In thin-walled drip irrigation pipe extrusion from linear low-density polyethylene at 300 kg/h without a gear pump, the stabilisation function normally provided by the melt pump is transferred to the extruder screw, the die geometry, and the pressure-control loop. The LLDPE grade selected must combine a density of 0.918–0.930 g/cm³ measured according to ISO 1183-1:2019 with a melt flow index of 0.8–1.2 g/10 min measured at 190 °C/2.16 kg according to ISO 1133-1:2022. Pipe dimensions for drip irrigation are typically 12–20 mm outer diameter with wall thickness of 0.6–1.2 mm, and the emitter may be a flat tape inserted and welded during extrusion or a cylindrical emitter punched or thermally bonded in a secondary operation. The absence of a gear pump means that screw-induced pressure ripple, feed-zone variation, and melt-temperature asymmetry are transmitted directly to the die land and therefore to pipe wall thickness. This configuration can operate at 300 kg/h only if the screw pressure capacity, die resistance, and cooling calibration are designed together as a single pressure-damping system.

What pressure-generating capacity must the screw retain when the gear pump is absent?

At 300 kg/h LLDPE throughput, the extruder screw must be able to deliver a die-head pressure between 200 bar and 280 bar, because the die itself becomes the only significant pressure-drop element that can damp screw-induced disturbances. A 75 mm diameter, 30:1 L/D grooved-feed extruder running at 90–110 min⁻¹ is commonly used for this throughput, while a 90 mm, 30:1 machine may run at 65–75 min⁻¹. The drive must be sized for a specific mechanical energy input of 0.20–0.25 kWh/kg, giving a motor rating of 90–110 kW when the higher die pressure and the absence of a gear pump are taken into account. The screw should be a barrier design with a spiral Maddock mixer in the metering section and a feed-zone cooling bore covering the first 4–6 L/D; the grooved feed bushing should extend for 3–4 L/D and be water-cooled to 40–60 °C to maintain solids conveying. Without a gear pump, the screw must compensate for leakage flow across the flight clearance. As flight clearance increases from 0.15 mm to 0.45 mm due to wear, the pressure-generating capacity at 300 kg/h can fall by 25–40 bar, forcing a higher screw speed that raises melt temperature and can initiate surface defects. This is the central operational boundary for a pump-free line: the screw is both the melting device and the only pressure source, and any wear or instability in the feed section is transmitted directly to the die. The control loop uses a die-entrance pressure transducer with a 0–500 bar range and ±0.25% full-scale accuracy; the controller adjusts screw speed to hold die pressure, but the screw residence time imposes a dead time of several seconds, making the loop too slow to correct short-term pressure ripple. A static mixer placed between the screw tip and the die can reduce radial temperature variation to ±1–2 K but adds 20–40 bar pressure drop, and this additional pressure requirement must be included in screw capability calculations because it cannot be absorbed by a gear pump.

A pump-free screw configuration for LLDPE drip tubing at 300 kg/h differs from a melt-fed gear-pump line in that compression ratio, metering depth, and mixer length must deliver pressure rather than simply melt and mix. Barrier flights improve solids-melt separation and reduce the pressure variation caused by solid-bed breakup in the compression zone; the barrier clearance should be 0.5–0.7 mm and the barrier profile should start at 6–8 L/D from the feed opening. A spiral Maddock mixer with a length of 2–3 L/D and an outlet clearance of 0.5–0.6 mm improves thermal homogeneity and reduces radial temperature differences that cause local viscosity changes in the die. The metering channel depth for a 75 mm screw is typically 3.0–4.0 mm; a deeper metering channel reduces viscous heating but also lowers pressure-generation efficiency, so it is suitable only if the die pressure demand remains below 220 bar. The feed-zone cooling bore temperature must be held at 45–55 °C; if it is allowed to rise above 70 °C, pellet surfaces become tacky, the grooved bushing loses conveying efficiency, and output can drop by 5–8% over 20–30 min. The screw drive should maintain speed within ±0.5% of set point because speed variation is directly translated into pressure and wall-thickness variation when no gear pump is present. Gravimetric dosing of LLDPE and masterbatch should be maintained within ±0.25% to reduce composition-induced viscosity shifts, and the hopper should be fitted with a level-control device to prevent low-level bridging events from causing abrupt feed interruptions.

Pressure ripple, die land length, and wall-thickness stability

Short-term pressure ripple observed on a pump-free LLDPE line at 300 kg/h typically has a frequency equal to screw rotation and can reach ±10–20 bar at the die entrance. If the mean die pressure is 250 bar, a ±15 bar ripple corresponds to a ±6% pressure variation. Because output through an annular die is approximately proportional to pressure drop, the instantaneous wall thickness can vary by roughly the same percentage unless the die land length and melt compressibility dampen the fluctuation. This is why the die land length in gear-pump-free operation should not be shorter than 35–45 mm for a 0.8 mm wall tube, and the die-head pressure should be kept above 200 bar; if pressure falls below 180 bar, the same ripple amplitude becomes relatively larger and the resulting wall variation may exceed the tolerance required for emitter insertion. The die pressure drop also increases with land length. For a 16 mm outer diameter tube with a 1.2 mm annular gap and 40 mm land length, the calculated pressure drop at 300 kg/h is 230–260 bar when using an apparent viscosity of 35–50 Pa·s at the corresponding shear rate. This pressure drop is the only meaningful damping mechanism in the absence of a gear pump, and it must be balanced against the viscous heating it generates. A further practical instability source is hopper bridging or feed-throat temperature rise above 60 °C, which changes pellet conveying and creates low-frequency output drift; if the feed zone is not cooled consistently, the screw pressure delivery becomes erratic and the die cannot mask it. The breaker plate screen stack of 60/80/120 mesh contributes 15–30 bar of additional pressure drop and acts as a coarse dampener by increasing total system resistance, but it is not a substitute for a gear pump because its pressure loss varies with contaminant loading.

When LLDPE enters the die at 8.85×10³ s⁻¹, sharkskin onset is no longer hypothetical

At a volumetric throughput of 1.07×10⁻⁴ m³/s, based on 300 kg/h and melt density 780 kg/m³, a 16 mm diameter die with 1.2 mm gap produces an apparent wall shear rate of approximately 8.85×10³ s⁻¹ using the thin-slit approximation. LLDPE at 210–230 °C has an apparent viscosity at this shear rate in the range 35–50 Pa·s, giving a wall shear stress of 0.31–0.44 MPa. This is above the commonly observed sharkskin onset stress of 0.14 MPa for linear polyethylenes; therefore, the die exit surface is prone to sharkskin unless a fluoropolymer processing aid is used at 300–800 ppm or the die geometry is modified to reduce shear stress. The die gap and land length must be selected so that either the wall shear stress remains below the critical value or the fluoropolymer additive migrates to the die steel surface quickly enough to establish slip. Without a gear pump, the die cannot be designed as a low-resistance gap because that would reduce the pressure drop and thus weaken the damping function; this is the central conflict in pump-free thin-wall LLDPE extrusion. If sharkskin occurs, the first corrective actions are to raise the die temperature by 5–10 K, reduce screw speed slightly, or increase the fluoropolymer processing aid concentration; however, raising die temperature also lowers viscosity and die pressure, which can destabilise wall thickness. A die land with a hard chrome or electroless nickel plating and Ra 0.05–0.10 µm roughness is required to allow the processing aid to coat uniformly. The final pipe wall of 0.8 mm is obtained from a die gap of 1.0–1.4 mm; the drawdown ratio must be controlled because excessive drawdown raises molecular orientation and can create splitty weld lines. Die pin and bushing eccentricity of ±0.05 mm at the die gap will produce local wall-thickness variation that the calibrator cannot fully correct.

Selecting LLDPE Rheology for Thin-Wall Pump-Free Extrusion

Material selection is governed by the need for high melt strength at the die exit and uniform viscosity at the die land. LLDPE grades based on hexene or octene comonomers with narrow molecular weight distribution and a melt flow index of 0.8–1.2 g/10 min provide better toughness and environmental stress crack resistance than butene-based grades, but their higher viscosity at high shear increases die pressure and torque. Viscosity measured according to ISO 11443:2021 at 190 °C and 100 s⁻¹ should be screened for batch consistency; if different batches vary by more than ±10 Pa·s, the pressure at 300 kg/h may shift by 15–25 bar. The resin should have a density of 0.920–0.930 g/cm³ according to ISO 1183-1:2019, a tensile yield stress of 11–14 MPa according to ISO 527-2:2012, and an elongation at break above 600% to survive punching and weld operations. Carbon black masterbatch is added at 2.0–2.5 wt% to meet outdoor UV resistance requirements; its dispersion must be monitored according to ISO 18553:2002 to avoid gels and agglomerates that can block emitter passages.

The property-compliance matrix for the compound is summarised below. The values are not derived from a single supplier datasheet; they represent the acceptance window typically applied when a pump-free line is qualified for drip-irrigation pipe with wall thickness below 1.0 mm.

Die Land Length Calculation and Its Role as the Only Damping Element

The die land length is calculated from the target die pressure and the maximum allowable wall shear stress, not merely from the final pipe dimensions. For a thin annular gap, pressure drop is ΔP = (12 Q η L)/(π D H³), where Q is volumetric throughput, η is apparent viscosity, L is land length, D is mean die diameter, and H is die gap. At 300 kg/h, with Q = 1.07×10⁻⁴ m³/s, D = 16 mm, H = 1.2 mm, L = 40 mm, and η = 40 Pa·s, the calculated pressure drop is 236 bar, which is within the required damping range. If the land length is shortened to 25 mm, pressure drop falls to 147 bar; the same ±15 bar screw ripple then becomes ±10% of total pressure, and wall-thickness variation becomes unacceptable for emitter insertion. If the land length is lengthened to 55 mm, pressure drop rises to 325 bar, which may exceed the die clamp rating and raises adiabatic melt temperature by 17–18 K; this is not viable without a gear pump or a lower-viscosity resin. The practical land-length window is therefore narrow, typically 35–45 mm, and it must be re-evaluated when the resin MFI or die temperature changes. The adiabatic temperature rise through the die is approximately ΔT = ΔP/(ρ Cp). With ΔP = 250 bar, ρ = 780 kg/m³, and Cp = 2300 J/(kg·K), the calculated rise is 13.9 K; this consumes a large part of the available temperature window before 245 °C degradation begins.

Start-up and shutdown transients are more severe when no gear pump is installed. At start-up, the cold die and calibrator create high backpressure; the screw must be ramped slowly while die pressure is monitored. If die pressure exceeds 350 bar, the adapter burst disk opens. The screw should be started at 10–15 min⁻¹ and ramped over 15–20 min until melt temperature and die pressure stabilise. During shutdown, residual LLDPE must be purged from the die before the barrel cools, or oxidised material will form gels on restart. The feed hopper should be emptied or dried, and the screw should run at low speed until die pressure falls below 50 bar before stopping. Restart after an emergency stop in a gear-pump-free line requires particular attention to the die land because partially frozen LLDPE in the die gap can behave as a temporary plug and cause pressure overshoot if screw speed is restored too quickly.

Vacuum calibration, cooling gradients, and emitter weld integrity

After the die exit, the tube enters a vacuum calibrator that sets the outer diameter and cools the outer surface while the bore is still at melt temperature. The vacuum level is typically 0.02–0.06 MPa for a 16 mm tube with 0.8 mm wall, and the calibrator stack contains a first cooling zone at 20–30 °C followed by a water bath at 30–50 °C to reduce frozen-in stress. If the calibrator vacuum is too high, the tube may stick and chatter; if too low, the outer diameter drifts and the die gap must be adjusted. Without a gear pump, any diameter drift caused by pressure variation is more difficult to correct because the die-gap adjustment is manual or slow-acting. The cooling gradient across the wall of an 0.8 mm tube is steep; the outer surface can cool below the LLDPE crystallisation temperature while the inner surface is still molten, and this generates asymmetric shrinkage. Emitter welding during extrusion requires the inner wall to be partially molten when the flat emitter tape contacts it; if the inner surface is too cool because of calibrator overcooling, the weld peel strength falls below the value required by the emitter-retention test. For flat tape emitters, the weld is made 20–80 mm upstream of the calibrator or immediately after die exit; the tape is fed through a crosshead channel and pressed against the inner tube wall by a mandrel or vacuum slot. Wall thickness around the weld is locally increased by 0.05–0.15 mm, and this asymmetric thickness affects cooling shrinkage and may produce a visible line. A gear pump would reduce short-term wall-thickness variation, but it would not eliminate this local weld-related asymmetry; the weld geometry itself must be controlled by tape width and contact pressure. Subsequent punching or laser perforation of the welded emitter locations requires a wall-thickness tolerance of ±0.05 mm at the weld; if wall thickness varies by more than this, the punch may cut through the tape or leave an occluded hole.

At what point does melt temperature exceed 245 °C and initiate gel formation?

LLDPE begins to degrade through a combination of oxidative chain scission and crosslinking when melt temperature exceeds 245 °C for even short residence times. At 300 kg/h and die pressure 250 bar, viscous heating can already raise temperature by 12–15 K; if barrel set points are too high or the screw is worn and operating at higher speed, die-exit melt temperature can cross the 245 °C threshold. The result is the formation of gel particles that appear as lumps on the inner tube wall or as weak spots at the emitter weld interface. The antioxidant package, typically a hindered phenolic primary antioxidant plus a phosphite secondary antioxidant, is consumed faster at elevated temperature; oxidative induction time according to ISO 11357-6:2018 at 210 °C should remain above 20 min for the finished compound. If OIT falls below 10 min, the resin is being processed too hot or the antioxidant masterbatch is below specification. Barrel zone temperatures should therefore be set to 180–210 °C in the feed and compression zones, 210–230 °C in the metering zone, and 230–240 °C at the adapter and die; the die temperature itself may be raised to reduce sharkskin but only if the melt thermocouple confirms die-exit temperature below 245 °C. Screw cooling at the feed zone must not be reduced to increase output because this raises frictional heating later in the screw. At 300 kg/h in a 75 mm machine, mean residence time may be 120–180 s; stagnation zones in the adapter or die can extend local residence time to several minutes, making temperature-sensitive degradation worse. When weld failure occurs during emitter insertion trials, the first measurements should be die-exit melt temperature and OIT of the pipe wall; if both are acceptable, the failure is more likely due to tape contamination or insufficient contact pressure than to resin degradation.

The Limits of ISO 9261 for Emitter Insertion in Gear-Pump-Free LLDPE Pipe

Relevant standards for emitting pipe include ISO 9261:2004 for emitters and emitting pipe and ISO 1167-1:2006 for thermoplastics pipe resistance to internal pressure. The drip-irrigation product must also meet dimensional stability, burst pressure, and emitter flow-uniformity requirements. The pipe wall-thickness tolerance in many commercial specifications is ±0.05 mm for a nominal wall of 0.8 mm, which is tighter than general plastic pipe standards; even this tolerance may be insufficient for emitter insertion if the variation occurs over short axial distances. A wall-thickness change of 0.05 mm over 50 mm of pipe length changes the inner diameter and the local flow cross-section, affecting emitter discharge rate and attachment geometry. The coefficient of variation of emitter flow rate along a drip line is used as a measure of line uniformity; if wall-thickness variation changes the inside diameter by more than 0.1 mm, the emitter flow path may be partially occluded or the weld may peel under field pressure. The gear-pump-free line can meet these tolerances only if die pressure is kept above 200 bar, screw speed variation is below ±0.5%, and die land length is within the calculated window. Published data for this specific gear-pump-free LLDPE drip-pipe configuration at 300 kg/h is limited; the values here represent engineering estimates from standard polymer-processing relationships and typical single-screw extrusion practice rather than a specific single-source field trial.