Precision glass moulding
The tools used must withstand high temperatures and pressures, and need to be resistant to chemical interaction with the glass.
[3] The following section describes basic traits of preform choice: “Used specifically for lenses with positive power: biconvex, plano-convex, and meniscus where the convex side is stronger than the concave side, this only works for a relatively small volume of material.”[4] “As a lens changes to negative in power biconcave, plano-concave, and meniscus where the concave side is stronger, an alternative preform shape, plano-plano, is required for the molding process.
The Lenslet is traditionally ground and polished to a near net shape of the final lens, and then pressed.
The table below gives an overview of achievable manufacturing tolerances in precision glass moulding at different companies.
[7] Specifications for aspheres:[6] Due to the fast cooling after moulding, the part retains a small amount of residual stress.
A lower cooling rate could circumvent the index drop, but would be less cost-efficient[4] Many glasses can be used with PGM.
However, there are some limitations:[4] So-called "low-Tg-glasses" with a maximum transition temperature of less than 550 °C have been developed in order to enable new manufacturing routes for the moulds.
Since they cannot withstand the high temperatures required for regular optical glasses, heat-resistant materials such as carbide alloys have to be used instead in this case.
In this process, the metallic binder improves the toughness of the mould as well as the sintering quality in the liquid phase to fully dense material.
Once process and tool have been developed, precision glass moulding has a great advantage over conventional production techniques.
In the case of aspherical lenses the measurement of optical surfaces is very difficult and connected to high efforts.
Additionally, when working with tactile measurement systems there is always a risk that the optical surface might be scratched.
“The materials that have been selected for the antistick coatings can be divided into 5 groups including: (1) single layer carbides, nitrides, oxides and borides such as TiN, BN, TiAlN, NiAlN, TiBC, TiBCN, NiCrSiB and Al2O3, (2) nitrides or carbides based gradient and multilayers, (3) nitrides based superlattice films, (4) amorphous carbon or diamond-like carbon and (5) precious metal based alloys”[21] Experiments carried out by Ma et al. yield the following results:[21] “The higher the temperature, the smaller the wetting angle between glass gob and substrate could be observed.
This indicates that severe interface chemical reaction occurred and resulted in the loss of transparency in glass appearance.
