Distance measurements on inaccessible wires in plastic parts with X-ray computed tomography

This work presents a process chain established to inspect plastic parts with metallic wires. In the region of interest, these wires are almost straight, aligned side by side and the distance from wire to wire has to be in conformance with a given tolerance for every location (ISO 14253-1). The paper covers the planning of the X-ray computed tomography (XCT) scans, the development of software algorithms that perform the metrological evaluations, the establishment of decision rules for “good” and “bad” parts and finally suitability tests for the complete measurement process. For the investigated specimen the traceability of measurements is not achievable by a standardised and already accepted procedure, since the XCT measurements are performed on inner and inaccessible structures without the possibility of reference measurements. We address this issue by scanning an additional calibrated reference standard together with the actual specimen and using numerical simulations for the assessment of the measurement uncertainty (ISO 15530-3). Achieved measurement results, expanded measurement uncertainties as well as limitations concerning the traceability of measurement are discussed in detail.