Embossed PET strap explained: elongation, retained tension and joints

Polyester (PET) strap is a common choice for palletising and unitising medium and heavy loads. It is also one of the easiest strapping materials to buy badly, because two rolls that look alike can behave quite differently on the line. A green, embossed, 16 mm strap from one supplier is not automatically the same product as a green, embossed, 16 mm strap from another.

This guide explains what the usual PET strap terms mean, why elongation figures are easy to misread, what retained tension tells you, and what to fix in a specification before comparing offers.

What “embossed polyester strap” describes

The description combines three separate facts:

• Material. Polyester, usually abbreviated PET. Resin source and processing affect how the finished strap behaves, so the material name alone is not a grade.
• Surface. Embossing is a pattern pressed into the strap surface. It changes the surface that a tool’s feed wheel and the inside of a seal bear on. If your clips or tools were approved on embossed strap, a smooth strap is a change.
• Size. Nominal width and thickness. Width is what most people quote; thickness matters just as much, because a seal or clip has to close over the strap’s actual cross-section.

Colour is a supplier’s convention, not a performance class. Do not use it as a shortcut for grade.

ASTM D3950-16 is the specification for nonmetallic strapping, the category polyester strap belongs to, together with its joining methods. Its abstract classifies the strapping into five types by material and lists breaking strength and elongation among the properties tested, along with joint strength where applicable. Citing it in an enquiry is a way to ask how a supplier’s values were obtained; it is not, on its own, evidence that a particular strap meets it.

Elongation: two different numbers

Elongation is how much the strap stretches under load, expressed as a percentage of its original length. The trouble is that data sheets can quote it at different points:

Term	What it describes	Why it matters
Elongation at break	Stretch at the moment the strap fails in a tensile test	Shows how much the strap can stretch before it breaks
Elongation at a stated load	Stretch at a defined working tension	Closer to what happens on a pallet in service
Recovery	How much of the stretch is recovered when load is reduced	Influences whether the strap keeps hugging a load that settles

A value from one row cannot be compared with a value from another. When two offers quote elongation, ask which measurement each one is, at what load and by which method. If the answer is not available, treat the numbers as unknown rather than comparable.

Retained tension

Retained tension is the tension still in the strap some time after it was applied. It is what actually holds the load during storage and transport, and it is usually lower than the tension applied at the tool. Several things reduce it:

• The load settles or compresses, especially cartons, bags and green timber.
• The strap relaxes under sustained load. Polyester is less prone to this than polypropylene: the background to a strapping patent application published in January 2011 describes polyester strap as very strong and not easily stretched, and polypropylene as cheaper but liable to stretch and loosen under high tension.
• The joint slips slightly before it locks.
• Temperature and moisture in the load change its dimensions after strapping.

Because these factors interact, retained tension is best judged in a trial: strap representative loads with the intended tool and joint, store them as they would be stored, and check them after a defined interval. A strap tension gauge gives a number; even a recorded hand check and a note on movement is better than assuming.

Joints for PET strap

PET strap can be joined in several ways, and each needs its own evaluation:

• Friction weld. Battery-powered and pneumatic tools melt the two strap layers together without a separate closure. The strap grade and the tool settings must suit each other.
• Metal seal. A sheet metal clip is crimped over the overlapping ends with a sealer. Signode’s overview of seal joints notes that plastic strap joints use a crimp-type seal or a buckle, and that crimp joints depend on friction rather than cutting into the strap.
• Buckle. A wire buckle can join some PET straps, but the buckle, strap and tensioner must be approved as a combination.

Whichever method you use, the strap’s breaking strength does not describe the joint. Ask for joint evidence on the specific strap, closure and tool. Our comparison of clips and buckles sets out the differences.

A PET strap specification checklist

Before comparing offers, write down:

• Nominal width and thickness, with the tolerance you need.
• Surface: embossed or smooth.
• Hand grade or machine grade, and the tool or machine it must run on.
• Core size and roll length, in metres.
• The test basis for breaking strength and elongation, and the measurement point for elongation.
• The joint method, closure part and tool used in your approval.
• What happens if the supplier changes resin or process: who is told, and whether a new sample is needed.

Our sample approval guide shows how to record what was accepted so that a repeat order is the same product.

Reviewing strap and joint together

To discuss strap and joint as one system, see YAKU’s PET strap and sheet metal clips pages. Breaking strength and roll length are on request. Share your width, thickness, tool and application through the enquiry page.