Meaning
Change in direction of an energy wave as it passes through a boundary between two substances with different propagation speeds alters the calculated position of internal features. When an ultrasonic pulse moves from a metal mould into a polymer melt, beam refraction causes the wave to bend at the interface. This shift must be accounted for when determining the exact depth of a void or the thickness of a wall.
Errors in this calculation lead to incorrect part dimensions or false rejections of good stock.
Propagation Deviation
Deviation from the original path follows the relationship between the sound velocities in the two media. If the velocity in the polymer is much lower than in the steel, beam refraction becomes more pronounced at steep angles. High angles of incidence increase the error in depth measurement.
Geometry Correction
Correction of the measured distance requires a mathematical model of the interface angles and the known velocities of the materials. In thick walled injection moulded parts, beam refraction can make a centered defect appear closer to one surface. Precision in tooling design often includes compensations for these wave paths to ensure that sensors are positioned correctly.
Modern monitoring software applies these corrections in real time to provide an accurate map of the internal part geometry.
Thickness Accuracy
Accuracy in final part inspection relies on the wave returning to the transducer after hitting the far wall. If the wave bends too far, it might not return to the sensor at all. This boundary condition limits the effective thickness that a single probe can measure.
