Pouch Seal Inspection With Ultrasound Technology

A pouch seal is a structural element that joins flat film material, and a component designed to close
a pouch airtight. Traditional test methods e.g. peel strength or burst, can measure the joint strength
of the seal, using destructive methods. However, this type of testing is often unreliable. A strong seal
is not necessarily an airtight one, and the flow of gas or liquid through the seal (leak) is just one type
of seal defect. As such, there is a fundamental challenge to any leak testing method, whether it is a
fish tank-bubble, vacuum deferential, or helium test: none of these methods detect non-leaking seal
defects.

No physical theory conclusively links the transparency or reflectivity of pouch material to seal
quality. Yet, visual inspection methods e.g. human vision (ambient light) or machine vision (IR, UV,
laser), are still widely used. The assumption is that defects are visible (at least if transparent material
is used to make a pouch).

In order to use ultrasonic inspection effectively, it must be possible to place the seal in a direct line
between a pair of transducers and move it (or move the transducers) along the seal. The ultrasound
is transmitted and reflected at the transition from one media to the next. The greater the acoustic
difference between mediums (most evident at a gas to solid transition) the more sound is reflected
and the less sound transmitted through. The variation of the material thickness has only a minor
effect.