When a seal leaks, the first thing almost everyone does is turn the temperature up. Sometimes that works. Often it makes the film shrink, sticks it to the jaw, and adds a second problem to the first one.
A heat seal is four variables working together: temperature, pressure, dwell time, and cleanliness. If you only ever adjust one of them, you will spend a lot of time chasing the other three.
What is actually inside a sealing jaw
A jaw is a metal body, a heater cartridge, a thermocouple, a layer of PTFE or a jaw cover, and a mechanism that applies pressure. The seal forms where all of those meet the film, so a fault anywhere in that chain looks identical from the outside: a seal that peels open.
Cause 1: a cold spot along the jaw
Heaters fail at one end first, and a single failed cartridge leaves a ten-centimetre strip that never reaches temperature. The packs look perfectly sealed until you open one from that side of the bag.
Find it with a thermal camera, or with temperature-sensitive strips laid across the jaw. Both take a minute and both tell you the truth that a single thermocouple cannot.
Cause 2: uneven pressure
Jaws are set with springs, shims, or air pressure, and they go out of adjustment. A jaw that presses hard at the centre and lightly at both ends produces seals that hold in the middle and fail at the corners, which is exactly where a bag is handled.
The cheap test: close the jaws on a strip of pressure-sensitive paper, or on a piece of carbon paper over white paper. The imprint should be even from end to end. If it is not, adjust before you touch the temperature.
Cause 3: worn PTFE
PTFE stops the film sticking to the jaw. When it wears thin, the film sticks slightly, pulls a thread of sealant away as the jaws open, and leaves a seal that is thinner than it looks.
Look for a shiny, polished patch in the centre of the jaw, a brown tint, or a visible step where the tape has worn through. Replace on a schedule rather than on failure, and change both jaws at once.
Cause 4: contamination in the seal zone
Powder, oil, crumbs, sauce, or a single grain of sugar in the seal area will create a channel. On a bag of coffee this is obvious; on a bag of powder it is invisible until the pallet arrives at the customer.
Fix the cause, not the symptom: a brush or air knife before the jaws, a product guard that keeps the fill stream centred, and a wipe-down at every break. On sticky products, a small jet of filtered air aimed at the seal zone pays for itself in a week.
Cause 5: thermocouple drift
The controller says 180 degrees. The jaw is at 168. Nothing is broken, the sensor has simply drifted, and every seal on that machine is 12 degrees colder than the recipe assumes.
Check with an external probe once a month, at production temperature, with the jaw closed on film. Write the offset on the maintenance sheet. This one check prevents a surprising number of “mystery” seal failures.
Cause 6: the wrong film for the jaw
Sealing layers have different melting ranges. A film with a high-seal-initiation temperature will not seal on a jaw set for a standard PE laminate, no matter how long you run it. If you changed film suppliers and the seals failed the same week, this is your answer.
Ask the film supplier for the seal curve: strength against temperature at a fixed dwell and pressure. Two minutes of reading replaces two days of guessing. Our guide to matching film to machine covers the compatibility side in more detail.
The 15-minute test routine
- Run five bags at production speed with the current settings
- Peel test each one, slowly, and note where it fails: at the seal, at the edge, or the film tears instead
- Cut a cross-section through the seal and look at it under a magnifier. A good seal shows a clear, even bond line with no channels
- Check jaw evenness with pressure paper
- Check actual jaw temperature with a probe
- Look at the PTFE for wear and the seal zone for contamination
That is the whole diagnostic. The failure mode usually tells you which of the six causes to look at first: edge failures point to pressure, random channels point to contamination, and a clean peel with no strength points to temperature or film.
The trade-off you cannot ignore
Temperature, pressure, and dwell trade against each other, but only within limits. More temperature and less dwell gives you a seal that looks fine and fails later. More pressure and less temperature gives you a seal that is mechanically squeezed rather than fused.
Change one variable at a time, write down what you changed, and verify with a peel test rather than a visual check. A seal that looks good and holds nothing is the most expensive kind.
What to keep in the drawer
PTFE tape in the right width, one spare heater cartridge, two thermocouples, a roll of pressure-sensitive paper, and a jaw cover set. Under 300 dollars, and it turns a two-day wait into a twenty-minute job.
Frequently asked questions
What temperature should I run the sealing jaws at?
Whatever the film supplier’s seal curve says for your dwell and pressure, typically 130 to 190 degrees C for PE laminates. Start at the middle of the recommended range and adjust one variable at a time.
Why do my seals fail only after a few hours of running?
Heat soak. The jaw gets hotter than the setpoint after an hour of continuous running, the film shrinks, and the seal thins. Check the actual temperature after two hours, not at startup.
Can I seal through product contamination?
Some films are designed to seal through light powder or oil, but none seal reliably through a crumb or a liquid film. If the seal zone is dirty, fix the filling and guarding rather than buying a more forgiving film.
How often should jaws be replaced, not just taped?
When the jaw surface is no longer flat, when the heater channel is eroded, or when pressure cannot be evened out with adjustment. Typically every three to five years on a single shift.
Related reading: the full maintenance schedule and troubleshooting film jams.