A film jam is the most ordinary emergency in a packaging plant and the one most often diagnosed by guesswork. Someone climbs up, pulls the film out, threads it again, and the machine runs for another forty minutes. Nobody asks why it happened, so it happens again tomorrow.
The trick is to work in order of cost. Start with the things that take thirty seconds to check, and only then take the machine apart.
1. The roll itself
Start here because it is free. Is the roll the right width? Is it telescoped, with the layers pushed to one side? Was it wound too tight or too loose at the factory? Is there a splice in the middle that you have just reached?
A roll that ran perfectly for two days and then starts jamming at the same point every time is usually a bad roll, not a bad machine. Note the batch number before you throw it away.
2. Unwind tension and the roll brake
As a roll gets smaller, its inertia drops and the brake has to work differently. Machines with a manual brake need adjustment through the roll; machines with a dancer or a load cell do it automatically, unless the sensor has drifted.
Symptom: jams and registration drift that get worse towards the end of a roll.
3. The dancer and the accumulation
The dancer absorbs speed changes. If it sticks, the film gets a sudden pull, and a sudden pull means a wrinkle, and a wrinkle means a jam at the sealing jaws.
Check by hand with the machine stopped: the roller should move freely through its full stroke with almost no resistance. Any stickiness, and you have found something.
4. Eye mark sensor and registration
A registration fault usually shows as bags cut in the wrong place rather than a jam, but on printed film a badly registered bag will fold over in the jaws and jam.
Wipe the lens, check the sensitivity against a printed and an unprinted area, and check that the eye mark is where the artwork says it is. Film printers do move marks between runs.
5. The forming shoulder and tube
The film should slide over the shoulder with an even tension on both sides. If one edge is tighter, the tube walks, and a walking tube jams the end sealers.
Look for film dust or adhesive build-up on the shoulder, a shoulder that is not centred, or a former that is the wrong size for the film width. A 2 mm misalignment is enough.
6. Pull belts
Worn or glazed pull belts slip, so the film feed lags, so the registration goes out, so the bag is long, so the jaws crush it.
Check belt condition and tension, and check for product dust on the belt surface. In a dusty plant, this is the single most common cause of a slow, intermittent jam nobody can reproduce.
7. Jaw timing
If the jaws close before the film has stopped moving, they will grab it and tear it. On a servo machine this is a timing parameter someone may have “optimised”. On a mechanical machine it is a cam or a clutch that is starting to slip.
Symptom: jams at exactly the same point in the cycle, at high speed only.
8. Static
Dry, cold rooms and fast film speeds generate static. The film sticks to itself, to the shoulder, and to the guides, and it jams in places that make no mechanical sense.
If jams happen more in winter or in an air-conditioned room, suspect static. Anti-static bars, tinsel, or a humidifier all help; the cheapest fix is often raising the room humidity slightly.
9. Environment
Cold film is stiff. Humid rooms make film tacky. A draught from a door or a chiller unit blows the film off the path. None of these are machine faults, and all of them look like one.
Symptom table
| Symptom | Check first |
|---|---|
| Jams at startup, runs fine after | Threading path, film stiffness, cold film |
| Jams at the end of a roll | Brake adjustment, roll inertia, splice |
| Jams at high speed only | Jaw timing, dancer response, pull belt slip |
| Jams after a film change | Film width, thickness, coefficient of friction, eye mark position |
| Jams in one lane of a dual lane machine | That lane’s shoulder alignment, belts, and sensor |
| Jams with wrinkles before the jaws | Shoulder centring, film tension balance, static |
The ten-minute routine
When it jams, before you clear it: photograph the jam exactly as it is. Then note the bag counter, the time, the roll diameter, and the film batch. Clear it, run it, and see whether the next jam happens at a similar counter value. Three data points usually identify the cause, and none of them require a technician.
Prevention that actually works
- Store film at room temperature, off the floor, in its wrapper until use
- Train one threading method and photograph it on the machine door
- Wipe the film path at every shift change
- Keep a spare set of pull belts and a spare dancer spring on the shelf
- Log every jam with the counter value, and review the log weekly
If jams are accompanied by bad seals or a knife that tears rather than cuts, the film itself may be the problem. See our notes on film and machine compatibility.
Frequently asked questions
Why does my machine jam only in the morning?
Cold film and cold room. Film stored overnight in an unheated space is stiffer and has different friction. Let rolls temper in the packing room, or store them there permanently.
Can a jam damage the machine?
Yes. A jam caught in the jaws can bend the jaw arm, damage the knife, or overload a servo. Always clear the film completely before restarting, and never force the jaws open with a bar.
How tight should the film be?
Tight enough to track straight, loose enough that the dancer sits mid-stroke. If the dancer is bottomed out, the film is too tight and you will get registration drift.
Do I need a technician for a recurring jam?
Not for the nine causes above. If you have checked all nine and it still jams, that is when a technician earns their fee, because the next layer is timing parameters and mechanical wear.
Related reading: seal failures and what causes them and the preventive maintenance calendar.