Stress in glass

Residual stress in pharmaceutical glass packaging increases the risk of delamination and breaking. To remove the stress and reduce the risk, the glass goes through an annealing process in a lehr oven. To continuously monitor if the stress is removed Vimec developed an in-line inspection system to detect residual stress in pharmaceutical packaging glass.

Stress in a vial visualized using polarized light

For Vials for example ISO 8362-1:2018(E) states: “The injection vials shall be annealed so that when the vials are viewed in a strain viewer, the maximum residual stress does not produce an optical retardation exceeding 40 nm per millimetre of glass thickness.” The unit of nm/mm refers to the retardation of light in nanometres per millimetre of glass thickness.

Vimec’s residual stress measurement detects this retardation as a darker or brighter light intensity with respect to a base level intensity. It is possible to convert the observed light intensity to an absolute value by the aid of a calibration procedure with a tension-free product.

Stress in a vial visualized using polarized light

Detection Limits

The detection limit of the offered measurement is given in nm retardation. The residual stress that can be detected is dependent on the thickness of the glass: thicker glass results in a better stress detection limit. The operator must enter the wall thickness of the product in the Vimec software interface.

  • Flint: Residual stress in flint glass can be reliably detected for retardations of 10 nm and above. The example images in the figures are made with flint vials with a wall thickness of 1.2 mm, which gives a stress detection limit of 10 nm / 1.2 mm = 8.3 nm/mm. This is well below the ISO limit of 40 nm/mm.
  • Amber. The detection limit for amber glass is different from flint glass due to its light absorbing nature. Like residual stress, light absorption can lead to dark or bright areas in the vial which are then incorrectly interpreted as retardation. Residual stress in amber glass can be detected for retardations of 15 nm and above also well below the ISO limit.

Measurement Methods

The following measurement methods are available:

  • Maximum stress level
  • Axial uniformity: the stress level is averaged in the rotational direction for every line
  • Average stress level

The System

The optical set-up consists of a camera, diffuse light source and multiple polarization filters. The system is placed after oven. It is available as an option for the Rhea system and can be integrated into several versions of existing Rhea systems. For more information about the implementation and compatibility please contact us via the contact form or your Vimec account manager.

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