Linear low-density polyethylene (LLDPE) is a copolymer of ethylene and α-olefins—commonly but-1-ene, hex-1-ene, or oct-1-ene—with density between 0.915 g/cm³ and 0.940 g/cm³ when measured under ASTM D1505-18. Extrusion grades are specified by melt index values of 0.5 g/10 min to 2.0 g/10 min at 190 °C and 2.16 kg load under ASTM D1238-20 or ISO 1133-1:2022. Unlike branched low-density polyethylene, LLDPE has a linear backbone with short-chain branches and no long-chain branching. This structure produces a narrow molecular weight distribution, high shear viscosity at processing shear rates, and a solid bed that resists compaction and melting. The processing problem commonly classified as “LLDPE screw design: barrier screws, compression ratio and output” therefore cannot be solved by applying an LDPE screw profile without modification.

Why Do LLDPE Melting Characteristics Favour Barrier Screws?

At extrusion shear rates between 100 s⁻¹ and 1,000 s⁻¹, a hexene-LLDPE grade with melt index 1.0 g/10 min exhibits higher apparent viscosity than an LDPE grade of equivalent melt index. On a conventional 65 mm, 30:1 L/D metering screw, the solid bed frequently fractures in the compression section because the linear polymer does not soften gradually. Solid-bed breakup releases partially melted particles into the melt channel; these particles travel into the metering zone and produce head-pressure pulsation, melt-temperature heterogeneity, and output instability. Field measurements on smooth-bore film extruders have shown head-pressure variation exceeding ±3% when the solid bed collapses on a conventional screw.

A barrier screw separates the solid bed from the melt pool by a secondary barrier flight located in the compression and melting zone. The barrier flight is undercut relative to the main flight, allowing melt to pass through the barrier gap while retaining the compacted solid bed. For a 65 mm screw, the main flight clearance is typically 0.08 mm to 0.12 mm, while the barrier flight clearance is 0.45 mm to 0.55 mm. The solids channel cross-section decreases along the barrier section while the melt channel cross-section increases. This geometry actively compresses the solid bed against the heated barrel wall, completes melting before the metering zone, and supplies a fully molten stream to the metering channel. The barrier section length for LLDPE is normally 8 L/D to 10 L/D.

Barrier screw geometry also changes the compression-ratio function. A conventional LLDPE screw relies on channel-depth reduction alone to generate melting pressure; a barrier screw uses the decreasing solids-channel volume and the barrier gap to separate molten polymer. This permits a lower feed-to-metering compression ratio of 2.5:1 to 3.0:1 for unfilled LLDPE on smooth-bore extruders. The lower compression ratio reduces shear heating, and the melt-temperature rise at the barrier discharge is commonly 8 °C to 15 °C lower than that of a conventional LLDPE screw at the same output.

The table values are not universal. Octene-LLDPE with MI 0.5 g/10 min may require a 0.05 mm to 0.10 mm larger barrier gap to avoid excessive pressure drop; high MI rotomoulding or injection LLDPE with MI 20 g/10 min uses a feed-to-metering compression ratio below 2.5:1 because melt viscosity is low and pressure generation is easier.

At 24:1 to 30:1 L/D ratios, compression ratio for LLDPE is a function of pellet bulk density, channel geometry, and die restriction. LLDPE pellet bulk density is generally 0.52 g/cm³ to 0.56 g/cm³, while LDPE pellet bulk density is often 0.48 g/cm³ to 0.53 g/cm³. The feed channel depth must be sufficient to convey pellets without hopper-throat plugging; a feed depth below 6.5 mm on a 65 mm screw starves the screw and limits output, while a feed depth above 10.5 mm raises the feed-to-metering compression ratio and can over-compact the solid bed.

Compression ratio is conventionally expressed as feed-channel depth divided by metering-channel depth. For barrier screws, the solids-channel depth at barrier entry and the melt-channel depth at barrier exit are also monitored. Unfilled LLDPE film and coating lines on smooth-bore extruders use feed-to-metering compression ratios of 2.5:1 to 3.0:1. A compression ratio below 2.2:1 can leave unmelted polymer in the metering zone, especially with octene-LLDPE of MI 0.5 g/10 min. A compression ratio above 3.5:1 increases shear work and raises melt temperature; at 230 °C and above, oxidative chain scission and gel formation become significant in blown-film dies with long residence time. The processing window for melt temperature is therefore narrow: 210 °C to 230 °C for many hexene-LLDPE film formulations, with a practical tolerance of ±5 °C across the die.

Many LLDPE film-line failures occur when a processor substitutes LLDPE into an LDPE screw with a compression ratio of 3.5:1 to 4.0:1. The high shear viscosity of LLDPE creates excessive viscous heating in the deep feed-to-metering transition. Melt temperature rises 10 °C to 20 °C above the set-point at the die, and the bubble in blown-film becomes unstable because melt strength decreases and the frost line height changes. The same effect appears in cast-film lines as edge tear and draw resonance. For this reason, the compression ratio must be reduced or the screw replaced with a barrier design when the assignment is to run LLDPE above 65% of total extruder capacity.

When Output Targets Climb Beyond 350 kg/h on a 75 mm Extruder

High-output LLDPE film lines using a 75 mm, 30:1 L/D smooth-bore extruder at 120 rpm commonly produce 320 kg/h to 400 kg/h with MI 1.0 g/10 min hexene-LLDPE and a die pressure of 20 MPa to 30 MPa. Specific output under these conditions is 2.7 kg/h/rpm to 3.3 kg/h/rpm. Beyond 350 kg/h, the screw design must manage torque, melt temperature, barrier clearance, and residence time together. The barrier flight clearance on a 75 mm screw is normally increased to 0.55 mm to 0.65 mm. Smaller clearances reduce the tolerance for screw deflection and thermal expansion; larger clearances allow solid particles to leak into the melt channel.

Motor load is a limit. LLDPE requires specific energy of 0.25 kWh/kg to 0.30 kWh/kg at high output, compared with 0.20 kWh/kg to 0.25 kWh/kg for LDPE of equivalent melt index. A 75 mm extruder processing 350 kg/h at 0.25 kWh/kg consumes approximately 87.5 kW, excluding drive losses. If the drive is sized below 120 kW, the maximum screw speed may be torque-limited. Barrel cooling must be active in the melting and metering zones because the high shear viscosity of LLDPE generates local melt temperature rises, and sustained operation above 240 °C increases gel content and reduces film impact properties measured under ASTM D1709-16ae1.

At output above 350 kg/h, melt-temperature control becomes the primary quality constraint. Barrier screws reduce melt temperature by completing melting early, but the metering channel still generates viscous heating. Backpressure at the screw tip is kept between 10 MPa and 15 MPa for barrier screws feeding gear pumps on blown-film lines. If head pressure exceeds 35 MPa, melt temperature can rise by 5 °C to 10 °C, the gel count increases, and film-thickness uniformity degrades. Published multi-zone temperature measurements on 75 mm LLDPE blown-film screws show that the highest melt-temperature increase occurs in the last 3 L/D of the metering section, not in the barrier section.

Compression Ratio Constraints in Low-Pressure LLDPE Film Lines

Barrier flight clearance is a primary wear and melt-quality variable. For unfilled MI 1.0 g/10 min hexene-LLDPE, barrier clearance on a 65 mm screw is set to 0.45 mm to 0.55 mm; on a 90 mm screw, the range is 0.70 mm to 0.80 mm. The barrier land is generally hardfaced with Stellite 6 or Colmonoy 56 because compacted solids are abrasive. If the barrier clearance increases beyond 0.80 mm due to wear, solid particles pass into the melt channel and produce optical defects in film. If the clearance is below 0.35 mm on a 65 mm screw, screw deflection and barrel misalignment can cause land contact and metal pick-up.

The screw substrate is usually AISI 4140 hardened to 50 HRC to 55 HRC; the barrier land receives a nickel-based hardfacing. If an abrasive additive is present, the barrier clearance is opened by 0.05 mm to 0.10 mm and the screw is inspected for wear more frequently. The barrier gap itself produces a local shear rate that can exceed 500 s⁻¹ at high screw speed. For LLDPE, this is acceptable if the gap is sized correctly; an undersized gap elevates melt temperature at the solid–melt interface, while an oversized gap dilutes the melt pool with unmelted solids.

Melt channel depth controls pressure-generating capacity. Drag flow varies approximately linearly with metering depth, while pressure flow varies with the cubic power of metering depth. Therefore, a deeper melt channel reduces pressure generation rapidly. For LLDPE at 230 °C and 100 s⁻¹, apparent viscosity of an MI 1.0 g/10 min grade is approximately 800 Pa·s to 1,200 Pa·s. If the melt channel on a 65 mm barrier screw is increased from 3.4 mm to 4.2 mm, the screw may lose the ability to overcome the die and screen-pack restriction at high speed, causing surging and transverse film-thickness variation. Conversely, a melt channel below 3.0 mm raises shear rate and melt temperature but increases pressure stability. This trade-off determines the output limit of a given barrier screw.

Barrier section length also defines the operational boundary. For 30:1 L/D extruders, a barrier section of 8 L/D to 10 L/D is typical for LLDPE. Below 6 L/D, melting is incomplete and unmelted particles pass the barrier. Above 10 L/D, the melt channel residence time lengthens, increasing gel formation in octene-LLDPE at die temperatures above 225 °C. The screw should be inspected every 3,000 h to 5,000 h in high-output LLDPE service; barrier-clearance wear of 0.10 mm to 0.15 mm is commonly used as a maintenance trigger.

In grooved-feed extruders, the compression ratio requirement is altered because axial or helical grooves in the feed barrel generate high solids-conveying pressure before melting begins. Grooved-feed barrels for LLDPE typically use groove depths of 4 mm to 6 mm and widths of 8 mm to 12 mm. The high feed-zone pressure allows a lower feed-to-metering compression ratio of 2.0:1 to 2.5:1 and reduces reliance on channel-depth reduction in the barrier section. A 75 mm grooved-feed extruder running LLDPE at 100 rpm can reach specific outputs of 4.0 kg/h/rpm to 5.0 kg/h/rpm. Barrier screws for grooved-feed lines use a shorter feed section of 4 L/D to 6 L/D and a longer melt-dominant barrier section. The groove zone must be intensively cooled; if the feed-zone metal temperature exceeds 80 °C, pellet bridging and output loss occur. Published data for this specific groove-barrier combination at 90 mm is limited, and screw-design simulation is required before scaling from 75 mm to 90 mm.