INEOS HDPE ELTEX CAP508 is a high-density polyethylene copolymer supplied in pellet form for injection-moulded caps and closures, specifically positioned for carbonated soft drink (CSD) closures. The grade is formulated to address thin-wall filling, resistance to environmental stress cracking, and low organoleptic contribution in direct beverage contact. Typical melt flow rate (MFR) at 190°C under 2.16 kg load is 5.0 g/10 min when measured according to ISO 1133-1:2022. Density at 23°C is 0.958 g/cm³ according to ISO 1183-1:2019. The CAP designation identifies the INEOS closure resin family. The material is supplied as a natural pellet; colour is normally introduced by masterbatch at the injection machine, and the base resin is not flame-retardant modified. Its narrow molecular weight distribution reduces warpage in shallow closures and improves dimensional consistency after ejection.

Specifications and Typical Property Window

The following table reproduces typical physical properties published for the unfilled natural material. These values are not specification limits and may vary with production campaign, test laboratory, and sample preparation. The current supplier datasheet should be consulted for commercial specification purposes.

The MFR of 5.0 g/10 min is a low-shear value. Under injection shear rates of 10³–10⁵ s⁻¹, viscosity decreases substantially, permitting filling of multicavity tools with reduced injection pressure. The density of 0.958 g/cm³ places the grade in the upper range for closure HDPE; it increases modulus and oxygen barrier relative to HDPE at 0.945–0.952 g/cm³, but may reduce stress crack resistance relative to lower-density copolymers if other variables remain constant. The elongation at break above 250% supports thread deflection without brittle fracture in standard tensile geometry, although closure thread performance is part-dependent.

On production-scale single-screw injection moulding machines with general-purpose screw L/D ratios of 20:1–25:1, CAP508 is processed with a nozzle melt temperature of 220–260°C. The optimised window for multi-cavity CSD closure tools is 235–245°C. Barrel profile from feed throat to nozzle typically follows 190–210°C, 210–230°C, 220–240°C, and 230–250°C. Mould temperature is held at 10–30°C to balance rapid solidification with thread filling. Processing at 260°C can reduce melt viscosity and improve thin-wall fill, but residence times above 5 min may generate odour/taste active species and should be avoided for sensitive beverage applications. Processing below 220°C demands higher injection pressure and can cause under-pack in closures with wall sections below 0.6 mm. Hot runner valve-gate systems are common for multi-cavity closures; gate diameters below 1.0 mm may lead to premature gate freeze-off and reduced top-load performance. Clamp force requirements depend on projected area and cavitation, but machines with clamp force class 100–250 t are typical for multi-cavity CSD closure tools. Cooling time for cap walls of 1.0–1.5 mm is estimated at 3–7 s at 20°C mould temperature. Part weight variation across cavities should be held below 0.5% to avoid capping torque variation.

Does Environmental Stress Cracking Control the Service Life of Carbonated Soft Drink Closures?

The closure service environment for CSDs combines internal carbonation pressure of 0.3–0.5 MPa, acidic pH in the range of 2.3–3.5, lipophilic flavour compounds such as limonene, and residual moulded-in stress from injection and capping torque. Environmental stress cracking proceeds by slow crack growth when stress, aggressive agent, and polymer resistance align. CAP508 is formulated with a higher molecular weight tail than high-flow HDPE grades; the result is an ESCR F50 above 200 h in ASTM D1693-15 testing at 50°C in 10% Igepal CO-630. This value should be interpreted only as a ranking metric, because actual closure ESCR depends on gate position, mould cooling rate, capping torque, and carbonation level. Published data for specific closure geometries is limited. Top-load and thread integrity are not universal material properties; they are part-dependent and must be measured on the production tool using internal or customer methods. However, the combination of 0.958 g/cm³ density and 5.0 g/10 min MFR supports closures with sufficient stiffness for carbonation pressure retention while maintaining injection productivity.

In the INEOS closure resin portfolio, CAP508 sits between lower-flow HDPE grades for large-diameter closures and higher-flow HDPE grades for thin-wall still-water closures. Compared with extrusion blow-moulding HDPE with MFR 0.2–0.4 g/10 min, the melt viscosity of CAP508 is substantially lower at injection shear rates, reducing fill pressure and enabling higher cavitation. Compared with high-flow injection grades with MFR above 20 g/10 min, CAP508 retains a higher molecular weight fraction, which is reflected in longer ESCR under ASTM D1693-15 and improved stress crack behaviour in thread roots. For non-carbonated water closures with very short cycle time, a higher-MFR grade may lower peak injection pressure and increase output, but the trade-off is typically lower ESCR and lower top-load at equivalent part weight. For aggressive CSDs or hot-fill flavour systems, CAP508 is preferred over general-purpose injection HDPE if ESCR is the controlling requirement. Published direct comparative data between CAP508 and competitor grades is limited; material selection should be confirmed with cap tool trials and laboratory ESCR tests using the actual product matrix.

Thermal and Rheological Data Support Thin-Wall Fill at Reduced Cycle Time

Vicat softening point A50 of 126°C per ISO 306:2013 and heat deflection temperature B at 0.45 MPa of 78°C per ISO 75-2:2013 describe short-term thermal behaviour relevant to capping operations and hot packaging contact, not continuous service temperature. The density of 0.958 g/cm³ produces higher crystallinity than low-density ethylene copolymers, increasing tensile modulus to 1,200 MPa and reducing oxygen transmission. This supports carbonation retention but also increases mould shrinkage anisotropy. Mould shrinkage for unfilled HDPE of this density is typically 1.5–2.0% in the flow direction and 1.0–1.5% transverse; tool designers compensate by dimensional scaling. Cap ovality after ejection is controlled by uniform mould cooling and core temperatures not exceeding 35°C in local hot spots. Rheologically, CAP508 exhibits shear-thinning behaviour; the viscosity under injection shear rates is substantially lower than the low-shear value measured by MFR. The exact high-shear viscosity curve is available in supplier technical service data and should be used for mould-filling simulation. Published generic values may not capture lot-to-lot variation.

For direct food contact in the European Union, the grade falls within the scope of Regulation (EC) No 10/2011 on plastic materials and articles intended to come into contact with food. Overall migration limit is 10 mg/dm² for plastic materials, with specific provisions depending on article geometry. In the United States, the grade is used under FDA 21 CFR 177.1520 for olefin polymers, subject to density and extractables limits. Under REACH Regulation (EC) No 1907/2006, supplier declarations state that substances of very high concern are not present above 0.1% w/w. Under RoHS Directive 2011/65/EU, where assessed as a homogeneous material in electrical equipment, cadmium is restricted to 0.01% w/w, while lead, mercury, hexavalent chromium, PBB, and PBDE are restricted to 0.1% w/w. Organoleptic performance for beverage closures is typically assessed by sensory panel methods specific to the customer; no single ISO method is sufficient. The material is not supplied as a medical-grade resin, and ISO 10993 biocompatibility testing has not been completed by the producer for this specific grade.

When Pre-Drying and Nitrogen Blanketing Are Necessary in High-Humidity Injection Moulding

Storage in unopened bags in a dry area below 50°C does not require pre-drying for HDPE. However, if pellets are stored in outdoor silos at relative humidity above 60%, condensation on cold pellet surfaces can introduce surface moisture that appears as splay, voids, or inconsistent part weight. In such cases, a desiccant dryer set to 80°C for 2–4 h is used. Nitrogen blanketing of hoppers is not normally required for CAP508, but it may be applied when colour changeovers demand purging or when oxygen-sensitive masterbatches are introduced. Prolonged exposure to ultraviolet radiation during outdoor storage reduces molecular weight and increases gel formation; covered storage is required for periods beyond 6 months. The resin is incompatible with strong oxidising acids at elevated temperature and with chlorinated solvents under stress; contact with such agents should be avoided. When recycled material is added, the user must assess migration limits and mechanical property retention under EU Regulation (EC) No 10/2011 and any local packaging requirements. The material should not be incinerated without suitable emission controls because standard hydrocarbon combustion products are generated.