Stretch Wrap, Strapping and Getting a Pallet to Arrive Intact

A palletised load has to behave as a single object through forklift handling, road vibration, weeks of ship motion and being stacked on. Most failures are not exotic. The load slides off the pallet as one piece, or the top two layers walk off, or the stack leans until the cartons crush.

All three come back to the same thing, and it is not how thick the film is.

Containment force, not micron count

Buyers specify stretch film by gauge or micron, which measures thickness. Thickness is an input. The output that determines whether the load survives is containment force: the actual force the wrapped film applies to the load, measured in kilograms or newtons at a point on the pallet with a simple pull plate gauge.

Containment force is a product of the film, the number of revolutions, and above all the pre-stretch applied by the wrapper. A good machine pre-stretches film by 200 to 300 percent, which means one metre of film off the roll becomes three or four metres on the load. Thin film that is properly pre-stretched and applied outperforms thick film applied slack, and it costs less.

As a rough guide, light stable loads need around 5 kg of containment force, typical loads 8 to 10 kg, and heavy or unstable loads considerably more. Measure it rather than assuming it, and measure at the top, middle and bottom of the load, because they are usually different.

Wrapping pattern

1. Tie the load to the pallet. Start at the base and take several revolutions that capture the pallet itself, not just the bottom carton layer. If the film does not grip the pallet, the whole load slides off the base intact, and that is the single most common pallet failure in transit.
2. Overlap each layer by roughly half its width as you climb.
3. Reinforce the top. The top layers have nothing above them and the least restraint, so add extra revolutions there.
4. Do not wrap the film into the pallet openings so tightly that a forklift tears it, and do not leave a long loose tail that catches on conveyors.

For loads exposed to weather or condensation, a top sheet under the final wraps keeps water off the top cartons. Wrapping alone does not shed water, and water pooling on a top layer is a familiar way to lose a pallet.

Corner boards

Vertical angled boards placed on the four corners under the film or strapping. They are cheap and disproportionately effective:

• They distribute strapping pressure instead of letting a strap cut into the corner cartons.
• They add vertical compression strength, which is what lets a pallet be stacked without the lower cartons crushing.
• They transfer containment force across the face of the load instead of concentrating it at the edges.

If a load is going to be double stacked in a container, corner boards are close to mandatory. See pallet sizes and container fit.

Strapping, and which type

Strapping restrains in tension where film restrains by containment. Three materials in common use:

• Polypropylene (PP). Light duty. Cheap, high elongation, and it loses tension over time and with temperature change. Fine for bundling, not for securing a heavy load for a five week voyage.
• Polyester (PET). The workhorse for heavy loads. High break strength, and critically it has elastic recovery: it stretches under shock and returns, so it stays tight as a load settles. It does not rust, it is safe to cut, and it has replaced steel in most applications.
• Steel. The highest break strength and almost no elongation, which is both its strength and its weakness: it cannot absorb shock, so it snaps or the load shifts under it. It rusts, it stains cargo, and it is genuinely dangerous to cut under tension. Still used for very heavy, rigid, high temperature loads such as steel coil and hot products.

For a settling load, PET beats steel, because a load that compresses slightly leaves steel strapping loose and PET strapping still tight.

Why pallets fail

1. The film never gripped the pallet. The load leaves the base as a unit.
2. Overhang. Cartons past the pallet edge lose all corner support and are crushed first. Underhang wastes space and destabilises the stack.
3. Poor column alignment. Carton strength is in the vertical corners. Interlocked or brick-stacked patterns look stable and are structurally much weaker than aligned columns, sometimes by half.
4. Too much load height for the footprint. A tall narrow pallet tips.
5. Slack strapping after the load settles, which is a PP problem and a steel problem alike.
6. No top sheet under condensation.

A specification worth writing down

Give your packer a written standard rather than leaving it to the wrapping machine's last setting: target containment force, number of base wraps capturing the pallet, overlap, top reinforcement, corner boards yes or no, strapping material and number of straps, top sheet yes or no. Then check a finished pallet against it occasionally.

That specification is also your evidence if a claim is declined for insufficient packing, which is an exclusion under every cargo policy. See what cargo insurance does not cover.