Sixpacks, in which six PET bottles are shrink-wrapped with plastic film, are – especially with larger bottles – sometimes provided with a carrying strap, which is usually stuck on. It sometimes happens that the carrying strap fails. Generally speaking, the defect is either due to the bond being poor or the strength of the shrink-wrap film being too low. In this particular case, the sequence of customer complaints indicated that only a few batches were affected and that these batches also had only a certain percentage of defective carrying straps.
In order to dispel these uncertainties and be able to describe the failure behaviour qualitatively and quantitatively, a tensile testing machine and a device adapted to the real load situation is used to determine the deformation pattern of an outer packaging for bottles when they are lifted and transported. With the help of a perforated sheet, a dot matrix is sprayed with a non-dissolving coating onto the film. These prepared film packs are then subjected to a realistic tensile load on a universal testing machine.
During the test, both the force and the deformation of the film were recorded with an optical measuring system. The optical measuring systems used here also offer significant advantages over other methods for determining deformation. Mechanical holders are difficult to mount on plastic films, and when strain gauges are applied, they falsify the measurement because of their inherent stiffness. Optical, contactless systems, on the other hand, ensure that interference is minimised. With the aid of the test setup, it is thus possible to directly relate the acting forces to the deformation of the film packaging. The possibility also exists of approximately determining a measure for the level of film elongation.
The investigations performed with optical measurements produced important findings on displacements and elongations in the film, as can occur under normal treatment. They also thus produce findings on the cause of failure of the outer packaging. On this basis, improvement measures can be developed and specifications can be drawn up for the plastic film and the adhesive bond.