The practical resolution of PP fish eyes, gel bodies, and melt fracture in production—frequently framed as “Solved: PP Fish Eyes, Gel & Melt Fracture in Production”—depends on separating optical defects by size, thermal behavior, and shear history rather than treating them as a single quality deviation. On a 3.2 m-wide cast-film line using a 120 mm single-screw extruder with 30:1 L/D and a 1.0 mm die gap, fish eyes generally appear as translucent or opaque circular spots between 0.10 mm and 0.50 mm in diameter; gel bodies often exceed 0.50 mm and exhibit irregular or elongated shapes; melt fracture appears as periodic surface ridges with amplitude below 0.05 mm but frequency high enough to reduce gloss. Automated camera systems with 20 µm to 50 µm pixel resolution classify defects according to size and aspect ratio, but optical inspection alone cannot distinguish a high-molecular-weight fish eye from a crosslinked gel.

Polypropylene homopolymer films are typically extruded from grades with a melt flow rate of 2 g/10 min to 10 g/10 min measured at 230 °C under 2.16 kg according to ISO 1133-1:2022 or ASTM D1238-20. The melting peak occurs between 160 °C and 165 °C by ISO 11357-3, but barrel set temperatures from 200 °C to 250 °C are required to overcome the high-molecular-weight tail and to reduce melt viscosity sufficiently for die forming. A fish eye can therefore coexist with otherwise acceptable melt temperature if the heat transfer in the pellet bed is insufficient to fully melt large granules, reactor skin particles, or high-molecular-weight fractions.

What Distinguishes Crosslinked Gel Particles from High-Molecular-Weight Fish Eyes in PP?

A fish eye consists primarily of an unmelted or partially melted high-molecular-weight isotactic polypropylene fraction. Hot-stage optical microscopy shows that a typical fish eye softens and flows when heated above 170 °C, although the surrounding matrix may melt at 163 °C. In contrast, a crosslinked gel particle remains infusible or only decomposes above 220 °C. Fourier transform infrared microscopy of gel bodies often shows a carbonyl band between 1710 cm⁻¹ and 1740 cm⁻¹, reflecting thermo-oxidative degradation products, while unoxidized fish eyes do not. Solvent extraction according to ISO 16152:2005 can quantify xylene-soluble fraction; both high-molecular-weight isotactic fish eyes and crosslinked gels may contribute to insolubles, so this test alone is insufficient without thermal or spectroscopic confirmation.

In production, the difference determines the corrective action. If the defect is a high-molecular-weight fish eye, the response is a higher barrel profile or a change in resin lot with lower extruder back-pressure; if the defect is a crosslinked gel, the response is a purge, screw inspection, or antioxidant package adjustment. On a 90 mm coextruder running a PP skin layer, gel defects have been observed to increase above 240 °C during start-up when oxygen is not fully displaced from the feed throat.

Thermo-Oxidative Degradation During Repetitive Screw Shear

Barrier-screw extruders with 30:1 L/D and 3:1 compression ratio process PP film-grade resins at specific energy inputs between 0.15 kWh/kg and 0.25 kWh/kg. In the compression zone, melt temperatures can exceed the barrel set point by 10 K to 30 K because of viscous dissipation. This zone is a source of gel bodies when dead spots occur at screw roots, mixing pins, or thermocouple wells. Thermo-oxidative degradation of polypropylene generally proceeds by chain scission, reducing melt viscosity, but in oxygen-starved high-residence-time regions secondary reactions can generate infusible crosslinked material. Stabilizer packages containing a hindered phenolic antioxidant at 0.05 wt% to 0.15 wt%, a phosphite stabilizer at 0.05 wt% to 0.15 wt%, and an acid scavenger at 0.02 wt% to 0.10 wt% are typical. Oxidative induction time measured by ASTM D3895-19 should remain above 20 min at 210 °C for film-grade PP; lower OIT values indicate poor stabilizer dispersion or repeated heat history from regranulated edge trim.

When barrel set temperatures are reduced below 210 °C to reduce smoke or off-taste, high-molecular-weight fish eyes increase, especially with random copolymers containing ethylene. A reduction of 5 °C in the feed zone may be sufficient to alter pellet bed compaction and increase the apparent unmelt fraction; the exact torque rise depends on screw design and resin crystallinity. The effective processing window for defect-free PP film is often narrower than the bulk melt-processing range. When melt temperature at the die is below 220 °C, fish eyes become more frequent; when melt temperature exceeds 260 °C, gel bodies and yellowing increase. A practical melt-temperature window of 220 °C to 250 °C is therefore maintained, with alarm limits at ±5 °C.

In narrow flat-die gaps from 0.4 mm to 1.0 mm, polypropylene melts can exhibit two regimes of melt fracture: sharkskin and gross melt fracture. Sharkskin initiates at the die exit when the melt surface is subjected to high extensional stress as the velocity profile rearranges from the die land; gross melt fracture originates upstream and produces severe volume distortions. Capillary rheometry according to ASTM D3835 or ISO 11443 is used to determine the critical wall shear stress. For linear polyolefins, the critical value is commonly reported in the range of 0.10 MPa to 0.30 MPa, but polypropylene can shift this boundary based on melt temperature, comonomer content, molecular weight distribution, and die-entry geometry.

Melt fracture is not a gel even though it appears as haze. It is solved by changing die geometry or coating, by reducing extrusion rate, by raising die temperature by 5 °C to 10 °C, or by adding a fluoropolymer processing aid at 200 ppm to 800 ppm. On a 75 mm single-screw extruder running a BOPP adhesion layer, raising the die-zone set point from 230 °C to 240 °C reduced melt fracture without increasing gel count. The critical shear rate is resin-specific and should be measured on the production resin batch because regranulated material can broaden the molecular weight distribution and lower the onset.

When Pressure Fluctuations Exceed ±0.5% at the Die Lip

Die-lip pressure fluctuation is a direct indicator of unstable melt delivery. On a cast-film line equipped with a gear pump, the pump inlet pressure should be stabilized between 5 MPa and 10 MPa; the pump outlet may reach 15 MPa to 25 MPa depending on die restriction and throughput. A fluctuation of more than ±0.5% at the die lip can produce periodic thickness variation and can initiate melt fracture even if the time-averaged shear stress is below the critical value. The gear pump decouples die pressure from extruder screw pulsation, but excessive screen pack loading can starve the pump and introduce pressure oscillation.

A typical film line uses a screen pack of 100/150/250 mesh or finer. When differential pressure across the screen pack exceeds 8 MPa, gel particles and degraded material can deform and pass through the breaker plate, or screen damage can release accumulated debris. Replacement at 5 MPa to 8 MPa differential is common to avoid pressure surging. For optical-grade PP film, a final filtration rating of 25 µm to 50 µm is used, but too fine a pack raises melt temperature and can shear-degrade polymer.

Additive selection for defect reduction in PP film is constrained by the same compatibility limits that influence optical clarity. Calcium stearate is used as an acid scavenger at 0.05 wt% to 0.15 wt%, but excess levels above 0.20 wt% can migrate and create die-lip deposit. Hydrotalcite-based neutralizers at 0.02 wt% to 0.10 wt% reduce residual catalyst residues without the same deposit tendency but can hydrolyze if stored at relative humidity above 60%. Fluoropolymer processing aids for melt fracture control are added by masterbatch because the active level is low and uniform dispersion across the die lip is required. These additives do not solve fish eyes caused by high-molecular-weight resin fractions; they only alter surface slip at the die wall. A purge compound with an MFR above 20 g/10 min is used after lot changes to displace dead zones, but purging cannot remove a severely crosslinked deposit that is mechanically attached to a screw root or thermocouple well.

Acceptance Thresholds, Automated Defect Classification and MFR Drift Limits

Routine production acceptance for PP cast film often uses automated surface inspection maps to count defect classes by number per square meter. General-purpose film may tolerate up to 20 defects/m² of size 0.10 mm to 0.30 mm, while optical or capacitor-grade film requires fewer than 5 defects/m² above 0.10 mm. However, published data for this specific configuration is limited because acceptable defect densities are proprietary and depend on film thickness, layer structure, and end-use. MFR drift should be monitored according to ISO 1133-1:2022 or ASTM D1238-20; a reduction of more than 0.5 g/10 min from the reference lot may indicate high-molecular-weight contamination or insufficient melt blending, while an increase above 1.0 g/10 min often indicates chain scission from repeated heat history.

Melt fracture onset should be measured with the production resin batch because capillary rheometry at 230 °C with a 1 mm die can produce critical wall shear stress values that differ by 0.05 MPa to 0.10 MPa between lots. A narrower molecular weight distribution tends to reduce sharkskin but may not suppress gross melt fracture if die-land residence time is short. The Bagley correction and Rabinowitsch correction must be applied when comparing shear stress data; otherwise entrance pressure losses are misinterpreted as melt fracture sensitivity.

For food-contact PP, FDA 21 CFR 177.1520 applies to olefin polymers, and EU compliance uses EU 10/2011 with overall migration limits. These regulatory standards do not directly address fish eyes, but they constrain the chemical systems that can be used to solve them. RoHS 2011/65/EU and REACH SVHC screening apply to electrical and electronic film applications and exported packaging when specified.