Protecting carton corners under PET strap: edge boards, tension and joint position

Corner damage under strap is one of the most familiar complaints in palletised carton shipping. The strap is tight at dispatch and the cartons look square. At the receiving end the top corners are crushed, the strap is slack and the stack has started to lean. PET strap is not the cause in itself. The problem is the contact between a tensioned strap and a corrugated corner that was never designed to carry a concentrated load.

Why corners fail under strap

When a strap is tensioned around a pallet load, its force is concentrated where it changes direction: the top edges of the upper cartons and the bottom edges near the pallet. Corrugated board resists load spread across its faces far better than load pressed into an edge. Several things follow:

• Crushing. The strap presses into the corner and flattens the flutes.
• Tension loss. Once the corner deforms, the path around the load is shorter than the strap. A strap that was tight becomes loose without any failure of the strap or the joint.
• Cutting. A narrow strap under tension can cut through the outer liner, especially when vibration rubs it along the edge.
• Instability. Slack straps stop compacting the load, and cartons can shift.

The CTU Code makes a closely related point for lashings: the securing values it gives assume material passing over smooth edges, and sharp edges and corners substantially reduce them. It recommends edge protectors wherever practicable. The code also treats strapping and wrapping as auxiliary measures, to be combined with proper securing in the transport unit. Corner protection keeps the strap doing its job; it does not turn a strapped pallet into a secured load.

Protection options and where each fits

Option	What it does	Points to check
Edge boards on vertical corners	Spread strap pressure along the corner and stiffen the stack	Length relative to stack height; board thickness; present under every strap crossing
Short corner protectors at strap crossings	Protect the exact contact point	Can move during strapping; need placement discipline
Top frame or top board	Spreads load across the top layer	Must cover the edges the strap crosses
Layer pads or tier sheets	Even out load between layers	Help stability; do not protect the edge on their own
Protection at the pallet edge	Stops strap cutting into lower cartons and the pallet	Depends on pallet design and the strap path under the deck

Cordstrap’s edge protector page makes the point that the right protector depends on the application and the type of strap or lashing, with cardboard and plastic types among the options. YAKU does not list edge protectors; specify and source them as part of the packing design.

Tension: enough to compact, not to crush

Many hand and battery-powered tools for plastic strap allow tension to be adjusted. That adjustment is part of the corner question. A tension chosen for a pallet of bricks will crush a stack of light cartons. Agree a tension setting per product family, write it into the packing instruction, and check it whenever the tool is serviced or replaced.

A simple trial is worth more than any general rule. Strap a representative stack with and without edge protection at the planned tension. Leave it under realistic storage conditions, handle it as it would be handled in distribution, then inspect the corners and test the strap tightness by hand. If possible, repeat the inspection at the end of a real route. Record the result with photographs so the decision can be revisited.

Joint position and strap path

Where the joint sits matters as much as the protection:

• Keep the joint, whether a friction weld or a crimped sheet metal clip, on a flat face rather than on a corner or an edge protector, so the closure is not bent over an edge.
• Make sure the clip’s edges do not bear on the carton. A metal clip pressed against a printed surface can mark or cut it.
• Route straps so they cross protected corners squarely, not at an angle that lets them slide off the protector.
• On the bottom path, check that the strap runs through the pallet entries without dragging across sharp deck edges, and that the lower cartons are protected where the strap turns under the deck.

Our comparison of sheet metal clips and wire buckles explains why clip thickness and joint arrangement need to be named when a closure is specified.

What to tell a supplier

When asking for PET strap or clips for carton loads, these details let a supplier respond usefully:

• carton dimensions, board grade if known, and the stack pattern;
• load mass per pallet and the transport route;
• the tool type and whether its tension is adjustable;
• current strap width and thickness, closure and joint type;
• the edge protection used now, and where damage appears;
• photographs of damaged corners taken at receipt.

Our article on strapping over sharp edges covers the contact-path information to collect for harder loads, and the receiving checklist helps confirm that the strap and clips delivered match what was approved.

Sharing carton load details

YAKU’s catalogue lists PET strap on request and sheet metal clips for joining PET and PP strap. Share your carton load details through our enquiry page. Strap strength and joint performance are on request and depend on the complete system, including the edge protection you choose.