This machine tutorial explains how to operate and troubleshoot film unwinder tension control on t-shirt bag machines on plastic bag making machines—T-shirt, courier, pouch, and non-woven lines. It is written for shift supervisors, maintenance technicians, and application engineers who need repeatable procedures—not theory alone.
Machine scope and operating context
Yaoshg field teams use this discipline on presses and converting lines built in Wenzhou—from early stack flexo units through CI, gravure, laminating, slitting, bag making, and paper container equipment. The steps below assume normal safety lockout rules, OEM manual limits, and documented substrate specifications for each job.
T-shirt bag quality begins at the unwinder, yet many lines are tuned starting at the seal bar. When parent roll tension is unstable, downstream dancer loops chase disturbances that show up as bag length variation, punch misalignment, and intermittent stacker jams. Unwinder discipline is the first control layer.
Brake or motor-tension control should be calibrated against roll diameter models, not fixed manual settings. As diameter decreases, inertia drops and the same brake command produces different web tension. Lines without diameter compensation often run well on full rolls and poorly on cores.
Step-by-step machine procedure
Dancer position is a diagnostic tool as much as a buffer. If the dancer rides consistently high or low through a run, the unwind zone is biased and downstream sealing windows compensate incorrectly. Center the dancer in its travel band before fine-tuning seal temperature or knife timing.
Roll-change events are high-risk periods. Splice tails, off-center cores, and temporary slack loops introduce tension spikes that propagate to punch stations within seconds. Standard work should define splice speed, handoff verification, and a short re-stabilization window before returning to rated output.
Bag making converts printed roll stock into sealed packs. Dancer and accumulator settings must match upstream unwind variability. Seal window—temperature, dwell, pressure—depends on film gauge and ink coverage.
Courier mailer and coex programs need seal-strength validation at line speed, not only on static samples. Auto splicers reduce downtime but require tension taper tuning to avoid transient seal defects.
Operator shift checklist
- Centerline seal temperature, dwell, and pressure for film gauge.
- Verify dancer response and accumulator limits on infeed.
- Check cutoff length, punch alignment, and stack height.
- Seal-strength spot check per shift on coex or printed film.
Common defects and corrective adjustments
Film gauge and slip characteristics change effective tension response. HDPE and LDPE blends, recycled content, and coex structures each need documented baseline brake profiles. Copying tension settings between material families without validation is a common source of handle asymmetry complaints.
Integrate unwind alarms into shift reviews. Trends such as repeated dancer saturation near roll end often indicate core quality issues or brake pad wear rather than seal problems. Fixing upstream tension saves hours of downstream troubleshooting that never addresses the actual disturbance source.
For commissioning, run tension mapping at low, nominal, and high speeds with data logging enabled. Compare unwind zone setpoints to actual bag length statistics over 200 consecutive pieces. Correlation between tension stability and length CpK gives objective evidence that unwinder tuning is production-ready.
Gusset asymmetry usually means former misalignment or unequal nip on fold rails. Non-woven ultrasonic seal issues often trace to horn wear or insufficient web clamp force.
Maintenance records and when to call service
Seal bar cleaning and punch alignment checks belong on daily checklists for e-commerce bag lines. Centerline cutoff length after film supplier changes.
If mechanical adjustment, drive parameter changes, or repeated defects exceed on-site scope, log serial number, job recipe, and photos before contacting Yaoshg service. Commissioning engineers can remote-review HMI trends when VPN or data export is available—faster resolution when shift records are complete.