Length determination on industrial polymer parts from measurement performed under transient temperature conditions

A way to reduce the cost of metrology in manufacturing is to perform dimensional verification directly in the production environment, avoiding a long and expensive acclimatization phase. In this work the effect of a transient temperature state, typical of the production environment, was investigated on commercial polymer parts. Two points length measurements were performed before the stabilization of the temperature and length at standard conditions was estimated. The experiments consisted of synchronized measurements of length and temperature of the part over several minutes during the cooling phase, from 27 °C to 20 °C approximately. The length variation was measured by means of an inductive probe and the temperature with an RTD surface sensor. The frame of the system was composed by elements in Zerodur and Invar to minimize the thermal deformations of the structure. Uniform temperature in the part was assumed. The reference length at 20 °C (L20) was calculated with an a posteriori regression of the data from the complete cooling curve. A prediction of L₂₀ was then performed exploiting partial segments of the curve. Several segments with different time spans and starting points were evaluated. The procedure was repeated 5 times on 5 nominally identical parts. An expanded uncertainty (k=2) below 2μm was evaluated for the predicted length at 20 °C