Strapping, also known as bundling and banding, is the process of applying a strap to an item to combine, stabilize, hold, reinforce, or fasten it. The strap may also be referred to as strapping, strapping is most commonly used in the packaging industry. Strapping is most often used in complete horizontal or vertical bands.
Edge protectors are used to help spread the load at corners and reduce damage to the load by the tensioned strap. Strapping can also be used in loops attached to holding locations on rail cars, skids, etc.
- Bundling items together for handling and shipment: newspapers, pipe, lumber, concrete block, etc.
- Attaching items to pallets, skids, and crates
- Reinforcing wooden boxes, crates, and corrugated boxes, such as gaylords
- Attaching items to flatcars, flatbed semi-trailers,
- Securing a unit load of bricks, packaged glass, metal parts, etc.
- Closing corrugated boxes and shipping containers
- Securing coils of steel or paper
- Holding bales of agricultural products or textiles
- Load securing items within intermodal containers, boxcars, and semi-trailers
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 it is 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 0.73 mm (0.029 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. Polypropylene and Polyester strapping can also be sealed in a similar manner with the heat being produced by sound in the form of Ultrasonic waves passing through the materials and exiting the molecular structure. They are then compressed as in a heat seal. In ultrasonic welding of plastics, high frequency (15 kHz to 40 kHz) low amplitude vibration is used to create heat by way of friction between the materials to be joined. The interface of the two parts is specially designed to concentrate the energy for the maximum weld strength.
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.
For pallets strapping based on norm ASTM D3953 Standard Specification, the number of the strapping procedure is around 10 times for a minimal.
There are two major types of equipment available for strapping: hand tools and strapping machines. A dispenser is used with both types of equipment to dispense the strap. Some strapping machines have the dispenser built-in, others (typically those on large arch machines) have a separate powered dispenser, which can automatically load or thread new coils. Non-powered separated dispensers have a pulley which releases and activates a brake to stop the inertia of the coil to keep the strap from over-running onto the floor. Typically, dispensers used on large arch strapping machines have multiple pulleys to act as a way to store strap in a way that can be quickly dispensed. Smaller arch machines such as table top machines will feed strap into a pool box/accumulator instead. In both cases this is required otherwise the Strapping Machine will draw the strap faster than the dispenser can pay off.
Within hand tools there are manual and automatic tools. Manual tensioners and crimping systems are available for low volume applications. Automatic hand tools are used for higher volume applications. Automatic tools are available using a battery power source or compressed air. Both manual and automatic tools are available as combination tools, which perform the tensioning, sealing and cutting, or as separate tensioners, sealers, and cutters. Manual sealers for plastic strap always use a seal and a compression (serrated seals) or crimp joint (smooth faced seals). Manual and automatic hand sealers for steel can use a seal or a seal-less joint. Automatic hand tools for plastic strap use a friction weld.
Strapping machines, also known as bundlers, are used for higher volume applications or special applications. They are available in horizontal and vertical orientations; however, the vertical orientation is far more common. They are also available in bottom seal, side seal, and top seal orientations, which define where the joint is located on the bundle. They are available in semi-automatic and automatic varieties. The automatic versions can additionally to be used in line with conveyor systems. A photocell is used to detect when the bundle is in the correct location, stop the conveyors, and activate the strapping machine. Most however will be found in standalone applications.
- No sharp edges
- Can be cut while under tension without the possibility of recoil injury
- withstand additional shock loading (18% more elasticity than steel) without failure
- Ease of disposal
- Safer work environment
- Ability to re-tension and reuse strap
- Polyester strap shrinks or expands with loads
- High retention memory
– stays strong and tight on product
- Will not rust or damage product
- The plastic strapping system is lighter than the metal strapping system.
Depending on the Load or Product being secured when it comes to securing heavy loads, it is very important that we have deeper understanding how different types of strapping works, which one will serve our purpose best because in the end we don’t want any accidents, injuries and damages to the operators and to the products we transport.