High-density polyethylene (HDPE) is a semicrystalline polyolefin classified by solid density between 0.941 g/cm³ and 0.965 g/cm³ under ASTM D792-20, and by melt-flow rate at 190 °C/2.16 kg under ISO 1133-1:2022. The crystalline melting peak of HDPE lies between 130 °C and 137 °C, but injection moulding requires melt temperatures of 180 °C to 250 °C to lower viscosity and ensure complete plastication. The process window described by HDPE Moulding Temperature Settings: Barrel, Mould and Cycle is therefore graded across three interconnected zones: barrel temperature sets the melt condition and residence-time risk, mould temperature sets the cooling rate and crystallization pattern, and cycle timing sets the pressure-history and ejection sequence. On production-scale injection machines from 500 kN to 5,000 kN clamp force, the typical failure modes associated with this window are screw torque spikes from incomplete melting, drool from an overheated nozzle, sink marks and voids from premature gate seal or packing removal, and warpage from non-uniform mould heat transfer. Because HDPE undergoes large volumetric contraction during solidification, the packing phase must be referenced to gate freeze-off rather than to a fixed timer. The nominal setpoints are also constrained by the degradation threshold near 260 °C, the practical mould temperature range of 15 °C to 60 °C, and the coolant system capacity. The following processing criteria apply to unfilled injection-moulding grades of HDPE and are verified through thermal, gravimetric and dimensional measurements rather than inferred from machine settings alone.

What Determines the Barrel Temperature Profile for HDPE?

For a general-purpose single-screw injection unit with an L/D ratio of 20:1 to 24:1 and a compression ratio of 2.5:1 to 3.5:1, the barrel zone settings are typically ascending from the feed zone to the metering zone. The feed throat is cooled at 40 °C to 70 °C to prevent pellet bridging and to protect the screw shank. The rear zone is set between 170 °C and 190 °C; the compression zone between 190 °C and 215 °C; and the metering zone between 200 °C and 230 °C. The nozzle is often set at 200 °C to 225 °C, or 5–10 °C below the metering zone, to minimise drool while keeping the gate area molten. These settings produce a melt temperature of 190 °C to 240 °C in most grades; the gap between barrel setpoint and actual melt temperature increases with screw speed due to viscous dissipation.

For high-flow HDPE with MFR above 20 g/10 min, barrel temperatures are normally reduced by 10–15 °C because low-viscosity melt does not require the same shear-heating contribution. For high-molecular-weight HDPE with MFR below 1 g/10 min, the metering zone may need 220–230 °C, but the nozzle should remain below 230 °C to limit oxidative degradation. The barrel-zone thermocouples should be calibrated against a traceable probe because an error of 10 °C in the metering zone can shift the melt temperature sufficiently to cause either unmelted material or degradation. During start-up, the barrel should be heat-soaked for 20–30 min at a temperature 20 °C below the operating setpoint before screw rotation; this practice reduces screw torque spikes and prevents brittle fracture of the solid bed. When an air-shot pyrometer shows a melt temperature 15 °C or more above the metering zone setpoint at screw speeds above 120 min⁻¹, the zone setpoints should be reduced rather than relying on the nozzle heater band to cool the melt.

After the melt leaves the nozzle, the cavity wall temperature becomes the dominant variable governing solidification. HDPE moulds are normally connected to chillers or temperature-control units with setpoints from 15 °C to 40 °C for thin-wall closures and packaging, and from 40 °C to 60 °C for thick-wall technical parts with wall sections greater than 3 mm. The lower range maximises cooling rate and reduces cycle time; the higher range delays frozen-layer growth so that cavity pressure can transmit through the flow path and weld-line regions. Mould temperatures above 70 °C are generally avoided because the slower crystallisation schedule increases total mould shrinkage and cycle time. Mould temperatures below 10 °C can produce condensation on the cavity surface, visible cold-slug defects and weak weld lines. The cooling circuits should maintain the coolant inlet-to-outlet temperature difference within 3 °C to 5 °C. A difference larger than 5 °C indicates insufficient turbulent flow; a Reynolds number above 10,000 is the usual design criterion for drilled cooling channels. Shrinkage measured under ASTM D955-21 for unfilled HDPE typically falls between 0.015 mm/mm and 0.040 mm/mm. Low mould temperatures reduce initial mould shrinkage by suppressing crystallinity, but can leave residual stresses that produce post-mould shrinkage and warpage when parts are exposed to service temperatures above 60 °C. Ejection should be delayed until the part surface temperature is below the heat-deflection temperature range of 65 °C to 85 °C under ISO 75-2:2013 Method B at 0.45 MPa; ejection above this range can cause sink marks, penetrating ejector pins and post-ejection warpage. For core-cavity differential cooling on flat parts, a temperature difference above 10 °C between the core and cavity can bend the part toward the colder side because the warmer side continues to crystallise and shrink after demoulding.

Injection, Packing-Hold and Screw Recovery Sequencing

The injection phase for HDPE is controlled primarily by velocity with a pressure limit. A slow initial injection speed of 30–50 mm/s at the sprue or gate prevents jetting and entrapped air; the machine then ramps to 100–250 mm/s to fill thin sections before the flow front freezes. The resulting peak injection pressure is not a fixed setpoint but a response variable; on production machines it commonly falls between 60 MPa and 100 MPa for unfilled HDPE. Switchover to packing is made by screw position 2–5 mm before complete fill, or by cavity-pressure trigger in sensor-equipped moulds. Packing pressure is set at 60–80% of the peak injection pressure. The pack time must exceed the gate seal time, and the screw should maintain a melt cushion of 3–6 mm in front of the check ring. Screw recovery is conducted during cooling. Screw speed between 50 min⁻¹ and 120 min⁻¹ and back pressure between 0.5 MPa and 1.0 MPa provide sufficient plastication without excessive melt-temperature rise. Back pressures above 1.5 MPa may be required for uniform dispersion of colour concentrates but can increase melt temperature by 10 °C or more. The cooling phase occupies 60–80% of the total cycle. A plate-like section of 2 mm wall in a 20 °C mould may cool in 10–20 s, but the production cooling time is validated by part weight stability and dimensional checks rather than by timer alone. In laboratory practice, specimens moulded under ISO 294-1:2017 require the melt temperature, mould temperature and hold pressure to be recorded because HDPE mechanical properties are sensitive to cooling history.

Cavity-pressure monitoring provides a more robust verification than machine timers. For HDPE, peak cavity pressure is commonly between 30 MPa and 60 MPa depending on wall thickness and gate dimension. The cavity-pressure curve should flatten during packing and then decay after gate freeze; a sudden drop immediately after switchover indicates that the gate is still molten and backflow is occurring. Automated part weighing can detect gate-seal failure when shot-to-shot variation exceeds ±0.02 g for small packaging parts. If screw recovery time is longer than the cooling time, the cycle is recovery-limited; this occurs with high-viscosity HDPE grades, undersized screws or low back-pressure setpoints.

When Cycle Time Reduction Conflicts with Gate Freeze-Off

When cycle time is shortened below the gate-seal threshold, the moulding process becomes unstable because the gate remains fluid while holding pressure is removed. HDPE exhibits a volumetric contraction during crystallisation that can exceed 15% by volume; if the gate is not sealed, backflow can pull melt from the cavity and create sink marks, internal voids and part-weight drift. Gate freeze-off time is governed by gate diameter, gate geometry and the local temperature difference between melt and mould. A subgate with diameter 0.5–1.5 mm may freeze in 0.5–2 s; a full edge gate with thickness 2–4 mm can remain open for 3–10 s depending on grade and mould temperature. The standard validation procedure is to plot part weight against hold time at constant pack pressure and identify the plateau point. If the target cycle cannot accommodate that hold time, the gate geometry must be modified, the mould temperature lowered, or the melt temperature adjusted. Lowering mould temperature accelerates gate freeze-off and can permit shorter hold time for thin-wall parts, but it may reduce weld-line tensile strength measured by ISO 527-2:2012 because the two melt fronts solidify before molecular diffusion and chain entanglement are complete. Raising the nozzle temperature delays gate freeze-off but should not push the melt above 250 °C for more than several minutes. Published data for the gate freeze-off time of a specific HDPE grade in a given mould is limited; the value is therefore determined empirically on the production tool rather than assumed from material data sheets.

Regrind level, moisture, and residence-time constraints impose additional boundaries on HDPE moulding temperature settings. Clean HDPE regrind is commonly incorporated at levels up to 30 wt% in non-critical applications when the source and previous thermal history are known. Higher regrind fractions can shift melt-flow rate and reduce tensile properties; the melt-flow rate should be rechecked under ISO 1133-1:2022, and tensile properties under ASTM D638-14 if the regrind content exceeds the grade supplier recommendation. HDPE is not hygroscopic, but condensation on pellets stored below dew point or exposed to relative humidity above 60% can introduce surface moisture that appears as splay or surface streaks. When condensation is observed, the material is pre-dried at 80 °C for 1–2 h in a desiccant or hot-air hopper dryer; this does not replace the need for dry storage. At the melt-processing end, residence time above 260 °C should not exceed approximately 10 min. Longer exposure accelerates thermo-oxidative degradation, colour shift and molecular-weight loss, and can reduce impact performance. Screw and barrel wear also shifts the effective barrel temperature profile by increasing clearance and reducing shear efficiency; if the screw/barrel clearance exceeds the machine builder tolerance, the original zone setpoints may no longer produce the same melt temperature. For food-contact applications, HDPE grades must meet FDA 21 CFR 177.1520 and applicable European Union food-contact requirements, and the processing temperature record is part of the compliance dossier because degradation products can migrate to the food simulant. These operational boundaries, rather than the nominal barrel setpoints alone, determine whether a moulding cycle is robust.