The right pre-stretch: why under-stretched film costs you twice
What happens when film is stretched
A stretch film goes through three phases as it is pulled longer:
- Elastic phase. The film is still close to its original thickness. It takes a lot of force to stretch, uses a lot of material and keeps a strong elastic memory: it wants to return to its original length.
- Plastic phase. The film starts to flow. It gets thinner, but not evenly: this is where you can see stripes of thick and thin film on a wrapped pallet.
- Strain hardening. The film’s molecules line up. The thickness becomes even, the film gets stronger and stiffer, and it locks the load. This is the working region a machine film is designed for.
Past that region the film approaches its breaking point and tears more easily, especially at sharp corners.
Why too little pre-stretch costs twice
- It costs film. Every metre of film that is not stretched to its rating is material the customer pays for and does not use. The difference between a film run at its rating and the same film run far below it can be a large share of the film per pallet.
- It costs stability. Under-stretched film keeps its elastic memory. After wrapping it pulls back, which can crush the corners of boxes and trays within the next 24 hours.
Example from the app: a pallet used 270 g of film. After the film was stretched into its working zone, the same load used 230 g. At 650 pallets a day and 220 working days, that is about 5.7 t of film a year, and the load still passed the stability tests.
Why the machine setting is not the answer
The pre-stretch shown on the wrapper is what the machine is set to do. What reaches the pallet can be very different:
- the film slips on the pre-stretch rollers,
- the roll is mounted the wrong way round,
- the rollers are dirty or worn,
- the film’s stated pre-stretch is wrong.
Wrappers also differ: a brake system stretches the film by holding it back against the turning load and rarely reaches high pre-stretch, while a powered pre-stretch carriage stretches the film between two rollers running at different speeds before it reaches the load.
How to check it at the wrapper
- Note the film: brand, thickness, width and its rated pre-stretch.
- Weigh the roll, wrap one pallet, and weigh the roll again.
- Count the revolutions of the wrapping cycle.
- Compare the real pre-stretch with the film’s working zone.
Palletseal does the calculation and shows the result on a red, amber and green scale: below the zone (there is a saving to find), in the zone, or above it (check the film and the rollers). Then it compares the customer’s film with yours on the same load and checks that the load still holds.
Questions we often get
What is pre-stretch?
How much the film is elongated before it reaches the load. 250 % pre-stretch means one metre of film becomes 3.5 metres.
Why is the real pre-stretch different from the machine setting?
Film can slip on the pre-stretch rollers, a roll can be mounted the wrong way round, rollers wear, and the film's stated pre-stretch can be wrong. The setting shows what the machine is told to do, not what reaches the pallet.
What happens if film is stretched too little?
It uses more film per pallet, the thickness varies, and the film keeps an elastic memory. It tries to pull back after wrapping and can squeeze the corners of boxes in the following hours.
What happens if film is stretched too much?
It gets close to its breaking point and tears more easily, especially at sharp corners.
How do I check the real pre-stretch?
Weigh the film used on one pallet and count the revolutions on the wrapper. Palletseal turns those measurements into the real pre-stretch and shows whether it is in the film's working zone.
