Precision Glass Moulding - Glass Material

Glass Material

Many glasses can be used with PGM. However, there are some limitations:

• The glass transition temperature Tg must not exceed the maximum heating temperature of the mould
• Many lead oxide flint glasses are not compliant with the RoHS EU directive (Restriction of certain Hazardous Substances)
• The glass composition influences moulding tool life
• Chalcogenide materials require certain preform shapes
• The coefficient of thermal expansion (CTE) of mould and glass should match. High CTE difference means high deviation between the moulded glass and the mould. High CTE glasses are also critical in terms of non-uniform temperature distribution in the glass. This means that especially fast cooling can not be applied. In addition to this, the temperature difference between the warm lens directly after moulding and the surrounding air can easily cause cracks.
• The internal and external quality of the blank must be the same or better than the requirements of the finished lens since the precision glass moulding process is not able to improve the glass quality.
• The glass exhibits a change in refractive index, called index drop, during the annealing process. This drop is caused by fast cooling of the mould insert, inducing a small amount of residual stress in the glass. As a result, the glass exhibits a small index change when compared to its fine anneal state. The index drop is small (usually .002-.006), but the optical design needs to be optimized to compensate for this change. As an example, the index drop for different glass types is displayed in the picture on the right for different annealing rates. Note that the annealing rate is not necessarily constant during the cooling process. Typical “average” annealing rates for precision molding are between 1000 K/h and 10,000 K/h (or higher). Not only the refractive index, but also the Abbe-number of the glass is changed due to fast annealing. The shown points in the picture on the right indicate an annealing rate of 3500k/h.

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. Mould materials such as steel can be used for moulding low-Tg-glasses whereas high-Tg–glasses require a high-temperature mould material, such as tungsten carbide.