Geometrical metrology on vacuum cast silicone rubber form using computed tomography

Ramona Pacurar, Pavel Müller, Leonardo De Chiffre

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An investigation on geometrical measurements of silicone rubber cake form and polyamide molds using three measuring techniques - CMM, optical scanner and CT scanner - was carried out. The only measurand was diameter of a cone measured at specified levels. An uncertainty budget for all three techniques was assessed and expanded combined uncertainties calculated. The measurements on CMM were considered as reference measurements. When measuring silicone rubber form, this was supported by the molds due to its the high flexibility. Investigation on deformation effect of the silicone rubber cake form and the molds was carried out. It was found that the higher probing force was used, the smaller coefficient of variation was obtained. This means that higher reproducibility was achieved when higher probing force was applied. Two measuring strategies on CMM were realized: Single point probing (SPP) and probing in scanning mode (SMP). It was found that SMP results in smaller data variability. However, the difference compared to SPP was not big and since SPP measuring strategy is generally used for measuring a big variety of parts, this was used for further comparisons between CMM and the other two measuring techniques. It was found that when the silicon rubber form was measured on the supported bottom mold or the bottom mold was measured itself, the diameter measurements performed on optical scanner and CT scanner were bigger compared to CMM measurements. On the other hand, the diameter resulted in smaller values when the silicon rubber form was measured on the supported top mold or the top mold was measured itself. A procedure for measurement of highly deformable part, such as silicone rubber form, was developed. Uncertainties from measurement on the optical scanner were big. This was mainly connected with low reproducibility of scanning individual positions of the cone. Measuring uncertainties for CT measurements were calculated in a range which is reasonable when considering such deformable sample. Therefore, CT scanning is highly recommended measuring technique, resulting in short measuring times and measuring of complicated features. In connection with the silicone rubber form, CT scanning could be also used for example for failure analysis, 3D volume analysis and other non-destructive and metrological analysis.
Original languageEnglish
PublisherDTU Mechanical Engineering
Number of pages68
Publication statusPublished - 2011

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