Effect of Al2O3 coated glass preform on glass-mold sticking behavior

Molding of low Tg glasses with high refractive index (nd) is very difficult due to the existence of a high concentration of active elements in the glasses. This research tried depositing a very thin layer of aluminum oxide (~ 20 nm) on various low Tg glass-preforms (with nd > 1.80) using a water based sol–gel process to reduce sticking between the glass and the mold material. High temperature glass wetting and molding tests were carried out to investigate the high temperature interfacial reaction between the coated glass gobs and the mold materials.When the uncoated glass-preforms were brought into contact with WC/Co (3%), the contact angle decreased with increasing heating duration at the softening point (Sp). Owing to the severe interfacial chemical reaction, all transparent glasses gradually turned translucent or opaque and severe sticking with the mold materials was observed. In the case of Al2O3 coated glass-preforms, the variation of the contact angles was very limited, and most of them showed no sticking and no wetting behavior. It was found that the acceptable molding temperature range is very narrow for molding uncoated low Tg glasses even with a Pt–Ir film on the mold surface. The molding conditions become more flexible when using glass preforms with an Al2O3 film on the surface, and the surface qualities of molded glass lenses were improved at the same time. The optical transmission of those lenses molded from the coated glass-preforms exhibited very little or no change after the molding process at a moderate molding temperature.

► The Al2O3 coated glass preforms demonstrated an excellent anti-sticking behavior.
► There is no obvious interaction between the Al2O3 coated glass and WC/Co substrate.
► An Al2O3 film on glass preform can extend the service life of the optical molds.
► Glass preform with an Al2O3 film allowed a wider range of molding conditions.