LyondellBasell HDPE 4261 A IM is a high-density polyethylene resin supplied for injection-moulding operations where rigidity, melt flow, and ambient-temperature impact resistance must be balanced against cycle time and part mass. The IM suffix identifies the injection-moulding formulation within the 4261 A product family; extrusion and blow-moulding variants are not interchangeable because molecular mass distribution, additive package, and rheological response are controlled differently. The grade is typically specified for caps, closures, crates, pails, housewares, thin-wall packaging, and technical components that require dimensional stability and selected food-contact compliance. The values discussed below are representative of the product family unless a specific certificate of analysis is cited; batch-to-batch variation can occur within the manufacturer’s published specification limits.

What processing conditions are observed on reciprocating-screw injection machines?

On a 200- to 350-ton hydraulic reciprocating-screw machine fitted with a general-purpose HDPE screw having 20:1 to 24:1 L/D and a compression ratio of 2.0:1 to 2.5:1, the recommended melt temperature is 220 to 260 °C. Barrel zone settings typically rise from 180 °C at the feed throat to 240 °C at the metering zone, with nozzle temperature held at 230 to 250 °C. Mold wall temperature is maintained at 10 to 30 °C for simple geometries, but a setting of 50 °C is used where gloss, weld-line strength, or thin-section filling is critical. Injection velocity is adjusted to maintain a melt-front velocity of 200 to 300 mm/s in cavities thinner than 2.0 mm; lower velocity reduces orientation but encourages premature skin solidification. Hold pressure is normally set at 50 to 80 MPa hydraulic pressure or adjusted to gate-seal time determined by part-weight stabilization. Back pressure is limited to 0.5 to 1.5 MPa to avoid excessive shear heating and molecular weight degradation, while screw decompression after recovery is set to 2 to 5 mm. No routine pre-drying is required for unopened packaging stored below RH 60%; if exposure to humid air exceeds 2 h, tray drying at 70 to 80 °C for 2 to 4 h is recommended. Plant-scale observation indicates that melt-pressure variability greater than 10% during screw recovery is an early indicator of feed-bridge disruption, screw wear, or regrind particle-size segregation.

In the melt phase, the injection-moulding formulation exhibits shear-thinning behaviour typical of linear high-density polyethylene. Apparent melt viscosity at 190 °C and a shear rate near 1000 s⁻¹ is lower than that of a blow-moulding grade of equivalent density, which permits shorter injection time and reduced gate pressure. At low shear rates, however, the narrow molecular mass distribution reduces melt strength; this limits the use of the material for parison extrusion, film blowing, or large unsupported flow fronts. Capillary rheometry according to ISO 11443:2021 is the appropriate method for lot-to-lot comparison if a converter requires a defined viscosity window. Published data for this specific configuration is limited to the manufacturer’s rheological curves.

Specification block tied to ISO 1133-1 and ISO 178

Representative values for unfilled HDPE 4261 A IM are shown below. They are not specification limits; converters should request the current product datasheet and certificate of analysis for the production batch.

Compared with blow-moulding HDPE grades of similar density, HDPE 4261 A IM has a higher melt-flow rate and lower melt strength, which improves cavity filling but makes continuous extrusion and parison control less stable. The distribution of molecular mass is narrower than that of a standard extrusion or sheet grade, reducing die swell and gate stringing but also lowering environmental stress crack resistance when measured against a slower-flow high-molecular-weight HDPE. High-flow thin-wall HDPE grades with MFR above 20 g/10 min can fill thinner sections at lower injection pressure, but they sacrifice notched impact and top-load retention; 4261 A IM retains a higher average molecular mass and is therefore preferred where closures must survive drop impact and top-load testing. Compared with isotactic polypropylene used for similar caps and closures, this HDPE grade has lower heat deflection temperature and lower flexural modulus but better resistance to aqueous surfactant stress cracking and less hinge whitening at low temperatures.

When thin-wall pails require warp reduction and controlled shrinkage

Mould shrinkage of 1.5 to 2.0% in the flow direction and 1.8 to 2.4% transverse to flow is typical for unfilled HDPE injection-moulding grades. Differential shrinkage exceeding 0.3 percentage points between flow and transverse directions produces observable warpage in flat pail lids and large rectangular crates. A diaphragm gate, a balanced hot-runner system with valve gates, or multiple edge gates arranged symmetrically reduces flow-length asymmetry. A minimum wall thickness of 2.0 mm is recommended for pail bodies with a projected area above 0.10 m². On production lines, post-mould shrinkage of 0.5 to 1.0% can continue for 24 to 48 h at ambient temperature; dimensional inspection should therefore be delayed or compensated. Use of a cooled post-mould fixture reduces warpage but increases auxiliary labour and may induce surface marking if the part is ejected too hot. Insert moulding and overmoulding of HDPE 4261 A IM are generally not recommended with polar substrates because surface energy is low and corona or plasma treatment is required for adhesion.

Venting depth of 0.02 to 0.03 mm is used on the parting line for HDPE because the low viscosity of the melt can cause flash if depths exceed 0.05 mm. Vent land length is kept at 1.0 to 1.5 mm. Gate land length of 0.5 to 1.0 mm and gate diameter of 0.8 to 1.5 mm are typical for direct and tunnel gates on caps and closures. Ejector pins should be positioned outside high-gloss visible surfaces; draft angle of 0.5 to 1.0° per side is necessary for untextured sidewalls, and textured surfaces require at least 1.5° per side.

Environmental stress crack resistance is the primary operational boundary in applications that combine external load with aggressive liquid media. For HDPE 4261 A IM, ESCR measured by ASTM D1693-15 Method B in 100% Igepal CO-630 at 50 °C is generally lower than that of a slower-flow high-molecular-weight HDPE blow-moulding grade. Closures for detergent bottles should be evaluated under simulation of the actual product headspace and not judged only by top-load testing. Continuous service with strong oxidizing acids, aromatic hydrocarbons, or halogenated solvents is outside the recommended envelope because the semi-crystalline matrix swells and loses tensile strength. For aqueous surfactant exposure at temperatures above 60 °C, a specific ESCR qualification programme is required; published data for this specific configuration is limited.

Food-contact status and regulatory compliance matrix

The grade is typically supported by regulatory documentation for selected food-contact and product-safety regimes. Certifications are lot- and production-site-specific; a supplier certificate is required before commercial use.

Regrind addition of 20 to 30 weight percent is common in pails, crates, and housewares, but multiple heat histories increase oxidation and reduce low-temperature notched impact. Field experience indicates that melt flow index can increase by 10 to 20% after three regrind cycles, while Charpy notched impact at −20 °C decreases by 15 to 25%. A conservative limit of 20% regrind is therefore applied in applications requiring mechanical shock resistance. The feed stream should be free of polypropylene contamination above 0.5 weight percent because immiscible PP domains reduce weld-line strength and create surface splay.