Some commercial software packages that simulate injection processes are available. It is possible to design moulds and to optimise the processes by using computer-aided engineering (CAE) tools. In addition, we employ C-MOLD, one of the best commercial packages, to determine the limiting thickness in polycarbonate lenses manufactured by injection. In this work we use a simple geometry to study the variation of different magnitudes as a function of the lowest thickness in biconcave and biconvex lenses. The parameters studied are the variation of temperature, pressure and piece mass during the filling time, air traps, melt lines and weld lines. The flow problem of the plastic in the mould is very different for biconcave and biconvex lenses. While the limiting thickness is in the centre for the biconcave lens, the perimeter is the limiting zone for the flow in the biconvex lens. For both geometries, however, the minimum thickness is limited by a mechanical and an aesthetic factor. If we take into account the variation of temperature, pressure and mass with minimum thickness, the mechanical limiting thickness can be estimated as 500 mum. But, if we also consider the evolution of the air traps and the melt and weld lines, which greatly influence the aesthetic factor, the resulting limiting thickness is 650 mum in the centre of biconcave lenses and 500 mum in the peripheric area of biconvex lenses. (C) 2003 Elsevier Science B.V. All rights reserved.
