Optical glass is widely used in optical components such as lenses, prisms, and optical fibers, and its performance directly affects key indicators such as imaging quality and laser transmission efficiency. During the molding process (such as compression molding and hot pressing), glass materials undergo high-temperature softening, forming, and cooling. Due to temperature gradients and mechanical forces, residual stresses are generated internally. If these stresses are not eliminated, they can lead to problems such as birefringence, uneven refractive index, and even cracking. Therefore, thermal annealing has become a necessary process for post-treatment of pressed optical glass.

2. Mechanism of Hot Annealing

2.1 Eliminating internal stress

During the molding process, when the glass rapidly cools from a high temperature state, the cooling rates of the surface and interior are different, resulting in residual thermal stress (caused by temperature gradients) and mechanical stress (caused by mold constraints). These stresses can cause uneven distribution of density and refractive index in glass, affecting its optical properties.

Thermal annealing restores the uniformity of the material by heating the glass to near its glass transition temperature (Tg), causing the molecular structure to relax again, and uniformly releasing stress during slow cooling.

2.2 Stable optical performance

Residual stress can cause changes in the refractive index of glass, and even result in birefringence (i.e. when light passes through the glass, it splits into two polarized light beams). This is extremely detrimental to precision optical systems such as microscopes and laser optical systems. After annealing, the refractive index distribution of the glass becomes more uniform and the optical properties become more stable.

2.3 Improve mechanical strength

There are micro stress concentration points inside unannealed glass, which are prone to cracking during subsequent processing (cutting, polishing) or use. Annealing treatment can significantly improve the mechanical strength and thermal shock resistance of glass, extending its service life.

2.4 Optimizing microstructure

Some special optical glasses, such as fluorophosphate glass and high refractive index glass, may have component segregation or microscopic defects after molding. The annealing process helps to rearrange atoms/molecules, improving the uniformity and stability of the material.

3. Hot annealing process parameters

The effect of hot annealing depends on key parameters such as temperature, holding time, and cooling rate:

Annealing temperature: usually around the glass transition temperature (Tg) (such as Tg ± 20 ° C) to ensure sufficient relaxation of molecules without causing deformation.

Insulation time: Depending on the thickness and composition of the glass, it usually takes several hours to tens of hours to ensure sufficient stress release.

Cooling rate: It needs to be strictly controlled (such as 1-5 ° C/min). If it is too fast, new stress will be generated, and if it is too slow, production efficiency will be reduced.

4. Application of Thermal Annealing in Optical Manufacturing

Precision lens manufacturing: eliminates residual stress in molded lenses to ensure imaging quality.

Laser optical components: reduce birefringence and improve the transmission efficiency of laser beams.

Fiber preform: Improve the uniformity of glass and reduce optical signal loss.

5. Conclusion

Hot annealing is a key process for post-treatment of pressed optical glass materials, which can effectively eliminate internal stress, improve optical uniformity and mechanical strength. Reasonable annealing parameters can ensure that glass meets the requirements of high-precision optical components and are an indispensable part of optical manufacturing. In the future, as optical glass develops towards higher performance, the optimization of annealing processes (such as computer simulation temperature control) will further improve material quality.