Joining Methods
After the strap is tensioned, the strap is often sealed to itself. Steel strap is sealed with a seal and notch joint, a seal and crimp, seal-less joint, or via welding. Seals come in various shapes depending on the application; some can be put on after the strap is tensioned, while others must be installed before tensioning. To create the seal, the seal and strap are notched on both sides; if one set of notches are created then it is called a single notch joint, if two sets of notches are created then its called a double notch joint. Crimp joints also use a seal, but crimp undulations are used instead of formed notches. These crimps form great frictional forces which keep the joint from seeping. Seal-less joints are created by forming interlocking keys into the middle of the strap.
Plastic strap is most commonly joined by melting the straps together by adding heat to the joint. For lighter gauge strap, usually 16 mm (0.63 in) and narrower, heat is introduced with a hot knife system. First, the hot knife moves in between the straps. Then a platen comes up and compresses the straps and hot knife against an anvil on the order of tens of milliseconds. The platen then drops and the hot knife is removed. After the hot knife is completely clear of the straps the platen comes back up and compresses the joint so that the melted portions of the strap mix and harden.
For thicker plastic strap, usually 0.9 mm (0.035 in) and thicker, friction is used to create heat within the joint. The process starts with the strap compressed between the platen and the anvil. There is a vibrator built into either the platen or the anvil, which has teeth; there are also teeth on the opposite component that does not move. The vibrator is quickly vibrated, which creates enough heat to melt the interface between the two strap, because of friction. To complete the weld the vibrator is stopped and the pressure held until the joint has solidified.
It may also be joined manually by crimping a seal or via a buckle.
The joint is the weakest part of the system, therefore the type of joining method used is very important if strength is an issue. The strength of a joint is defined as the force required to break the strap in uniaxial tension. This is then compared to the uniaxial strength of the strap and recorded as the percent difference (e.g., a sample of steel strap may have a 5,000 lb (2,300 kg) break strength and the seal may fail at 3,000 lb (1,400 kg), so the seal is said to have a 60% strength). Hot knife welds have a minimum break strength of 55%. Friction welds have a minimum break strength of 65%. Well set up strapping machines or friction weld tools will aim for around 80% of break strength. Metal clips/seals are surprisingly low with circa 100 – 120 kg slip resistance.
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