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Sealing trouble shooting

One of the most important functions of packaging is to ensure the protection of the product. This implies (among others) that the pack must be securely sealed (either lock seal or easy open). With flexible packaging, this can be achieved on the packing line by heat sealing, or by the use of some form of adhesive system (cold and heat seals coatings). Product protection and hence effective shelf life are a function of the quality of sealing of the package. One of the most overlooked factors in the production line is the efficient performance of the complete packaging system (pack-machine/pack-line/packaging-film). There are several packaging machine types {Vertical Form Fill Seal (VFFS), Horizontal Form Fill Seal (HFFS), Horizontal Pouch/Sachet Forming, etc}, however below we will try to present a sealing-trouble shooting for a VFFS machine. Usually similar approach applies to solve some issues to all machine types.
In the below drawing a VFFS machine description can be found.

To make a heat seal, the sealant layer must be melted together then allowed to cool down enough to become what is called “crystalline”, or solid. The three interdependent elements to heat-sealing are: Heat/Pressure/Dwell Time. In general think of them as a triangle: Think of it like a triangle: If more pressure is applied, less heat and dwell time is needed. Less heat and pressure are necessary if dwell time is increased. If less pressure is applied, more dwell time and heat are needed. More dwell time is required if heat and pressure are decreased.
Sealing issues which are quite frequent at VFFS machines are seal-integrity, seal-strength (or peel-force), and hot-tack (hot-seal strength) issues.
Hot-tack issues
If the bottom seal of the pack cannot withstand the weight of the packed article while it is still hot, then the following steps should be taken:
  • Increase sealing temperature in the seal-area itself. Usually we check temperatures on the temperature controller only. This could not be enough. Machine operators require to check and control the temperatures of the sealing jaws and find the correct setting to apply sufficient energy to the sealant layer, to fuse together and become one medium.
  • Increase if possible the pressure of the jaws.
  • Sometimes (and where the VFFS line allows to) a cooling air blast helps the hot-tack strength.
  • Finally a new sealant design maybe is needed.
Seal integrity
When we cannot achieve 100% tight seals then the following steps should be taken:
  • Check if the sealing jaws are not parallel. An A4 sheet paper can help, by sealing on it and see the trace.
  • Check temperature settings. See if the set ones are applied to the sealing jaws. Icrease the temperatures step-by-step. Note that sometimes reducing the temperatures may help. Adjust pressure and time accordingly.
  • If a gusseted pack-type is applied, and a leak problem is on the edges of the gusset areas, then the pressure is a very critical parameter to check.
  • Check if the seal areas are contaminated. If yes, and there is no way to avoid it, then a new sealant design is obligatory.
  • Check if the sealing tools are clean. If not, clean them.
Seal integrity may be evaluated as part of the in-line quality function by using standard instruments to test the pack under pressure or vacuum and identify if air or oxygen will pass through the seals.
Seal/Peel strength
Usually seal strength issues can be overcome by applying the correct temperature settings. Peel force the same (when we need less opening strength we need to decrease the temperatures, and the opposite when harder peel is required). Sealing strength is influenced by the type of the seal layer and thickness of the film web. Take into consideration that with the same sealant layer (type and thickness), usually a doubling of the base film thickness almost doubles the seal strength, however the thicker the material, the narrower the temperature sealing range under normal sealing conditions.
Overview of Possible causes of imperfect sealing (Check – List)
Wrong setting of sealing temperature
Wrong setting of sealing pressure
Sealing tools not parallel
Sealing tools not clean
Contaminated sealing areas
Defective heating elements
Melted sealant deposit on sealing tools
Damaged sealing tools