Film quality complaints rarely say "your defect count per square meter exceeded spec." They say "the film has fisheyes," or worse, "there are gels everywhere." These words get used loosely — and misidentifying the defect type sends the corrective action in the wrong direction. This guide defines the defect types that matter in PE, PP, PET and PC film, where each one comes from, and what it takes to detect them reliably.

The five defect families

Gel (fish-eye, crystalline point). A small, translucent or transparent blob visible against the film. Gels originate from high-melting-point impurities, crosslinked resin, or unmelted polymer fragments surviving the extrusion screen pack. They are the most common defect complaint on optical-grade film.

Black speck. An opaque dark particle — typically carbonized resin, degraded polymer stuck in dead zones of the extruder, or contamination from conveying lines. Black specks are the most damaging defect commercially: customers can see them at arm's length on clear film.

Fiber. Thin, elongated contamination — usually from packaging (jute, paper), drying air filters, or operator clothing. Fibers align with the machine direction and are often missed by area-based sampling because they cover little total area.

Wrinkle / fold. A processing defect rather than contamination: visible streaks or creases from uneven chill-roll contact, tension imbalance, or gauge variation. Wrinkles rarely cross the "defect" threshold on their own but compound with thickness variation to fail downstream converting.

Hole (pinhole). A full-thickness breach — from oversized contaminants pushed through the die, bubbles, or from a gel or speck that tore out during drawdown. Holes are catastrophic for barrier film and laminate grades.

Why correct identification changes the fix

The countermeasure for each family is different: gels point to resin quality and melt filtration; black specks point to thermal history and dead zones in the extruder and to contamination upstream; fibers point to plant hygiene and filtration; wrinkles point to process control, not cleanliness at all. A defect log that only counts "defects per m²" cannot drive any of these decisions — a classified defect log can.

Detection thresholds that hold up in practice

Manual visual inspection catches what a trained inspector sees under controlled lighting — typically defects from roughly 200 μm upward, with performance degrading over a shift. Automated line-scan systems change the game:

  • Gels and fisheyes: reliably detected from 20 μm upward (GF-20 Online with an 8,192-px line-scan camera)
  • Black specks: same 20 μm class on line-scan systems, and they are the defect type where automation pays back fastest, because human fatigue matters most for dark-point detection
  • Fibers: length-based classification — flag anything above your customer's visible threshold
  • Holes: detected by transmission change, effectively never missed by a line-scan system regardless of size

Results should be reported in the units your customer contracts in — defects per m², defect area in mm²/m², or PPM of defect area — all of which a classified, sized defect database produces automatically.

The take-away

Before escalating a film quality complaint, classify it. A two-minute look through a classified defect log tells you whether the fix belongs in resin purchasing, melt filtration, plant hygiene, or process control — four completely different conversations with four different owners. If you don't have that log yet, that's the capability to build first.