RSF testing can be performed on a low force Instron system such as a 68SC1 with a 500 N or 1,000 N 2580 Series load cell. Instron also offers a specialized fixture (CP131410) designed to exert a compressive load around the edge of a vial until the lower lip of the aluminum cap breaks away and begins to move in a direction opposite to the loading. The top compressive fixture is spherically seated, which allows for self-pivoting in order to maintain near perfect alignment, even if the cap itself is misaligned. Automatic measurement of the residual seal force requires software capable of measuring the decline in load associated with the sealing process. The force where the aluminum cap breaks away from the top and moves in the direction opposite to loading is visible in the software as a plateau seen in the graph. The residual seal force can be calculated using either a manual cursor select or an automatic calculation in Bluehill Universal software.

• Reducing variability when testing residual seal force can be difficult to control. Since elastomeric vial seals relax in the short term and age in the long term, it is important to make sure that all specimens in a sample are capped at approximately the same time to minimize variability during testing. Alignment is also critical for repeatable results. Fixturing should help the operator maintain concentricity between the vial and the platen surface, as off-center loading can greatly increase the variability between tests.
• By placing a compressive load on the vial cap, a clear inflection point will appear in the curve at the point where the RSF point has been surpassed causing separation of the crimped cap from the head of the vial.
• To learn more about mechanical testing for drug delivery devices, click here.