Extrusion blow moulding of 25 L jerrycans in high molecular weight high-density polyethylene (HMW-HDPE) is governed less by solid-state tensile properties than by the transient elongational rheology of the parison. A resin with a melt flow rate of 0.20–0.40 g/10 min measured at 190°C under 2.16 kg load in accordance with ASTM D1238-20 or ISO 1133-1:2022 is typically specified, and density is maintained between 0.945 and 0.955 g/cm³ when measured by ISO 1183-1:2019. These ranges are not arbitrary; they reflect the necessity of a high molecular weight fraction that resists gravity-driven elongation during the parison hang time between die exit and mould closure. On a production-scale accumulator-head machine with a 80–120 mm grooved-feed extruder and a 24:1 to 30:1 L/D barrel, the parison mass for a 25 L container commonly falls between 900 and 1300 g, producing a parison length of approximately 450–650 mm. The unsupported parison experiences tensile stress that, if the melt strength is insufficient, results in localised necking and wall-thickness variation exceeding the ±0.2 mm tolerance required for consistent hydraulic pressure testing at 250 kPa for 10 min under UN 3H1 certification. High molecular weight contributes directly to the storage modulus and zero-shear viscosity governing this resistance, but it also increases extruder head pressure and shear heating, so melt strength control is a compromise between parison stability and output rate. The rheological requirement is further sharpened by the fact that a 25 L jerrycan mould has a relatively deep draw and a multi-axis parting line, both of which demand that the molten tube remain dimensionally stable through the entire mould closing sequence and blow phase.

What Limits Melt Strength Retention When Barrel Temperatures Exceed 220°C?

In HMW-HDPE, melt strength at a fixed molecular weight is not a constant material property; it is a processing-history dependent variable. As the melt temperature at the die entry is raised from 190°C to 220°C, the zero-shear viscosity decreases by approximately 8–12% per 10°C increment for a typical bimodal blow moulding grade, which reduces parison hang time before unacceptable sag. However, the more severe risk above 220°C is thermo-oxidative chain scission, which permanently reduces the high molecular weight tail of the molar mass distribution and therefore lowers melt strength irreversibly. Extrusion blow moulding processors observe this as a gradual increase in melt flow rate, a reduction in head pressure, and greater parison length variation. Oxidation induction time measured by ISO 11357-6 at 200°C is a quality-control proxy; when OIT falls below 20 min in processed regrind, the residual stabilizer package is no longer sufficient to inhibit β-scission during the next heat history. Barrel set points are therefore normally kept between 180°C and 210°C, while melt temperature is checked with an insertion thermocouple at the accumulator head and recorded to maintain a window no wider than ±5°C. Pre-drying is not required for HDPE due to equilibrium moisture contents below 0.01 wt%, but pellets stored in unheated silos at high relative humidity can condense surface moisture; if the pellet surface temperature is below the dew point and ambient humidity exceeds 60% RH, a hopper-air dehumidification step or drying at 70–80°C for 2 h is applied to prevent splay defects. The same temperature constraints apply to the die head, because a die temperature above 230°C can accelerate gel formation even when the barrel remains inside the normal window.

Accumulator-head extrusion blow moulding of 25 L jerrycans relies on axial and radial parison programming to compensate for gravity-driven thickness taper. A servo-hydraulic die gap actuator with 100-point axial profiling adjusts the annular die gap during extrusion, typically between 1.5 mm and 5.0 mm, to place additional material in the upper parison where hang time is longest and less material in the lower sections that are compressed by the accumulated parison weight. The melt strength of HMW-HDPE determines how much correction is required: for a 0.25 g/10 min bimodal grade, the programmed profile might require a 30–45% increase in die gap at the top of the parison, whereas a 0.35 g/10 min material may require a 50–70% increase to achieve the same final wall distribution. This gap correction is not a substitute for melt strength; excessive programmed gaps amplify swell and can create die-flow instabilities. The target wall thickness for a 25 L jerrycan is normally 1.2–1.8 mm in the sidewall and 2.0–3.0 mm at the pinch-off, with a minimum wall of 1.0 mm after shrinkage. Mould temperature is maintained at 12–18°C by chilled water, and blow pressure between 0.6 MPa and 0.8 MPa is applied for 60–90 s depending on wall thickness and coolant flow rate. Processors use drop tests at -18°C after conditioning to ASTM D2463-15 or ISO 6603-2 to confirm weld-line and pinch-off integrity.

Bimodal HDPE Architecture and Parison Sag Resistance

The bimodal molecular weight distribution is produced in cascaded slurry/gas-phase reactors and is the dominant architectural strategy for large-part blow moulding HDPE. The low molecular weight fraction, typically with Mw below 50,000 g/mol, imparts shear thinning and reduces extruder torque, while the high molecular weight fraction, often above 1,000,000 g/mol, supplies the long relaxation times that resist extensional deformation. The comonomer—commonly 1-butene or 1-hexene—is preferentially incorporated into the high molecular weight chains, increasing the concentration of tie molecules between lamellae and improving environmental stress crack resistance as measured by ASTM D1693-15 Condition B in 100% Igepal CO-630 at 50°C. A bimodal grade with a melt flow rate of 0.30 g/10 min commonly exhibits an ESCR value greater than 600 h, whereas a unimodal HMW-HDPE of the same melt flow rate may fail before 200 h. Tie molecular weight distribution also affects the elongational viscosity; the bimodal resin maintains a higher or at least slower-decaying extensional viscosity at Hencky strain rates between 0.1 and 1.0 s⁻¹, which are relevant to parison sag. Some producers modify the high molecular weight fraction with slight long-chain branching using metallocene or post-reactor peroxide modification. That approach increases melt strength by 15–25% at equal melt index, but it narrows the processing window and can raise die swell, so it is used only when parison stability cannot be achieved with reactor bimodality alone.

The following comparative data are aggregated from producer technical bulletins for three representative HMW-HDPE blow moulding grades used in 25 L jerrycan moulding.

When Regrind Concentration Exceeds 30 wt%, the Linear Melt Strength Blend Rule Breaks Down

In plant practice, flash, tail, and start-up waste are reground and reintroduced into the extruder feed. The melt strength of a virgin/regrind blend is often assumed to follow a linear rule-of-mixtures by weight fraction, but this assumption fails above 30 wt% regrind when the regrind has experienced multiple heat histories. Chain scission reduces the high molecular weight tail, and residual stabilizer consumption shifts the oxidation induction time downward. As regrind content is increased from 0 wt% to 50 wt%, the melt flow rate may shift upward by 0.02–0.06 g/10 min depending on the number of heat histories and the type of size-reduction equipment; knife mills with low rotor speed generate less shear heating than high-speed grinders and preserve more of the original molecular weight distribution. Published data for this specific configuration is limited because each plant blends different tops and tails, but the operational control point is consistent: if the blended MFR exceeds the virgin specification by more than 15%, the parison sag correction must be increased and the drop test failure rate rises in thin-wall sections. To compensate, processors either reduce regrind to below 30 wt%, increase the virgin resin molecular weight, or add a processing stabilizer masterbatch at 0.5–1.0 wt%. The regrind stream should be sieved to remove fines below 500 μm because fine particles melt prematurely in the feed section and create gels that act as stress concentrators in the sidewall. Batch-to-batch variance in regrind quality remains a primary processing bottleneck on high-output 25 L jerrycan lines.

How High-Load Melt Index Governs Accumulator-Head Machine Output

The high-load melt index at 21.6 kg and 190°C, commonly designated HLMI or I₂₁, is more informative for extrusion blow moulding than the standard 2.16 kg melt index because it loads the melt into the shear rate range encountered in the extruder and die. A bimodal HMW-HDPE with an MI of 0.25 g/10 min typically has an HLMI between 7 and 9 g/10 min, giving an HLMI/MI ratio of 28–36. The ratio is an indirect indicator of shear thinning breadth; higher ratios correspond to broader molecular weight distributions and lower extruder torque at equivalent melt strength. In a 90 mm grooved-feed extruder running at 60–80 rpm, the head pressure may stabilise between 25 MPa and 35 MPa for a 0.30 g/10 min grade, and the melt temperature at the accumulator is controlled to 195–210°C. If the HLMI falls below 7 g/10 min, the extruder motor load often exceeds 85% of rated current, and the accumulator fill time becomes longer than the cooling time, limiting cycle output. If the HLMI exceeds 11 g/10 min, the melt strength is generally insufficient for the parison hang time needed in a 25 L mould. Capillary rheometry according to ASTM D3835-16 at 190°C provides a full shear viscosity curve; the zero-shear viscosity of a suitable bimodal grade is normally between 80,000 and 120,000 Pa·s, which decays to below 2,500 Pa·s at 100 s⁻¹. This shear thinning is what permits the high molecular weight tail to coexist with industrially acceptable extruder output.

Processing Stabilizer Packages That Delay Oxidative Degradation in HMW-HDPE

The stabilizer package in HMW-HDPE for large blow moulding is typically a combination of a hindered phenolic primary antioxidant and a phosphite secondary antioxidant, with an acid scavenger such as calcium or zinc stearate. The phenolic antioxidant terminates propagating alkylperoxy radicals during high-temperature processing, while the phosphite decomposes hydroperoxides before they initiate chain scission. In a 25 L jerrycan application, the virgin resin is usually supplied with an oxidation induction time above 40 min at 200°C when tested by ISO 11357-6, and after five heat histories the value should not fall below 20 min. If post-industrial recrystallized material or certain color masterbatches are added, the package must be supplemented because pigments based on transition metals can catalyse oxidation. Amine-based additives are incompatible with the food-contact status of the finished article where direct food approval under FDA 21 CFR 177.1520 or EU No 10/2011 is required, and they can produce yellowing during prolonged warehouse storage. Calibration of the feed hopper is critical because an underfeeding of stabilizer masterbatch by 0.1 wt% can reduce OIT by 5–10 min and shift the melt flow rate upward after three extrusion passes. Operational boundaries therefore include a maximum melt temperature of 230°C, a maximum barrel residence time below 5 min, and purging with a lower-viscosity HDPE when the line is stopped for more than 15 min. These limits are set because melt strength loss from oxidative degradation is cumulative and cannot be recovered by lowering temperature after the high molecular weight fraction has been damaged.

Pinch-off geometry in 25 L jerrycan moulds determines the weld-line strength that limits the service performance of the container. The pinch-off insert is typically a hardened steel land with a width of 0.3–1.0 mm, designed to produce a compressive squeeze flow that removes degraded polymer and contaminants from the weld zone before the flash is trimmed. With high molecular weight HDPE, insufficient melt strength leads to excessive flash thickness and incomplete pinch-off compression, resulting in a weak seam that fails during drop impact at -18°C according to ASTM D2463-15. The blow ratio, defined as the ratio of maximum mould diameter to parison outer diameter, is maintained between 2.0:1 and 2.8:1 for uniform sidewall orientation; higher blow ratios improve stiffness but reduce melt strength at the far sidewall and increase the risk of pinholes. Mould vents with depths of 0.02–0.05 mm along the parting line prevent air entrapment, but excessive vent depth creates flash and reduces the effective melt strength at the mould pinch. A high molecular weight tail also raises the cooling time because the thicker crystalline lamellae and higher plateau modulus delay dimensional stabilisation; at a wall thickness of 1.5 mm, cooling times of 60–90 s at a mould temperature of 15°C are typical. Incompatibility arises with certain silicone release sprays and external lubricants that can migrate into the pinch-off and lower interfacial adhesion; such agents must be avoided where UN certification requires weld integrity.