In glass tempering production, “white haze” is a common surface quality defect. It presents itself as microscopic scratches or cloudy marks on the tempered glass surface, appearing in various forms: longitudinal white stripes in the centre, transverse haze marks at both ends, hazy areas near the corners, or equally spaced, repetitive streaks.
Fundamentally, white haze is caused by excessive mechanical friction between the glass, while in its high-temperature, softened state, and the ceramic rollers of the tempering furnace. The specific root causes involve multiple variables, including equipment precision, heating temperatures, and process operations.
Drawing upon extensive production experience, LandGlass has systematically analysed the three core causes of white haze along with their corresponding troubleshooting methods for industry reference. As the second article in this series, this piece focuses on heating temperature control.
Improper heating temperature control is a primary processing factor that induces white haze, which typically involves the following three scenarios.
1. Central white haze caused by excessive lower heating
As the glass travels inside the furnace, if the lower surface temperature is significantly higher than the upper surface, the bottom of the glass expands more. This causes the glass edges to warp upward. During the oscillating movement, the centre of the glass bears its entire weight, leading to severe pressure concentration and friction. This generates the most common longitudinal central white haze. The corrective action is to properly lower the set temperature of the lower heating zone or increase the upper temperature to balance the thermal distribution between the top and bottom surfaces.
2. End white haze caused by excessive upper heating
Conversely, this type of white haze usually occurs on large-format, nearly square glass sheets (typically exceeding 1.5 square metres. When the upper heating temperature is too high, the top surface of the glass expands more significantly than the bottom, forcing the glass into a convex warp. As a result, the centre of the glass lifts off the rollers, leaving only the leading and trailing ends (approximately 100 to 200 millimetres) in direct contact with the ceramic rollers. During oscillation, these two ends bear the entire weight of the glass sheet. This severe load concentration creates intense friction, resulting in white haze at both ends.
Operational Note for Heating Adjustments: For both scenarios caused by overall temperature imbalances, temperature adjustments should be made incrementally—by roughly 5 degrees Celsius at a time. Observe the results over two to three batches before making further corrections to prevent over-adjustment.
3. White haze caused by localised overheating
When a heating element fails, a solid-state relay (SSR) shorts out, or a thermocouple malfunctions, the temperature in a specific zone of the furnace will spike abnormally. This leads to non-uniform warping of the glass or localised overheating of the rollers, which triggers white haze. Unlike the first two scenarios caused by overall temperature differentials, localised overheating is an equipment fault. If the white haze appears asymmetrical or at inconsistent locations, and conventional temperature adjustments yield no results, priority should be given to troubleshooting the heating elements and temperature sensors in the corresponding zones, followed by a scheduled shutdown for maintenance or replacement if necessary.