Last updated: August 17, 2026. A vertical form-fill-seal (VFFS) machine turns flat rollstock film into a filled, sealed bag in one continuous vertical cycle. The sequence is fixed: unwind the film, fold it into a tube around a forming shoulder, seal the vertical overlap, drop the product through the tube, then close and cut the top and bottom of each bag with heated horizontal jaws. Most field failures — leakers, wandering seals, film-tracking drift, weight variation — trace back to one of these steps.
This entry covers the working principle. For the practical VFFS-vs-HFFS decision, see VFFS vs HFFS Packaging Machines, and browse the Form-Fill-Seal equipment category for manufacturers and representative machines.
The film path
Rollstock film unwinds from a reel, passes a tension-control device, and is folded around a forming shoulder into a vertical tube. The order is: unwind → tension control → forming shoulder → vertical seal → horizontal seal jaws → cut-off. Every stage feeds the next: a tension surge at the unwind shows up later as a wandering vertical seal or a short bag length.
Forming the tube
The forming shoulder (or forming collar) folds a flat web into a tube without creasing the film edges. Its geometry is matched to the bag width and the film, so a bag-width change means a different shoulder and tube. Because the film wraps around a fixed collar, the tube diameter is set by the shoulder, not by the film itself.
The vertical seal: lap vs fin
The seam that closes the tube is either a lap seal or a fin seal. A lap seal overlaps the two film edges (sealant to outer surface); a fin seal brings the two inner sealant faces together edge-to-edge. The choice is driven by the film structure, the barrier requirement and shelf presentation. A fin seal generally gives a more consistent sealant-to-sealant bond, while a lap seal keeps more printable surface facing the consumer.
The horizontal seal: temperature, pressure and dwell
The horizontal jaws close on the tube to make the bottom seal of the bag above and the top seal of the bag below, then cut between them. Three variables control the seal:
- Temperature — must reach the sealant layer’s sealing range without melting through the outer layer or degrading the barrier.
- Pressure — must force the molten sealant layers into intimate contact; too little leaves leak channels, too much squeezes melt out and thins the seal.
- Dwell time — how long the jaws stay closed at temperature and pressure; a shorter dwell needs a hotter seal, and a longer dwell reduces output.
These three work as one system: raising temperature can buy back dwell time, but only within the film’s seal range. Operating at the edge of that range is where intermittent leakers come from.
Seal range: laboratory vs manufacturing
A film’s seal range is the temperature window inside which it forms an acceptable seal. The laboratory seal range is wider than what a running machine can actually use — the manufacturing seal range is narrowed by jaw design, machine speed and how evenly the film is heated and pressed. A machine running near the top of a film’s laboratory range can produce acceptable samples on a bench sealer yet fail intermittently at production speed.
Film tension
Tension must stay constant from the unwind through the forming tube. A dancer arm or a load-cell feedback loop absorbs speed changes as the jaws cycle; if tension swings, the vertical seal wanders, the film stretches unevenly, and bag length drifts. Registration marks and a photosensor keep the printed film in step with the cut-off so each bag carries the artwork in the same place.
Filling through the tube
Product is metered from above and falls through the forming tube into the partially sealed bag. Because the product free-falls, VFFS suits free-flowing goods — granules, powders, snacks, frozen pieces — and works against products that bridge, smear, dust or break on impact. The fill must finish before the horizontal jaws close, or product traps in the seal area and creates a leaker.
Where it goes wrong
- Wandering vertical seal — usually film tracking or tension, not the sealer itself.
- Intermittent leakers — running at the edge of the seal range or with worn jaws.
- Short or long bags — film registration, tension or draw-length drift.
- Weak seals — an imbalance of temperature, pressure or dwell for that film.
- Product in the seal — filling not finished before jaw close, or dust drawn into the seal area.
