The film ran great in March. Now it won’t run at all.
Same spec. Same supplier. Same machine.
Your production team is frustrated. Your plant manager wants answers. You pull the spec sheet, and nothing has changed on paper.
So what happened?
Often the answer is three letters: COF. Most people think of COF as a fixed number on a spec sheet. It is a number that moves. It can change after the film is made, after it is laminated, and even while it sits in your warehouse.
Once you know why it moves, you can stop it from stopping your line.
What is COF?
COF stands for coefficient of friction. In plain words, it tells you how slippery a film is.
A low number means the film is slick. A high number means the film grabs.
Slip level | Typical COF | How it feels |
|---|---|---|
High slip | About 0.10 to 0.20 | Slick, slides easily |
Medium slip | About 0.20 to 0.40 | Some grip, some glide |
Low slip | Above 0.40 | Grabby, resists sliding |
These ranges are a rough guide. Suppliers draw the lines a little differently.
There are two kinds of COF:
Static COF is the force it takes to get the film moving.
Kinetic COF is the force it takes to keep it moving.
Labs measure both with a test called ASTM D1894. A small weighted sled is pulled across the film, and a machine records how hard it had to pull.
You need the right number for your line. Too slick, and rolls telescope and bags slide off the pallet. Too grabby, and the film drags on your machine and jams.
Why COF changes
Most films get their slip from an additive mixed into the plastic. The additive does its job by rising to the surface, a bit like oil rising to the top of salad dressing. The industry calls this blooming. The most common slip additive is a fatty amide called erucamide. Because it travels through the film, it is known as a migratory slip.
Here is the catch. The additive never fully stops moving. Four things push it around.
1. Time
Fresh film is not slippery yet. The slip needs time to reach the surface. That is why film makers often wait about a day before they test COF. A roll tested too early gives a number that is unstable.
2. Lamination
This is the big one. Many packages are made of two or more films glued together. The laminating adhesive, often a polyurethane, and the outer film, often PET or nylon, can pull the slip away from the sealant surface. Engineers call this slip scalping.
The result can be dramatic. Plastics Technology reports that a polyethylene film with erucamide slip can climb to a COF of 1.0 or more after lamination. A film that was slick on Monday can be sticky by the time it ships.
3. Heat
Heat speeds everything up. Laminated rolls often cure in a warm room. Then they ride in a hot truck and sit in a summer warehouse. Each warm stop gives the slip another push, and it does not always move in the direction you want.
4. Contact in the roll
In a wound roll, the inside of the film presses against the outside. Slip can transfer from one side to the other. Inks and varnishes on the printed side add their own friction too.
Step | What happens | Effect on COF |
|---|---|---|
Film is made | Slip starts rising to the surface | Drops over the first day |
Lamination | Glue and outer film pull slip away | Can rise sharply |
Curing and hot storage | Heat speeds up slip movement | Can keep shifting |
Sitting in the roll | Slip transfers between sides | Either side can change |
What a COF shift costs you
A COF problem rarely shows up as a COF problem. It shows up as something else:
Jams and drag. Grabby film fights the forming collar on a vertical form-fill-seal machine.
Tracking trouble. The film wanders, and your seals and print drift out of place.
Feeding issues. Pouches stick together or will not open cleanly.
Sliding loads. Film that is too slick lets bags and cases shift on the pallet.
Downtime. Every stop costs labor, product, and schedule.
The hard part is that the film still meets its spec on paper. The number was right on the day it was tested. It was wrong on the day you ran it.
Five questions to ask your packaging partner
You do not need to be a film chemist. You need a partner who plans for COF to move. These questions will tell you if yours does.
When do you test COF? A test after lamination and curing tells you far more than a test on fresh film.
Which surfaces do you test? Inside to inside, outside to outside, and film to metal can all give different numbers.
What kind of slip is in the film? Erucamide and other migratory slips keep moving. Newer non-migratory slips stay put and hold COF steadier through lamination.
How should we store the rolls, and for how long? Heat and time both matter.
Can you send COF results with each shipment? Data makes a problem much faster to solve.
Share your side too. Tell your partner what machine you run, how fast you run it, and where the film touches metal. The more they know, the better they can match the film to your line.
Same film, different day
Back to that film that will not run.
The film from March and the film on your line today matched on paper. But COF lives on the surface of the film, and that surface kept changing. Time, glue, heat, and pressure all had a turn.
The spec sheet could not show you that. A good packaging partner can. Their job is to know how the film will behave on the day you run it, long after the day it was made.
That is how a film runs great in March and every month after.
Have a film that will not behave? Talk to the Admiral team. We are happy to take a look.
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