The influence of humidity on accuracy length measurement on polymer parts

The work deals with an experimental study of the influence of humidity on accurate length measurements on ABS parts. Polymer parts absorb water from the ambient until they reach hygroscopic equilibrium. Water content causes an expansion of the polymer part. The relationship between the water content and this expansion has been barely studied, especially from a metrology point of view including its contribution to the measurement uncertainty. The experimental set-up includes a humidity chamber, an invar fixture with 8 inductive probes and a scale. The humidity chamber was used to create the experiment ambient conditions. The invar fixture with the inductive probes, with a resolution of 0.1 μm, allows length measurement on 8 ABS parts at the same time. The scale was used to collect the water content variation during the experiment with a resolution of 0.1 mg. The length and weight of the ABS parts were measured at 5 levels of relative humidity from 50 %RH to 90 %RH, and constant temperature, 20 ±0.2˚C. Water content equilibrium with the ambient was achieved at each level by acclimatization of the parts for 24 hours. An average length variation over the humidity range of 15 μm was found. A condition specific coefficient of moisture expansion of 69±4 μm/(Δ of weight in mg) was estimated.