Mathematical modelling of contact lens moulding

Compression moulded contact lenses are produced by placing fluid between two moulds and squeezing the fluid outwards to form the shape of the lens. A common problem seen in this process is that at times the fluid moves outwards asymmetrically, resulting in partially formed lenses. In this article, the system is modelled using the thin film equations and the results are analysed to find the optimal operating setup to reduce asymmetrical flow. A simple model with one curved surface and one flat surface is considered first. This assumption is verified by a more realistic model that investigates the effects of curvature on the dynamics of the fluid. The simple model is modified to include the effect of surface tension. The results of this model show that surface tension plays no role in the fluid dynamics for this particular fluid. A second modified model allows for lateral movement of the lower mould. The model shows that allowing the lower mould to slide hinders the symmetrical flow of the fluid.