Heat sealing

Heat sealing is how flexible packs are closed: heated jaws press two film surfaces together until the inner sealant layer, usually PE or CPP, melts and fuses into one seam. Pouch edges are sealed during manufacture and the top is usually sealed after filling.

The three variables

• Temperature: too low and the seal will not hold; too high and the film thins or the outer layer is damaged.
• Pressure: ensures full contact across the seal.
• Dwell time: how long the jaws stay closed, which shrinks at high machine speeds.

Key terms

Seal initiation temperature is the lowest temperature at which a seal forms. Hot tack is the strength of a seal before it cools, critical on vertical form-fill-seal lines. Seal strength is the force needed to pull a cooled seal apart.

Common problems

Powder or oil in the seal area, wrinkles and channels all weaken seals and reduce barrier. Tell your converter what sealing equipment and speed you use, and always trial a new structure on your own machine with samples.