Zhejiang Mingli Machine Technology Co., Ltd.
Most downtime and scrap on a bag making line doesn't come from a machine that suddenly breaks. It comes from tension, temperature, or alignment drifting a little at a time until bags start failing. This guide covers six common problems, with the symptoms to look for, the usual causes, what to check first, and how a machine can be designed to prevent them. The causes listed here are general industry experience, so always follow your own machine manual for specific procedures.
| What You See | Check First |
| Seals peel or leak | Sealing parameters against the film, and contamination in the seal area |
| Wrinkles or bubbles | Unwind tension and roller alignment |
| Bag length varies | Feed rollers, sensors, and the active parameter set |
| Film keeps breaking | Tension changes and film roll quality |
| Valves sit off position | Positioning reference and valve consistency |
| Quality differs between shifts | Whether settings are recorded and reused |
| 1 |
Weak or Leaking SealsLikely causes: temperature, dwell time, or pressure that no longer matches the film; product dust or liquid in the seal area; worn sealing elements. Check first: confirm settings against the current film structure, clean the seal area, and run a seal strength test. Prevention: pulse heat sealing, which needs no preheating and cools quickly after heating, helps keep sealing consistent on pure PE film. |
| 2 |
Wrinkles and Bubbles in the FilmLikely causes: uneven tension, rollers that are not parallel, static, curled film edges. Check first: unwind tension, roller alignment, and the condition of the film roll. Prevention: servo-driven unwinding with ultrasonic sensing and automatic tension control keeps multi-layer film steady, and a vacuum suction port holds the film flat in the bag making area. |
| 3 |
Bag Length Drift and Inconsistent CuttingLikely causes: film slipping at the feed, tension swings, dirty or loose sensors, worn feed rollers, a parameter changed by mistake. Check first: feed roller surfaces, sensor lenses, and whether the active settings match the intended recipe. Prevention: a PLC control system that shows speed, bag length, temperature, and counting on one touch screen makes drift easier to spot early. |
| 4 |
Film Breakage and Unplanned StopsLikely causes: sudden tension changes, roll diameter changes that aren't compensated for, film defects or poor joins, rough film edges. Check first: where the film broke, the quality of that roll, and the tension behavior before the break. Prevention: ultrasonic measurement of roll diameter, floating roll position feedback for consistent film pressure, and automatic film breakage detection with an alarm, so a break is caught immediately instead of running on. |
| 5 |
Valve or Spout MisalignmentLikely causes: positioning error, a valve not fully seated, contamination in the weld area. Check first: the positioning reference, consistency of incoming valves, and cleanliness of the weld area. Prevention: CCD vision positioning with robotic valve mounting takes placement out of the operator's hands. See how this works in our article on CCD vision and liquid bag leakage, or the ML-911 valve adding robot. |
| 6 |
Slow Changeovers and Repeated Setup ErrorsLikely causes: changeovers done by manual adjustment, settings kept in memory, different habits on different shifts. Check first: whether your team has a recorded parameter set and a changeover checklist for each product. Prevention: recipe-based production stores sealing temperature, film tension, bag length, and module position, so a format change is one touch rather than a manual re-setup. |
Clean the seal area at the start of every shift
Inspect sealing elements, heat-resistant layers, and rollers for wear
Wipe sensor lenses and check that sensors are firmly mounted
Record the parameters used for every product and reuse them
Spot-check seal strength during each shift
If the checks above don't solve it, send your supplier the following so they can diagnose faster:
Machine model and the product you were running
Film structure and supplier
Photos or a short video of the problem
The parameters in use
Any alarm message on the screen
Ask three questions before you order. Which problems does the machine detect and alarm on automatically? How are parameters saved and recalled? And how quickly does the supplier respond when something goes wrong, including remote support? Clear, specific answers say a lot about the support you'll get later. Our supplier checklist and spec comparison guide cover how to evaluate both.
Mingli's machines are designed around controlling the variables above: automatic tension control, ultrasonic sensing, film breakage detection and alarm, recipe storage, and CCD-guided valve positioning on liquid bag lines. None of this removes the need for routine maintenance, but it makes problems easier to prevent and faster to spot. For more on reducing stoppages, see our article on less downtime in bag-in-box production.
Dealing with a problem on your line? Send us your machine model, film structure, and what you're seeing, and our technical team will help you work out where to look first.
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