Many leak detectors rely solely on vacuum decay. If a leak is present when the vacuum is pulled, air leaves the flawed blister
cavity and creates a concave surface in the lidstock. If the card is leak-free, the lidstock over each cavity domes slightly
from the internal air pressure. There are various ways to measure these surface changes, including contact methods such as
proximity sensors and load cells and noncontact imaging systems. Each has advantages and disadvantages.
The ideal blister-pack leak tester
Proximity sensors require complex tools to position them above each blister cavity. Hence, each blister format requires its
own array. This requirement is an expensive proposition in tooling, operation, and validation if multiple blister formats
must be inspected.
A load cell measures how much the lidstock pushes against it. Load cells can be difficult to adjust and validate because each
cavity requires a corresponding cell. The technology also may be impractical for blister formats with cavities that are arranged
extremely close together.
Some imaging systems use a laser as a light source. These systems can be affected by variations in reflectivity caused by
printing on the lidstock. The systems also require considerable precision because the center point of each blister must be
identified to reveal changes in position.
Another camera-based system combines vacuum technology with indirect imaging. Key to this patented and patent-pending equipment
is a flexible bladder positioned over the blister as it rests in the test fixture. The flexible membrane serves several purposes.
During the test, it supports the seal and prevents deformation and peeling. It also increases the sensitivity of the pressure
measurements. Finally, covering the lidstock not only eliminates problems the imaging system might have with printed or glossy
surfaces, but also provides a clear indication of the flaw locations and identifies leaks as tiny as 3.5 μm.
After a test cycle of less than 10 s, results are presented in numerical, statistical, and image formats. The touch-screen
operator interface and personal-computer–based controller also network with a printer and database to print and archive test
results. Changeover is almost instantaneous and requires lifting out one fixture and setting a new one in its place. Once
in position, the machine reads a notched identification code on the fixture and sets test parameters for the new blister card
("VeriPac 225/BLV," vacuum leak-testing system, Packaging Technologies and Inspection, LLC, Tuckahoe, NY,
To increase throughput, test fixtures can be designed to hold four blisters. In addition, the semiautomatic machine could
be equipped with a shuttle system that would allow the operator to remove a tested card and replace it with an untested card
while a test cycle is running. With suitable integration and material-handling hardware, an on-line model could perform 100%
inspections. Moving leak testing on-line would require integration with the blister-packaging machine and related material
handling. Most likely, this would take the form of a pick-and-place device that would move the blister cards to the leak tester
and then back onto the line. In addition, the tester itself would probably require multiple stations to keep up with line
speeds. This means on-line testing could cost significantly more than off-line testing, perhaps as much as 10 times more.
Experts say it also would be harder to validate.