ExxonMobil HDPE HD6908 is a high-molecular-weight high-density polyethylene resin supplied in pellet form for extrusion blow moulding of rigid hollow parts, including industrial containers, large-format bottles, and fluid reservoirs. The published typical density is 0.953 g/cm³ when determined according to ASTM D1505-18. The melt index is 0.08 g/10 min at 190 °C under 2.16 kg load according to ASTM D1238-20. The high-load melt index at 190 °C under 21.6 kg load is 6.0 g/10 min, giving a melt flow ratio of approximately 75. This places HD6908 in the high-molecular-weight blow moulding envelope rather than the injection moulding envelope, where melt index values commonly exceed 2 g/10 min. The grade is a copolymer, and the short-chain branching distribution is controlled to raise environmental stress crack resistance relative to a homopolymer HDPE of equivalent density while retaining the modulus associated with a density above 0.945 g/cm³. Published data for the specific branching distribution is limited, but the property profile is consistent with a high-melt-strength, high-viscosity blow moulding resin.

At receiving, pellets should be stored in a sealed hopper or silo at ambient temperature below 50 °C and protected from ultraviolet radiation. Moisture uptake in high-density polyethylene is generally low, but condensation on cold pellet surfaces can introduce surface moisture. If the resin has been exposed to relative humidity above 60% for more than 12 h, hopper-air drying at 80 °C for 2 h to 4 h is necessary to prevent splay and parison surface roughness. The drying air dew point should not exceed -20 °C. This requirement is less severe than for hygroscopic resins, but process feedback from accumulator-head machines with 80 mm single-screw extruders shows that wet pellets produce visible parison bubble defects at melt temperatures below 200 °C.

Does a Narrow Melt Temperature Window Govern Parison Sag in HD6908?

Parison sag is controlled by the high-load melt index and the zero-shear viscosity of the melt. For HD6908, the high-load melt index of 6.0 g/10 min at 190 °C is lower than that of general-purpose HDPE blow moulding grades with high-load melt index values above 8 g/10 min, so the parison retains more green strength during mould closing. The processing consequence is a narrower melt temperature window than a lower-viscosity general-purpose blow moulding grade. Below 180 °C the melt is too viscous for uniform parison formation, and above 230 °C the parison elongates too rapidly under its own mass, causing wall thinning in the pinch-off area. On shuttle machines with a 60 mm extruder and a 30:1 L/D ratio, barrel zone set-points should follow a reverse profile: 180 °C at the feed throat, 190 °C in the compression zone, 200 °C in the metering zone, and 190 °C at the die head. Melt temperature measured by an immersion probe should remain between 190 °C and 220 °C. Residence time should not exceed 6 min at 220 °C to avoid crosslinking and gel particle formation. Carbonyl index measured by FTIR increases once the melt has remained in the accumulator for more than 10 min at 230 °C.

Accumulator-head machines require slow decompression of the melt accumulator to prevent melt fracture. A backpressure set-point of 5 MPa to 10 MPa is typical for HD6908. Higher backpressure increases shear heating and may shift the melt temperature upward by 3 °C to 5 °C. Screw speed should maintain a fill time of less than 2 s for a 20 L container parison. Longer fill times on a 90 mm accumulator head increase the risk of melt stagnation in the die spider legs. The die gap should be adjusted to 1.5 mm to 2.5 mm for a finished wall thickness of 2 mm to 3 mm. Die swell in HD6908 is higher than in an injection moulding grade because of the high molecular weight and broad molecular weight distribution. The blow-up ratio should be limited to 2.0:1 to 2.5:1 to avoid uneven wall distribution and excessive thinning in container corners.

Extrusion Blow Moulding Equipment Set-Points and Melt Temperature Stability

For single-station shuttle machines, clamp force must be sufficient to resist blow pressure without mould flash. At a blow pressure of 0.6 MPa to 0.8 MPa, the projected area of a 20 L container of approximately 0.25 m² generates a mould-opening force of 150 kN to 200 kN. A machine rated at 25 ton clamp force provides a safety factor of 1.2 to 1.7. The mould should be cooled with water at 8 °C to 12 °C to achieve a cycle time of 45 s to 60 s for a 2 mm wall thickness. Mould temperatures above 15 °C extend cooling time and increase post-mould shrinkage. Shrinkage of HD6908 after 24 h is approximately 1.5% to 2.5% in the machine direction and 1.0% to 2.0% in the transverse direction when conditioned at 23 °C and 50% relative humidity according to ASTM D955-21.

The grade is not intended for injection moulding. Its melt index of 0.08 g/10 min is far below the 5 g/10 min to 20 g/10 min range typical for thin-wall injection moulding polyethylenes. If substituted into an injection moulding press with a 25 mm diameter reciprocating screw, short shots and gate freeze-off occur because the shear viscosity at 1000 s⁻¹ remains above 300 Pa·s at 220 °C, whereas a high-melt-flow-rate HDPE falls below 100 Pa·s at the same shear rate. The difference is intrinsic to the high molecular weight and is not corrected by raising melt temperature alone. At 240 °C, the viscosity of HD6908 is still too high for spiral flow lengths greater than 30 cm in a 1 mm wall. Therefore the resin is restricted to low-shear, thick-wall processes such as extrusion blow moulding, sheet extrusion, and profile extrusion.

Typical physical properties reported in current manufacturer technical literature are reproduced below. These values are typical properties and are not to be construed as specification limits. The current product certificate of analysis and the latest technical data sheet remain the controlling documents for production lots.

When HD6908 is precoloured with 2 wt% carbon black masterbatch, the melt viscosity decreases slightly because of the lubricating effect of the masterbatch carrier, but tensile yield strength may drop by 3% to 5% compared with natural resin. If a white masterbatch containing 50 wt% titanium dioxide is added at 3 wt%, the dry-batch blend should be agitated for 20 min to avoid colour streaks. The screw speed should be reduced by 20 rpm relative to natural resin to limit shear heating. Field data from 80 mm grooved-feed extruders indicate that carbon black concentrates with a melt index below 2 g/10 min cause melt pressure fluctuations of ±0.5 MPa in the accumulator head if pellet size is uneven.

When HD6908 Is Compared with High-MFR Injection Moulding and Unimodal Blow Moulding Grades

Compared with a unimodal HDPE blow moulding grade of density 0.945 g/cm³ and high-load melt index of 8 g/10 min, HD6908 exhibits higher melt strength but lower flow. The melt flow ratio of approximately 75 indicates a broader molecular weight distribution than a standard unimodal grade with a flow ratio of 50 to 60. This broad distribution increases shear thinning, so the viscosity under high shear in the die is lower than the zero-shear value. The result is adequate extrusion output while parison strength is retained at low shear after the melt exits the die. The trade-off is an increase in die swell and a more pronounced tendency for parison curl if the die is not centred. Published data for this specific comparison is limited, but the measured high-load melt index and flow ratio support the distinction.

In contrast to a high-density polyethylene injection moulding grade with a melt index of 8 g/10 min, HD6908 cannot be processed in thin-wall moulds without excessive pressure loss. The difference is visible in the spiral flow length, in which an injection moulding HDPE may reach 60 cm in a 1 mm channel at 220 °C, while HD6908 would fail to fill a 30 cm channel under equivalent pressure. The high molecular weight also produces a higher die swell, which is beneficial for parison formation but harmful in injection moulding because it causes dimensional instability at the gate. Therefore the specification sheet for HD6908 does not include injection moulding conditions. The intended conversion technology is extrusion blow moulding, where the melt is shaped at low shear and then inflated under low pressure.

Hot-plate welding of HD6908 moulded parts at a plate temperature of 200 °C to 220 °C produces a weld strength of 85% to 95% of the parent tensile yield strength when a melt seal of 0.3 mm to 0.5 mm is developed. The grade is not recommended for continuous contact with strong oxidizers such as concentrated nitric acid at temperatures above 40 °C, nor for aromatic solvents such as xylene above 25 °C, because solvent uptake above 5 wt% reduces the flexural modulus below 800 MPa. For diesel fuel contact in fuel tank applications, the finished part must be validated by permeation testing according to SAE J2659 or an OEM-specific 48 h SHED test. The raw resin alone does not confer fuel-system approval.

Regulatory statements commonly attached to this grade are compiled in the matrix below. Compliance of the finished article remains the responsibility of the converter and brand owner because processing aids, colourants, and secondary operations modify the final composition and migration behaviour.

In blow moulding operations where a multilayer structure is required, HD6908 is used as the structural or regrind layer but not as the barrier layer. The melt temperature of the structural layer should remain below 220 °C at the die junction to avoid thermal degradation when ethylene vinyl alcohol or polyamide barrier layers are present. If regrind containing barrier polymer is blended with HD6908, the addition level should not exceed 20 wt% unless the accumulator residence time is reduced below 4 min. Above that addition level, the melt becomes prone to irregular flow lines and reduced pin weld strength in the container base. These limits are derived from the processing consistency required for large-part blow moulding rather than from a single property threshold.

For sheet extrusion, HD6908 can be processed on a single-screw extruder with a 30:1 L/D barrier screw and a polished three-roll stack maintained at 60 °C to 80 °C. The melt temperature should not exceed 210 °C to avoid surface oxidation. The extruded sheet retains a matte finish because the high molecular weight reduces melt drawdown but also limits line speed to approximately 2 m/min to 4 m/min for sheet thicknesses from 2 mm to 4 mm. Published data for this specific configuration is limited, and the line speed must be established on the target equipment.