In extrusion blow moulding of small- and medium-volume rigid packaging, Sinopec Zhongke HDPE 5502 is processed on continuous shuttle machines and intermittent accumulator-head machines. The grade is characterized by a melt flow rate of 0.30–0.40 g/10 min under 190°C and 2.16 kg (ISO 1133-1:2022) and a nominal density of 0.955–0.957 g/cm³ (ISO 1183-1:2019). Because the melt flow rate is low, parison sag is limited and layer thickness can be maintained at blow-up ratios between 2:1 and 3:1; however, the higher viscosity requires tight control of die head temperature. The melt temperature at the die head is maintained at 180–200°C, while the feed zone is kept at 160–175°C and the compression zone at 180–195°C. Mould coolant inlet is set to 8–15°C, which accelerates solidification of the pinch-off zone and reduces cycle time. Blow air pressure is set at 0.6–0.8 MPa and blow time is 10–25 s depending on container wall thickness. Parison programming is used to shift wall thickness into the sidewalls away from the pinch-off, because the pinch-off is the highest-stress zone after flash trimming. Pinch-off clearance is set to 0.10–0.35 mm; excessive clearance creates a weak weld, while insufficient clearance can cause brittle tearing at the parison pinch. In household chemical containers, the environmental stress crack resistance requirement under ASTM D1693-15 Condition B with 10% Igepal at 50°C commonly requires F50 greater than 60 h, and for aggressive surfactant formulations F50 greater than 100 h is specified. For food-contact bottles, compliance with FDA 21 CFR 177.1520 and EU 10/2011 is verified on the finished article, not on pellets alone. Production-scale failures on shuttle lines are usually observed as die-lane weld lines, parison curl at the die exit, and ovality caused by insufficient mould venting; die-lane weld lines are reduced by adjusting melt temperature and die gap, while ovality is corrected by increasing mould clamp pressure and verifying vent slots at the parting line.

What Melt Viscosity Window Governs High-Speed Injection Moulding of HDPE 5000S?

The 5000S grade is processed on high-speed reciprocating-screw injection moulding machines with L/D ratios of 22:1–25:1 and compression ratios of 2.5:1–3.5:1. Its nominal melt flow rate of 0.8–1.0 g/10 min (ISO 1133-1:2022) places it above HDPE blow moulding grades but below low-viscosity injection grades used for small closures, thereby keeping injection pressure high enough for thin-wall pails and totes without producing excessive melt turbulence. The melt temperature window is 180–230°C, with the lower limit defined by incomplete packing and the upper limit by gate blush, odour generation, and excessive sink formation. Mould wall temperature is held at 10–30°C; lower temperatures improve cycle time but can freeze the flow front prematurely in wall sections thinner than 1.5 mm. Peak injection pressure is set at 70–100 MPa, and packing pressure is maintained at 50–70% of the injection peak, with packing time 5–12 s selected to hold gate seal. Clamp force per projected area is typically 3–6 kN/cm², and vent depths at the parting line are kept below 0.025 mm to avoid flash while allowing gas exit. Mould shrinkage for HDPE 5000S after 24 h is in the range 0.015–0.040 mm/mm (ASTM D955-21) depending on wall thickness and gate design; thick bosses and rib intersections generate higher sink. In thin-wall pails with a flow length to wall thickness ratio above 250:1, if the lot MFR drifts below 0.8 g/10 min, short shots and gate blush appear on fast-cycle lines. If the MFR rises above 1.0 g/10 min, low-temperature impact toughness measured by ISO 179-1:2010 can fall below the specification for cold-storage containers. The following table summarizes typical class data for the two grades; lot-specific certificates may differ.

Processing of 5000S for pails and crates is not interchangeable with 5502 without adjusting melt temperature and injection profile. On high-speed pail lines with wall thickness between 1.2 mm and 2.5 mm, the transition from 5502 to 5000S requires a reduction in melt temperature of 10–20°C and an increase in injection velocity to avoid premature freeze-off at the gate. The lower viscosity of 5000S also improves filling of rib corners and allows shorter packing time, but the mould must be designed with sufficient venting because the faster flow front can trap gas and create burn marks at the end of fill.

Closure and cap moulding from HDPE 5000S imposes tighter organoleptic and dimensional boundaries than open-top thin-wall containers. A multi-cavity hot-runner mould with valve-gate nozzles is typically used; gate diameter is 0.5–1.5 mm, melt temperature is 190–210°C, and cooling time is 4–8 s for closures in the 3–8 g range. Thread diameter tolerance is held to ±0.15 mm and ovality below 0.30 mm to maintain seal integrity; these dimensions are measured after 24 h of conditioning at 23°C and 50% RH. Residual stress at the gate region, if not relaxed, can initiate environmental stress cracking when closures are exposed to fatty food simulants or detergent solutions; ESCR is therefore evaluated under ASTM D1693-15 Condition B in 10% Igepal at 50°C. Organoleptic requirements for food-contact closures made from 5000S require low odour and taste transfer; converters commonly conduct sensory panels after extraction in distilled water at 60°C for 24 h, while compliance is anchored to FDA 21 CFR 177.1520 and EU 10/2011. Because HDPE stress relaxation can reduce back-off torque, sealed closures are conditioned at 40°C for 24 h before torque release is measured. Published data for this specific closure configuration is limited; therefore, converter trials on the target mould are required to establish the acceptable processing window.

When Parison Pinch-Off Integrity Governs Heavy-Duty Container Performance

When the target container must survive drop impacts at low temperature and maintain environmental stress crack resistance after chemical exposure, extrusion blow moulding from HDPE 5502 is preferred over lower-viscosity injection grades. The moulded part is produced on an accumulator-head machine; the accumulator volume is selected between 5 L and 20 L for containers from 20 L to 1000 L. Die gap at the accumulator lips is set at 4–10 mm, and parison programming adjusts wall thickness over 40–100 positions depending on the control system. Melt temperature is held at 190–210°C with a tolerance of ±5°C at the head; excursions beyond this band create parison sag instability and weld-line thinning at the pinch-off. Mould clamp force on large parts is typically above 2000 kN depending on projected area; insufficient clamp force leads to flash at the parting line and increases the risk of blow-out at the pinch-off. The pinch-off zone is designed to compress the parison into a weld bead with thickness 0.10–0.35 mm; if the bead is too thick, trimming leaves a sharp notch that initiates low-temperature failure. If the bead is too thin, leakage occurs along the parting line. Drop impact is tested at -18°C under ASTM D2463-15, and dangerous goods packaging is qualified under 49 CFR Part 178 for UN-rated containers. ESCR after chemical exposure is evaluated under ASTM D1693-15 Condition B in 10% Igepal; industrial specifications for agricultural chemicals and automotive coolants often require F50 above 100 h. Published drop-height data for this specific configuration is limited; UN drop tests require pass heights based on filling density and container capacity.

Melt Filtration and Additive Masterbatch Dispersion in HDPE 5000S and 5502

Across extrusion lines running HDPE 5000S and 5502 with in-line colour concentrates, UV stabilizer masterbatches, or processing aids, melt filtration is used to remove agglomerates and incidental contamination. Screen packs are staged from 40 mesh to 120 mesh, corresponding approximately to aperture sizes of 0.38 mm to 0.125 mm; the pressure differential across the screen pack is limited to 20–25 MPa because higher differentials indicate gel accumulation and can raise melt temperature by viscous heating above 5°C. Such overheating can create oxidation gels in HDPE and reduce ESCR. Masterbatch let-down ratio is typically 2–4 wt% for colour concentrates and 1–2 wt% for UV stabilizer packages; the resulting melt flow rate shift is kept below 10% (ISO 1133-1:2022). Carbon black dispersion in polyethylene can be assessed by ISO 18553:2002, Method A, in which a compression-moulded plaque is examined for agglomerates; acceptance limits are defined in the relevant product standard, and no agglomerate larger than 0.50 mm is generally acceptable for exposed surfaces. Titanium dioxide and coloured masterbatches are checked by pressure-rise measurement across the screen pack and by optical microscopy on 50 µm sections. Although HDPE is not hygroscopic in the bulk, surface moisture at storage relative humidity above 60% can generate splay and surface pitting; conveying air is dried to a dew point below -40°C on production lines.

Regrind and post-industrial rework streams introduce viscosity drift and contamination risk that affect both HDPE 5000S and 5502. Reworked HDPE from edge trim, start-up scrap, and rejected containers is granulated, densified, and blended with virgin pellets at 20–40 wt%; the allowable ratio is established by melt flow rate drift less than 10% from the virgin lot and ESCR retention above 80% of the virgin value under ASTM D1693-15 Condition B. Metal contamination from granulator wear or conveying equipment is controlled with magnetic traps rated for 10,000 gauss and by periodic screen pack inspection. Multiple heat cycles shift melt flow rate upward and can broaden molecular weight distribution; therefore, rework streams are tested after every 24 h of production for MFR (ISO 1133-1:2022) and for gel content on cast film or compression-moulded plaques. In blow moulding of HDPE 5502, rework above 40 wt% can degrade parison melt strength and pinch-off toughness, even if MFR remains within specification. In injection moulding of HDPE 5000S, rework above 40 wt% can increase warp due to changes in crystallinity and molecular weight distribution. A single-screw extruder with L/D ratio 25:1 and vacuum venting at -0.08 MPa is commonly used to re-pelletize rework before blending; vent vacuum below this level can leave residual volatiles that cause splay in the finished article.

Assessing Food-Contact Compliance and Recyclability for HDPE 5000S and 5502

For food-contact applications, the regulatory status of HDPE 5000S and 5502 is not determined by the grade name alone; converters must verify lot-specific certificates of compliance from the resin producer, additive package composition, and migration test results on the finished article. The base olefin polymer is covered by FDA 21 CFR 177.1520 when the finished article meets extraction limits for the intended food type. Under EU 10/2011, the overall migration limit for plastic food-contact materials is 10 mg/dm² for food simulants, and specific migration limits for additives and catalyst residues are declared in Annex II. In China, GB 4806.7-2016 applies to food-contact polyethylene and establishes sensory requirements, total migration, and specific migration thresholds. Recyclability is assessed in the HDPE bottle recycling stream; HDPE is coded as resin identification code #2 under ASTM D7611-19, and unpigmented or lightly pigmented HDPE 5000S and 5502 containers are generally compatible with high-density polyethylene bottle reclaim streams. The presence of metallic closures, silicone gaskets, oxygen barrier coatings, or adhesive label systems modifies recyclability and should be evaluated under the relevant plastic recycling scheme.