Glass Defect

Optical deformation (pot spot)

Optical deformation, also known as “even spot”, is a small four resistance on the surface of glass. Its shape is smooth and round, with a diameter of 0.06 ~ 0.1mm and a depth of 0.05mm. This kind of spot defect damages the optical quality of glass and makes the observed object image dark, so it is also called “light cross change point”.

The optical deformation defects are mainly caused by the condensation of SnO2 and sulfides. Stannous oxide can be dissolved in liquid and has great volatility, while stannous sulfide is more volatile. Their vapor condenses and gradually accumulates at lower temperature. When it accumulates to a certain extent, under the impact or vibration of air flow, the condensed stannous oxide or stannous sulfide will fall on the surface of glass which is not completely hardened and form spot defects. In addition, these tin compounds may also be reduced to metallic tin by the reducing components in the shielding gas, and the metallic tin droplets will also form spot defects in the glass. When tin compounds form spots on the surface of glass at high temperature, small craters will be formed on the surface of glass due to the volatilization of these compounds.

The main ways to reduce optical deformation defects are to reduce oxygen pollution and sulfur pollution. Oxygen pollution mainly comes from trace oxygen and water vapor in protective gas and oxygen leaking and diffusing into tin gap. Tin oxide can be dissolved in liquid tin and volatilized into protective gas. The oxide in the protective gas is cold and accumulated on the surface of tin bath cover and falls on the glass surface. The glass itself is also a source of oxygen pollution, that is, the dissolved oxygen in the glass liquid will escape in the tin bath, which will also oxidize the metal tin, and the water vapor on the glass surface will enter the tin bath space, which also increases the proportion of oxygen in the gas.

Sulfur pollution is the only one brought into tin bath by molten glass when nitrogen and hydrogen are used. On the upper surface of the glass, hydrogen sulfide is released into the gas in the form of hydrogen sulfide, which reacts with tin to form stannous sulfide; On the lower surface of the glass, sulfur enters into the liquid tin to form stannous sulfide, which dissolves in the liquid tin and volatilizes into the protective gas. It can also condense and accumulate on the lower surface of the tin bath cover and fall on the glass surface to form spots.

Therefore, in order to prevent the occurrence of existing defects, it is necessary to use high-pressure shielding gas to purge the condensate of oxidation and sulfide sub couple on the surface of tin bath to reduce the optical deformation.

Scratch (abrasion)

The scratch on the surface of a fixed position of the original plate, which appears continuously or intermittently, is one of the appearance defects of the original plate and affects the perspective performance of the original plate. It is called scratch or scratch. It is a defect formed on the glass surface by annealing roller or sharp object. If the scratch appears on the upper surface of the glass, it may be due to a heating wire or thermocouple falling on the glass ribbon in the back half of the tin bath or in the upper part of the annealing furnace; Or there is a hard building like broken glass between the rear end plate and the glass. If the scratch appears on the lower surface, it may be broken glass or other prisms stuck between the glass plate and the tin bath end, or the glass belt rubs on the tin ellipsoid outlet end due to low outlet temperature or low tin liquid level, or there is broken glass under the glass belt in the first half of annealing, etc. the main preventive measures for this kind of defect are to frequently clean the drive lift to keep the roller surface smooth; What’s more, we should often clean the glass slag and other debris on the surface of the glass to reduce scratches.

The sub scratch is the scratch on the glass surface caused by friction when the transmission is in contact with the glass. This kind of defect is mainly caused by the contamination or defects on the surface of the roller, and the distance between them is just the circumference of the roller. Under the microscope, each scratch is composed of dozens to hundreds of micro cracks, and the crack surface of the pit is shell shaped. In severe cases, cracks may appear, even cause the original plate to break. The reason is that individual roller stop or speed is not synchronous, roller deformation, roller surface abrasion or pollution. The solution is to timely repair the roller table and remove the impurities in the groove.

Axial pattern is also one of the surface scratch defects of glass, which shows that the surface of the original plate presents spots of indentation, which destroys the smooth surface and light transmittance of glass. The main reason for the axle pattern is that the original plate is not completely hardened, and the asbestos roller is in contact. When this kind of defect is serious, it will also cause cracks and cause the original plate to burst. The way to eliminate the axle pattern is to strengthen the cooling of the original plate and reduce the forming temperature.