Low melt flow polyethylene grade 2426H, characterized by a melt flow rate of 1.9 g/10 min under 190 °C and 2.16 kg in accordance with ISO 1133-1:2022 and a density of 0.924 g/cm³ per ISO 1183-1:2019, belongs to the high-molecular-weight segment of low-density polyethylene homopolymers used in extrusion blow moulding. Published datasheet values for 2426H typically list a tensile stress at break of 15 MPa and an elongation at break of 400% under ISO 527-2:2012, with a Vicat softening temperature near 96 °C under ISO 306/A120. In narrow-neck squeeze bottle blow moulding, the polymer is extruded through an annular die to form a hollow parison, which is then enclosed by split mould halves, inflated through a blow pin inserted into the neck opening, and cooled under internal air pressure. The application imposes narrow processing boundaries because the neck diameter is substantially smaller than the body diameter, the parison must remain dimensionally stable during mould closing, and the base pinch-off weld must withstand repeated compressive squeezing and flexure without stress cracking. Process optimization for 2426H therefore centres on maintaining melt strength without exceeding the oxidative degradation threshold, while controlling wall thickness distribution in the neck, shoulder, sidewall, and base regions.

Why Does Narrow-Neck Geometry Amplify Parison Melt Strength Requirements?

During production of a narrow-neck squeeze bottle with a neck inner diameter between 14 mm and 24 mm and a body diameter from 40 mm to 80 mm, the parison must remain suspended from the die for an interval that typically ranges from 1.5 s to 8.0 s depending on part mass, mould closing speed, and accumulator refill dynamics on reciprocating-screw machines. The parison length-to-annular-gap ratio can exceed 100:1; because the neck finish is formed by cutting and calibrating the parison end around the blow pin, any diameter variation greater than 0.3 mm in the neck region produces closure-leakage defects or inconsistent thread engagement. Low melt flow 2426H has sufficient molecular weight to resist uniaxial drawdown under gravitational stress, but the melt strength requirement is not uniform along the parison. The upper neck segment experiences the longest residence time and the highest gravitational load, while the lower base segment is compressed and welded during pinch-off. The die swell ratio for LDPE of this molecular weight typically lies between 1.25 and 1.60 at shear rates relevant to blow moulding, depending on head tooling geometry and melt temperature; die swell contributes to final neck wall thickness but also complicates concentricity around the blow pin. A material with a melt flow rate above 4 g/10 min would be susceptible to excessive drawdown and uneven wall distribution in this configuration, whereas 2426H at 1.9 g/10 min provides a wider sag-resistance window. Specific published rheological data for 2426H in narrow-neck tooling configurations are limited; therefore, processor trials using a capillary rheometer per ISO 11443:2021 and parison sag measurements under actual hang time should be performed before setting final process limits.

Continuous shuttle machines with single-screw extruders of 60 mm to 90 mm diameter and L/D ratios of 24:1 to 30:1 are commonly operated for narrow-neck LDPE squeeze bottles; their performance with 2426H is determined by barrel temperature profile, screw rotation speed, and die-head pressure. Barrel zones are typically set from a feed zone at 150 °C to 160 °C, a compression zone at 165 °C to 180 °C, and a metering zone at 180 °C to 195 °C, while the die head is held at 185 °C to 200 °C to maintain a homogeneous melt. Screw speeds of 20 rpm to 60 rpm on a 60 mm extruder generating die pressures between 20 MPa and 35 MPa correspond to melt residence times that are short enough to avoid oxidative degradation and long enough to erase pellet boundaries. A grooved feed throat is generally unnecessary for LDPE but can improve throughput stability when regrind content exceeds 20%. Excessive screw speed produces shear heating above 200 °C, at which point long-chain branching may undergo thermomechanical scission, reducing melt strength and shifting the effective melt flow rate upward beyond the design envelope. For this reason, melt temperature should be measured with an immersion probe at the die exit and maintained within the range specified by capillary rheometry per ISO 11443:2021 for the specific batch.

Melt Temperature, Extruder Speed, and Die Swell Operating Envelope

Rheological characterization of 2426H under shear rates representative of blow moulding when measured by capillary rheometry in accordance with ISO 11443:2021 shows pronounced shear thinning; apparent viscosity at 190 °C declines from values on the order of 3000 Pa·s to 600 Pa·s between 10 s⁻¹ and 1000 s⁻¹. In extrusion blow moulding, the annular die gap commonly ranges from 0.8 mm to 2.0 mm, and the apparent shear rate at the die lip for a parison output of 5 kg/h to 25 kg/h per die can be calculated from the volumetric flow rate and the annular geometry; for many narrow-neck tools it falls between 80 s⁻¹ and 400 s⁻¹. Under these conditions, the die swell of 2426H is a function of exit shear stress, melt temperature, and die-land length. Increasing die-land length from 10 mm to 20 mm reduces die swell and stabilizes the parison outer diameter. Conversely, reducing melt temperature from 195 °C to 175 °C raises die swell and improves parison hang resistance but increases head pressure and energy consumption. The practical operating envelope for 2426H is bounded on the lower side by extruder motor amperage and melt fracture onset, and on the upper side by thermal degradation and loss of parison strength. Melt temperatures above 220 °C should be avoided due to rapid oxidation, while temperatures below 160 °C may produce melt fracture, poor surface finish, and inconsistent pinch-off welds.

Parison programming is not optional for narrow-neck squeeze bottles made from 2426H. A closed-loop parison controller with wall-thickness profile resolution of at least 25 points per cycle permits zone-specific die gap changes that compensate for the different drawdown histories of neck, shoulder, sidewall, and base. The neck zone is usually programmed with a narrower die gap to reduce dilation around the blow pin; the shoulder and sidewall zones are opened 20% to 40% relative to the neck gap to provide enough material for blow-up ratios of 2.0:1 to 3.5:1. The tail zone is narrowed again to limit flash at the pinch-off. Head tooling for narrow-neck bottles requires a mandrel and bushing whose annular diameter is smaller than the neck inner diameter because the parison outer diameter must fit around the blow pin and inside the neck insert during calibration. For a neck finish with an outer diameter of 24 mm, a die bushing diameter between 14 mm and 18 mm and a mandrel diameter between 12 mm and 16 mm are representative of production tooling; exact values depend on neck finish dimensions, part weight, blow pin length, and die swell.

When Blow-Air Pressure Is Applied Through the Narrow Neck

After the split mould closes on the parison, inflating air is introduced through a blow pin that enters the parison at the neck opening. In narrow-neck squeeze bottles, the restricted neck passage governs the maximum permissible annular clearance between the blow pin and the neck insert, typically 0.1 mm to 0.4 mm. Blow-air pressure is usually maintained between 0.4 MPa and 0.8 MPa, with a preblow phase of 0.2 MPa to 0.4 MPa used to stabilize the parison and prevent webbing at the shoulder. Low-air-pressure preblow is particularly important for 2426H because its high melt viscosity delays expansion; an immediate high-pressure blast can create localized thinning where the parison first contacts the chilled mould wall. Final blow pressure must be high enough to force the semi-molten polymer against the cavity surface before the melt temperature falls below the crystalline freezing onset near 90 °C to 95 °C. Mould cooling water is typically controlled at 8 °C to 15 °C to accelerate solidification while avoiding condensation. Cycle times for narrow-neck squeeze bottles from LDPE of this melt flow range typically fall between 10 s and 20 s for wall thicknesses of 0.6 mm to 1.5 mm, depending on part weight, mould temperature, and chiller capacity.

Pinch-off weld integrity at the base of a narrow-neck squeeze bottle is a critical failure site. During mould closing, the tail of the parison is compressed between opposing die edges, creating a weld line at the bottom. For 2426H, adequate weld strength is achieved when the melt temperature at the closing edges remains above 170 °C and when mould clamping force is sufficient to displace molten polymer rather than merely press two cooled skins together. Mould pinch-off geometry should include a sharp but rounded edge with a land angle between 40° and 60°; too sharp an edge severs the weld, while too blunt an edge produces a thick flash pad that distorts the base. Drop impact testing per ASTM D2463-15 is the standard method for evaluating finished container resistance to fracture, and environmental stress crack resistance testing per ASTM D1693-21 on compression-moulded plaques is used to evaluate susceptibility under surfactants such as nonylphenol ethoxylate. In production, the base weld should be sampled and sectioned for polarized-light stress analysis; pronounced birefringence near the pinch-off line indicates localized orientation from overstretching and predicts premature stress cracking at that seam. Specific ESCR values for 2426H in narrow-neck bottle configurations are not always published; verification under ASTM D1693-21 is required before qualifying the container for surfactant-containing products.

Uncontrolled regrind fraction in 2426H narrow-neck production is a common source of batch-to-batch variability in melt flow and parison drawdown. After multiple regrind cycles, thermo-oxidative chain scission increases the effective melt flow rate; at regrind contents above 30% in the dry blend, the melt flow rate may drift from 1.9 g/10 min toward values above 2.5 g/10 min, narrowing the sag-resistance window. The processor should cap regrind addition at 20% to 30% and verify melt flow rate per ISO 1133-1:2022 after every silo transfer. Moisture absorption in LDPE is relatively low, but surface condensation on regrind stored outdoors or in unheated silos can produce splay and surface defects; if visible moisture is present, drying at 80 °C for 1 h to 2 h using a desiccant bed or hot-air hopper dryer is recommended. Combine 2426H with peroxide-based masterbatches or high levels of acid-neutralizing stearates only after rheological verification, because certain additives can reduce melt strength and shift the process window. Avoid processing 2426H in equipment previously used for PVC, acetal, or fluoropolymer melts unless purging with a viscous LDPE purging compound has been completed and the barrel is free of acidic residues, which catalyze degradation of polyethylene. For narrow-neck squeeze bottles that will hold aggressive solvents, essential oils, or high-pH formulations, the converter must evaluate stress-cracking resistance of the finished container under actual product contact, as the relaxation of moulded-in orientation in the neck and base regions may be slower than in injection-moulded samples. If a product formulation contains more than 5% nonionic surfactant, ESCR failure at the base pinch-off seam becomes the governing design limit; material substitution or a barrier liner may be required.