A heat seal is made by temperature, time and pressure at the jaws. Most weak, leaking or burned seals trace to one of those three, or to something on the film surface that keeps the two layers from fusing. Look at how the seal failed before you change anything, because a cold seal and an overheated seal need opposite corrections.
Also described as: weak seals, leaking seals, seals peeling open, burn-through.
Quick diagnosis
| What you see | Likely cause | Check first |
|---|---|---|
| The seal peels apart cleanly and the two faces look smooth and unchanged | Not enough heat reached the sealing faces. The jaw is too cool, the dwell is too short, or the line speed went up and the dwell went down with it. | Jaw face temperature, measured, not the controller reading; dwell time; line speed |
| The film is thinned, wrinkled or holed at the seal, or it breaks along the edge of the seal | Too much heat or pressure. Melt is squeezed out of the seal and the film beside it is left thin. | Jaw temperature and pressure; sharp jaw edges; the condition of the release cover |
| The seal is good along part of the jaw and weak along the rest | Uneven temperature or pressure: a worn or misaligned jaw, a failed heater, a worn backing pad or residue built up on the jaw. | A pressure impression along the full jaw; temperature at several points along the jaw |
| Leaks at a fold, a gusset or where two seals cross | The number of film layers changes there, and the step leaves a channel the melt did not fill. | Find the channel with a dye or a pressure test; the backing pad; jaw pressure |
| The seal fails where product, powder, liquid or dust was in the seal area | Contamination between the faces. The melt cannot fuse through it. | Fill level and drips; dust; static pulling powder into the seal area |
| Thick bands seal weakly and thin bands burn through, at the same settings | Gauge variation across the web. Thick film needs more heat to reach the sealing face, and thin film takes less to burn through. | Gauge profile across the web, at the positions that fail |
| Film that sealed well before now seals weakly, with no change at the sealer | The sealing surface changed. More slip has bloomed, antiblock went up, the seal side picked up corona treatment, or the resin density or melt index moved. | Film age; slip and antiblock level; a dyne test on the seal side; COA of the good lot against the bad lot |
Checks in order, cheapest first
- Look at how the seal failed. Peel several seals by hand and mark where each one came from across the web and along the jaw. A clean peel points to a cold seal. A break in thinned film at the seal edge points to a hot one. A failure at the same place on every bag points to the jaw.
- Measure the jaw temperature at the face. Use a contact probe at several points along the jaw. The controller reads the sensor, not the sealing face, and the two can differ. The jaws are hot enough to burn, so use a probe made for the job.
- Find the sealing window, one setting at a time. Make seals at stepped temperatures with dwell and pressure fixed. Note where the seal first holds and where burn-through begins, and run between the two. More pressure does not make up for too little heat, and too much pressure squeezes the melt out of the seal.
- Check the jaws. Look at alignment, wear, the release cover and the backing pad, and clean off built-up residue. Lock out the sealer and let the jaws cool before you work on them.
- Keep the seal area clean. Stop product, liquid and dust from reaching the seal area. On powders, check for static that pulls dust up onto the film.
- Measure film gauge across the web. A film that averages on gauge can still have thick and thin bands. A narrow sealing window cannot cover both.
- Check the film surface, then compare lots. Confirm which side is corona treated and that the seal side is not. Compare the age, slip level and COA of film that seals well with film that does not. As a quick test, wipe the seal face of a sample with isopropyl alcohol, well away from the hot jaws because it is flammable, let it dry fully and seal it. If the seal improves, something on the surface is in the way.
- Change the sealant resin last. Seal failures are usually settings, jaws or surface condition and not the base resin. If the window is still too narrow for the machine, look at the sealant layer. The options and their costs are below.
The numbers
Density and melt index decide how a polyethylene seals. A lower density resin melts at a lower temperature, so it starts to seal sooner. A higher melt index flows more easily into folds and gussets, and gives a weaker seal while it is still hot.
| Resin | Melt index (g/10 min) | Density (g/cm³) | In sealing |
|---|---|---|---|
| LLDPE, blown film grades | ~0.5–2 | ~0.916–0.926 | The usual sealant resin. Lower density grades start to seal at a lower temperature. Metallocene grades seal best. |
| LDPE film grades | ~0.2–4 (general-purpose film ~1–2; coating grades higher) | ~0.918–0.930 | Seals easily, with less seal strength than LLDPE. |
| HMW-HDPE film grades | Very low, often <0.1 at 190°C/2.16 kg. High-load melt index ~5–15 at 190°C/21.6 kg | ~0.945–0.960 | Higher melting, so it needs more heat at the jaw than low density film. |
| EVA, film and heat-seal grades | See the grade sheet | Not listed. Sold by vinyl acetate content: ~2–9% VA for film, ~9–20% VA for heat-seal layers | Lower melting as vinyl acetate content rises, with better adhesion. |
Typical published ranges from our LLDPE, LDPE film, EVA and HMW-HDPE pages. Melt index is measured at 190°C/2.16 kg unless stated. Grades vary, so confirm against the data sheet and the COA for your lot. This site publishes no seal temperatures, dwell times or pressures, because they depend on the sealer, the film structure and the gauge. Find the window on your own machine as described in step 3. See melt flow index explained.
How a heat seal forms, and why it fails
- Two faces have to melt and mix. The jaws heat the film from the outside. The seal forms only when the two inside faces melt, touch and stay together long enough for the polymer chains to cross from one to the other.
- Heat has to travel through the film. Thicker film, and film with a higher-melting outer layer, takes longer to bring the sealing faces up to temperature. A faster line gives it less time.
- Anything on the surface is in the way. Slip and antiblock sit at the surface by design. Product, dust and moisture get there by accident. Corona treatment oxidizes the surface, and an oxidized face needs more heat to seal.
- Hot tack is a separate property. On a form-fill-seal machine the product drops onto the seal while it is still hot. The strength of the seal at that moment is its hot tack, and it can be poor in a film whose cooled seals test well.
- Too much heat has its own failures. An overheated seal thins, the film shrinks back beside it, and the weak point moves from the seal to its edge.
Sealant changes, and what each one costs you
| Change | Why it helps | What it costs you |
|---|---|---|
| A lower density LLDPE in the seal layer | It starts to seal at a lower temperature, which widens the window. | A softer film that blocks more readily, and lower stiffness on the customer’s machine. |
| Metallocene LLDPE in the seal layer | Better sealing than conventional LLDPE, with better dart impact and puncture resistance. | Harder to process, with higher melt pressure and motor load. Usually a higher price, more antiblock needed, and a new grade to approve. |
| EVA in the seal layer, or a higher vinyl acetate content | Lower melting and better adhesion. | A tackier film that blocks more, lower heat resistance in use, and a lower limit on extrusion temperature than polyethylene. Follow the data sheet, because overheated EVA breaks down and gives off acidic fumes. |
| A dedicated sealant grade in a coextruded film | A low-melting inner layer under a higher-melting outer layer seals before the outside softens, which resists burn-through. | A coextrusion line, or buying the film in. Sealant grades usually carry a higher price. |
| Less slip or antiblock, or a slip that does not migrate | A cleaner sealing surface. | Higher friction on the packaging machine and more blocking in the roll. |
| A melt index and density range on the specification | Sealing that drifts by lot often traces to one of the two moving between lots. | Nothing in the film. It takes an incoming check, and it may narrow your choice of sources. |
Any blend change moves film properties. Slip, blocking, clarity, stiffness and strength all move with the sealant. Run the new film on the customer’s sealer, at their speed, before you approve a swap.
Often confused with
- Weak film. A bag that splits in the body, away from the seal, has a film strength or puncture problem. The seal itself is sound. See film tearing or puncturing.
- Blocking. A bag mouth that will not open is stuck, not sealed, if the layers come apart undamaged along their whole width. See film blocking.
- Pinholes from gels. A leak through a lump or speck in the film, in the seal or away from it, starts with the gel. See gels and fisheyes in film.
Quick answers
Why are my heat seals failing?
Most weak, leaking or burned seals trace to temperature, time or pressure at the jaws, or to something on the film surface that keeps the two layers from fusing. Look at how the seal failed first. A clean peel points to a cold seal, and a break in thinned film at the seal edge points to a hot one.
How do I fix weak seals on bags?
Start by measuring jaw temperature at the face with a contact probe, because the controller reads the sensor and not the sealing face. Then make seals at stepped temperatures with dwell and pressure fixed, and run between where the seal first holds and where burn-through begins. More pressure does not make up for too little heat.
Why do my seals leak or peel open?
A seal that peels apart cleanly, with smooth and unchanged faces, did not get enough heat: the jaw is too cool or the dwell is too short. Leaks at a fold, a gusset or where two seals cross come from a channel the melt did not fill. Product, powder, liquid or dust in the seal area also stops fusion.
Are heat seal failures caused by the film resin?
Usually not. Seal failures are most often settings, jaws or surface condition and not the base resin. The film matters when the sealing surface changed: more slip has bloomed, antiblock went up, the seal side picked up corona treatment, or the resin density or melt index moved. Change the sealant resin last.
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