Common Filling and Sealing Issues for Flexible Packaging
Learn common filling and sealing issues for flexible packaging pouches and how to review sealing temperature, pressure, material and pouch design.
Published: 2025-07-15 | Updated: 2026-05-21
Why Sealing Problems Are Not Always Only Bag Problems
When filling and sealing issues occur on the production line, the packaging material is often the first suspect. In practice, sealing problems can result from a combination of factors: the heat seal layer of the packaging material, the filling machine's sealing temperature and pressure settings, product residue in the seal area, film tension during feeding, and environmental conditions. Effective troubleshooting requires checking all of these factors, not just the packaging material.
Heat Sealing Layer, Temperature and Pressure
The heat seal layer is the innermost layer of the packaging laminate — typically PE (polyethylene) or CPP (cast polypropylene). Proper sealing requires the right combination of temperature, pressure and dwell time. If the temperature is too low, the seal layer does not melt sufficiently and the seal is weak. If the temperature is too high, the film may shrink, wrinkle or burn through. The optimal sealing window depends on the specific film structure and should be confirmed during sample testing with your actual filling machine.
Filling Contamination and Product Residue
One of the most common causes of seal failure is product residue in the seal area. When powder, liquid droplets, oil, or product particles land on the seal area during filling, they prevent the heat seal layers from bonding properly. This is especially common with dusty products (milk powder, flour, protein powder) and oily products (fried snacks, nuts). Solutions include adjusting fill nozzle position, using seal-through-contamination films, and ensuring proper dust extraction on the filling line.
Roll Film Tracking, Eye Mark and Tension Control
For roll film on automatic packaging machines, proper film tracking is critical. The eye mark tells the machine where to cut and seal. If the eye mark is misprinted, inconsistent, or misread by the sensor, the machine cuts at the wrong position, causing misaligned seals and wasted film. Film tension must also be balanced — too high stretches the film and distorts seals; too low causes film wandering and inconsistent bag lengths.
Pouch Size Tolerance and Accessory Position
For premade pouches, dimensional consistency directly affects filling line performance. Pouches that vary in width or height may not feed correctly through automatic filling equipment. Zipper and spout positions have specified tolerances — a zipper placed too high makes the pouch difficult to open; too low interferes with the seal area. Quality pouch suppliers control these dimensions within tight tolerances and provide tolerance specifications for machine setup.
Machine Compatibility Checks Before Production
The best way to avoid filling and sealing issues is to verify machine compatibility before bulk production. Key checks include: confirming the heat seal layer is compatible with the machine's sealing jaw type and temperature range, testing film COF (coefficient of friction) for smooth machine feeding, verifying eye mark sensor compatibility, and running a small production trial with actual product to identify any issues early.
How to Reduce Bulk Order Risk
Several steps help reduce the risk of filling and sealing problems in bulk production: request a pre-production sample and test it on your actual filling line with real product, confirm sealing parameters (temperature, pressure, dwell time) with the film supplier, discuss product characteristics that may affect sealing (oil content, dust generation, filling temperature), and agree on seal strength testing methods and acceptance criteria before bulk production begins.
Practical Buying Checklist
- Specify product type, filling temperature and filling weight
- Confirm packaging machine brand, model and sealing jaw type
- Specify heat seal layer type (PE or CPP) and required seal strength
- Discuss product characteristics: dusty, oily, liquid, or frozen
- Request pre-production sample and test on actual filling line
- Confirm eye mark type, position and sensor compatibility for roll film
- Specify roll width and COF range if using roll film
- Agree on pouch size tolerances and accessory position tolerances
- Confirm seal strength testing method (ASTM F88 or equivalent)
- Run a small production trial before committing to full bulk order
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Frequently Asked Questions
Why do my pouches leak even though the material looks fine?
Leakage can result from insufficient sealing temperature, product residue in the seal area, incorrect sealing pressure, film tension issues, or pouch dimensional tolerances. Check all machine parameters and test with clean, empty pouches to isolate the cause.
Can the same film work on different filling machines?
Not necessarily. Different machines have different sealing jaw types, temperature ranges, and film feeding mechanisms. Film that works on one machine may not seal properly on another. Always test film on the specific machine that will be used for production.
What is seal-through-contamination film?
This is film with a specially formulated seal layer that can form a strong seal even when small amounts of product residue are present in the seal area. It is particularly useful for powder and oily products but may cost more than standard seal layers.
How can I test seal strength before bulk production?
Seal strength is typically tested per ASTM F88 using a tensile tester. The supplier can provide seal strength data from production samples. For the most reliable results, test pouches on your own filling line with actual product and then measure seal strength.