Glass annealing Notes

 GlassThe thermal conductivity is relatively small, the performance of both thermal expansion and contraction when subjected to temperature changes, there is inevitably the glass temperature gradient in different parts of the resulting glass has a different molecular size, and thus the thermal stress occurs in the glass. The existence of thermal stress, damage of optical properties, mechanical properties, such as glass, serious, glass production line to burst occurred. Therefore, in order to improve the quality of glass products, the production process must reduce this stress, resulting in the production of bulk glass products, is an important step of - annealing.

Precautions glass annealing system developed

Formulated glass annealing system needs attention to the following issues.

① Lehr uneven temperature distribution in the affected section of the annealing furnace annealing temperature distribution is often uneven, causing the temperature of the glass products are not uniform. For this reason, the design of the annealing curve, slow cooling rate required to determine the stress values ​​selected lower than the allowable stress values, generally take half of the allowable stress.

② thickness of the article and the thickness of the shape of the article, the greater the temperature gradient in the glass outside the heating and cooling process, the annealing temperature range in the glass, the glass thickness, the higher the holding temperature, the faster the cooling rate of stress relaxation, the article permanently greater stress; complex shape glassware stress tends to concentrate. Therefore, the thick-walled products, complex shape products should be lower holding temperature, appropriate to extend the holding time, heating and cooling rate should be slower.

③ different products in the same annealing furnace annealing in the same chemical composition, different thicknesses products annealed glass annealing temperature is determined in accordance with minimum wall thickness of products in the same annealing kiln to avoid deformation of thin-walled products; heating, cooling rate by the maximum wall thickness determined to ensure that products are not thick too large due to thermal stress cracking. The chemical composition of different glass products in the same annealing furnace annealing holding temperature should be determined at a lower annealing temperature of glass products prevail, when asked insulation appropriate extension.

When the effects of stress inherent ④ rapid heating products, in addition to the temperature difference between temporary stress calculation, we must also consider the inherent stress of the situation.

⑤ relative impact points meet changing the nature of the partial products, glass products, such as Pyrex glass, the annealing temperature is too high due to the glass, resulting in a long time after the Q-phase separation of the glass, the water resistance of glass is reduced, the annealing temperature of the glass Pyrex no more than 570. C (the general control 560.C), the residence time at this temperature is not more than 30min.