Pharma glass defects – Part 30. Glass Stringer

Hello everyone – welcome to Part 30 of an ongoing series devoted to visual defects in pharmaceutical glass vials.  Today’s defect is the “Glass Stringer”, also known as a “Heel Sticker”.  I briefly mentioned the Glass Stringer in my previous post on the “Lehr sticker” as belonging to a related family – i.e., defects involving extraneous bits of glass adhered to regions/surfaces of the vial.  As the name implies, a Glass Stringer often consists of a filamentous strand of glass extending out from the heel of the vial, although glass beads may also be observed.  Refer to Figure 1 for an illustration of a hypothetical Glass Stringer defect.

The tubular conversion process involves spinning a length of glass tubing that is sequentially heated and shaped with tooling to create individual vials.  The last stage of this process involves the greatest amount of heating that is used to separate the vial from the remaining glass tube and form the bottom of the vial.  The relatively low viscosity of the glass at this point increases the possibility of a stray mass spinning out from the heated region to form a Glass Stringer defect.  I might draw an analogy to making cotton candy (a.k.a., candy floss or fairy floss), although this is a little misleading – that process involves the spinning of molten sugar through tiny orifices to intentionally produce filaments (as opposed to creating filaments from a free surface).

PDA TR 43 lists the Glass Stringer as being a Major B defect because of it’s potential to impact vial machinability on fill-finish lines.  Interestingly, the defect lexicon doesn’t offer any guidance in the “Method” column to help assess the defect and doesn’t include the possibility of classifying it as an Acceptable Imperfection.  I might normally suggest a visual limit sample in such cases, but I could see the difficulty of creating a limit sample in practice given the potential variability in the appearance of a Glass Stringer.  It suggests that identifying a Glass Stringer should be straightforward, but I’d be curious to hear from anyone with more experience in classifying this particular defect. Is it always obvious?

Questions or comments?  Please leave them below or feel free to directly contact me.

About the Author

Matthew Hall is Technical Affairs Director for Corning Pharmaceutical Technologies , a manufacturer of primary glass packaging for parenteral drug products.  Based in upstate New York, Dr. Hall serves as a technical expert supporting business operations, sales, and marketing and educating customers on pharmaceutical glass packaging.  He is a member of the Parenteral Drug Association and the International Society of Pharmaceutical Engineering.