PETpla.net Insider 01+02 / 2016

INSPECTION 18 PET planet insider Vol. 17 No. 01+02/16 www.petpla.net Part 2: Metrology principles for the PET packaging industry Base and wall thickness measurement by Chris Freshman MEng.(Hons) CEng MIMechE, Torus Measurement Systems, Telford, UK This article explores the PET industry’s changing requirements from process monitoring to ‘true’ measurement systems. The differences between accuracy, resolution, precision, repeatability, reproducibility and traceability are detailed and each clearly defined. It then demonstrates how discernible metrology is coupled with technological improvements, such as those described in part 1 (see PETplanet insider 12/2015), to produce the highest level of measurement systems. Quality control Process monitoring evaluates part to part variation, focusing on the dif- ference between one result and the next as opposed to the determination of a ‘true’ value. The PET packaging industry, due to the initial requirement for stable production, originally imple- mented inspection systems based on process over metrology. This aided the creation of reliable manufactur- ing techniques with minimal variation, thus allowing performance optimisa- tion, material resource reduction and design improvements. However, qual- ity requirements are changing with the emphasis now on ‘true’ metrology. Recorded, traceable and accurate measurement results are essential to ensure parts are correct and con- sistent, regardless of the country of manufacture. There has also been an increase in the number and variety of measured features with a greater amount of the bottle and preform being inspected, calling for more sophisticated methods. An undesirable scenario If a batch of bottles were unchar- acteristically splitting in the base area, it would be important to determine if they had been blown and stretched correctly. This could be assessed by checking the material thickness in that area. For example, captured data may indicate parts checked during that pro- duction period tested within tolerance and were verified against a reference sample. However, how do we know instruments accuracy it must first be quantified by using one of two meth- ods. The first method, categorised as a ‘Type A’ evaluation, statistically analyses repeated measurements, often referencing values from equip- ment of a higher traceable accuracy. The second method, categorised as a ‘Type B’ evaluation, uses a calcu- lated uncertainty value by following the ISO Guide to the Expression of Uncertainty in Measurement. Gauges supplied by Torus Measurement Sys- tems, such as the B300 Wall Thick- ness, B303 Bottle Burst or B304 Top Load & Volume, are supplied with a proven accuracy, giving the end user confidence in their measurement results. This is determined through a Correlation study (Type A), where six parts are each measured five times and compared to the same number of measurements from a higher accu- racy instrument. Resolution is not accuracy Resolution is defined as the small- est discernible change in the instru- ment or value of interest; a system may have a better resolution than accuracy. For example, the camera configuration used on Torus’ Preform Inspection Gauge, Figure 1 has a resolution equating each virtual pixel to a physical 17μm, but has a proven system accuracy of 30μm. Accuracy, as previously defined, is affected by factors such as lens aberration, calibration uncertainty, part variation, algorithm programming, view angle, and focal distance. the average thickness result reflects the physical size and has not been affected by linearity errors? Addition- ally, how do we know the reference standard used for setup is correct? Furthermore, how do we know, if re-checked, the results would repeat within an acceptable range? The answers to these questions define the fundamental differences between process monitoring and traceable measurement. Discernible metrology A measurement value can never be exact or perfect. There is always a degree of error associated with any type of inspection. Measurement is its own discipline with internationally recognised definitions, methodology and standards. Only through strict adherence to these rules and guide- lines can quality of measurement be ascertained. Clearly defining terms such as accuracy, precision, resolu- tion, repeatability and reproducibility, and their associated methodologies, is important for a fair comparison between systems. Additionally, trac- ing measurements back to reference instruments and international stand- ards can ensure, for instance, that test results obtained in Brazil correlate to those obtained in Poland. System accuracy Accuracy is a qualitative term defined as the deviation between the measurement result and the ‘true’ value. In order to determine an