LLDPE Machine Matching: Screw, Die and Cooling by Process Type is determined by the material’s narrow molecular weight distribution, shear-thinning index, and melt strength relative to low-density polyethylene. LLDPE film grades characterized under ISO 1133-1:2022 at 190°C with 2.16 kg generally fall between 0.5 g/10 min and 2.0 g/10 min; density measured by ASTM D792-20 is 0.918 g/cm³ to 0.940 g/cm³. The power-law index at film-extrusion shear rates is commonly 0.5 to 0.6, which means die pressure drops are less throughput-sensitive than in LDPE but the melt remains highly viscous at low shear rates. A single-screw extruder with a barrier flight profile and a Maddock or spiral mixer mounted after the compression zone is preferred because LLDPE resists reorientation of its short-chain branches and can produce unmelted micro-gels if the screw is run too hot. The feed throat should be maintained below 50°C, and the barrel profile for a 24:1 to 30:1 L/D screw is ordinarily flat-to-reverse from 190°C to 220°C to offset viscous heating. Screw drive power for LLDPE is commonly 0.25 kWh/kg to 0.35 kWh/kg on commercial film lines. Head pressures above 35 MPa should not be used to control output because melt fracture appears at die-wall shear stresses near 0.14 MPa.

Material conditioning is process-specific. Cast film lines seldom require predrying below 60% RH, but high-humidity storage can introduce surface moisture that causes bubble defects; hopper drying at 60°C for 1 h is applied where dew point exceeds 15°C. LLDPE does not require the aggressive drying used for PET or PA, but condensation on cold pellets fed into a hot barrel can reduce output by 3% to 5%.

What Screw Configuration Prevents Melt Fracture in LLDPE Blown Film?

On commercial blown film lines, LLDPE machine matching starts with the screw because low melt index resin requires dispersive mixing at moderate melt temperatures. A barrier screw with 24:1 to 30:1 L/D and a compression ratio of 2.5:1 to 3.5:1 is used; the barrier flight clearance is typically 0.8 mm to 1.2 mm, and a Maddock mixer at the metering end reduces melt-temperature variation to below ±1.5°C. The die is a spiral mandrel design rather than a spider design to avoid weld-line streaks in LLDPE. Die gap is held between 1.8 mm and 2.5 mm; reducing gap below 1.5 mm raises die-wall shear stress and promotes sharkskin. Die land length is 10:1 to 15:1 relative to gap. The bubble is inflated to a blow-up ratio of 2.0:1 to 3.0:1, and for 50 μm film the frost line height is 500 mm to 900 mm at line speeds near 80 m/min. Cooling is provided by a dual-lip air ring at 5°C to 15°C; internal bubble cooling at 10°C to 15°C increases heat transfer coefficient to 25 W/m²·K to 50 W/m²·K. Gauge variation on lines without mixing elements can exceed ±7% at 0.5 g/10 min, while a mixer-equipped screw reduces variation to ±3% to ±4%. Dart drop impact tested by ASTM D1709-16a and secant modulus by ASTM D882-18 are common release criteria, but they are influenced more by bubble cooling than by screw geometry.

On blown film lines producing more than 250 kg/h, a gear pump between extruder and die is often installed. The pump inlet pressure is kept at 3 MPa to 6 MPa, and die pressure is maintained at 25 MPa to 30 MPa. This decouples screw speed from die resistance and reduces film thickness variation attributable to pressure pulsation. Screen packs are 200 μm to 400 μm; finer mesh raises head pressure and increases melt temperature.

Machine sizing for LLDPE cast stretch film on a 2.0 m wide line at 50 μm gauge requires a die gap that is intentionally narrow because draw-down rather than bubble expansion controls thickness. A coat-hanger die with lip gap 0.5 mm to 0.8 mm and land length-to-gap ratio of 20:1 to 40:1 is common. The melt curtain is drawn over an air gap of 100 mm to 150 mm; longer air gaps increase neck-in and reduce edge trim yield, while shorter gaps reduce time for stress relaxation. The first chill roll is polished chrome with surface roughness Ra 0.025 μm to 0.05 μm, maintained at 15°C to 30°C to prevent blocking on the winder. Vacuum-box or air-knife pinning is required above 250 m/min to avoid air entrapment. Electrostatic pinning is used on high-speed cast lines because vacuum boxes can mark the melt surface above 300 m/min; the discharge electrode is set at 5 kV to 10 kV and the charge is neutralized before the winding section. For 500 kg/h extrusion rate, chill roll heat load is approximately 150 kW to 180 kW, requiring closed-loop water circulation of 2 m³/h to 4 m³/h per meter of roll width. Screw design for cast LLDPE favors 28:1 to 32:1 L/D with compression ratio 2.8:1 to 3.5:1; melt temperature at the die adapter is 205°C to 260°C. Haze measured by ASTM D1003-21 and tear strength measured by ASTM D1922-19 are strongly dependent on roll cooling uniformity.

Extrusion Coating Dies and Chill Roll Quench Dynamics

When LLDPE is processed as an extrusion coating or laminating web, machine matching shifts toward high melt temperatures because adhesion to paper, board, or oriented polymer film requires controlled oxidation of the melt curtain. Coating grades with melt index 4 g/10 min to 15 g/10 min are run at 260°C to 315°C. The screw is a 28:1 to 32:1 L/D design with compression ratio 3.0:1 to 4.0:1; the temperature profile is flat from feed to metering to avoid dead spots that can produce oxidized gel particles. The die is an internal-deckle slot die with lip gap 0.6 mm to 1.0 mm. The air gap between die exit and nip is 100 mm to 250 mm, exposing the curtain for 40 ms to 80 ms. Excessive oxidation increases gel formation and reduces seal strength, so the melt curtain is quenched immediately at the mirror chill roll, which operates at 10°C to 20°C with water flow high enough to keep roll surface temperature variation below ±2°C. The nip pressure roll applies 15 N/mm to 25 N/mm linear load. Adhesion tested by ASTM D1876-08 below 1.5 N/15 mm typically indicates insufficient melt oxidation or a contaminated chill roll. Because LLDPE exhibits more neck-in than LDPE, die edge deckles are set 2% to 5% wider than the desired coating width and edge trim is removed downstream. For form-fill-seal structures, heat seal initiation temperature is verified by ASTM F2029-16 or an equivalent hot-tack method.

Neck-in is quantified as width loss between die exit and nip; for LLDPE at 15 g/10 min, neck-in can reach 40 mm to 60 mm per edge at 200 mm air gap. Blending with LDPE reduces neck-in but may lower the seal initiation temperature. Adhesion to aluminum foil is tested by ASTM D1876-08 after 24 h conditioning at 23°C and 50% RH.

On injection molding machines, LLDPE is processed with a reciprocating screw that functions as both plasticator and injection ram. A general-purpose screw with 20:1 to 24:1 L/D and compression ratio 2.0:1 to 3.0:1 is appropriate; the non-return valve clearance should be below 2.0 mm to prevent backflow. Melt temperature at the nozzle is 180°C to 220°C, and injection pressure is 60 MPa to 100 MPa. Mold cooling with water at 10°C to 30°C controls shrinkage; LLDPE mold shrinkage measured by ASTM D955-21 ranges from 1.5% to 2.5%. The clamping force requirement is 4 kN/cm² to 6 kN/cm² of projected mold area, and the shot volume should be 30% to 70% of barrel capacity to avoid residence-time degradation. Vented barrels are not recommended above 50% shot volume for LLDPE because the material does not release enough volatiles at normal molding temperatures to justify the additional residence time. Gates are sized at 50% to 75% of the nominal wall thickness, and cold runner diameters below 4 mm increase pressure loss. Cooling time for a 2 mm wall at 20°C mold temperature is approximately 8 s to 12 s; the cooling time scales with the square of wall thickness. Mold vents of 0.02 mm to 0.05 mm depth prevent gas traps at the flow front.

When LLDPE Is Blended with HDPE for Extrusion Blow Molding

Continuous extrusion blow molding of LLDPE-HDPE blends is constrained by parison sag and melt fracture at the die bushing. The parison die gap is set at 1.0 mm to 3.0 mm, and the blow-up ratio is 1.5:1 to 3.0:1 for bottles and technical parts. The screw is a 20:1 to 28:1 L/D metering screw with compression ratio 2.0:1 to 3.0:1; a grooved feed section is not necessary because LLDPE-HDPE blends have a lower coefficient of friction than HDPE homopolymer. Blow mold cooling uses water channels at 5°C to 15°C and blown air at 0.5 MPa to 1.0 MPa and 10°C to 15°C. Cycle times for 1 L bottles are typically 12 s to 20 s; faster cycles require a separate mold chiller with 10 kW to 20 kW cooling capacity per cavity. LLDPE addition improves environmental stress-crack resistance tested by ASTM D1693-15b, but it reduces parison stiffness, so accumulator-head machines require a 2 mm to 3 mm thicker die gap than for HDPE homopolymer to compensate for draw-down. Melt temperature at the head is held between 180°C and 210°C; temperatures above 220°C accelerate loss of parison integrity and increase plate-out on the die land. Parison programmers are typically set to open the die gap from 1.0 mm to 2.5 mm during extrusion to compensate for wall tapering.

Rotational molding does not use a screw or a slot die, but cooling by process type remains the dominant machine variable for LLDPE because polymer powder or micropellet is sintered against a hot mold wall. The mold is heated in a forced-air oven at 260°C to 300°C, and internal air temperature is held at 190°C to 200°C until the desired wall thickness is fully coalesced. Cooling is staged: forced air for 5 min to 10 min, then water mist at 1.5 L/min to 3.0 L/min per square meter of mold surface, followed by final air cooling to below 70°C before demolding. Cooling rates above 15°C/min through the crystallization plateau increase warpage and can increase part shrinkage by 2% to 4%. Published data for this specific configuration is limited, but the effect of cooling rate on LLDPE rotomolded part impact strength is routinely evaluated by low-temperature drop-weight methods under ISO 6603-2:2023 or equivalent internal plant criteria.