The direct answer
Product is kept out of the cross seal by giving the dose a controlled path and enough usable pack volume to finish moving before the jaws close. The solution may involve dose, nozzle or auger timing, headspace, cut-off, dust control, film motion and product-feed stability.
Seal settings cannot compensate reliably for product trapped between the film layers. Begin by identifying when contamination appears and what product mechanism caused it, then confirm the corrected process with an agreed pack-integrity method and retained samples.
Why does product enter the sachet seal area?
Common causes include overfill, insufficient headspace, late product fall, liquid dripping, paste stringing, foam rise, powder dust, particle bounce and movement of the pack before the dose has settled.
The cause may be continuous or linked to a transient such as refill, stop, restart or temperature change. Inspect sequential packs and the machine timing rather than treating every contaminated seal as a film or heater fault.
How do headspace and fill timing reduce contamination?
Headspace creates a clear zone between the product and the cross seal, while timing allows the dose to enter and settle before the film advances or the jaws close.
More headspace can improve margin but increases pack length and material use. More settling time can reduce cycle output. The optimum must be proven with the most demanding product/dose/pack combination, not chosen from an empty-film test.
What should be checked when contamination appears after refill?
Check whether refill changes product pressure, aeration, bulk density, hopper level, agitation, feeder flow or the timing of the next dose.
Liquids may entrain air or surge; powders may become aerated or compacted; granules may avalanche. Retain packs immediately before, during and after refill and record the relevant level or feeder state. Later stable packs cannot explain the transient.
How do powder dust and liquid drips need different controls?
Powder dust is controlled through product flow, drop containment, air movement and film/jaw cleanliness, while liquid drips require valve and nozzle closure, pressure control and enough time for cut-off.
A paste tail may need a different closure or suck-back action, and granules may need a wider path or reduced bounce. The acceptance test should use product-specific observations and not one generic statement that the seal “looks good”.
Why can increasing seal temperature hide the real problem?
Higher temperature may temporarily bond around light contamination, but it does not remove product, restore film-to-film contact or correct a dosing and timing fault.
Excessive heat can reduce process margin, distort film, increase residue or create inconsistent seals after a stop. Return to the approved film and seal window, clean and inspect the jaws, then correct the product path before revalidating the seal.
Seal-contamination fault table
| Evidence pattern | Likely process area | Next controlled check |
|---|---|---|
| Every pack | Pack volume, fill timing or constant cut-off problem | Reduce to known baseline and observe dose-to-jaw sequence |
| Only after refill | Feed pressure, aeration, compaction or surge | Record levels and sequential packs around refill |
| One lane only | Lane-specific nozzle, auger, chute or seal alignment | Keep lane identity and compare neighbouring lanes |
| Only at higher speed | Insufficient settle/cut-off time or unstable film motion | Compare accepted packs across controlled setpoints |
| After a pause | Drip, relaxation, settling or first-dose response | Inspect first packs and stopped product position |
Acceptance and enquiry inputs
- Product condition, dose, pack volume and required headspace.
- Film structure and the agreed seal-process window.
- Nozzle, auger or chute geometry and product feed conditions.
- Samples from start-up, stable running, refill, pause, restart and upper intended output.
- Agreed visual, strength or leak-test method and pass criteria.
- Lane identification, reject response and retained-sample plan.
For food applications, the pack owner should use packaging suitable for the intended food-contact use and retain the appropriate supporting evidence. Machinery trials establish runnability and seal-process evidence but do not replace material compliance or shelf-life assessment. Read Food Standards Agency packaging guidance.