How to Eliminate Gels, Fish Eyes, and Pinholes in Blown Film Extrusion: A Practical Guide for Ghana Factories

Running a blown film extrusion line in Accra, Tema, or Kumasi brings a specific set of daily problems. High ambient humidity, voltage swings, and heavy reliance on locally reprocessed pellets to keep costs down all push against film quality, and the defects that show up most often are gels, fish eyes, and pinholes.

When hard, unmelted specks run across a polythene roll, they do more than spoil the clarity of pure water sachet film or shopping bags. Each fish eye is a weak point: at the bag-making or sachet-filling stage it stretches, tears, and leaks, and the batch comes back.

Why Gels, Fish Eyes, and Pinholes Appear on a Ghana Blown Film Line

Start by working out which defect you actually have, because the fix differs. A clear, oval, lens-shaped speck is unmelted resin. A speck with a dark or amber centre is burnt, cross-linked material. An irregular, opaque particle that looks nothing like the surrounding film is outside contamination. If the defect count changes when you switch pellet supplier, contamination or resin quality is the likely driver; if it stays the same across batches, look at the machine.

Moisture is the first cause to rule out. Locally washed and pelletized scrap often carries residual water from the washing tanks or from the rainy season, and that water turns to steam inside the barrel. The steam disrupts melt flow, leaves pinholes and small bubbles, and keeps the surrounding granules from melting evenly.

Heat is the second. Running the barrel hot to chase output degrades the polymer, and overheated plastic cross-links into hard, rubbery carbon specks that will not melt however high you set the zones. The opposite also happens: cold zones, or a worn screw past its mixing capacity, let hard cores of unmelted pellets pass straight through the die lip.

Voltage belongs on that list in this market. When the supply dips, the heater bands deliver less than the controller believes, so the effective melt temperature falls while the display still reads normal. Gels and unmelted cores then appear without anyone touching a setpoint. A clamp meter on each band, read during a full-load shift, shows whether the zones are actually getting their current; logging defects against power events usually makes the pattern obvious.

Fixing Pinholes and Gels at the Hopper, the Zones, and the Screen Pack

Begin at the hopper. Pre-dry recycled pellets in a hopper dryer for at least two hours, and confirm the dryer is reaching its set temperature rather than assuming it. Dry resin removes the steam source, which is what stops pinholes and steadies melt pressure.

Then set the thermal profile to match the defect. For unmelted resin, raise the compression and metering zone temperatures by 5–10°C to improve homogenisation. For burnt resin, lower the die head and adapter temperatures and inspect for stagnant material pockets where polymer sits and cooks.

Filtration matters more with local regrind than with virgin resin. Replace coarse screens with a multi-layer stainless pack, typically 80, 100, and 120 mesh, to catch fine contaminants before they reach the die. Change screens on a schedule rather than on failure, and watch the pressure trend: a pack that is too fine for the pump creates backpressure of its own.

Preventing Gels When Running Recycled PE: Die Lip Care, Mixing, and Screen Changes

Keep the die lip clean, and treat that as a hot job. Carbon builds at the lip and drags vertical lines into the film, so soft brass scrapers are the right tool — but the die is above 200°C. Wear heat-resistant gloves and a face shield, have a trained person do the work, and do not stand or reach directly in front of the lip opening.

Screen changes carry their own risk. Hot melt sits behind the pack under pressure, so release pressure and follow the lockout procedure before opening the changer, and remember that hydraulic changers store pressure even when the line looks idle.

For plants scaling up on post-consumer material, the equipment side is worth planning. Barrier screws and Maddock mixing sections improve melt consistency and reduce unmelted cores, and a hydraulic screen changer lets the crew swap a clogged pack without a long shutdown — though there is still a brief pressure and flow interruption while it indexes, so plan the change around the run rather than mid-order.

Clean resin, correct zones, and a screen pack that matches the material do most of the work. Add the die lip routine and the voltage check, and the roll comes off clear through the humid season instead of only in the dry months.

If your line is losing yield to gels, fish eyes, or pinholes, or you are specifying blown film equipment for Ghana’s humid, unstable-power conditions, our technical team would be glad to review your setup.