Pharma glass defects - Part 25. Belt marks

Hello everyone – welcome to Part 25 of an ongoing series devoted to defects in pharmaceutical glass vials.? Today’s post concerns “Belt Marks”, a term that evokes corporal punishment but in fact has nothing to do with striking vials (I refer you instead to Bruises, Bump Checks, etc. for defects of this type).? In this case, Belt Marks are referring to permanent marks accidentally impressed into glass containers by contact with the belt of an annealing lehr.? You may recall from an earlier post in this series that newly formed glass containers should be heated for some period of time to remove residual stress through a process called annealing (pronounced uh-NEEL-ing).? Failure to sufficiently remove this residual stress results in a container that is more prone to cracking and breakage.

Annealing schedules are developed by considering a number of factors, including viscosity, thermal expansion, and dimensions of the glass article.? The so-called “Annealing Point” is a standard reference point that can be defined on the basis of the viscosity-temperature curve for a glass – i.e., the temperature at which the viscosity is 1013 Poise (see Footnote 1).? As the name suggests, the Annealing Point provides guidance on selecting an appropriate temperature for relieving residual forming stress.? However, this may also be a source of confusion – is the Annealing Point the only temperature that is appropriate for annealing?? The simple answer is “no”.? Temperatures lower than the Annealing Point can result in the removal of stress, albeit at a slower rate that can impact the throughput of the container manufacturing process.? Annealing can also be performed at temperatures above the official Annealing Point, although another practical limit will be encountered.? Annealing glass at too high of a temperature will result in obvious physical deformation due to viscous flow, which leads us to the Belt Marks defect.?

Figure 1 shows an example of Belt Marks on the side of a glass vial.? Their appearance is defined by relatively smooth, regularly spaced indentations caused as glass begins to sag over the metallic links making up the conveyor belt that transports vials through the annealing lehr.? The sort of the Belt Marks illustrated in Figure 1 only occur if the vial is annealed on its side.? Annealing upright can also can lead to the appearance of Belt Marks on the vial footprint.

Belt Marks are considered a Minor aesthetic defect that can impact visual inspection of glass containers.? In my experience, Belt Marks are rare and should be relatively easy to fix.? Their appearance points to the annealing lehr being set too high or perhaps temperature spikes within the lehr.

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Footnotes

1.?????? This is just one potential definition for the Annealing Point.? Other definitions rely on measurement of the glass transition temperature using differential thermal analysis, differential scanning calorimetry, or thermomechanical analysis.? Another practical definition states that the Annealing Point is the temperature at which a majority of stress is relaxed within “several minutes”.? None of these definitions are necessarily the most correct – it can however be useful to understand the basis for a quoted Annealing Point value and then be consistent in its application.