How to Fix Uneven Film Thickness on Blown Film Lines: A Guide for Ghana Film Plants

A roll that is thick on one side and thin on the other looks like a minor annoyance until it reaches the printer. Then it becomes uneven winding tension, hard bands and baggy edges, and registration that will not hold at speed. For a plant in Accra, Kumasi, or Tema, the gauge problem usually comes from three places — the die, the heat zones, and the air ring — and Ghana’s humid air adds a fourth that most guides leave out.

Where Uneven Film Thickness Starts: Die Lip, Heat Zones, and Air Ring

Start at the die lip. The gap between the mandrel and the outer ring is what sets how much melt goes to each point on the circumference. After a colour change, a purge, or a rough clean-up, that gap shifts. A few microns of difference changes the flow resistance, and the melt takes the path of least resistance, so more material leaves the wide side and a thick band forms along that axis of the bubble.

The second place is the heat zones. The die body is wrapped in several zones so the melt viscosity matches at every exit point. If one heater band burns out, or a thermocouple loses contact and reports a false number, a cold spot develops. Cooler plastic flows slower, so the film above that zone leaves thinner while the hotter zones deliver more.

The third is the air ring. It has to deliver an even curtain of air around the bubble base. If it sits off-centre, or a channel is partly blocked by wax and dust, one side gets higher velocity. That side cools faster, locks earlier, and stretches less — which leaves it thicker.

In Ghana, drafts belong on that list too. An open door, a wall fan, or a workshop vent that hits one side of the bubble does the same thing as an unbalanced air ring, and it changes with the weather.

Correcting Gauge Variation: One Variable at a Time

Correcting gauge is a process of elimination, and you change one thing at a time so you can tell what moved.

The die comes first, and it has to be checked at operating temperature — which means this is a hot-job. Wear heat-resistant gloves and a face shield, work with a trained technician, and never stand directly in front of the lip opening. Do the coarse gap work cold, and keep hot adjustment to small trims.

Measure the lip opening around its full circumference with a feeler gauge, then adjust the push-and-pull centering bolts in quarter turns. Let the flow settle for several minutes between adjustments, and judge the result by measuring the film, not by watching the gauge — the film is the only honest reading.

Next, validate the thermal profile. Check the current draw on every die zone to confirm each band is drawing what it should, and check that the thermocouples are making proper contact. A band that is still “on” can be half dead and drawing less than it should.

Then balance the air ring. Use a digital anemometer at several points around the lip, clean the channels if velocity drops in one sector, and re-center the ring on the mandrel if the pattern follows the ring’s position. On a line with Internal Bubble Cooling, confirm the intake and exhaust blowers are in step, because internal pressure swings distort the bubble shape and show up as gauge error.

Ghana’s Humidity and Gauge Drift: Moisture, Resin, and Power

Damp air removes less heat. Ghana’s coastal humidity means the cooling air arriving at the ring carries moisture, so the heat transfer rate drops and the frost line drifts through the day. As the frost line moves, the drawdown ratio changes, and the thickness moves with it — often as a lengthwise trend rather than a one-sided band.

A moisture trap or desiccant on the air ring blower intake is a cheap way to steady this. Keep it on the maintenance list during the rainy season, when the problem is worst.

The resin matters as much. Recycled LDPE from water sachet stock often carries moisture, and moisture becomes vapour in the bubble, which disturbs both gauge and bubble stability. Dry the material before it reaches the hopper, and the gauge problem gets simpler.

Power is the other local variable. Voltage swings and Dumsor interruptions change what the heater bands actually deliver, so the thermal profile moves even when the controller display does not. Check band current with an ammeter, consider a stabiliser on the control side, and log gauge against power events — the pattern is often obvious once you write it down.

Condensation is worth watching in the wet months as well. Chilled rollers that sweat will drag or mark the film, and that gets misread as a thickness problem.

Automatic Gauge Control and Haul-Off: What They Can and Cannot Fix

Automatic gauge control does not do what many brochures imply. On most lines it works through either segmented air ring airflow or automatic die-bolt heating at the lip — not through heaters built into the air ring. It narrows the error, but it still needs a sound mechanical baseline underneath.

An oscillating haul-off or a rotating die head does not remove thickness variation. What it does is spread any thick band around the roll instead of stacking it in one place, which is what turns a thick band into a hard roll that will not convert cleanly.

Uneven gauge is a mechanical, thermal, and aerodynamic problem before it is anything else. Settle the die, confirm the zones, balance the ring, then control the moisture and the power supply, and the roll will hold its tolerance through the humid season rather than only in the dry one.

If your plant is fighting one-sided rolls, hard bands, or gauge that drifts through the shift, or you are comparing blown film lines for Ghana’s humid, unstable-power conditions, our technical team would be glad to review your setup.