Japan Polyethylene Corporation supplies HDPE HJ221 within the Novatec HD grade slate, identifying it as an injection-moulding high-density polyethylene. The producer's published technical data sheet places the nominal density at 0.953 g/cm³ when tested to ISO 1183-1:2019 method A and the nominal melt flow rate at 5.2 g/10 min under ISO 1133-1:2022 at 190 °C and 2.16 kg. Certificates of analysis commonly record lot-specific MFR values in the 4.8–5.6 g/10 min band, and this band is sufficient to produce measurable cavity-pressure variation of approximately ±4 MPa in a 32-cavity cold-runner closure tool on a 1,200 kN toggle press. Target applications are injection-moulded caps, screw closures, pails, housewares, and general-purpose rigid containers with wall thickness between 1.5 mm and 4 mm. Tensile yield stress is reported at 28 MPa using ISO 527-2:2012 type 1B specimens at 50 mm/min, with nominal tensile modulus near 1,050 MPa. Notched Charpy impact at 23 °C is referenced at 6 kJ/m² under ISO 179-1:2020; at -20 °C the value drops to 3 kJ/m², establishing a lower service boundary for frozen-food packaging impacts. The Vicat softening temperature is listed as 124 °C under ISO 306:2022 A50 and the peak crystalline melting point as 132 °C under ISO 11357-3:2018. These thermal limits mean that hot-fill or retort conditions above 95 °C exceed the practical continuous-use threshold for unfilled polyethylene unless the part is mechanically supported. Because the density sits below 0.954 g/cm³, the grade retains a degree of short-chain branching that improves environmental stress-cracking resistance relative to homopolymer HDPE grades with density above 0.960 g/cm³. The producer does not specify an external lubricant package for HJ221; converters requiring controlled coefficient of friction should add an FDA-compliant slip masterbatch rather than assume a surface-modifying additive is present.

When Is HJ221 Preferred Over a Blow-Moulding HDPE Grade?

Selection between HJ221 and a blow-moulding grade is governed by the relationship between melt flow rate under 2.16 kg and high-load melt index under 21.6 kg, not by density alone. Blow-moulding grades with a high-load melt index of 5–10 g/10 min are optimized for parison hang strength and exhibit lower screw recovery rates in injection moulding, causing cycle-time extensions in thick-walled pails. HJ221, with its moderate 2.16 kg flow, plasticates faster and fills multi-cavity tools at lower barrel-temperature settings, typically 190–230 °C. The trade-off appears in environmental stress-cracking resistance: blow-moulding grades with densities below 0.950 g/cm³ can outperform HJ221 in long-term detergent bottle exposure, whereas HJ221 provides higher top-load stiffness and more predictable ejection because of its crystallinity at the specified density. Converters switching from a blow-moulding grade should not expect equal parison melt strength; the grade is unsuitable for continuous extrusion blow moulding where parison hang strength is the critical process variable. For injection-blow applications, a preform injection step can be used, but published data for this specific configuration is limited.

On a water-cooled multi-cavity tool with a 25 mm reciprocating screw and 24:1 L/D, a barrel profile of 180/200/210/220 °C from feed throat to nozzle is commonly set for thin-wall closures. A melt temperature below 180 °C elevates apparent viscosity and shifts the maximum shear stress at the gate, producing short shots and flow-line defects at the knit line. The hold-pressure profile must compensate for crystallisation shrinkage of approximately 1.5–2.0% in the flow direction and 1.2–1.6% in the transverse direction after 48 h at 23 °C under ISO 294-4:2018 specimen preparation. A cushion of 3–5 mm and a back pressure of 0.8–1.2 MPa on the screw are necessary to maintain shot-weight consistency; lower back pressure produces visible unmelted pellets in the gate region. Injection speeds are typically set at 80–120 mm/s for wall sections of 1.5–2.5 mm. A mould temperature of 10–20 °C accelerates solidification and minimises cycle time, but mould temperatures below 8 °C can cause condensation on the core, resulting in surface splay and poor dimensional repeatability in the first 50 shots after start-up. Pre-drying is not required when bags are stored below 60% RH; if condensation is visible, a desiccant dryer at 80 °C for 2–3 h is recommended. Residence time in the barrel should not exceed 300 s at melt temperatures above 200 °C because oxidative degradation increases yellowness and reduces notched impact strength.

If the Melt Is Processed Below 180 °C, the Following Injection-Moulding Defects Emerge

At suboptimal melt temperatures, the first observed failure is gate freeze-off before hold-pressure transfer; this defect appears as sink marks opposite rib intersections in closures with bosses. The pressure transducer inside a 4 mm diameter cold sprue typically records an early peak of 70 MPa and then a sharp decay of 25 MPa/s, which indicates that the gate has sealed before packing. In this condition, the cavity-pressure trace never reaches the hold plateau of 35–40 MPa required for surface replication of a textured lid. The resulting tensile yield stress may remain within specification at 28 MPa, but the notched Charpy impact can drop by 15–20% because the frozen molecular orientation at the gate is higher. In multi-cavity tools, this defect is not uniform: cavities fed by longer hot-runner channels show shorter shots because the melt has lost heat during flow. The root cause is the increase of viscosity at low shear rates; at 180 °C the apparent viscosity at 100 s⁻¹ is roughly 1,200 Pa·s, while at 220 °C it falls below 700 Pa·s. A processing window of ±5 °C around the recommended melt temperature of 210 °C is therefore applied in production when wall thickness is below 2 mm. Barrel zones should be checked with a needle pyrometer every 8 h, and nozzle temperature should not exceed 230 °C to avoid drool between cycles.

Comparative Mechanical and Shrinkage Data Across High-Density Grades

Table 1 compares HJ221 with a generic blow-moulding HDPE and a high-flow injection HDPE using published datasheet values. The data are representative values, not a lot-specific guarantee. Differences in notched impact should be read with sample thickness and notching method; values generated under ISO 179-1:2020 are not directly interchangeable with ASTM D256 values. Table 1 is excerpted from producer technical bulletins and does not account for regrind content, pigment, or nucleating additives.

The most significant difference is that the high-flow injection grade reaches a melt flow rate of 20 g/10 min, enabling thinner-wall filling but reducing notched Charpy impact to approximately 4 kJ/m² at 23 °C and lowering Vicat softening to 121 °C. HJ221 is therefore positioned between high-flow injection grades and blow-moulding grades: its impact retention is higher than the former, while its mould filling is faster than the latter.

A compliance review for food-contact closures should begin with 21 CFR 177.1520(c) when the finished article is intended for distribution in the United States. The base olefin polymer in HJ221 meets the general requirements for polyethylene defined in that section when the extractable fraction and end-use condition match the listed food-type and temperature limits. For the European Union, a final article must comply with EU Regulation 10/2011, including overall migration limits of 10 mg/dm² or 60 mg/kg depending on container geometry; the processor must verify migration under the intended food simulant. A RoHS declaration under Directive 2011/65/EU is normally limited to electrical and electronic equipment, but a cadmium, lead, mercury, and hexavalent chromium screening value of less than 100 mg/kg per homogeneous material is often requested by brand owners. The grade is not supplied with an antimicrobial additive, and no claim of sterility should be made from the resin alone. Storage in a cool, shaded warehouse below 40 °C avoids oxidation of the pellet surface; opened bags should be consumed within 6 months to prevent dust and moisture absorption from shifting the MFR by more than ±0.2 g/10 min. Converters blending post-industrial regrind should limit regrind to 20% by weight for applications requiring notched impact above 5 kJ/m² at 23 °C, because repeated heat history increases the carbonyl index and reduces Charpy performance.

Dimensional Tolerances in Multi-Cavity Cap Tools Require Anisotropic Shrinkage Control

Caps with a 30 mm diameter, 1.8 mm side wall, and a 0.9 mm tamper-evident band are injection-moulded on hot-runner systems with 48–96 cavities. HJ221 exhibits differential shrinkage along the flow and cross-flow axes because the frozen-in molecular orientation is highest near the gate. In practice, the post-mould diameter measured at 24 h can be 29.55–29.70 mm when the tool is cut with a uniform allowance, which is unacceptable for a 30.00 ±0.10 mm cap thread. Mould makers compensate by applying a non-uniform cavity expansion: the cap shell is enlarged by 1.5% in the flow direction and 1.3% in the transverse direction, with the gate region cut 0.2% larger than the edge. The hot-runner nozzle temperature is balanced across all cavities within ±2 °C; a drift of 5 °C between the first and last nozzle changes fill speed and cap ovality by 0.04–0.08 mm. For the tamper-evident band, the tool is fitted with a collapsing core and the injection hold time is extended by 0.3–0.5 s to prevent underfilling the hinge of the band. After ejection, caps are cooled on a plug conveyor with forced air at 10–15 m/s for 12–15 s; parts dropped directly into a bulk box without cooling develop post-mould deformation at the band hinge. These dimensional controls are often more important than the absolute MFR difference between HJ221 and higher-flow injection grades.