Plastic strapping is widely used in the packaging industry to bundle items together, and secure them to pallets for transport. They are most commonly made of polypropylene or polyester. Polyester is the stronger of the two and is a very attractive alternative to steel strapping because it does not corrode (rust) and stain products, is much lighter, safer and easier to dispose of, while still having a tensile strength rivaling that of steel. Polyester strapping is also resistant to ultraviolet degradation and able to withstand prolonged exposure to the elements during storage and transport.
When approached by a producer of strapping materials for a solution to their tensile testing needs, we were faced by a number of challenges. The gripping solution had to be able to provide a consistent gripping force, hence a manual (hand-tightened) approach was ruled out.
The expected break forces are typically greater than 12 kN for 25 mm wide polyester straps. Since these forces are higher than most standard pneumatic (air powered) side acting grips can withstand, hydraulic (oil powered) grips have traditionally been used in the industry. However, our client preferred a pneumatic solution due to the high cost of hydraulic side acting grips.
Preliminary testing had revealed that regular serrated jaw faces would not work because they caused the polyester strap to split longitudinally and fail prematurely. We ran the test successfully on our 5900 series load frame (3300 series can be used as well) with a custom 30 kN pneumatic side acting grip and jaw faces specifically designed for plastic strapping. The results obtained were consistent as shown in the graph, and good breaks were observed in 9 out of 10 of the specimens as shown in the picture. There was a longitudinal split on specimen 3 but this did not affect its test results. Note the curves on the graph are offset for clarity.