CMM measurement of injection-moulded plastic lenses and steel moulds: strategies for the determination of the reference system

This work aims to measure the macro- and micro-geometry of plastic lenses and steel mould inserts with a Coordinate Measuring Machine (CMM). The investigation intended to study the geometrical influence on known optical properties of the lenses. Plastic lenses are produced by injection moulding, which involves two inserts mounted on either side of a mould. The near-perfect central symmetry of the inserts complicated their measurement, specifically their planar orientation. A key cut on the insert's side was inaccessible while measuring with a CMM and auxiliary datums were required on the mould. The lenses too exhibited near-perfect central symmetry—a concave and a convex spherical cap interfaces—only broken by two tiny lateral protrusions, which were used for planar orientation. A special holder was designed and 3D-printed at INRiM to measure both sides of the lenses in one run. A common system of coordinates (CSY) for inserts and lenses was crucial for the overall comparability of the measurements. A common CSY of the two mould halves was established through their mating parts in the closure; the CSYs of the inserts was centred to the insert and planarly oriented to the cuts forming the lens lateral protrusions; the lens CSY was established by means of pins contacting the lens isostatically and a CSY transformation established by simulating the pin positions while measuring the inserts.