Braskem HDPE SGF4960 is a high-density polyethylene injection molding grade. The nominal melt flow index is 20 g/10 min when measured at 190 °C under 2.16 kg load by ASTM D1238-20; the nominal density is 0.956 g/cm³ by ASTM D1505-18. The resin is used for thin-walled closures, caps, housewares, and food and nonfood containers where a balance of flow, stiffness, and rapid gate freeze is required.

The grade is positioned as a high-flow injection molding HDPE. In comparison with lower-flow HDPE grades, SGF4960 reduces injection pressure and cycle time in multi-cavity tools but carries lower notched Izod impact and lower slow crack growth resistance than lower-flow blow molding or pipe grades. The product is not intended for pressure-pipe or long-term structural service.

These values are typical published data for the base resin and are not lot-release specification limits. Additives, regrind, color concentrates, and molding conditions can shift each value.

On a high-cavitation closure line using a hot-runner stack, barrel setpoints typically begin at 180 °C in the feed zone and 220 °C at the nozzle, with melt temperature raised to 260 °C only when required for thin-wall fill. Mold temperature is maintained between 15 °C and 40 °C to freeze gates rapidly. Injection velocity is set so that the flow front completes filling before gate freeze; in a cap with a gate land below 0.5 mm, this can require fill times below 0.3 s. Holding pressure is applied until the gate seals, after which cooling dominates the cycle. Screw recovery is normally completed during cooling because the 20 g/10 min melt index reduces plastication torque. Back pressure between 0.5 MPa and 1.0 MPa is a common starting point. No predrying is required for sealed containers; if surface moisture is present, drying at 80 °C for 2 h in a desiccant hopper is sufficient.

The processing window is constrained by the conflict between fill pressure and impact retention. If melt temperature is raised above 260 °C to reduce fill pressure, notched Izod impact can decrease because of chain scission and stabilizer depletion; if temperature is reduced below 200 °C, flow marks and cold slugs become more likely. The effective operating band is therefore 200 °C to 260 °C for most injection equipment. Injection pressure is mold-dependent, but high-flow grades generally move the required machine pressure away from the upper limit of smaller molding machines. Process validation should include short-shot progression, gate seal time by weight increase, and cavity pressure measurement with flush-mounted transducers.

Cooling time follows part thickness squared. For a 0.8 mm wall closure, cooling time may be between 4 s and 7 s at a mold temperature of 20 °C; parts should be ejected below the Vicat softening point to prevent gate-area distortion. If ejection is premature, residual flow orientation can relax and produce cap ovality. Tooling with forced water cooling near the gate reduces localized heat accumulation from high-flow melt.

What Rheological Boundaries Define the SGF4960 Processing Window?

The 20 g/10 min melt flow index is a low-shear, low-stress measurement. It should not be used to calculate injection pressure at the shear rates of 1,000 s⁻¹ to 10,000 s⁻¹ that occur in closure gates. Published capillary rheometry data for SGF4960 across this shear-rate range is limited; mold trials are required to measure the viscosity at the target gate geometry and melt temperature. High-flow HDPE grades generally show pronounced shear thinning, so the viscosity at thin-wall filling can be substantially lower than a melt flow index comparison suggests. This allows narrower gates and lower injection pressure, but if the gate becomes too small, local shear heating may raise the melt temperature enough to cause yellowing and gate stringing. The practical melt-temperature window in high-speed closure molding is bounded at the lower end by flow marks and at the upper end by drool and thermal degradation in hot-runner components.

Tooling should use positive shut-off nozzles and thermal or valve gates because the low viscosity increases nozzle drool. Valve gates are preferred for thin lids and closures because they leave a clean vestige and reduce decompression artifacts at the gate. With cold-runner systems, the runner diameter must be balanced against sprue freeze time; cold slugs in the main runner can become brittle if the nozzle is too cold. First-stage injection velocity is usually increased while second-stage pack pressure is held at 60% to 80% of the injection peak. Regrind use up to 20 wt% is common in nonfood packaging; food-contact applications must use regrind that complies with the intended food-contact standard. Hoppers should be cleaned between product changes because high-flow pellets may bridge in warm hoppers.

Differences from Lower-Flow HDPE and Blow Molding Grades

Compared with a blow molding HDPE with melt flow index between 0.8 g/10 min and 2.0 g/10 min, SGF4960 fills thin sections at much lower injection pressure. The blow molding grade retains a larger high-molecular-weight fraction for parison melt strength, but that fraction would restrict flow in a multi-cavity closure tool. SGF4960 does not require high melt strength because it is not blown into a parison before mold closure. Compared with a lower-flow injection HDPE of 8 g/10 min, SGF4960 shows lower cavity pressure at the same wall thickness but also lower notched Izod impact and lower stress crack resistance. The melt strength difference is visible in extrusion blow molding versus injection molding: a blow molding grade with melt flow index of 0.8 g/10 min to 2.0 g/10 min can sustain a parison, while SGF4960 cannot. In injection molding, that limitation is largely irrelevant because the melt is confined by the cavity.

On a 2,000 kN machine running a 96-cavity closure stack, the available clamp force per cavity is approximately 20.8 kN. A 20 g/10 min grade may permit filling such a stack at lower injection pressure than a 4 g/10 min grade, leaving a larger margin against flash. Published data for this specific mold configuration is limited; production trials generally show that cycle time becomes limited by gate freeze or cooling, not screw recovery. Closure designs with SGF4960 should include generous radii at gate areas and thread transitions because lower molecular weight increases notch sensitivity.

Shrinkage control on the target tool is more decisive than generic plaque data. High-density polyethylene closure grades typically exhibit mold shrinkage between 0.015 mm/mm and 0.025 mm/mm on a 3.2 mm end-gated plaque under ASTM D955-08. SGF4960 may fall within this range, but published shrinkage values for the exact grade and plaque geometry are limited. Thin wall sections reduce flow-direction shrinkage and increase the influence of gate freeze time. Differential shrinkage in a cylindrical cap can produce ovality after 48 h of relaxation; localized mold cooling near the gate can reduce this distortion. If the part is exposed to elevated service temperatures, post-mold annealing below the Vicat softening point may be required to stabilize dimensions.

When ESCR Resistance and Long-Term Hydrostatic Strength Are Priority Criteria

Parts exposed to detergents, alcohols, or continuous stress should not be specified with SGF4960 solely because of flow. High-density polyethylene pipe and blow molding grades derive slow crack growth resistance from the high-molecular-weight tail and inter-crystalline tie molecules; the high melt flow of SGF4960 implies a lower molecular weight distribution that sacrifices some of those tie molecules. Accelerated environmental stress crack resistance testing under ASTM D1693 can rank materials, but published failure times for this exact grade are limited. The grade is not formulated for long-term hydrostatic strength under ISO 9080 or ASTM D2837; it should not be used for pressure pipe, gas distribution, or chemical storage where hoop stress is sustained. In closure and container applications, stress is usually intermittent and imposed by thread engagement or stacking; designs should avoid sharp thread roots, uncontrolled weld lines, and large gate vestiges that can initiate cracking.

Food-contact suitability must be confirmed under 21 CFR 177.1520 and, for the European market, EU No 10/2011. The final article’s surface-to-volume ratio, food simulant, time, and temperature can alter specific migration results, so the converter must verify compliance for the finished part. Colorants, processing aids, and regrind must also comply with the same end-use requirements. For nonfood applications, ingredients should be reviewed against REACH restrictions. Processing temperatures should remain below 300 °C to avoid excessive thermal decomposition in the barrel and hot runner.

Batch-to-batch control is monitored by melt flow index and density. A fluctuation in melt flow index of more than 10% from the nominal 20 g/10 min can alter fill time and gate freeze; incoming lots should be tested before use. On the molding floor, screw cushion stability should be recorded; a drifting cushion at constant melt index can indicate check-ring wear or barrel temperature errors. Because the high melt flow makes the melt more compressible, transfer from injection to holding pressure should be based on screw position rather than time alone. Machine barrel capacity should not exceed 70% of the shot size to avoid prolonged residence time. Hot runners should be purged before shutdown, and purgings should be segregated from the product stream.