Thermoforming mechanism of precision glass moulding.

Precision glass moulding (PGM) enables the production of an aspherical lens and irregular optical products in a single step, but its product quality depends highly on the control of both material properties and process parameters. This paper investigates the thermoforming mechanism of a glass lens in PGM. To precisely describe the material behavior in PGM, a modulus-based constitutive model was framed and integrated with the finite element analysis. This model can be parameterized conveniently by an impulse excitation technique. Key processing parameters that influence the final profile and residual stresses of a lens were identified with the aid of dimensional analysis. The study found that the cooling stage above the glass transition temperature can bring about large geometry deviations of a lens. The residual stresses in a lens depend mainly on the temperature history in the supercooled liquid region caused by the variability and heterogeneity of thermal expansion. However, the stresses can be reduced effectively by decreasing the cooling rate from moulding temperature to glass transition temperature.

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