LyondellBasell HDPE M5372 is a high-density polyethylene injection molding grade supplied as pelletized reactor product. Its release specifications are controlled by melt mass-flow rate determined at 190 °C under 2.16 kg load using ASTM D1238/ISO 1133-1:2022, and by density determined with ASTM D1505/ISO 1183-1:2019. Typical material certificates list a melt mass-flow rate of 7.0 g/10 min and a density of 0.953 g/cm³. The grade is therefore classified as a medium-flow HDPE injection molding resin. In practice, HDPE M5372 is used for crates, pails, caps, houseware components, and thin-wall packaging where a combination of short cycle time, stiffness, and resistance to environmental stress cracking is required. The balance between melt flow rate and density positions the product between low-flow blow molding grades, which require high melt pressure, and high-flow thin-wall grades, which exhibit lower melt strength and may show higher shrink orientation.

Lot-to-lot melt flow variation is controlled within ±0.5 g/10 min in typical release documentation. This narrow variation is significant in high-cavitation molds where a melt flow shift of 1 g/10 min changes fill pressure by 2–4 MPa and may alter gate seal time. Incoming quality checks for HDPE M5372 usually include density, melt flow, and visual pellet cleanliness; shipments are supplied with a certificate of analysis that lists the batch-specific values for the same test methods used in product specifications.

How Does HDPE M5372 Differ from Higher-Flow and Lower-Flow HDPE Grades?

Higher-flow HDPE grades with melt mass-flow rates above 20 g/10 min are typically chosen for high-cavitation thin-wall packaging because they fill long flow paths at lower injection pressure. However, the reduction in molecular weight that produces the higher melt flow also lowers stress crack resistance and can increase warpage due to orientation. Lower-flow HDPE grades with melt mass-flow rates below 2 g/10 min provide improved creep rupture behavior in pipe and blow molded containers, but their higher melt viscosity limits injection moldability to thick-wall parts and requires higher melt temperatures. HDPE M5372 occupies an intermediate position: the 7.0 g/10 min melt flow is low enough to retain melt strength during cap and closure injection-compression molding, but high enough to allow cavity filling in crates and pails on standard injection molding machines with clamp force from 80 t to 250 t. The grade is not intended for film or blow molding because the molecular architecture is optimized for injection molding solidification rates.

Injection molding of HDPE M5372 on a 100 t hydraulic injection molding machine with a 35 mm barrier screw and 20:1 L/D ratio follows the processing envelope shown below. The barrel temperature profile from feed throat to nozzle is maintained at 190 °C, 210 °C, 220 °C, and 220 °C; nozzle temperature is held at 210 °C to 230 °C. Holding pressure is set to 50–80% of the injection pressure. Back pressure is maintained at 0.5–2.0 MPa to avoid excessive shear heating without creating splay from unmelted pellets. Screw recovery speed is set between 50 min⁻¹ and 120 min⁻¹, with cushion position controlled to 3–8 mm to maintain consistent holding pressure transfer.

At nozzle settings above 230 °C, residence time should not exceed 5–8 min; prolonged residence leads to viscosity loss and yellowing. On machines with accumulator-assisted injection, the fill time for a 2 mm wall pail may be 1.2–2.0 s; if fill time exceeds 2.5 s, the melt front may solidify before packing, producing a surface blush at the end of flow. The injection velocity profile is therefore set with an initial fast stage of 80–100 mm/s and a reduced final stage of 20–40 mm/s to avoid jetting and air entrapment. In hot runner systems, manifold temperature should be maintained within ±5 °C of the nozzle set point; greater deviations produce viscosity imbalance across cavities. For a 32-cavity cap mold, cavity-to-cavity fill imbalance should be controlled within 2–3% of shot weight to maintain roundness after ejection.

When Mold Temperature Is Kept Below 40 °C, Crystallinity Gradients Alter Shrinkage

In cold-mold operation between 10 °C and 40 °C, HDPE M5372 develops a non-uniform shell-core morphology. The skin freezes quickly, while the core continues to crystallize after ejection. Total linear shrinkage measured according to ASTM D955 is typically 1.5–2.5%, with higher shrinkage in the flow direction due to orientation release. Gate location therefore determines warpage direction: an edge-gated crate sidewall may bow inward if the packing pressure is insufficient to compensate for volumetric shrinkage at the intersection of the sidewall and base. To minimize warpage, hold pressure is applied until gate freeze; for a 1.5 mm wall, gate freeze time is 3–6 s. For thicker bosses above 4 mm, sink marks are controlled by reducing local wall thickness or by adding a gas counterpressure channel, not by increasing hold pressure alone.

Crate and pail applications impose drop impact and stacking load requirements. HDPE M5372 moldings are evaluated by ASTM D638-14 for tensile yield stress; typical values for this product are near 26 MPa. Flexural modulus tested by ASTM D790-17 is approximately 1,350 MPa. Notched Izod impact tested at 23 °C by ASTM D256-23 is commonly reported near 2.5 kJ/m², but toughness depends on gate type, weld lines, and pigment dispersion. Black and white masterbatch additions below 2.0 wt% do not materially alter tensile yield stress, but excessive pigment above 4.0 wt% can reduce weld-line strength because pigment agglomerates act as stress concentration sites. At low temperatures, the ductile-to-brittle transition in unnotched geometries is near −60 °C; notched parts transition near 15 °C depending on notch radius and weld line. For freezer-grade crates operating at −20 °C, weld-line impact strength is controlled by maintaining melt temperature above 220 °C and avoiding cold mold spots below 10 °C.

Weld-line strength in HDPE M5372 is affected by melt temperature, mold temperature, and the angle at which melt fronts meet. In rectangular crates with multiple gates, weld lines formed by head-on flow at 90° are weaker than weld lines formed by flow-front convergence at angles above 135°. Drop impact failures in 20 L pails at −20 °C typically initiate at the bottom chine weld line when the mold temperature is below 10 °C; raising the mold temperature to 20–30 °C improves weld-line strength by allowing molecular interdiffusion across the frozen skin. Gate sequencing with valve gates can shift weld lines away from high-stress regions, but published data for this specific product is limited.

Outdoor crates and pails require ultraviolet stabilization. The base HDPE M5372 resin does not contain a heat or UV stabilizer package; weathering resistance is introduced through a carbon black or hindered amine light stabilizer masterbatch. Carbon black masterbatch at 2.5 wt% provides long-term outdoor opacity, while hindered amine stabilizers are used in colored parts at addition levels of 0.1–0.5 wt%. Weathering performance is evaluated by ASTM D2565 with xenon-arc exposure; failure is defined by loss of tensile elongation or surface cracking. Without stabilization, HDPE M5372 exhibits surface chalking and embrittlement after outdoor exposure; published data for this specific product is limited.

Extrusion Blow Molding Grades Differ from HDPE M5372 in Molecular Architecture

Extrusion-grade HDPE resins with melt flow rates below 1 g/10 min provide higher environmental stress crack resistance and are used in blow molded containers for aggressive fluids. HDPE M5372 is not a blow molding grade; its melt flow rate of 7.0 g/10 min and narrower molecular weight distribution reduce die swell and improve injection cavity replication. Compared with a broad molecular weight distribution extrusion HDPE, M5372 displays lower viscosity at high shear rates, but lower melt strength at low shear rates. This means that extrusion blow molding of HDPE M5372 would result in parison sag before mold closing. The environmental stress crack resistance measured under ASTM D1693-15 in 100% Igepal CO-630 at 50 °C is expected to be lower than that of a high-molecular-weight blow molding grade, although published data for this specific product is limited.

Cap and closure molding on high-cavitation hot runner systems uses HDPE M5372 at melt temperatures between 200 °C and 220 °C. Hot runner valve-gated systems with pitch diameters below 25 mm per cavity require stable melt viscosity to avoid gate drool and stringing. Torque removal for 28 mm PCO closures is influenced by closure design and liner thickness; published data for this specific grade is limited, but HDPE closure resins with similar melt flow typically show removal torque in the range 2–6 N·m at ambient temperature after 24 h conditioning. The low melt flow variation of the product supports consistent cap diameter roundness when cavity pressures are balanced. In injection-compression molding of thin-walled closures, the compression stroke is typically 1.5–3.0 mm after initial injection; this allows lower clamping force and reduces molded-in stress. Crystallization shrinkage in the cap skirt is compensated by holding the compression force for 2–5 s after filling. Mold temperatures for closure tools are often set at 20–30 °C to balance cycle time and surface gloss.

Moisture Uptake Limits and Desiccant Drying Requirements

HDPE pellets are not hygroscopic in the manner of polyamide; however, surface moisture from condensation at high relative humidity can produce splay and surface defects. In plants where silos or hoppers are exposed to conditions above 60% RH, pre-drying is applied at 70 °C for 2 h in a desiccant dryer with dew point below −30 °C. Drying time can be reduced to 1 h if pellets have been stored in sealed railcar compartments. The effect of moisture on melt viscosity is less severe than in condensation polymers, but steam bubbles in the melt can disrupt gate sealing and reduce screw recovery consistency.

In central conveying systems, pellet drying hoppers should be sized for 2–4 h residence time to allow moisture removal without overheating. Pellet temperature above 85 °C can soften surface and cause bridging in the hopper outlet. Dry air flow rate is maintained at 3.7 m³/h per kg/h through the desiccant bed. Return air dew point is monitored at the hopper inlet and should remain below −20 °C for consistent surface quality.

Regulatory filings for HDPE M5372 may include food-contact compliance under FDA 21 CFR 177.1520 for olefin polymers and the European Commission Regulation (EU) No 10/2011 for plastic materials intended to contact food. The product is considered a non-hazardous polymer for transport and handling; safety data sheets address dust explosion limits and thermal degradation products. Users should verify specific migration limits for the intended food type because fatty food simulants extract low-molecular-weight fractions at higher temperatures. REACH registration under Regulation (EC) No 1907/2006 is maintained upstream by the manufacturer.