A VFFS machine can continue producing bags at speed while making seals that look acceptable but fail under handling, compression or leak testing. That is why the question, why do VFFS seals fail, needs to be investigated as a process issue rather than treated as a simple temperature-setting problem. Reliable sealing depends on the interaction between film, product, jaw condition, machine timing and the environment around the sealing area.
For production teams, the practical objective is to identify whether the failure is occurring in the longitudinal seal, the top or bottom cross seal, or at a local point on the pack. The location and appearance of the defect usually narrow the likely cause considerably.
Why do VFFS seals fail during normal production?
A VFFS bag is formed from rollstock around a forming tube. The vertical sealing system creates the longitudinal seam, while cross-seal jaws form and cut the top and bottom seals. Each seal must receive sufficient, repeatable heat, pressure and dwell time for the film structure in the sealant layer to bond.
Failure occurs when one or more of these conditions falls outside the usable sealing window for the material. That window can be narrow, particularly with high-speed lines, recyclable mono-material films, metallised laminates or packs containing powders, oils or liquids.
A seal can be weak because it has not reached the required bond strength. It can also fail because the film has been overheated, distorted or cut by the jaws. These are different faults and require different corrective actions. Increasing jaw temperature without understanding the failure mode can make an unstable process worse.
Heat, pressure and dwell time must work together
Temperature is often checked first because it is accessible from the HMI, but the displayed setting is not necessarily the temperature at the jaw face. Worn heaters, failing thermocouples, loose wiring and poor jaw insulation can all create a gap between the setpoint and the actual sealing condition.
Insufficient heat may leave seals that peel apart cleanly with little resistance. Excessive heat can cause film thinning, burn-through, wrinkling or seal squeeze-out. On printed films, it may also damage the outer web or affect pack appearance.
Pressure matters equally. The jaws must close squarely and apply consistent force across the full width of the seal. If they are misaligned, one side may seal while the other remains weak. Worn bearings, loose linkages, incorrect pneumatic pressure or uneven jaw faces can produce this pattern.
Dwell time is the period in which heat and pressure are applied. At higher machine speeds, dwell time reduces unless the jaw design and motion compensate for it. A film that seals reliably at a lower output may therefore fail when production rate increases. Reducing speed temporarily is a useful diagnostic step, but it should lead to a review of jaw configuration, sealing parameters and film suitability rather than becoming a permanent workaround.
The effect of jaw condition
Contaminated, worn or damaged jaws transfer heat and pressure unevenly. Build-up on the sealing faces can insulate the film from the jaw, while damaged PTFE tape may cause product or film to stick. Serrated jaw patterns can also wear down over time, reducing their ability to hold the film securely during sealing.
The vertical seal system deserves the same attention as the cross-seal jaws. A worn sealing band, uneven contact pressure or contaminated former can create a longitudinal seam that opens when the bag is filled or handled. Because this seam is often hidden at the rear of the pack, it can be missed during visual checks.
Film specification and storage can cause seal faults
Not all films respond in the same way to a given VFFS setup. The sealant layer determines the heat-seal range, while the total laminate construction affects stiffness, heat transfer, friction and how the web tracks through the machine. A replacement roll may have the same dimensions and printed artwork but a different sealant formulation or coefficient of friction.
Film changes should therefore be controlled. Confirm the material specification, seal initiation temperature, recommended dwell conditions and whether the film is intended for the product being packed. This is particularly relevant where a site is changing from a conventional laminate to a lower-plastic or recyclable structure. The new material may require different jaw temperatures, pressure, cooling time or machine speed.
Poor roll storage can also affect performance. Film exposed to high humidity, temperature variation, dust or physical damage may not run consistently. Rolls should be allowed to acclimatise to the production area where necessary, especially when they have been moved from a cold warehouse into a warm packing room.
Printing and registration introduce another consideration. If the web is not tracking correctly, the seal may land across a gusset, a thick printed area or an inconsistent part of the film. This can be mistaken for a heat problem when the underlying issue is web alignment, dancer tension or registration control.
Product contamination is a frequent cause of leaking packs
A clean, correctly set jaw cannot make a reliable seal through product residue. Powders, crumbs, granules, oil, moisture and product fibres can all sit in the cross-seal area and create channels through which air or liquid escapes.
The appearance of the defect is useful here. Contamination-related leaks are often localised and irregular rather than extending evenly across the seal. On powder applications, a dusty line near the seal can indicate that product is falling too late or that the settling time is insufficient. With sauces, wet foods or oily snacks, product may be splashing or being drawn into the seal area during jaw closure.
Addressing contamination usually requires changes upstream of the jaws. Product delivery timing, discharge height, dosing accuracy and settling time should be assessed together. A suitable anti-static arrangement, dust extraction or a product knock-down device may help in dry applications. For liquid or particulate products, bag support and controlled product drop are often more effective than raising sealing temperature.
Check whether the bag is being pulled under tension
Seals can be damaged after they have been made. If the take-down belts pull the web excessively, or if the bag is unsupported as it fills, stress can be applied to a warm seal before it has cooled and developed full strength. This is common on heavier packs, longer bags and applications with inconsistent product weights.
Incorrect belt pressure, poor belt condition or misalignment can cause intermittent tension. The problem may only appear at certain pack lengths or speeds. Similarly, a product that shifts suddenly after filling can strain the bottom seal.
Look at the complete bag path, not only the sealing station. The former, pull belts, gusset system, discharge chute and transfer conveyor all affect how the pack behaves during and immediately after sealing.
A practical process for finding the cause
Randomly adjusting several settings makes it difficult to establish what corrected the problem. A more controlled approach reduces wasted film and gives operators a repeatable troubleshooting method.
- Classify the defect. Record whether the issue is a peelable seal, burn-through, wrinkling, a channel leak, an open longitudinal seam or a failure at one edge. Keep samples from the beginning, middle and end of a production run.
- Confirm actual sealing conditions. Check jaw-face temperature with a suitable calibrated method, verify pneumatic pressure and inspect the timing of jaw closure. Compare these readings with the film supplier’s processing guidance.
- Inspect jaws and consumable surfaces. Check alignment, PTFE tape, serration condition, heaters, thermocouples and evidence of contamination. Replace worn parts rather than compensating indefinitely with higher settings.
- Review film and product variables. Confirm the correct rollstock is fitted, check web tracking and examine whether product is entering the sealing area. Review recent changes to film supply, product recipe, fill weight or line speed.
- Change one variable at a time. Run a controlled trial, then test seals using the site’s agreed method. Depending on the application, this may include visual inspection, burst testing, vacuum testing or tensile seal-strength testing.
Documenting the working parameters by film type, pack format and product is worthwhile. It gives operators a validated starting point at changeover and makes gradual drift easier to identify.
When seal problems point to a wider machine issue
Repeated failures despite stable film and product conditions can indicate that the VFFS machine needs a more detailed mechanical review. Intermittent faults are often linked to worn jaw drive components, inconsistent pneumatic supply, poor temperature control or movement in the forming set.
The age and duty cycle of the equipment matter. A machine running one shift on a straightforward dry product has different wear patterns from one operating continuously with abrasive powders or corrosive product vapours. Preventive maintenance should reflect that reality, with particular attention to sealing assemblies, air quality, heaters, sensors and moving components that control jaw alignment.
Where output requirements have increased substantially, the existing sealing system may no longer provide the dwell time or control required by the chosen film. In that case, the correct response may be a change in jaw arrangement, film construction or machine configuration rather than continual adjustment by operators.
A reliable VFFS seal is the result of a controlled process, not a single setting on a screen. Treat each failure as evidence from the pack: its position, shape and timing can show whether the issue lies with heat transfer, jaw mechanics, material behaviour or product control. That approach helps production teams restore pack integrity while protecting output and reducing unnecessary trial-and-error.