Hengli Petrochemical (Dalian) HDPE 7750M is a high-density polyethylene injection-molding resin supplied in pellet form. The product is classified by a nominal melt flow rate of 7.5 g/10 min measured at 190 °C with a 2.16 kg load under ISO 1133-1:2022 and by a nominal density of 0.950 g/cm³ determined under ISO 1183-1. In production terms, this melt flow range corresponds to a medium-flow HDPE injection grade: it is more fluid than low-flow extrusion resins with melt flow rates near 0.9 g/10 min and less fluid than thin-wall packaging grades above 20 g/10 min. The resin is used in multi-cavity injection molds for caps, closures, containers, pails, crates, housewares, and technical components in which a balance of processability, stiffness, and impact resistance is required.

Typical injection molding practice for HDPE 7750M uses a barrel profile from 180 °C in the feed zone to 220 °C in the metering zone, with nozzle temperature held at 200–230 °C. Mold temperature is generally set between 20 °C and 50 °C; the upper end improves surface replication and reduces orientation, while the lower end shortens cooling time. Screw rotation of 80–120 min⁻¹ and back pressure of 0.5–2.0 MPa are acceptable starting parameters for screw sizing. The resin does not require routine drying when stored in sealed hoppers below 60% relative humidity. If pellet surface moisture exceeds 0.10 wt% due to condensation, drying at 80 °C for 2 h is recommended to prevent splay and shot-weight variation.

Which rheological differences define substitution boundaries with extrusion HDPE grades?

The medium melt flow of 7750M directly reduces melt strength compared with low-melt-flow HDPE grades used in blown film, pipe, and large-part blow molding. In extrusion-grade HDPE with a melt flow rate near 0.9 g/10 min, the high molecular weight fraction maintains the parison or film bubble under low shear. The same molecular weight fraction is partly absent in a 7.5 g/10 min injection grade, so sag resistance under gravity is lower. As a result, HDPE 7750M is not a drop-in replacement for blow-molding or pipe grades where continuous melt strength and long-term hydrostatic creep resistance under ISO 4427 are required. Published data for direct substitution into extrusion blow molding with 7750M is limited, and a rheological melt-strength test should be run before any such change is attempted.

In injection molding, the lower melt viscosity at high shear becomes an advantage. Under cavity-filling shear rates above 10³ s⁻¹, the resin exhibits shear-thinning behavior that lowers effective viscosity and reduces peak injection pressure in thin-wall tools. This allows the grade to fill a 1.2 mm wall section with a flow length of 180 mm at melt temperature 220 °C and peak injection pressure below 90 MPa in a conventional cold-runner mold. For longer flow paths, increasing melt temperature by 5–10 °C is more effective than raising packing pressure because the melt front viscosity decreases without the increased residual stress that high pack pressures create.

Compared with a high-flow HDPE of 20 g/10 min, the 7.5 g/10 min melt flow of 7750M leads to slightly longer fill times but generally better notched impact and stress-crack resistance because of its higher average molecular weight. Compared with a low-flow extrusion grade, 7750M sacrifices long-term creep resistance and melt strength for improved moldability. This trade-off narrows the substitution window: the grade can replace a lower-flow injection grade when cycle time is a constraint, but it should not replace a pipe or large-part blow-molding grade in applications requiring pressure-pipe classification or high parison stability.

Mechanical property benchmarks and standardized test references

Commercial technical literature for HDPE 7750M lists the following typical property values. The values are nominal and should be confirmed against the current certificate of analysis and lot-specific test reports before final part qualification.

The tensile yield stress of 25 MPa supports stacking loads in rigid containers, while the flexural modulus of 1050 MPa maintains sidewall stiffness in deep-draw or thin-wall parts. The Charpy notched impact value of 4.5 kJ/m² at 23 °C is not to be interpreted as a guarantee of freezer impact performance; parts intended for use at or below -20 °C require additional low-temperature notch-impact testing, such as ISO 179-1/1eA conditioned at the intended service temperature. For applications involving continuous load or exposure to surface-active liquids, environmental stress-crack resistance testing should be performed on the finished article because stress-crack resistance is influenced by molded-in residual stress, wall thickness, and closure design.

Property differences between 7750M and other HDPE products follow from the melt-flow and density position. A 20 g/10 min thin-wall grade may offer shorter cycle time but lower notched impact and increased ESCR sensitivity. A 0.9 g/10 min film or blow-molding grade offers higher toughness and melt strength but cannot fill thin-wall injection molds without excessive pressure or thermal degradation. The 0.950 g/cm³ density of 7750M provides stiffness while retaining enough flexibility for snap-fit assembly and impact-loaded closures. The product does not contain slip or antiblock additives at high levels, so external colorant or additive masterbatch should be selected to minimize viscosity shifts and warpage.

Mold shrinkage for HDPE 7750M is generally in the range of 1.5–2.5%, depending on wall thickness, melt temperature, packing pressure, and gate geometry. A holding pressure of 50–70% of peak injection pressure and holding time of 0.5–1.0 s/mm of part thickness are common starting points. Gate design influences the practical difference between this grade and a higher-flow product. In a hot-tip gate for a cap with wall thickness 1.0 mm, a gate diameter of 0.8–1.2 mm is typically used; smaller gates cause high shear heating and may degrade the material at fill rates above 50 cm³/s. Weld-line strength under ISO 527-2 is commonly 60–80% of the original material strength, so gate placement should avoid weld lines at high-stress locations.

When the melt temperature exceeds 240 °C or residence time exceeds 5 min

At melt temperatures above 240 °C, oxidative degradation of the polyethylene backbone becomes measurable as a loss of melt viscosity and an increase in extractable low-molecular-weight species. Barrel residence time should be kept below 5 min at 230 °C. In hot-runner tools, manifold and tip temperatures should not exceed 230 °C; a local hot spot above 250 °C can degrade the resin even when the injection barrel is within the recommended range. Degraded material may appear as yellowing, gas splay, or black specks in the part. Purging with a polyolefin purging compound or HDPE is recommended for changeovers. Halogen-containing polymers and acidic decomposition products should not remain in the barrel when changing to HDPE 7750M, because they can accelerate degradation and corrode tooling surfaces.

The screw and barrel should be sized to avoid excessive idle residence time. A shot-to-cylinder capacity ratio below 20% increases the risk of extended residence and should be avoided on thermally sensitive colors or regrind blends. If regrind is used, granulate size should be uniform and humidity below 0.10 wt%. Copper-bearing components in unprotected injection-unit surfaces can catalyze polyethylene oxidation at processing temperatures; nitrided or stainless-steel screw and barrel surfaces are preferred for extended campaigns. The use of silicone or fluoropolymer purge compounds should be evaluated for food-contact packaging, as residues may affect regulatory compliance.

Regulatory documentation and conversion responsibility

Compliance of the finished article depends on the base resin, masterbatches, and processing aids used, as well as the conversion conditions. The following standards are commonly referenced when qualifying HDPE 7750M for commercial applications. The list is a starting point and does not replace end-use regulatory review.

Published data for the specific migration of additives from HDPE 7750M into food simulants is limited. Therefore, converters intending food-contact applications should perform overall and specific migration testing on the final article under (EU) 10/2011 or 21 CFR 177.1520 protocols. For outdoor applications, the base resin does not provide long-term UV weathering resistance; a minimum 2 wt% carbon black masterbatch or a hindered amine stabilizer package should be qualified. For medical device applications, no USP Class VI or ISO 10993 certification should be assumed for this grade in the reviewed literature.

Quality-release documentation should include lot-specific melt flow rate, density, and color. Converters should request the certificate of analysis for each batch and monitor lot-to-lot variation when switching between production campaigns. The product should be stored in a dry, contamination-controlled silo or hopper to avoid pellet corrosion, dust accumulation, and cross-contamination with other polymer types. Pellets contaminated with polypropylene, PET, or PVC should not be fed into the machine, as incompatibility can cause delamination, surface defects, and regulatory failure.