Braskem HDPE SGM7746C is a high-density polyethylene injection-molding grade supplied in natural pellet form. The resin occupies the 0.945–0.955 g/cm³ density class and the 4.0–10.0 g/10 min melt-flow class when determined according to ASTM D1238 at 190 °C and 2.16 kg. These values represent the product class rather than lot-specific release limits; each production lot should be checked against the Braskem certificate of analysis because melt-flow-rate and density tolerances are known to shift fill pressure, packing intensity, and warpage. The product is intended for thin-wall injection-molding applications such as caps, closures, dairy packaging, housewares, and industrial components in which short cycle time and consistent cavity filling are primary requirements. Compared with HDPE blow-molding grades with melt flow rates below 1.0 g/10 min, SGM7746C has lower melt viscosity, which reduces injection-pressure demand and permits thinner wall sections. The trade-off is a narrower upper-temperature processing boundary because lower molecular weight increases susceptibility to thermal degradation at elevated melt temperatures. The grade is not designed for blown film, blow molding, or pressure-pipe applications where high melt strength or long-term hydrostatic creep resistance is required. The operational boundaries that govern successful processing of this product are defined by melt temperature, residence time, gate freeze-off, and lot-to-lot melt-flow variation.

What Limits Hot-Runner Residence Time at the Upper Melt-Temperature Boundary?

On single-stage reciprocating-screw machines with 20:1 L/D general-purpose screws and check-ring non-return valves, the melt temperature should be profiled from 180 °C in the feed zone to 215 °C at the metering zone, with the nozzle held at 200–220 °C. Mold temperature is maintained between 15 °C and 40 °C. When the melt temperature exceeds 230 °C, residence time in the barrel and hot runner becomes the controlling variable. At 230–240 °C, total melt residence time should not exceed 15 min; above 240 °C, even short hold times can generate black specks, yellowing, and low-molecular-weight volatiles that condense on cavity surfaces. Field-observed failure in valve-gated hot runners is intermittent gate stringing and black-speck cycling with stable barrel temperatures; the cause is polymer degradation in hot-runner dead spots. The remedy is to reduce hot-runner tip temperature by 5–15 °C and purge with a lower-viscosity polyolefin purge compound. Hot-runner controllers should be monitored at each tip, not only at the manifold, because tip-to-tip temperature differences exceeding ±5 °C can produce non-uniform cavity filling despite stable machine settings. Moisture absorption is generally low, but when storage is at relative humidity above 60%, surface moisture can cause splay. Pre-drying in a dehumidifying hopper dryer at 60–80 °C for 2–4 h with a dew point below -20 °C is recommended under these conditions.

Mold-Filling Pressure, Gate Freeze-Off, and Dimensional Stability

For wall thickness between 1.0 mm and 1.5 mm, nozzle injection pressure typically falls within 30–70 MPa hydraulic on a 120-ton machine. Transfer from velocity-controlled fill to pressure-controlled pack should occur at 95–99% volumetric cavity fill, not by timer, to avoid overpacking. The holding-pressure setpoint of 25–60 MPa hydraulic is maintained until gate freeze-off is confirmed; for a 1.0–1.5 mm wall with a cold-runner gate diameter of 0.5–1.2 mm, freeze-off commonly occurs within 2–5 s. If gate freeze-off is not confirmed by part-weight trend, the holding time should be increased in 0.5 s increments until part mass stabilizes. Dimensional stability is evaluated under ASTM D955 for mold shrinkage and ISO 294-4 for specimen preparation; mold-temperature uniformity within ±5 °C across the cavity is required to control warpage. Lot-to-lot melt-flow variation within ±0.5 g/10 min may require 5–10% injection-pressure or holding-pressure adjustment on the same mold. Processors should use in-process part-weight checks rather than fixed pressure settings because melt-flow shifts change the fill profile even when machine parameters remain unchanged. Screw cushion should be maintained between 3 mm and 6 mm; cushion collapse below 2 mm is associated with sink marks and underpacking.

Table 1 summarizes the suggested operating window for thin-wall injection molding of SGM7746C. The ranges are starting points, not release limits.

Because the grade is formulated for injection molding, its melt strength is lower than that of HDPE grades designed for blow molding or blown film. This is not a product defect but a material design boundary. Blow-molding HDPE grades with melt flow rates below 1.0 g/10 min sustain parison stability under draw-down forces; SGM7746C is unsuitable for those operations because its lower molecular weight does not provide sufficient melt strength at typical extrusion temperatures. High-molecular-weight film grades with melt flow rates below 0.1 g/10 min provide bubble stability for thin-gauge blown film; this injection-molding grade would show excessive melt sag and poor bubble stability. In injection-molding comparisons, SGM7746C lies on the higher-flow side of the product family, reducing cycle time and pressure drop but generally lowering environmental stress-crack resistance measured under ASTM D1693 relative to lower-flow HDPE injection grades. The actual difference must be confirmed on finished-article specimens because ESCR data are highly geometry-dependent. Published comparative data for SGM7746C against every nearby Braskem grade are limited; direct comparative trials in the production mold are therefore required before substitution.

When a Lower-Flow HDPE Grade Is Replaced by SGM7746C in an Existing Mold

Substitution in an existing mold requires readjustment of fill, pack, and hot-runner settings. The lower melt viscosity of SGM7746C reduces pressure drop through the sprue, runner, and gate; therefore, the same pressure setpoint used for a lower-flow HDPE may overpack the cavity. Overpacking appears as flash, gate blush, elevated shrinkage variation, and dimensional distortion. The transfer position should be moved earlier, and holding pressure should be reduced in 5–10% steps while monitoring part mass and sink-mark appearance. For hot-runner molds, the hot-runner tip temperature should be lowered by 5–15 °C relative to a lower-flow HDPE because the reduced viscosity can cause drool and gate stringing at the same setpoint. Cooling time may be shortened because of the lower melt enthalpy at the same wall thickness. On a 120-ton hydraulic injection molding machine with a 20:1 L/D screw, screw recovery speed should be set between 60 rpm and 100 rpm. Lower speeds may cause feed-zone bridging in the hopper throat, while higher speeds can raise melt temperature by shear heating beyond the intended nozzle setpoint. Back pressure is maintained between 0.5 MPa and 1.5 MPa. A worn non-return valve is a common failure mode during substitution: high-flow HDPE leaks backward through an improperly sealed check ring, producing erratic shot sizes and part-weight variability. Replace or verify the check ring before running the new grade if shot-to-shot weight variation exceeds 0.3%.

Weld-line strength in molded parts is influenced by melt temperature, injection velocity, and gate geometry. For SGM7746C, weld-line tensile strength measured under ASTM D638 may be 10–25% lower than unfilled material strength if melt temperature at the weld front is below 190 °C. Raising nozzle temperature to 210–220 °C and increasing injection speed to 100–200 mm/s linear flow-front velocity improves weld-line fusion, but only up to the point where burn marks appear. Gate position should be balanced so that melt fronts meet at 90° or greater angles; knife-edge weld lines produced by opposite-flow fronts are more prone to brittle failure. Hot-runner gates smaller than 0.5 mm diameter can shear the melt excessively, producing localized temperature rises and flow marks; for thin-wall caps and closures, gate diameter between 0.6 mm and 1.0 mm is generally used. Masterbatch or pigment addition should be limited to the dosage range specified by the masterbatch supplier. Excessive color concentrate above 4 wt% can reduce melt-flow consistency and change shrinkage; the carrier resin of the color concentrate should be melt-compatible with high-density polyethylene to avoid splay and delamination.

Food-contact status is not a single statement. For olefin polymers, the applicable United States framework is FDA 21 CFR 177.1520, but the finished article must meet specified extractables and end-use limitations; color concentrates and additives can alter the status. In the European Union, plastic food-contact materials are evaluated under Regulation (EU) 10/2011, with migration testing according to the EN 1186 series and applicable simulants. REACH registration under Regulation (EC) 1907/2006 and RoHS recast under Directive 2011/65/EU are supplier-level obligations documented through Braskem product stewardship. The current regulatory status must be verified with the latest Braskem documentation because certification scope can differ by production site and grade formulation. Injection-molding HDPE of this class is not automatically qualified for potable-water pressure pipe under ISO 4427 or ASTM D3350; such qualification is the responsibility of the finished-article manufacturer. Table 2 lists the standards commonly applied to this product class, not an exhaustive regulatory certificate.

Processing limits are defined by melt temperature and residence time, not by a single setting. Avoid sustained operation above 240 °C; at this boundary, oxidation-induced gel formation and black specks become increasingly likely. If production interruption exceeds 15 min, reduce barrel temperature to 160–170 °C standby or purge with a commercial polyolefin purge compound. The product is not recommended for continuous contact with strong oxidizing acids or chlorinated solvents unless specific chemical-resistance testing has been performed. Natural SGM7746C without an ultraviolet stabilizer has limited weathering resistance under ASTM D4329; outdoor service requires a UV-stabilized version selected with the stabilizer package identified in the supplier documentation. Mold trials on the actual cavity geometry remain the final determinant of acceptable dimensional stability, impact performance, and cycle time because laboratory property values cannot capture gate-induced orientation, weld-line strength, or part geometry.