Challenge

An ultrasonic packaging sealer used a reference material as part of its calibration procedure. Significant inconsistencies were occurring between calibration results, affecting confidence in the repeatability of the process. The source of the variation was unknown.

Engineering Approach

A systematic investigation was undertaken to identify the parameters influencing calibration repeatability. Potential mechanical, process and environmental variables were considered and the calibration results compared with changes in ambient conditions. The investigation identified a strong relationship between calibration behaviour and atmospheric humidity. Further analysis established that the polyimide reference material was hygroscopic, absorbing moisture from the surrounding air and consequently changing material mechanical properties relevant to the ultrasonic calibration process. Psychrometric analysis was used to characterise the environmental conditions and establish the relationship between atmospheric moisture and calibration performance.

Outcome

Humidity was identified as the principal cause of calibration variability. Appropriate controls for the conditioning, storage and use of the polyimide calibration material were established, eliminating the previously unexplained environmental influence on the calibration process.