventilation. Prevention: Optimize firing atmosphere to ensure full oxidation of greenware; adjust glaze formulation to reduce calcium content; enhance kiln ventilation. |  |
Underfiring and Overfiring
Causes: Underfired products exhibit a yellowish appearance, high water absorption, poor glaze luster with rough texture, low strength, and a dull sound when tapped. Overfired products deform, with glaze bubbling or sagging.
Primary causes include excessive or insufficient firing temperatures, improper holding time control at high temperatures, unreasonable loading density, or large temperature variations during firing, leading to localized overfiring or underfiring.
Prevention: Conduct firing experiments (test firings) to determine the optimal firing temperature range (the safe zone between the start of sintering and the onset of overfiring) and holding time for different products and different body-glaze formulations. This forms the foundation of all control measures.
Lack of Luster
Cause: Also known as loss of brilliance. This defect arises from the formation of microcrystals in the glaze layer and inadequate glaze fusion, resulting in a dull surface. Implement rapid cooling during the initial cooling phase to prevent glaze crystallization and enhance surface gloss.
Prevention: Optimize glaze formulation and preparation. Precisely control firing schedules. Use temperature cones or thermal analyzers to determine the optimal glaze firing curve, including heating rate, maximum firing temperature, holding time, and cooling rate. Within the critical temperature range of the glaze's glass transition, appropriately control the cooling rate to avoid rapid cooling causing glaze crystallization.
Summary: Preventing ceramic forming defects requires a multi-faceted approach encompassing raw material selection, process parameter control, and equipment maintenance. Strict adherence to process specifications and timely adjustment of production parameters can effectively reduce defect occurrence.