Plastic strapping covers two materials with different behaviour. PP stretches and recovers, so it tolerates a softer protector. PET is stiffer, holds higher tension and behaves more like steel at the corner. Matching the paper wall to the band type is the difference between a strap that stays tight for the whole journey and one that is loose on arrival.
Specification at a glance
| Band | Wall | Leg | Where it fails |
|---|
| PP hand tool | 2.5 mm | 40 mm | Soft carton deforms before the board |
| PP machine | 3 mm | 50 mm | Ragged cut jams the feeder |
| PET hand tool | 3.5 mm | 50 mm | Band goes slack in steps |
| PET machine | 4 mm | 50-60 mm | Seal notch tears under vibration |
PP: the forgiving band
Polypropylene stretches under tension and relaxes slowly, which means it keeps pulling even as the pack compresses. That makes it the easiest band to protect: a 2.5-3 mm wall with 40-50 mm legs is normally enough, and the band will still be tight when the container is opened.
The exception is a soft carton. PP will keep pulling until it deforms the corner, so if the carton board is light, use a wider leg rather than a thicker wall. Width reduces pressure; thickness only resists cutting.
PET: closer to steel
Polyester band has much less stretch, so almost all of the tension arrives at the corner at once and stays there. A thin protector creeps and the band goes slack in steps. We quote 3.5-4 mm with 50 mm legs, and add a seal piece where a metal or heat seal bears on a narrow line.
PET is also more prone to cutting. A burred seal or a sharp corner will notch the protector and the notch becomes a tear under vibration.
Application notes that change the spec
Machine strapping at high cycle rates needs a protector that feeds without jamming, so a clean cut end and consistent wall matter more than the last half millimetre of thickness. Hand tools are more forgiving of a slightly ragged cut.
Where both band types are used on the same line, specify one protector for the heavier band and use it for both. Two paper grades in one packing area get mixed up within a week.
Questions buyers ask before ordering
Is one protector enough for PP and PET?
If you run both, specify for PET. The heavier board works on PP without a penalty other than cost.
Do plastic bands need protectors at all?
Yes. Plastic cuts carton corners just as steel does, only more slowly. Without a protector the band loses tension as the corner compresses.
Can I use a very thin board to save cost?
Only on light cartons with PP hand strapping. Below 2.5 mm the board creases at the band line and stops spreading load.
What length for a banded carton?
At least 100 mm past the band on each side. For a 400 mm carton that means a 250-300 mm piece at each corner.
Do you supply pre-cut short lengths?
Yes, from 300 mm up, cut to the millimetre, which is usual for carton corner pieces.
Samples and prices are issued from mona@hengnicepacking.com once cut length, wall thickness and quantity are known.
