Hengli Petrochemical (Dalian) HDPE 7750M2 is an injection-molding high-density polyethylene produced at the Dalian integrated refining and petrochemical complex. The resin is positioned in the high-flow HDPE range for thin-wall rigid packaging, industrial containers, crates, and pallets. The producer’s specification framework reports melt mass-flow rate under ISO 1133-1:2022, density under ISO 1183-1:2019 method A, tensile yield properties under ISO 527-2:2012, flexural modulus under ISO 178:2019, notched Charpy impact under ISO 179-1:2010, and Vicat softening temperature under ISO 306:2022. Nominal melt flow rate is commonly stated near 7.5 g/10 min at 190 °C under 2.16 kg load, and nominal density is in the 0.950–0.960 g/cm³ band; however, these are not purchase-specification guarantees. The certificate of analysis for the specific lot is the controlling document for mold design, drying, and incoming inspection. The pelletized resin is suitable for central vacuum conveying and silo transfer, but fines generation during high-velocity dilute-phase conveying can shift apparent bulk density and create feed-throat bridging on high-throughput injection machines. Converters should therefore verify pellet geometry, fines content, and hopper throat angle on the actual material-handling configuration before starting a large campaign.

Which melt rheological and thermodynamic boundaries define the processing window?

Processing of 7750M2 on an injection molding machine should not be controlled by the 2.16 kg melt flow rate alone, because shear rates in gates and thin runners can exceed 10,000 s⁻¹. Single-point melt flow data at 190 °C understates the viscosity reduction available under high shear in thin-wall filling. A general-purpose screw with 20:1 to 25:1 L/D and a compression ratio of 2.5:1 to 3.0:1 is adequate to disperse the as-supplied stabilizer package without excessive shear heating. A conservative barrel temperature profile from the feed zone to the nozzle is 180 °C, 210 °C, 220 °C, and 230 °C. Mold temperature should be maintained between 20 °C and 40 °C for rapid cycle times; raising the mold to 60 °C can improve weld-line strength in deep-flow crates but extends cooling time and may add 15–25% to total cycle time depending on wall thickness and tool cooling efficiency.

The resin is not hygroscopic in the manner of polyamide or polycarbonate; however, condensation on cold pellet surfaces after outdoor storage or short-term exposure to relative humidity above 70% RH can produce splay, gas tracks, and gate blush. A dehumidifying hopper set to 80 °C for 2 h is a practical corrective procedure for wet surface moisture, but continuous drying is not required for dry indoor storage at ambient temperature. Production-scale hot-runner evaluations have shown that nozzle tip diameters below 1.5 mm can cause premature gate freeze-off, and multicavity flow imbalance increases when fill time exceeds 0.4 s for wall sections below 1.2 mm. These are tooling constraints rather than inherent product failure modes. Processors using all-electric injection machines with velocity-controlled filling should set injection velocity to maintain a flow-front velocity of 100–250 mm/s through the thinnest wall; published data for this specific configuration is limited, so gate-seal time and hold-pressure decay should be mapped with short shots on the production mold. Back pressure above 8 bar is generally unnecessary and can increase shear heating in the metering zone if residence time exceeds 10 min at melt temperatures above 250 °C, leading to odor generation from additive decomposition.

For rigid packaging applications such as collapsible crates, beverage crates, and industrial pallets, 7750M2 is selected where the processing requirement is high melt fluidity at moderate molecular weight and the performance requirement is top-load stiffness after crystallization. On production-scale clamp force machines in the 800–1,200 t range molding a 3.2 mm wall crate, the material can often be filled at lower injection pressure than a lower-flow HDPE pipe grade with a melt flow rate below 1.0 g/10 min, but weld-line strength remains governed by melt temperature, venting, and flow-front convergence rather than by flow alone. Top-load creep in stacking crates should be evaluated under ISO 12048:2000 or an equivalent compression test on the actual pallet or crate geometry; material datasheet flexural modulus cannot predict buckling of ribbed sidewalls. For pallets with fork entry openings, impact testing at -20 °C is necessary because polyethylene undergoes a ductile-to-brittle transition at lower temperatures, and a room-temperature 23 °C Charpy value by ISO 179-1:2010 does not assure cold-room service.

In thin-wall closures, caps, and dispensing plugs, 7750M2 may be compared with polypropylene impact copolymers, but high-density polyethylene has lower flexural modulus and different stress-cracking behavior in detergent and surfactant environments. Removal torque on the actual closure design should be measured under the relevant application-specific specification, not inferred from the resin’s coefficient of friction. Environmental stress-cracking resistance can be assessed by ASTM D1693-21 or ISO 22088-1:2006; if the grade is used in contact with aggressive cleaning agents, the chosen masterbatch and colorant package must be included in the test. The material is not intended for extrusion blow molding of large technical parts or for pressure pipe applications, because its melt strength is lower than high-molecular-weight blow molding and pipe grades.

Oxidative stability, additive depletion, and storage limits

The as-supplied stabilizer and acid scavenger package is formulated for multiple melt histories, but regrind content above 25 wt% can change the spatial distribution of antioxidants and lower the oxidative induction temperature measured by differential scanning calorimetry under ISO 11357-6:2018. Converters should test oxidative induction time on dried virgin pellets and on molded parts after the intended regrind cycle; a significant reduction at 200 °C or 210 °C indicates stabilizer depletion. The material should not be dry blended with amine-based flame retardants, strong acid scavengers, or transition metal soaps without compatibility testing, because acid-base interactions can deactivate the primary antioxidant package and reduce long-term thermal stability. Storage at ambient indoor conditions below 40 °C is recommended; prolonged exposure to ultraviolet radiation, ozone, or oxidizing agents in warehouse environments can initiate surface oxidation. Outdoor service requires a UV-stabilized masterbatch in a separate melt-compounding step, with dispersion quality assessed by film gel count or by accelerated weathering under ISO 4892-2:2013. If the molded article is used for potable water contact, the specific formulation must be verified against the applicable national certification standard; the base resin alone is not a certification of compliance.

When a converter substitutes 7750M2 for a conventional unimodal HDPE of equivalent melt flow rate

Substitution is not a drop-in adjustment unless shrinkage, warpage, and impact retention are compared under the same tool boundary conditions. The product is reported to have a narrower or structured molecular weight distribution that can reduce orientation-induced warpage in flat parts and produce a different shear viscosity profile at gate velocities, but published comparative data for this exact grade are limited. A controlled substitution trial should mold ISO 294-3:2020 plaques at constant melt temperature, injection velocity, and mold temperature, then measure in-plane and through-thickness shrinkage after 48 h at 23 °C and 50% RH. Differences in mold shrinkage relative to a conventional 7.5 g/10 min unimodal HDPE are typically below 0.2 percentage points if mold temperature is maintained below 40 °C; above 60 °C, crystallization differences can amplify part warpage. Notched impact strength may show an inflection at low temperatures, but the specific ductile-to-brittle transition should be measured under ISO 179-1:2010 with notched specimens conditioned at -20 °C, 0 °C, and 23 °C rather than assumed from room-temperature data. Weld-line strength, stiffening rib fill, and gate blush also differ with molecular architecture, so process capability studies should include short shots and seal-time studies before full mold transfer.

Compared with high-load melt index pipe resins from the same producer, 7750M2 has a much higher melt flow rate and lower molecular weight, which reduces melt strength and makes it unsuitable for sag-controlled wall thickness in extrusion blow molding. Compared with HDPE film grades in the 0.946–0.952 g/cm³ density band, the grade shifts the property balance toward flexural modulus and top-load stiffness, but at the expense of dart impact and tear resistance measured under ASTM D1709-16 or ISO 7765-1:2004 on blown film. Compared with injection-grade polypropylene copolymers, 7750M2 has lower modulus, higher notched impact at low temperature in some configurations, and different hinge fatigue behavior; living-hinge designs are generally outside the reliable use of high-density polyethylene and should be prototyped with flexural fatigue testing under the intended angle and cycle count. When the application requires autoclave sterilization above 121 °C, neither unmodified HDPE nor standard polypropylene may be acceptable; the exact exposure time and pressure conditions must be tested on the finished article.

The certificate-of-analysis test matrix for HDPE 7750M2 should include the following standard designations and conditions. Actual numerical limits are lot-specific and must be taken from the producer’s current certificate of analysis, not from a third-party summary.

Incoming-inspection plans for 7750M2 should include visual or camera-based black speck and gel counts on molded plaques, because contamination introduced during transfer, blending, or regrind handling can reduce part appearance and impact retention. A typical in-house control for packaging-grade HDPE is a maximum of 10 black specks per 1,000 cm² on molded surface plaques and a metal contamination limit below 1.0 mm ferrous particle size, using a commercial metal separator or magnetic drawer before the feed throat. These values are not product specifications but common plant acceptance criteria; each converter must define limits according to the end-market surface requirement and the sensitivity of the mold hot-runner system to particle blockage. If low-gloss or high-purity surfaces are required, the as-supplied pellet lot should be tested for pellet-to-pellet color variation using a spectrophotometer under CIELAB conditions, and for moisture content by a Karl Fischer oven method if wet storage is suspected.

Regulatory documentation for 7750M2 should be requested from Hengli Petrochemical (Dalian) and cross-checked against the intended market and final article composition. For direct food-contact applications in the European Union, the finished article must comply with Regulation EU 10/2011, including overall migration and specific migration limits; the base resin alone is not sufficient for compliance. In the United States, food-contact use may fall under FDA 21 CFR 177.1520 for olefin polymers, subject to the stated conditions of use and end-testing on the finished article. For electrical and electronic equipment, RoHS compliance is primarily a function of the absence of restricted heavy-metal stabilizers and flame retardants; the producer should confirm that lead, cadmium, mercury, and hexavalent chromium are not deliberately added. REACH compliance requires confirmation that the grade does not contain a substance of very high concern above 0.1 wt%. The product is not specified for medical implants, long-term pressurized hot-water service, or safety-critical gas distribution components; these applications exceed the design envelope of a high-flow injection-molding HDPE. Published data for this specific configuration is limited for migration kinetics in polymer matrices, so food-contact approval must be performed on the final molded article with the actual colorants and masterbatches.