LyondellBasell HDPE M6138 is a high-density polyethylene injection-moulding resin supplied as 25 kg bags and 1,000 kg octabin lots. The grade is identified by a melt flow rate of 20 g/10 min at 190 °C under 2.16 kg load when tested in accordance with ASTM D1238, and a solid-state density of 0.954 g/cm³ under ASTM D1505. These two values position the material at the high-flow end of the LyondellBasell HDPE injection-moulding range. The narrow molar mass distribution is used to shorten fill time in thin-wall tooling, reduce screw recovery energy, and improve melt-front consistency in multicavity layouts. The resin is not designed for extrusion blow moulding, pipe extrusion, or long-term stressed-agent service typical of high-molecular-weight HDPE grades.

Typical property data published in supplier documentation are summarised in Table 1. The values are lot-average indications and do not represent batch-release minima. Certification of a specific shipment must be read from the certificate of analysis. Mechanical values are sensitive to specimen preparation; injection-moulded specimens may show yield stress differences of ±5% relative to compression-moulded plaques.

The listed values are not intended for specification purposes. Where ASTM and ISO methods are both shown, the results are not strictly interchangeable because specimen dimensions and test speeds differ.

Melt temperature, residence time, and screw recovery on a direct-clamp injection line

The processing window extends from 204 °C to 260 °C barrel melt temperature. For wall sections between 1 mm and 2.5 mm, a melt temperature of 218–240 °C is used to balance flow length and cooling time. Above 250 °C, the stabiliser package is consumed more rapidly; residence times above 10 min at 250 °C have been associated with melt pressure variation and yellowing on direct-clamp machines with 90 mm barrier screws. Below 210 °C, short shots and gate freeze can appear in thin lids when the mould temperature is below 15 °C.

General-purpose polyolefin screws with 20:1 to 25:1 L/D and compression ratios of 2.5:1 to 3.0:1 are suitable. A back pressure of 6–10 bar and screw surface speed of 0.8–1.2 m/s are adequate for melt homogeneity without excessive viscous heating. The non-return valve should be maintained with radial clearance below 0.05 mm; a worn check ring in high-flow HDPE creates cushion loss and cavity-to-cavity weight variation.

Lot-to-lot variation in melt flow rate is commonly bounded within ±1.5 g/10 min on the certificate of analysis. On fixed-shot-size machines, a shift from 19 g/10 min to 21 g/10 min may reduce cushion by 2–4 mm in thin-wall tools; shot weight and cushion should be checked after each lot change. Clean in-house regrind can be added up to 20% by weight without altering melt flow rate beyond typical part tolerance when the regrind is not degraded. Multiple heat histories consume the processing antioxidant; higher regrind levels require verification of melt flow rate and notched Izod retention.

The required clamp force can be estimated from projected cavity area and an expected cavity pressure of 350–500 bar for thin-wall HDPE. For a 5 L pail lid with a projected area of 450 cm², the clamp force requirement is between 1,575 kN and 2,250 kN. Moulds with long flow lengths and restrictive gates need higher internal cavity pressure; in such cases, a direct-clamp machine with position-controlled injection is preferred over a toggle machine to maintain consistent cushion.

Mould coolant temperatures of 10–20 °C are used for thin-wall containers to shorten freeze time. For wall stock above 3 mm, coolant temperatures of 25–40 °C reduce sink marks and surface stress whitening. If the coolant circuit temperature varies by more than 3 °C across the tool, differential shrinkage can produce part warp; water manifold balancing is required at the tooling design stage.

In thin-wall tub and lid moulding with hot-runner valve gates, M6138 is specified when flow length-to-wall-thickness ratios approach 250:1. The high melt flow rate permits filling from a central sprue to the rim at hydraulic injection pressures below 90 MPa, provided the tool has vent channels of 0.02–0.03 mm depth and a gate diameter of 0.8–1.5 mm for side-gated parts. Short-shots are not resolved by raising barrel temperature alone; injection velocity profile, switch-over position, and holding pressure decay must be set from cavity pressure readings. Cavity pressure at transfer should be held between 350 bar and 500 bar for lids with 1 mm nominal wall, depending on flow length.

The narrow molecular weight distribution reduces melt viscosity under shear but also produces lower melt strength. Therefore, drooling from open nozzles is controlled with a shut-off nozzle or spring needle shut-off. In hot-runner systems, thermal gating must be set to prevent stringing when the valve pin closes; pin seat leakage in high-flow HDPE leads to gate vestige variation and part weight drift above 0.5%.

Knit lines in multi-gated pails and crates are a further processing concern. Because M6138 has low melt strength, the converging flow fronts may not interdiffuse sufficiently when the melt front temperature falls below 210 °C. The result is a visible knit line and a local reduction in notched impact. Overflow wells, local heating, or gate sequencing can be used to place knit lines in low-stress regions.

Hot runner manifold temperatures for M6138 are typically set at 220–240 °C, with nozzle tips at 230–250 °C. The gate tip should not exceed 260 °C; local residence time in a heated manifold should be kept below 10 min to avoid gel formation. Colour changes from natural to white or black are slower than with lower-viscosity general-purpose polyethylene because of the high pigment loading needed for opacity; dedicated screws may be required for high-gloss black parts.

Dimensional stability after demoulding is influenced by post-mould crystallisation. Parts should be cooled uniformly on a flat surface or fixture; post-mould shrinkage of semicrystalline HDPE continues for 24–48 h. Final dimensions are therefore measured after 48 h at 23 °C, not immediately after ejection. In multicavity tools with asymmetrical cooling, cavity-to-cavity mass variance above 0.6% can create warp gradients that exceed 1.2 mm on a 300 mm diameter lid.

What distinguishes M6138 from higher-melt-strength blow moulding and pipe HDPE resins?

High-molecular-weight blow moulding HDPE grades are typically characterised by melt flow rates of 0.2–1.2 g/10 min and a multimodal molar mass distribution that produces high melt strength and high parison stability. M6138 has a melt flow rate of 20 g/10 min and low melt strength; it cannot maintain a stable parison in continuous or intermittent extrusion blow moulding of containers with parison hang times exceeding a few seconds. For PE pipe grades with long-term hydrostatic design bases, the higher molar mass and copolymer structure provide slow crack growth resistance. M6138 is not designed for pressure pipe or stressed-agent service.

Within the injection-moulding HDPE family, lower-flow grades in the 5–8 g/10 min MFR range are selected for thicker-wall crates, pallets and industrial containers where notched impact and durability under static load are more important than cycle time. M6138 shifts the balance toward high flow and rapid solidification, but the trade-off appears in reduced environmental stress crack resistance and lower melt strength. The suitability of M6138 for detergent or surfactant-containing closures should be tested by full-part contact with the intended filling system; published data for this specific configuration is limited.

The grade also differs from high-density polyethylene blow moulding resins in shrinkage behaviour. Narrow MWD and high flow yield directional shrinkage from 0.018 mm/mm to 0.025 mm/mm, while high-molecular-weight blow moulding grades are processed under entirely different cooling conditions and their shrinkage data are not transferable. Tooling dimensions for M6138 must therefore be derived from injection shrinkage experiments, not from blow moulding shrinkage tables.

Environmental stress crack resistance of high-flow HDPE is inferior to high-molecular-weight blow moulding grades. M6138 is not typically specified for containers holding ethoxylated surfactants, terpene-based cleaning agents, or strong caustic solutions at elevated temperature. Failure occurs by brittle crack propagation at stress concentrations such as sharp corners, weld lines, and gate vestiges. Radii below 0.5 mm and abrupt wall-section transitions increase the risk of environmental stress cracking; moulded parts should be prototyped and tested with the actual chemical formulation.

For rigid pails and tote boxes, the flexural modulus of 1,380 MPa is used to calculate short-term stacking deflection. Under continuous load at 40 °C, polyethylene exhibits viscoelastic creep; creep modulus at 1,000 h under ISO 899-1:2021 is lower than short-term flexural modulus. Unsupported spans in crate bases must be ribbed or derated to keep tensile strain below the yield point. The tensile stress at yield of 26.2 MPa is the short-term design ceiling; sustained stress above 10 MPa at 50 °C can lead to non-linear creep in semicrystalline HDPE.

The Vicat softening point of 127 °C under ISO 306:2022 A50 indicates short-term thermal resistance but not continuous use temperature. HDPE components made from M6138 are not recommended for continuous contact with hot fill above 80 °C because stress relaxation and distortion can occur under load. For microwave reheating or hot-fill above 90 °C, polypropylene or HDPE grades with higher Vicat and lower creep are generally selected.

Food-contact declarations for M6138 are based on high-density olefin polymer composition. The resin is typically covered under FDA 21 CFR 177.1520(c) 3.1a when the finished article density is not less than 0.94 g/cm³ and the intended use conditions are met. Migration testing of the final article under EU Regulation (EU) No 10/2011 is required for European food-contact compliance; overall migration limits are 10 mg/dm² for flat articles or 60 mg/kg for volume-based articles. REACH and RoHS statements are available in supplier documentation but do not replace application-specific testing. The grade is not supplied with an antimicrobial additive or long-term UV stabiliser as standard; outdoor use requires separately compounded ultraviolet stabilisation.

Pre-drying is generally not required for HDPE because bulk water absorption is below 0.01% at 23 °C. Surface condensation after cold storage can release water into the melt and create splay. If pellet surface moisture is suspected at ambient relative humidity above 60%, a hopper dryer at 65–75°C for 1–2 h is used. The resin should not be blended with copper-containing additives, strong oxidising agents, or aromatic and halogenated solvents that reduce molecular weight through radical mechanisms. Purge with LDPE or a commercial polyolefin purging compound after processing. PVC, acetals, and chlorine-containing purge compounds are incompatible because they can release acidic decomposition products that attack barrel surfaces and destabilise HDPE.

In high-speed closure moulding with unscrewing cores and cycle times below 8 s, M6138 is run with a melt temperature near 230 °C and mould coolant at 10 °C. The limiting process variable is core temperature; if the core exceeds 80 °C, the part can stick during unscrewing because the high-flow resin remains soft on the inner diameter. Published data for this specific configuration is limited; therefore production trials are required to set screw delay, core stroke, and ejection air pressure. The grade should not be selected for closures in continuous contact with aggressive hydrocarbon solvents without validating environmental stress crack resistance using the finished closure geometry.