The polyolefin designated CHN Energy Ningxia Coal HDPE 8008 is a high-density polyethylene injection-moulding resin manufactured by CHN Energy Ningxia Coal Industry Co., Ltd. The material is supplied as natural-colour pellets for reciprocating-screw injection machines. Melt mass-flow rate is 7.5 g/10 min at 190 °C under 2.16 kg, tested according to ISO 1133-1:2022. Density is 0.956 g/cm³ according to ISO 1183-1. The high melt flow permits filling of thin-wall sections and multi-cavity tools, while the density confers stiffness typical of high-density polyethylene.

Typical mechanical data from commercial certificates of analysis include tensile stress at yield of 25 MPa by ISO 527-2, flexural modulus of 1,050 MPa by ISO 178, notched Charpy impact strength at 23 °C of 4.5 kJ/m² by ISO 179-1/1eA, and Vicat softening temperature A50 of 125 °C by ISO 306. These values are representative; lot-specific certificates of analysis should be reviewed before mould commissioning because additive variations and polymerization lot variability can shift results.

The distinction from fractional-melt-flow high-density polyethylene used in blow moulding and pressure pipe is primarily molecular weight. Pipe-grade HDPE often exhibits an MFR below 0.5 g/10 min and higher slow-crack-growth resistance, but it cannot fill thin-wall injection moulds at comparable pressure. HDPE 8008 sacrifices some environmental stress crack resistance and melt strength for processability. Published data for this exact grade under long-term hydrostatic pressure testing to ISO 1167 is limited; therefore pressure-pipe substitution is not supported. Compared with lower-MFR injection HDPE from the same product family, HDPE 8008 is assigned to applications requiring wall sections below 2.5 mm or high cavitation counts. For structural parts with section thickness above 4.0 mm, a lower-flow grade should be evaluated for higher impact resistance.

How Does Low Molecular Weight Affect Melt Strength in High-Speed Injection?

The low melt strength of HDPE 8008 is a direct consequence of its relatively low molecular weight for an injection-moulding HDPE. In processing terms, low melt strength reduces die swell and prevents thread formation at the nozzle, but it also limits applicability in blow moulding, film extrusion, and profile extrusion. On reciprocating-screw machines with screw L/D ratios of 20:1 to 24:1 and compression ratios near 2.5:1, the resin melts homogeneously at barrel temperatures between 180 °C and 220 °C. The recommended nozzle temperature is 205 °C; higher settings may accelerate degradation and lower molecular weight further, while lower settings may raise injection pressure and cause short shots in thin sections.

When HDPE 8008 replaces a fractional-MFR HDPE in an existing tool, fill pressure usually falls owing to lower viscosity. Injection pressure reductions of 15–30% are commonly recorded on hydraulic machines with clamp forces between 1,000 kN and 8,000 kN, but the reduction is tool-dependent and is not a material specification. Hold pressure should be set at 50–70% of fill pressure and adjusted so that peak cavity pressure remains between 35 MPa and 50 MPa. Cavity pressure monitoring is preferable to machine pressure settings because pressure loss through sprue, runner, and gate varies with tool geometry.

For crates, trays, and industrial containers with wall thickness from 1.8 mm to 3.0 mm, a typical barrel profile from feed to nozzle is 170 °C, 190 °C, 210 °C, 210 °C, and 205 °C. Mould temperature is held between 10 °C and 40 °C. Lower mould temperatures shorten cycle time but can produce visible flow lines and reduce surface gloss. Screw back pressure is maintained at 0.5–1.5 MPa to ensure melt homogenisation without excessive shear heating. Screw speed is usually limited to 80–120 rpm for screw diameters of 35–70 mm. Cushion should be maintained at 3–6 mm to provide consistent hold pressure and avoid screw bottoming.

Injection speed is set to fill the cavity in 0.5–1.5 s for thin-wall food containers, with velocity profiling to reduce jetting at the gate. Gates should be designed so that shear rate at the gate does not exceed 50,000 s⁻¹ to avoid melt fracture. Direct edge gates with land lengths of 0.8–1.5 mm and gate thickness equal to 50–80% of the adjacent wall thickness are commonly used. Hot-runner systems should be naturally balanced across all cavities; artificially balanced systems may introduce pressure drops that affect cavity-to-cavity weight variation.

Typical applications include beverage crates, logistic trays, pails, thin-wall food containers, caps and closures, toys, and household articles. In load-bearing crates, stacking strength depends on moulded-in stress and wall thickness. Parts with wall thickness 1.8–2.5 mm moulded at moderate cavity pressure of 35–45 MPa show consistent stacking performance, but verification under ISO 12048 or ASTM D642 is required. For food-contact uses, converters must obtain lot-specific declarations and verify migration according to EU Regulation 10/2011, FDA 21 CFR 177.1520, and GB 4806.6-2016. The final article must be tested under actual time-temperature conditions because processing can generate oxidation products that affect overall migration.

When Moisture Uptake Exceeds 0.05% by Weight in Regrind Streams

Virgin HDPE 8008 does not normally require pre-drying before injection moulding. Surface moisture on pellets or regrind can create splay, silver streaks, or nozzle bubbles, especially when regrind fraction exceeds 20% by weight and has been stored in high humidity. If surface defects appear, desiccant drying for 2 h at 80 °C is a standard corrective action. The drying temperature should remain below 90 °C to prevent pellet fusion and hopper bridging. The moisture threshold of 0.05% by weight is a practical control limit derived from visual defect onset in thin-wall containers, not a formal material specification.

Regrind addition is typically maintained at or below 30% by weight for load-bearing crates. Repeated plastication increases melt mass-flow rate through chain scission and decreases notched Charpy impact. A shift in MFR of more than 0.5 g/10 min from the virgin value is an operational indicator of degradation and should trigger reduction in regrind level, melt temperature, or screw speed. Moisture analysis by ISO 15512 or Karl Fischer titration is recommended for regrind stored outdoors.

Gate design and pressure-drop criteria for multi-cavity tools

Flow-balance in HDPE 8008 becomes critical when cavity count exceeds 16. High melt flow creates lower pressure loss in the runner but also reduces melt strength at the advancing melt front; poor balance can cause gas traps and weld-line weakness. The runner system should be sized for a pressure drop of not more than 20–30 MPa from sprue to cavity at the maximum fill rate. Weld-line tensile strength should be verified by ISO 527-2 on specimens cut from moulded parts, because weld lines in high-flow HDPE can retain only 70–85% of the tensile strength of the bulk material.

For thin-wall caps and closures, valve gates with diameter 1.0–2.0 mm and delayed opening sequences improve cavity-to-cavity filling consistency. The injection screw should be equipped with a low-compression check ring and non-return valve to minimise melt leakage; excessive leakage causes cushion instability and short shots. Screw recovery should be completed within 60–80% of the cooling time to avoid cycle extension and melt residence-time changes.

Shrinkage in HDPE 8008 is anisotropic and thickness-dependent. Mould design should assume parallel shrinkage of 1.5–2.5% by ISO 294-4; tool trials are required because flow-induced orientation can produce perpendicular shrinkage differences of up to 0.5%. Post-mould shrinkage can continue for 24–48 h. Dimensional inspection should therefore be delayed accordingly.

Pigmentation and additive masterbatches alter melt flow and impact. A low-melt-index carrier is preferred to reduce viscosity dilution. Letdown ratios of 2–4% by weight for carbon black in outdoor crates are typical. Higher letdowns may reduce Charpy impact and must be verified. Residence time above 15 min at 220 °C may cause yellowing and MFR increase; purging is required after stoppages.

Natural HDPE 8008 is not intended for long-term outdoor exposure without a stabilisation package. For outdoor crates exposed to sunlight, a UV stabiliser masterbatch with 2–3% carbon black or an equivalent hindered amine stabiliser system is required. The material is not suitable for continuous immersion in strong oxidising acids, aromatic solvents, or hot-water pressure-pipe service. Chemical resistance of the final article should be evaluated under ISO 175 or ASTM D543 using the expected service fluid and temperature. In low-temperature impact applications below -20 °C, validation by notched Charpy impact at the service temperature is required, because the standard 23 °C value does not predict sub-zero behaviour.