Two Common Problems with Foil Pouch Sealing

On a foil pouch, the quality of the seal directly determines whether the finished bag will leak air or liquid. In day-to-day production, sealing defects look endlessly varied, but they fall into two broad groups: seals that are not tight enough, and seals that wrinkle or blister.

A weak seal is the more common complaint. One cause is a heat-sealing bar that runs too cool or applies too little pressure: the sealing layer never fully melts, the two layers merely touch at the surface, and the bond strength stays low. Another cause lies in the sealing material itself — its initiation temperature may be too high, or its sealing behaviour may be inconsistent, so different areas of the same roll perform differently and no single set of machine settings can control the process reliably.

The second group shows up visually: the seal edge wrinkles or blisters, and in serious cases the seal strength drops as well. The reasons behind it are more scattered, and four of them are seen most often.

First, the material cannot withstand heat and shrinks or distorts after hot sealing, which produces wrinkling. Second, the adhesive and the ink do not tolerate heat well enough, and wrinkling appears during sealing for the same reason. Third, too much residual solvent remains in the adhesive and ink; during sealing the solvent gathers and evaporates, forming bubbles along the seal edge together with wrinkling. Fourth, the machine itself may be at fault: the parallelism of the heat-sealing bars, the hardness and flatness of the silicone, or foreign matter stuck to the bar can all make heat and pressure uneven across the seal.

When a seal fails, it is more useful to identify which group the problem belongs to than to adjust parameters one by one. Is the sealing condition mismatched with the material, is the trouble in auxiliary materials such as adhesive, ink and solvent, or is it a matter of the accuracy and cleanliness of the bag-making equipment? Sorting the cause into the right category keeps the adjustment pointed in the right direction.