Introduction
Film width and tension adjustment is the most frequent fine-tuning task on a flow wrapper — and the one that most directly affects package appearance, seal quality, and material waste. A machine that is perfectly set up for one film type and product size will need re-tuning when either variable changes.
This guide covers the systematic approach to adjusting film width (through the folding box and forming shoulder) and film tension (through the unwind system and dancer arm), with specific values and verification methods our service engineers use during commissioning.
Key Takeaways
- Film width is set at the folding box, not the unwind station — The folding box determines the tube diameter; the unwind system only controls tension, not width
- The 3mm overlap rule — For a fin seal, the two film edges should overlap by exactly 3-5mm at the fin seal wheels; less causes weak seals, more causes wrinkles
- Tension must be consistent through the entire film path — Check tension at three points: after the dancer arm, before the forming shoulder, and at the fin seal wheels
- Different films need different tension — BOPP tolerates higher tension (2.0-3.0 kg); thin PE requires lower tension (0.8-1.5 kg) to prevent stretching
- Document your settings — Record film type, width, thickness, unwind tension, and folding box position for each product; this makes the next changeover 5x faster
How to Adjust the Film Width ?
1.1 Where Film Width Is Set
Film width on a flow wrapper is determined by two interacting components:
- Folding box: The primary width control. Adjusting the folding box width changes the diameter of the film tube before it enters the fin seal wheels. This is a mechanical adjustment — turning a handwheel or loosening/tightening set screws.
- Forming shoulder: The film path guide that transitions flat film into a tube shape. The shoulder size is fixed for a given machine model and film width range. Using a film width outside the forming shoulder’s design range will always cause problems regardless of folding box adjustment.
1.2 Why Width Adjustment Matters
Incorrect film width causes specific, diagnosable problems:
- Too wide (excessive overlap at fin seal): Wrinkles at the fin seal, uneven seal width, film bunching at the forming shoulder entrance
- Too narrow (insufficient overlap): Weak or open fin seals, film edges not meeting at sealing wheels, product exposure
- Film not centered: Fin seal offset to one side, inconsistent seal width along package length, product shifting during wrapping
1.3 The 3mm Overlap Rule
At the fin seal wheels, the two edges of the film should overlap by 3-5mm. This provides enough material for a strong seal without excess film that wrinkles. To measure:
- Stop the machine with film threaded but not running.
- Mark both film edges with a fine-tip marker at the fin seal wheel entrance.
- Measure the distance between the two marks — this is your overlap.
- Adjust folding box width until overlap is 3-5mm.
2. Step-by-Step Width Adjustment
Step 1: Prepare the Machine
Thread film through the full path but do not engage the fin seal wheels or end seal jaws. Run the machine at low speed (10-15 ppm) and observe the film at the forming shoulder. The film should enter the shoulder centered and leave it forming a smooth tube with no wrinkles.
Step 2: Measure Current Overlap
Stop the machine. Mark both film edges at the fin seal wheel entrance. Measure with a ruler — this is your current overlap. If it is 3-5mm, width is correct. If not, proceed to adjustment.
Related: Threading Film on a Flow Wrapper:
Step 3: Adjust the Folding Box
The folding box has a width adjustment mechanism (typically a handwheel or threaded rod). Changes are incremental:
Related: Reducing Film Waste in Flow Wrapper
- To decrease overlap: Widen the folding box (turn handwheel clockwise on most machines)
- To increase overlap: Narrow the folding box (turn handwheel counter-clockwise)
Make adjustments in 1-2mm increments. Re-measure overlap after each adjustment.
Related: Film Wrinkling: Causes, Prevention, and Correction
Step 4: Verify with Production Test
Once overlap is 3-5mm, run 20 empty bags. Check that the fin seal is centered, consistent in width (5-8mm), and wrinkle-free. For printed film, verify eye mark registration remains stable after width adjustment (width changes can shift the registration point slightly).
3. Film Tension Adjustment
3.1 The Three Tension Zones
Film tension is not a single value — it varies through the machine:
| Tension Zone | Location | What It Controls | How to Measure |
|---|---|---|---|
| Unwind tension | Between roll and dancer arm | Prevents over-unwinding, sets baseline tension | Set via unwind brake or motor torque |
| Path tension | Between dancer arm and forming shoulder | Film flatness, registration stability | Finger press test (10-15mm deflection) |
| Sealing tension | At fin seal wheels and end seal jaws | Seal consistency, package tightness | Visual: no film bunching at seal entry |
3.2 Adjusting Unwind Tension
For pneumatic brake unwind systems:
Adjust the air pressure regulator. Higher pressure = higher brake force = higher tension. Typical starting point: 0.15-0.25 MPa (1.5-2.5 bar) for 30-micron BOPP. Reduce pressure for thinner films, increase for thicker films.
For motorized unwind systems:
Adjust the torque setting in the HMI or drive parameter. Typical range: 10-30% of maximum motor torque. The correct setting produces consistent dancer arm position (mid-range, not bottomed out or fully extended).
3.3 Tension by Film Type
| Film Type | Typical Thickness | Recommended Tension (400mm width) |
|---|---|---|
| BOPP | 20-40 microns | 2.0-3.0 kg |
| CPP | 25-50 microns | 1.5-2.5 kg |
| PE (LDPE) | 30-60 microns | 0.8-1.5 kg |
| Laminated (BOPP/PE) | 40-70 microns | 2.5-4.0 kg |
| Metallized BOPP | 20-30 microns | 1.5-2.5 kg |
Note: These are starting values. Adjust based on film behavior at operating speed. Higher speeds require slightly higher tension to prevent necking-in.
3.4 Dancer Arm Position
The dancer arm should oscillate within the middle third of its travel range during normal operation. If the arm consistently bottoms out (minimum extension), unwind tension is too low — film is being pulled faster than it is supplied. If the arm stays fully extended, unwind tension is too high — film is being supplied faster than it is consumed.
Dancer arm troubleshooting:
- Arm oscillates wildly (more than 50% of range): Insufficient damping. Check pneumatic cylinder damping adjustment or add flow control valves.
- Arm drifts to one end over several minutes: Unwind brake or motor torque is drifting. Verify consistent air pressure or drive parameter stability.
- Arm position inconsistent between rolls: Different roll diameters or core fits. Verify core is seated properly on mandrel.
Frequently Asked Questions
Q: How do I know if my film tension is too high?
Three signs: (1) Film visibly necking-in (narrowing) as it enters the forming shoulder — measure film width before and after threading; a reduction of more than 2% indicates excessive tension. (2) The fin seal shows stretch marks or a “wavy” appearance under tension. (3) The dancer arm is fully extended and appears to be pulling hard. Additionally, if you measure package length and it is consistently 1-2mm longer than expected, the film is being stretched during wrapping.
Q: Why does my film wrinkle at the forming shoulder even after adjusting width?
Wrinkles at the forming shoulder usually have one of three causes: (1) The forming shoulder itself is misaligned — check that mounting bolts are tight and the shoulder is parallel to the machine frame. (2) The film roll is not centered on the mandrel, causing uneven tension across the film width. (3) A guide roller before the forming shoulder is out of parallel — use a spirit level to verify all rollers are level and parallel.
Q: Can I use the same tension setting for different film roll diameters?
Yes, if your machine has a closed-loop tension control system (dancer arm with feedback). The dancer arm automatically compensates for changing roll diameter by adjusting unwind brake pressure or motor torque. If your machine uses manual brake adjustment (open loop), you will need to reduce brake pressure as the roll diameter decreases — typically check and adjust every 25% of roll consumption for thin films (below 30 microns), and every 50% for thicker films.
Conclusion
Film width and tension adjustment is not a one-time setup — it is a recurring task that changes with every film type, film batch, and product configuration. Master the 3mm overlap rule, learn your machine’s tension sweet spot for each film type, and document every successful setting. These habits eliminate the most persistent source of package quality variation.
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