However, of course, only defects which already exist at the time of the inspection can be detected by such methods.
Seidenader Maschinenbau GmbH in cooperation with ilis gmbh now present a method by which specific pre-stages of crack formation may be identified.
Mechanical stress can reduce the fracture strength and processability of glass containers. Even small residual stress can increase the likelihood of breakage or cracking significantly.
In a relaxed state, glasses are optically isotropic, i.e. the refractive index is equal in all spatial dimensions. The propagation speed of light depends, among other things on the density of matter. Mechanical stress causes deformations in the material structure, i.e. different particle densities in the various spatial dimensions. This ultimately results in different speeds of light in the medium - the refractive index is dependent on the direction, the medium is birefringent. This birefringence can be analyzed with a polarimeter, which determines the angle of rotation of the polarization direction of linearly polarized light as it passes through a sample.
So far, correct measurements of the rotation angles with manual polarimeters were time-consuming and experienced laboratory personnel was required. Improvements could be achieved with the development of automated imaging polarimeters, which allow for inspection of not just one point, but of the whole object, and also exclude operator-induced measurement errors by automation.
Despite short measuring times of a few seconds, previously available systems are not able to inspect pharmaceutical products at clock rates of 600/min. Only now with the consistent further development of the automated imaging polarimeter is it now possible to measure and evaluate mechanical stress in glass containers in real time.
With the integration of such a system into a fully automatic inspection machine, objective and reproducible measurements at production speeds of up to 600 products/min. can be recorded so that production induced stress is detected early. Containers with residual stress can be automatically removed from the production and discharged into the defect channel. Therefore formation of stress-related cracks in the downstream production process or after product delivery is excluded to the largest possible extent.