Lianyungang Petrochemical HDPE STL 6888 is a high-density polyethylene injection-moulding grade supplied under the producer-specific designation STL 6888. It is not listed as a general-purpose HDPE, a bimodal pipe compound, or a blown film resin; the grade is positioned for high-flow injection moulding and thin-wall extrusion where fast cavity filling and short cycle times control manufacturing economics. Applications mentioned in trade literature include rigid food-compliant packaging, closures, caps, pails, crates, measuring cups, thin-walled housewares, and industrial mouldings. Distributed technical data associate the grade with a nominal melt mass-flow rate of 8.8 g/10 min at 190 °C under 2.16 kg and a nominal density of 0.968 g/cm³. These values are typical, not guaranteed release limits; the heat-specific certificate of analysis remains the controlling document for machine setup, lot acceptance, and food-contact declarations.

The combination of high fluidity and high density separates STL 6888 from high-molecular-weight HDPE grades used for blown film and pressure pipe. Its higher melt mass-flow rate permits shorter injection times and lower pressure loss, but its lower molecular weight reduces melt strength and environmental stress crack resistance relative to PE100 pipe and large-part blow-moulding resins. The grade is therefore selected when the application requires high productivity in injection moulding rather than long-term hydrostatic strength or parison hang strength.

The standard test architecture follows the ISO polyolefin chain. Melt mass-flow rate is measured under ISO 1133-1:2022. Density is measured by the immersion method under ISO 1183-1:2019. Tensile yield stress and elongation at yield are determined on Type 5A specimens under ISO 527-2:2012. Flexural modulus is measured under ISO 178:2019, and notched Charpy impact is determined under ISO 179-1:2023. Vicat softening temperature is recorded under ISO 306:2022 method A50. Distributor-reported typical values for STL 6888 are summarised in Table 1, but these are not guaranteed release limits.

What processing window must be maintained on high-speed injection moulding machines?

For thin-wall containers and closures with wall thickness below 0.8 mm, the typical melt temperature range is 210 °C to 250 °C. Mould temperature is usually held between 20 °C and 40 °C, although higher cavity temperatures may be used when low weld-line visibility or dimensional stability of technical mouldings takes priority over cycle time. The injection velocity is set high enough to keep the flow front moving before gate freeze, but not so high that shear heating at the gate causes surface defects or resin degradation. On a 1,200 kN electric injection moulding machine with a 35 mm general-purpose screw, high-flow STL 6888 generally lowers peak injection pressure relative to medium-flow HDPE grades; however, the same fluidity can produce overpacking and part distortion if holding pressure and cooling time are not adjusted separately from the medium-flow baseline.

Production-scale experience with high-flow HDPE grades shows that parting-line vent depth should be maintained between 0.02 mm and 0.03 mm. Deeper vents can flash; shallower vents can trap volatile residuals and cause burn marks, short shots, or weld-line weakness. Melt temperature should not exceed 280 °C during normal processing because prolonged residence above this threshold accelerates chain scission, yellowing, and the formation of plate-out on the screw root, nonreturn valve, and hot-runner channels. Screw recovery with a fast-flow HDPE is typically shorter than with blow-moulding or pipe-grade HDPE; the screw may idle during the last portion of the cooling timer. That idle is not a machine fault, but the cushion should be maintained between 2 mm and 5 mm to stabilise holding pressure and prevent sink marks.

Screw geometry should be selected for high-flow polyolefins. A general-purpose three-zone screw with an L/D ratio of 20:1 to 25:1 and a compression ratio of 2.0:1 to 2.5:1 is normally adequate. Shorter screws may not generate sufficient melt homogeneity; longer screws may increase residence time and shear history without improving melt quality. The nonreturn valve should be designed for HDPE and checked for wear because a leaking check ring can produce cushion loss, shot-size drift, and inconsistent packing on fast cycles. These equipment conditions are not specific to STL 6888 but are critical when the grade is used at the upper end of the producer’s recommended temperature window.

In multi-cavity hot-runner tools, the lower viscosity of STL 6888 can expose poorly balanced runner systems. If cavity-to-cavity fill time variation exceeds 5%, some cavities may overpack while others remain underpacked, producing inconsistent part mass, sink marks, and variable shrinkage. Moulding technicians may compensate by adjusting individual gate temperatures or injection velocity profiles; however, the underlying runner geometry should be corrected through flow simulation before production. For high-flow HDPE of this density, gate diameters for thin-wall parts are often between 0.5 mm and 1.0 mm, but the exact diameter depends on wall thickness and expected flow length. Published data for this specific configuration is limited, so a finite-element filling study is recommended for new tools.

Shear thinning, chill-layer formation, and shrinkage anisotropy in high-flow HDPE

High-fluidity HDPE melts are non-Newtonian and display shear thinning. In capillary rheometry near 190 °C, an HDPE homopolymer with density close to 0.968 g/cm³ shows apparent viscosity falling from the zero-shear plateau into the 100 to 10,000 s⁻¹ shear-rate region. STL 6888 has a relatively narrow molecular weight distribution by design, so its shear-thinning response is less pronounced than that of blow-moulding grades formulated for parison hang strength. The practical effect is a more Newtonian flow front during thin-wall filling, lower die swell, and reduced recoverable shear strain. This gives good dimensional predictability in hot-runner systems but also limits the grade’s melt strength in extrusion blow moulding and large-part blow-fill-seal operations.

Shrinkage is anisotropic and depends on gate location, wall thickness, cavity pressure, and hold time. For an unfilled HDPE with density near 0.968 g/cm³, moulding shrinkage is commonly in the 1.5% to 3.0% range in the flow direction and 1.0% to 2.5% transverse to flow, provided packing pressure is sufficient. A change from a medium-flow grade to a high-flow grade can increase flow lengths and reduce pressure decay, but it may also change gate freeze time and cooling channel requirements. Tooling-specific shrinkage trials are required before final cavity dimensions are released; published data for this specific configuration is limited, so the mould designer should not rely on generic HDPE shrink values alone.

The crystallinity of HDPE at this density is typically in the range of 65% to 75% as measured by differential scanning calorimetry. This high crystalline fraction contributes to the flexural modulus and chemical resistance of STL 6888, but it also raises moulding shrinkage relative to lower-density LLDPE grades. In thin-wall packaging, variations in crystallinity driven by cooling rate can produce warpage if cavity cooling is asymmetric. Mould cooling circuits should be balanced so that core and cavity temperatures differ by less than 10 °C on average; otherwise differential shrinkage may produce lid curl or container ovality. For high-flow HDPE, a melt temperature closer to 230 °C and a mould temperature near 30 °C often provide a reasonable compromise between crystallinity, flatness, and cycle time, but the specific gate geometry and part thickness control the final condition.

Thermal stability of HDPE STL 6888 is controlled by the producer’s antioxidant package. The oxidation induction time at 200 °C under ISO 11357-6 is often reported as a release criterion for high-density polyethylene; however, the specific OIT limit for STL 6888 must be taken from the certificate of analysis. For high-speed injection moulding, repeated recycling of regrind in thin-wall packaging can consume antioxidants and reduce long-term thermal stability. Moulders typically limit regrind levels to 20% to 30% by weight unless a higher level is validated by differential scanning calorimetry. At processing temperatures above 280 °C, the degradation rate increases sharply and can lead to acid-generating decomposition products that corrode hot-runner components and affect part odour and taste. This is particularly relevant in caps and closures for mineral water or sensitive beverages; lot-specific organoleptic testing should be reviewed before switching to high regrind fractions.

When STL 6888 is compared with PE100 pipe, extrusion blow-moulding homopolymer, or random-copolymer polypropylene

STL 6888 is not a substitute for PE100 pressure-pipe material. PE100 grades are usually bimodal HDPE with melt mass-flow rates below 0.5 g/10 min and are classified for long-term hydrostatic strength under ISO 9080 and ISO 12162. STL 6888 has much higher fluidity and a higher density, which provide stiffness and rapid mould filling but lower long-term hydrostatic strength and lower environmental stress crack resistance. Its resistance to slow crack growth under ASTM D1693 or ISO 22088-1 is expected to be below that of high-molecular-weight HDPE and PE100 pipe compounds. The material is therefore unsuitable for buried pressure pipe, industrial vessels, fuel tanks, or geometrically complex blow-moulded articles that must retain stress-crack resistance under sustained hoop stress.

Compared with HDPE extrusion blow-moulding grades in the 0.2 to 1.2 g/10 min range, STL 6888 has lower melt strength, lower die swell, and less parison stability. In shuttle or accumulator blow moulder trials, the parison may draw down excessively at temperatures above 220 °C. The grade should not be used for bottles, canisters, or tanks unless a dedicated extrusion trial confirms adequate hang strength. Compared with polypropylene random copolymers of similar melt flow, STL 6888 has a higher density and a lower melting range; it is not a direct clarity or heat-grade replacement. The choice between HDPE and PP should be based on part stiffness at low temperature, chemical exposure, hinge performance, and the required food-contact migration limits.

Within the HDPE injection-moulding family, the main difference between STL 6888 and a medium-flow grade with melt mass-flow rate near 3 to 4 g/10 min is the length of the molecular weight chains and the ease of mould filling. The higher flow of STL 6888 reduces pressure loss in long flow paths and permits lower melt temperature or lower clamp force; however, it also reduces resistance to slow crack growth and can increase notch sensitivity at very low temperatures. Applications that require repeated drop impact at sub-zero conditions, such as industrial tote bins, may be better served by a lower-flow high-density or a bimodal HDPE grade with higher notched low-temperature impact. For thin-wall food packaging and closures, the balance of stiffness and processability offered by STL 6888 is generally appropriate. The final selection should be validated against the specific distribution environment, stacking load, and closure torque requirements.

Food-contact use requires lot-specific verification under FDA 21 CFR 177.1520 for olefin polymers and, for the European Union, compliance with Regulation (EU) No 10/2011 and its overall migration and specific migration limits. The resin supplier may provide a declaration of conformity for the natural grade, but the declaration is heat-specific and must not be carried over across production campaigns. For electrical and electronic applications, RoHS compliance is assessed against Directive 2011/65/EU; HDPE itself does not normally contain the restricted heavy metals or brominated flame retardants above the maximum concentration values, but colourants, processing aids, and secondary additives must be verified in the final compound.

Storage and contamination boundaries must be observed when HDPE STL 6888 is introduced into a plant that previously ran other resins. HDPE is not hygroscopic, but surface condensation can occur when silo or hopper temperatures change rapidly and relative humidity exceeds 60%. If a cold silo discharges into a warm shop-floor hopper, surface moisture can produce splay, voids, or dimensional instability. In such conditions a desiccant hopper dryer set to 60 °C for 2 hours may reduce visible moisture, but pre-drying is not normally required for the grade. The resin should be kept away from strong oxidising agents, aromatic and chlorinated solvents, and direct flame. When a machine is switched from PVC, polycarbonate, or another polar resin to HDPE, the barrel should be purged with a high-flow HDPE or an acrylic purge compound. Residual polar polymer can degrade and cross-link at HDPE processing temperatures, producing black specks, gel particles, and plate-out. If a masterbatch is added, the carrier resin and dosing level should be matched to the base melt flow rate; excessive external lubricant or metal stearate can cause screw slip in the feed section and erratic recovery times. In high-flow HDPE, screw slip is typically corrected by lowering the rear zone temperature, reducing back pressure, or using a screw with a grooved feed section rather than by increasing the masterbatch level.