High Density Polyethylene HDPE 9255 is supplied as a high-molecular-weight, hexene-1 copolymer grade intended primarily for extrusion blow moulding of large-part industrial containers, intermediate bulk container inner vessels, and low-temperature fluid reservoirs. The grade is characterised under ISO 1183-1:2019 with a nominal density of 0.953 g/cm³ and under ISO 1133-1:2022 with a melt mass-flow rate of 0.06 g/10 min at 190 °C and 2.16 kg load. The 21.6 kg melt mass-flow rate is typically reported in the 5.5–6.5 g/10 min range, producing a flow ratio near 90–110. Published data for this specific configuration is limited; lot-specific certificate-of-analysis values should take precedence over general grade-class envelopes.

For extrusion blow moulding line trials, the grade has been run on a 90 mm grooved-barrier single-screw extruder with an L/D ratio of 30:1 and a barrier flight section covering 12D. The temperature profile from feed throat to die was set at 180 °C, 190 °C, 195 °C, 200 °C, 205 °C, and 210 °C; head and die temperatures were maintained at 195–205 °C. At a screw speed of 55 min⁻¹, melt pressure at the breaker plate was recorded between 280 bar and 340 bar. Surface melt fracture was observed at die gaps below 1.2 mm, while heavy shark-skin defects occurred at wall shear rates exceeding 800 s⁻¹.

What Distinguishes HDPE 9255 from Conventional HDPE Commodity Grades?

Unlike a 0.35 g/10 min blow-moulding HDPE commonly used for detergent and personal-care bottles, HDPE 9255 operates at roughly one-sixth the melt mass-flow rate under 190 °C/2.16 kg conditions. The lower value indicates a longer average macromolecular chain length, which increases environmental stress-cracking resistance but reduces screw throughput and increases head pressure at equivalent screw speed. In field trials on a 60 mm grooved-feed extruder, throughput of the 0.35 g/10 min control grade reached 190 kg/h, while HDPE 9255 required a 15–20 % reduction in screw speed to remain below the motor current limit and produced 160–170 kg/h at comparable melt quality.

The distinction from a 7 g/10 min injection-moulding HDPE is more pronounced. Injection-moulding grades favour short relaxation times, low pressure drop, and rapid cavity filling, whereas HDPE 9255 retains pronounced elastic recovery after die exit. Capillary rheometry at 190 °C shows shear viscosity of approximately 14,000 Pa·s at 10 s⁻¹ and 1,300 Pa·s at 100 s⁻¹, compared with values below 200 Pa·s at 100 s⁻¹ for a typical 7 g/10 min injection HDPE. The following comparative matrix summarises the practical difference across standard mechanical and thermal profiles.

Across a Capillary Rheometer, Shear Viscosity Declines Rapidly with Shear Rate

Capillary and dynamic shear measurements place HDPE 9255 near the upper end of commercial HDPE melt strength. Parallel-plate oscillatory rheometry at 190 °C gives a crossover frequency of approximately 0.15–0.25 rad/s, indicating a broad relaxation spectrum. The storage modulus exceeds the loss modulus below this frequency, characteristic of long-chain linear molecules. In uniaxial extension, a Hencky strain rate of 0.1 s⁻¹ produces tensile stress growth coefficients above 10⁵ Pa·s at 10 s; this contributes to parison stability on large accumulator-head machines. In shear, the grade exhibits pronounced shear thinning, with viscosity falling from roughly 14,000 Pa·s at 10 s⁻¹ to 1,300 Pa·s at 100 s⁻¹ and below 300 Pa·s at 1,000 s⁻¹. These representative values are useful for screw design and die land calculations but should not replace capillary data from a grade-specific lot.

On a 40 mm co-rotating twin-screw extruder with an L/D ratio of 44:1, carbon black masterbatch addition at 2.0–2.5 % by weight is performed with side-feed and distributive mixing elements. Excessively aggressive kneading blocks raise melt temperature above 240 °C, initiating oxidative chain scission and generating gel particles that appear as surface bumps on blow-moulded parts. Compounding trials show that limiting specific mechanical energy input below 0.25 kWh/kg and maintaining melt temperature below 220 °C preserves the high-molecular-weight fraction.

Extrusion Blow Moulding Die Swell and Parison Hang Time

Die swell for HDPE 9255 ranges between 30 % and 45 % at apparent wall shear rates of 100–500 s⁻¹ through an annular die with land length of 20–30 mm and land-length-to-gap ratio near 10:1. The high die swell is a direct consequence of the broad molecular weight distribution and slow relaxation after convergent die flow. Parison sag resistance is superior to that of fractional-melt blow-moulding grades; a 30 L canister parison with a shot weight of 1.8 kg can be held for 12–18 s without visible necking when die temperature is kept near 195 °C. At die temperatures above 210 °C, sag rate increases and the parison diameter becomes non-uniform.

Annular die gaps below 1.2 mm produce high shear at the wall and trigger melt fracture, especially with die land temperatures below 190 °C. The recommended starting die gap for a 20–30 L container is 1.8–2.4 mm, with blow pressure controlled between 0.70 MPa and 0.85 MPa. Preblow pressure should be set at 0.05–0.10 MPa to avoid parison flutter and premature contact with the mould parting line. The use of a converging die bushing with an entry angle of 15–20 ° reduces stagnation and carbon black streaking.

If HDPE 9255 Is Processed on Accumulator-Head Blow Moulders

For accumulator-head machines, the grade requires a head volume sized to the shot weight, with a fill rate sufficient to maintain uniform melt temperature. A 20 L container with a shot weight of 1.8 kg is typically run with an accumulator head volume of 3.0 L, push-out speed of 45 mm/s, die gap of 2.0 mm, and clamp force between 140 t and 180 t. Coolant temperature is maintained at 12–15 °C, and blow pressure is 0.75 MPa. At push-out speeds above 55 mm/s, the parison may develop oscillatory thickness variation due to high shear at the die land exit. At slow push-out speeds below 30 mm/s, the cycle time increases and the lower parison section can sag on large shot weights.

Regrind addition is acceptable up to 30 % by weight when the regrind is dry, film-free, and generated from unpigmented or internally pigmented containers. Post-consumer recyclate should be limited to 20 % by weight and screened for odour-generating residues. If regrind exceeds 30 %, the operator should reduce screw speed and verify head pressure stability, because repeated heat history shifts the molecular weight distribution toward lower viscosity and reduces parison hang time.

Environmental stress-cracking resistance of HDPE 9255 is evaluated under ASTM D1693-15 in 100 % Igepal CO-630 at 50 °C; compression-moulded plaques of 2 mm thickness typically exceed 600 h to F50. The performance results from high molecular weight, low melt flow rate, and limited short-chain branching in the high-molecular-weight fraction. The grade is routinely selected for containers storing aqueous agrochemical formulations, dilute acids, and alkaline cleaning agents below 40 °C. It is not recommended for prolonged contact with strong oxidizing acids above 60 °C, chlorinated solvents, or aromatic hydrocarbons, because swelling and accelerated stress cracking may occur. Permeation of nonpolar solvents is governed by solubility parameter differences; published data for specific formulations is limited.

Food-contact suitability is assessed under FDA 21 CFR 177.1520 and EU 10/2011 when the grade is unpigmented or contains cleared pigments. Stabilizer migration kinetics in polymer matrices govern organoleptic performance in food-contact containers; therefore only pigment and additive packages with migration limits confirmed under EU 10/2011 should be used. The polymer is not inherently UV-stabilized; outdoor service in high-UV environments requires a carbon black masterbatch at 2.0–2.5 % by weight or a co-fed hindered-amine stabilizer package. Pellets should be stored below 40 °C and protected from direct sunlight to minimize oxidative induction time drift. Pre-drying is not normally required because polyethylene is non-hygroscopic, but surface condensation should be removed if pellets are moved from cold storage into humid ambient air above 60 % relative humidity.

Processing without pre-drying is acceptable at pellet surface moisture below 0.02 % by weight. Above 60 % relative humidity, condensation should be removed with a desiccant hopper at 60 °C for 2 h. The grade should not be processed above 240 °C or held at melt temperature for more than 5 min, because oxidative degradation can reduce molecular weight and generate gel defects. If equipment shutdown is required, a fractional-melt HDPE purge compound should be used to displace the high-viscosity melt from the head and die.