High-density polyethylene (HDPE) screw extrusion is governed by the interaction between feed-zone solids conveying, transition-zone melting, and metering-zone melt pressurization. The design parameter set HDPE Extrusion Screw Design: Compression Ratio and Output refers to the selection of channel depth ratios and the resulting mass throughput in single-screw machines. Compression ratio is defined for constant-pitch screws as the ratio of the first feed-zone channel depth to the final metering-zone channel depth. For HDPE grades with a melt flow rate of 0.2 g/10 min to 2.0 g/10 min under ISO 1133-1:2022 at 190°C/2.16 kg, compression ratios between 2.5:1 and 4.0:1 are common in industrial screw drawings. Output is described by the metering-zone drag flow minus pressure backflow and leakage flow. In a 90 mm, 24 L/D smooth-bore extruder running a 0.95 g/10 min film grade at 80 rpm, a 3.0:1 screw with a metering depth of 4.8 mm typically delivers 250 kg/h to 350 kg/h at head pressures between 150 bar and 250 bar. Specific output declines with rising pressure because pressure backflow scales with the cube of the metering channel depth; increasing die pressure from 150 bar to 350 bar can reduce specific output by 0.7 kg/h/rpm to 1.1 kg/h/rpm depending on melt viscosity. Screw speed is the primary actuator for output, but the maximum screw speed is limited by the melting rate of the pellet bed. If screw speed exceeds melting capacity, unmelted solids reach the metering zone and produce pressure oscillations and die flow variation. For 60 mm screws, the practical upper screw speed for high-molecular-weight HDPE is 120 rpm to 140 rpm; for 90 mm machines, 100 rpm to 120 rpm is typical.

Solids conveying in smooth-bore extruders depends on the difference between the coefficient of dynamic friction on the barrel and on the screw at the feed-section temperature. Polyethylene pellet bulk densities of 0.54 g/cm³ to 0.60 g/cm³ are reached in the feed hopper. The feed-zone channel depth for HDPE screws is typically 10 mm to 13 mm on 75 mm machines and must be large enough to avoid bridging. A feed-zone depth that is too shallow reduces intake capacity and creates a specific-output plateau below 90 rpm. Pre-drying is not mandatory for HDPE at relative humidity below 60%, but surface moisture from outdoor storage above 60% RH can cause feeding interruptions and must be removed with a hopper dryer at 80°C for 1 hour to 2 hours. Melting capacity is determined by the transition zone length; a transition length of 5D to 8D is common for HDPE because its crystalline melting range is 125°C to 135°C with peak melting near 130°C by ISO 11357-3:2018.

What Limits Output When Compression Ratio Is Incorrectly Specified?

Compression ratio error limits output in two distinct ways. When the compression ratio is below 2.0:1 on a constant-pitch screw, the transition zone does not generate sufficient pressure to compact the solid bed and to force melt through downstream screen packs. The observed failure mode on production sheet lines is throughput surging of ±5% to ±8% at screw speeds above 60 rpm when the breaker plate is fitted with 100 mesh screens. When the compression ratio exceeds 4.0:1, the metering depth becomes too shallow relative to the feed depth, and viscous dissipation raises melt temperature beyond the stabilizer package limit. A 60 mm extruder processing a 0.7 g/10 min HDPE film grade at 100 rpm may show a melt temperature increase of 8°C to 14°C at the screw tip when the metering depth is reduced from 4.2 mm to 3.2 mm with the barrel set-point held at 210°C. The temperature increase is caused by shear heating in the shallow metering channel where the apparent wall shear rate at the barrel surface rises from 50 s⁻¹ to 70 s⁻¹. For HDPE melt at 190°C, apparent viscosity in the metering zone typically ranges from 400 Pa·s to 900 Pa·s depending on molecular weight distribution. Higher viscosity increases the pressure backflow term for a given die restriction, so output sensitivity to metering depth is stronger for bimodal-film HDPE grades than for unimodal injection-moulding grades.

Incorrect compression ratio is observed on production lines through pressure profile transducers. If the pressure peak occurs in the metering zone rather than in the transition zone, the compression ratio is too low. If the peak occurs too early and screw-tip melt temperature rises beyond target, the compression ratio is too high or screw cooling is insufficient. Flush-mounted pressure transducers with ±1% full-scale accuracy track the peak position along the barrel. Extruder manufacturers report pressure peak movement of 2D to 3D upstream when compression ratio is raised by 0.4 on 75 mm bimodal HDPE screws. Published data for a specific 75 mm screw with 30:1 L/D and compression ratio 3.8:1 are limited and machine-dependent.

Barrier-flight screws alter the compression mechanism by moving the solid bed away from the melt pool. The barrier flight has a larger radial clearance than the primary flight, allowing melt to pass while retaining solids. In 75 mm, 30:1 L/D barrier screws for blown-film HDPE, the melt channel is 0.5 mm to 1.0 mm deeper than the solids channel. The effective compression ratio is generated by reducing the solids channel cross-section over 6D to 8D in the melting zone instead of a single tapered transition. Production trials indicate that barrier screws achieve the same output as conventional three-zone screws at screw speeds 10 rpm to 15 rpm lower because the melting rate per unit length increases. Spiral Maddock mixers with radial clearance of 2.0 mm and helix angle of 30° produce melt temperature rises of 5°C to 8°C at 90 rpm on 75 mm machines. In HDPE pipe lines, gravimetric throughput stability with a barrier-plus-Maddock screw is reported at ±0.3% to ±0.6% over 24-hour runs when the extruder is equipped with an automatic screen changer. The main operational boundary is increased screw torque at start-up: cold HDPE granules in the barrier section may elevate torque by 10% to 15% until the barrel reaches 180°C. If the barrel temperature exceeds 220°C during start-up, the heat may melt pellets in the feed zone and cause feed blockage.

Thermal Homogeneity Limits in HDPE Melt Conveying

Thermal homogeneity limits output because melt temperature gradients affect die uniformity and oxidation. Melt temperature at the screw tip is measured with an immersion thermocouple; on a 90 mm smooth-bore extruder at 80 rpm, radial gradients of 2°C to 6°C are within normal range. For a 60 mm grooved-feed extruder processing 0.3 g/10 min blow-moulding HDPE at 120 rpm, radial melt-temperature differences across the die can reach 12°C when the compression ratio exceeds 3.8:1. Blow-moulding parisons produced with such a gradient exhibit visible wall-thickness redistribution due to local viscosity differences. HDPE oxidation is assessed by oxidation induction time under ASTM D3895-19 at 200°C; extrusion grades with OIT above 30 min tolerate melt temperatures up to 220°C for residence times near 15 minutes. A 10°C increase above 210°C in unstabilized reactor powder can reduce OIT by 40% to 60%, while stabilized pellet grades retain more than 20 min under the same conditions. Screw cooling through the screw core at 40°C lowers screw-tip melt temperature by 5°C to 7°C but thickens the solid bed and may require a larger compression ratio to maintain positive pressure in the metering zone.

Melt temperature is also affected by head pressure and screw speed. The viscous dissipation rate in the metering zone is proportional to the square of the shear rate. Doubling screw speed from 50 rpm to 100 rpm increases maximum shear rate by a factor of two and viscous heat generation by roughly a factor of four in the melt film. This explains why melt temperature rises nonlinearly with screw speed in shallow-metering screws. Processors measure melt temperature with a protruding thermocouple in the adaptor; if the reading exceeds 230°C for standard HDPE stabilizer systems, the screw must be redesigned or the barrel temperatures reduced. Extruder barrel cooling zones, particularly after the feed throat, must maintain a temperature profile of 180°C, 200°C, 210°C, 215°C, and 220°C across five zones for a 24 L/D machine. Air-cooled or water-cooled cast heaters are used; water cooling on the first and second zones can remove 5 kW to 8 kW of excess heat.

When a Grooved Feed Zone Changes the Pressure-to-Speed Relationship

Axial grooves of 0.5 mm to 1.5 mm depth in the barrel liner increase the friction coefficient between pellet and barrel, allowing the feed zone to generate 100 bar to 300 bar before the pellet bed reaches the compression section. Throughput on a 75 mm grooved-feed extruder processing 0.94 g/10 min HDPE rises nearly linearly with screw speed from 40 rpm to 150 rpm, with a slope of approximately 180 kg/h per 100 rpm. The compression ratio can be reduced to 2.5:1 because the feed zone already provides pressure generation; maintaining a higher ratio would overheat the melt. Output advantage over smooth-bore machines at the same screw speed ranges from 15% to 30% for high-torque drives. The liner hardness must exceed 58 HRC to resist wear from compacted pellets. In HDPE pipe extrusion, 90 mm grooved-feed machines deliver 380 kg/h to 420 kg/h at 120 rpm with melt pressures from 250 bar to 320 bar. High feed-zone pressure increases screw thrust bearing loads; the thrust bearing must be sized for 150 kN to 250 kN on 90 mm machines.

Grooved feed zones alter the compression ratio requirement because the material enters the transition zone as a highly compacted pellet bed. Feed-zone pressure for HDPE is typically 80 bar to 160 bar at 80 rpm on a 75 mm machine, measured through a transducer port located 2D to 3D from the hopper. Higher feed pressure improves melting but requires higher drive torque. Specific energy consumption of a grooved-feed extruder is 0.22 kWh/kg to 0.28 kWh/kg for HDPE pipe grades, compared with 0.18 kWh/kg to 0.25 kWh/kg for smooth-bore machines. The gearbox must be rated for the torque increase; a 90 mm grooved-feed extruder may require a motor of 200 kW to 300 kW. Barrel heating in the feed zone is normally switched off after start-up because frictional heat from pellet compaction maintains the liner above 80°C.

Metering Depth Sets the Output Ceiling at High Head Pressure

The output ceiling of a given HDPE screw is set by the metering depth and the head pressure of the downstream tooling. At head pressures above 300 bar, pressure backflow in a single-flighted metering section becomes dominant. For a 75 mm screw with 3.0:1 compression ratio and 4.5 mm metering depth, increasing head pressure from 250 bar to 350 bar reduces throughput by 8% to 12% when screw speed is held at 100 rpm. The effect is larger for lower-viscosity grades with melt flow rates above 1.2 g/10 min. Extruder operators compensate by increasing screw speed, but torque rises proportionally and may reach 90% to 95% of drive capacity. High-head-pressure applications such as blown film with tight screen packs or pipe dies with spiral mandrels therefore use longer 30:1 or 33:1 L/D screws with metering depths at the upper end of the recommended range to preserve output at pressure. A metering section length of 8D to 10D is used for pipe and blown-film HDPE where die pressure fluctuations must remain below ±1% of set point.

A gear pump between extruder and die changes the pressure map: the extruder can operate at 180 bar to 220 bar while the die receives 250 bar to 300 bar. This increases specific output by 10% to 20% because the screw no longer has to generate the full die pressure. The melt pump also reduces screw wear by lowering screw tip pressure. Gear pump operation requires a melt temperature limit of 230°C to avoid bearing damage, and start-up must be pressure-controlled to prevent overpressure above 350 bar. Published data for gear-pump-assisted HDPE extrusion with barrier screws at pressures above 320 bar are limited and require pilot validation.