First Trial Run on New LLDPE Fails: A Structured Troubleshooting Sequence

Linear low-density polyethylene (LLDPE) is an ethylene/α-olefin copolymer with density from 0.916 g/cm³ to 0.940 g/cm³ when measured by ASTM D792-20 and melt index from 0.5 g/10 min to 50 g/10 min when measured by ASTM D1238-20 at 190 °C and 2.16 kg. In a first trial run of a new butene-based LLDPE film grade on a 75 mm single-screw cast-film extruder with 30:1 L/D, 250 mm slot die, and downstream chill-roll haul-off, the line failed to reach steady state because melt pressure oscillated by more than ±0.5 MPa and die-lip gel accumulation increased haze beyond 8%. The response followed a structured troubleshooting sequence rather than immediate adjustment of the additive package or resin lot.

Does Melt Rheology Account for a New LLDPE Trial Run Pressure Oscillation?

LLDPE has a narrow molecular weight distribution and no long-chain branching. This molecular architecture produces lower shear sensitivity than high-pressure low-density polyethylene and a higher apparent viscosity at low shear rates. In a capillary rheometer operated to ASTM D3835-16 at 190 °C, a 1.0 g/10 min melt-index LLDPE typically exhibits an apparent viscosity near 800 Pa·s at 100 s^-1 and near 200 Pa·s at 1000 s^-1. The critical wall shear stress for sharkskin melt fracture in LLDPE is reported between 0.10 MPa and 0.20 MPa, whereas low-density polyethylene can tolerate higher wall shear stress before surface fracture. In a slot die with 0.8 mm lip gap, a throughput of 200 kg/h and melt density near 0.74 g/cm³ gives an apparent wall shear rate above 1000 s^-1; the die-lip shear stress therefore enters the melt-fracture zone if the melt temperature remains below 220 °C. The first corrective trial lowered the rear barrel temperature, which increased melt viscosity at the die and made the oscillation more severe.

Rheological testing with ISO 11443:2021 should include Bagley-corrected shear stress and Rabinowitsch-corrected shear rate. Entrance pressure loss below 0.2 MPa at 1000 s^-1 indicates that the failure is not dominated by the die entry region. When entrance pressure loss exceeds 0.5 MPa, the die-entry geometry, not the lip land, should be modified. The melt relaxation time of LLDPE is short, so the pressure oscillation amplitude can be used as a real-time proxy for melt-temperature variation at the die lip. In plant operation, a pressure oscillation exceeding ±0.5 MPa at 14 MPa die pressure is sufficient to disrupt melt curtain edge stability. The die-lip temperature should be profiled with at least five exposed-tip thermocouples across the width; a variation above 3 °C creates local viscosity differences that amplify edge neck-in. Published data for this specific grade configuration is limited; however, the observed pressure variation is consistent with melt-fracture onset rather than gross melt inhomogeneity.

When a 30:1 L/D Single-Screw Extruder Is Operated Below the Feed Throat Bridging Temperature

LLDPE pellets soften at lower temperature than high-density polyethylene. Feed-throat bridging occurs when the feed section reaches 60 °C to 70 °C, causing pellets to flatten and form a solid plug. The first corrective attempt reduced the rear barrel zone from 200 °C to 170 °C to improve solids conveying. This action starved the second barrel zone of molten film and raised pre-die pressure. For a 1.0 g/10 min melt-index LLDPE, barrel zone 1 should be held at 180 °C to 190 °C with a water-cooled feed throat at 40 °C to 50 °C, while zones 2 through 5 increase in 10 °C increments to a die temperature of 220 °C to 230 °C. Running below the melting point in zone 2 starves the barrier section and produces screw-speed-dependent pressure instability. A grooved feed section with compression ratio 2.8:1 to 3.5:1 improves pellet drag and reduces air entrapment. Pre-drying is not required below 60% RH.

Melt-pressure control required a gear pump between the screen changer and slot die. The suction-side pressure was maintained at 5.0 MPa to 8.0 MPa to avoid cavitation, and the discharge-side die pressure was targeted at 12.0 MPa to 16.0 MPa for the 250 mm slot die with 0.8 mm lip gap. Pressure fluctuation greater than 0.3 MPa at the die inlet corresponded to film thickness variation above ±5% at 200 kg/h. The screw speed was trimmed from 80 rpm to 65 rpm while the gear pump speed was increased to maintain throughput, but die-pressure oscillation persisted until the melt temperature at the die lip exceeded 215 °C. This indicated that the instability was thermal and rheological rather than purely mechanical. Because LLDPE is shear-sensitive in the high-shear region, increasing screw speed without raising melt temperature will increase die pressure and melt fracture.

Comonomer Type, Gel Bodies, and Additive Carryover in Film-Grade LLDPE

Butene-based LLDPE film grades generally contain 5 mol% to 10 mol% 1-butene. Under polarized light microscopy, translucent gel bodies between 50 μm and 300 μm were observed in the first trial film. These gel bodies were absent in the incoming pellets and formed during residence time in the screen pack and breaker plate. The screen pack was a 60/80/100 mesh combination, and the differential pressure across the pack rose from 3.5 MPa to 7.2 MPa over 45 min. Hot-stage FTIR microscopy showed no carbonyl absorption at 1715 cm^-1, ruling out gross oxidative degradation but not localized crosslinking. Oxidative gel formation was consistent with the measured oxidative induction time of 18 min at 200 °C by ASTM D3895-19; this is below the 25 min minimum typically required for long-residence-time cast-film start-ups. The stabilizer package was not altered before checking barrel residence-time distribution.

Comonomer length changes the tie-chain population and lamellar thickness. Butene-based LLDPE at 0.920 g/cm³ has lower dart impact than hexene-based LLDPE at the same density, with typical film dart impact values of 200 g to 400 g for butene-based and 400 g to 800 g for hexene-based at 50 μm in ASTM D1709-22. The observed gel bodies were differentiated from comonomer-rich areas by hot-stage FTIR microscopy. Because butene-based LLDPE has lower melt strength than high-pressure LDPE, cast-film edge neck-in and draw resonance become more sensitive to die-lip temperature imbalance during a first trial run.

The First Treatment Is a Resin Characterisation Panel, Not a Formulation Change

Before any additive change, the as-delivered resin was tested. Melt index by ASTM D1238-20 at 190 °C/2.16 kg, density by ASTM D1505-18, and differential scanning calorimetry by ISO 11357-3:2018 established that the pellets fell within the 0.920 g/cm³ to 0.922 g/cm³ density band and 1.0 g/10 min to 1.2 g/10 min melt-index band. The DSC thermogram showed a single melting peak at 122 °C and crystallization onset at 106 °C. A broad pre-melting shoulder was not observed, ruling out gross contamination by high-pressure LDPE but not high-molecular-weight gel bodies generated during pellet conveying.

For food-contact film, LLDPE resin must meet 21 CFR 177.1520(c) items 3.1a or 3.2a and EU Regulation 10/2011 overall migration limit of 10 mg/dm² for general food-contact conditions. A change in antioxidant or slip package during troubleshooting must be revalidated against these regulatory citations and not inferred from processing performance. Slip and antiblock additives, typically erucamide at 500 ppm to 1000 ppm and silica at 2000 ppm to 5000 ppm, were pre-compounded. The failure investigation required a 24 h purge and restart to separate additive decomposition from resin gel formation.

Applying a Structured Troubleshooting Sequence and Instrumentation Thresholds

The failure was resolved by applying the following sequence before any formulation change. Each corrective action was accepted only when the associated measurement crossed a defined threshold. The sequence assumes a single-screw cast-film configuration with a 30:1 L/D barrel, gear pump, and slot die; it does not apply directly to blown-film lines where bubble stability is controlled by blow-up ratio and frost-line height.

Following the sequence, the line returned to steady state at 65 rpm and 200 kg/h with die pressure 14.2 MPa, melt temperature 223 °C, and thickness variation ±2.5% on 50 μm cast film. The die-lip gel count fell below 10 bodies per square meter by polarized light microscopy after a 30 min purge with a 2 wt% silica LLDPE purge compound. No additive change was required. The same troubleshooting sequence may not transfer to hexene- or octene-based LLDPE blown-film lines without revalidation of frost-line height, blow-up ratio, and melt strength.