Chevron Phillips Chemical HDPE 9708 is a high-flow, high-density polyethylene injection moulding grade supplied in pellet form. The nominal density is 0.962 g/cm³ when determined under ASTM D1505, and the nominal melt mass-flow rate is 8.0 g/10 min at 190 °C/2.16 kg when measured under ASTM D1238. Primary conversion is on conventional reciprocating-screw injection moulding machines for rigid packaging, caps, closures, thin-walled storage containers, housewares, and industrial bins. The grade is not intended for extrusion blow moulding or for film applications in which melt strength, parison sag resistance, and low gel content are primary control variables.

Polyethylene 9708 Material Identity and Specification Set

The resin is classified as an ethylene homopolymer with short-chain branching low enough to produce a density of 0.962 g/cm³. This high density increases crystalline content relative to lower-density HDPE copolymers, contributing to higher short-term stiffness and higher flexural modulus. The melt mass-flow rate of 8.0 g/10 min places the grade in the high-flow segment of the injection moulding HDPE range, but it is not as low in viscosity as high-melt-index LLDPE grades. The values below are typical for the base resin and may vary between production lots; they are not contractual specification limits.

Mechanical values generated under ISO 527-2 or ISO 178 can differ from ASTM D638 and ASTM D790 because of specimen geometry, test speed, and conditioning history. Comparative studies should not mix test methods without documented conversion. Notched Izod impact values are highly sensitive to notch radius, specimen thickness, and temperature; published data for HDPE 9708 is not reproduced here unless the exact specimen preparation and conditioning protocol is fixed under ASTM D256.

Material handling for HDPE 9708 does not normally require predrying if the resin remains in unopened moisture-tight packaging and the warehouse relative humidity is maintained below 60%. Cold-stored pellets moved into a warm, humid conversion hall can develop surface condensation; in such cases a desiccant hopper dryer set between 60 °C and 80 °C for 2 h to 4 h is used to restore surface dryness. Screw and barrel configurations for this grade employ general-purpose or mixing screws with 18:1 to 24:1 L/D and compression ratios from 2.5:1 to 3.5:1. A non-return valve with adequate shut-off clearance minimizes shot-weight variation when the melt is highly fluid.

Barrel profiles are generally set with a feed-zone temperature near 180 °C and a nozzle temperature of 220 °C to 230 °C. Mould temperatures in the range of 10 °C to 40 °C support ejection and dimensional stability. Residence time at melt temperatures above 250 °C should be limited to reduce oxidative chain scission and yellowing. The high density of 0.962 g/cm³ contributes to mould shrinkage values in the range of 1.2% to 2.0% depending on wall thickness, gate type, packing, and cooling; tool design should rely on data generated under ISO 294-4 or ASTM D955 for the specific cavity geometry.

Rheological behaviour of HDPE 9708 under low shear is captured by the melt mass-flow rate of 8.0 g/10 min; however, the melt index test is a single-point low-shear measurement and does not substitute for capillary rheometry or filling analysis. In injection moulding, apparent melt viscosity falls with increasing shear rate through gates, runners, and thin sections. The high density of 0.962 g/cm³ raises the crystalline melt transition compared with lower-density copolymers, but it does not alter the fundamental requirement that the flow front remain above the solidification temperature until cavity fill is complete. Thermal conductivity and heat capacity of the melt control the rate of heat removal; converters should not apply amorphous-polymer cooling-time rules to a semi-crystalline HDPE because the latent heat of crystallization can extend cooling demand in thick bosses and ribs.

Why Does the 8.0 g/10 min Melt Index Constrain Wall Thickness and Cycle Time?

The melt mass-flow rate of 8.0 g/10 min lowers resistance to flow and allows short filling paths in thin sections, but it also affects molecular weight distribution and resistance to slow crack growth. In multi-cavity hot runner tools, the benefit is reduced injection pressure and improved replication of fine features; the limitation is that thin sections below 0.8 mm can still short-shot if the flow length-to-thickness ratio exceeds the capability of the runner system and gate diameter. Pressure drop across valve pins and hot runner channels should be measured with the actual tool because published data for this specific configuration is limited.

Cooling and crystallization control are central to cycle time. High-density polyethylene of 0.962 g/cm³ develops a relatively high crystalline fraction, which increases heat removal demand and solidifies more rapidly than lower-density copolymers. That higher crystallinity raises flexural modulus but can create shrinkage anisotropy between flow and transverse directions. In moulding trials, melt temperature variation should be kept within ±5 °C for thin-wall articles; excessive variation produces observable differences in fill, sticking, and part weight. Control of mould surface temperature is similarly constrained, with adjustments in increments of no more than 5 °C to avoid warp or sink mark variation.

There is a process conflict between high-flow filling and packing. The low melt viscosity that permits filling at lower pressure also shortens gate freeze time, which may reduce the effectiveness of packing at the gate. Injection speed should be high enough to fill before the flow front solidifies, but excessive velocity can generate shear heating and surface defects. For 2.0 mm wall sections, holding pressure is typically applied until gate freeze is complete; the precise holding time is gate- and melt-temperature dependent and must be determined with the actual mould.

Lot-to-lot variation in melt flow and density can be expected within the supplier specification window. In multi-cavity or hot-runner production, small differences in melt index or density can produce shifts in shot size, part mass, and clamp force demand. Processors commonly set control limits on melt flow rate and density from the certificate of analysis. If a lot is near the upper melt flow boundary, barrel temperatures may need reduction of 2 °C to 5 °C; if a lot approaches the lower boundary, packing pressure or time may require increase. These adjustments are empirical and tool-specific.

Observed failure modes in high-flow HDPE injection moulding include short shots in thin wall sections when the hot runner manifold is not balanced, jetting from undersized pinpoint gates, and warp due to non-uniform cooling in boxes with thick base and thin sidewall intersections. HDPE 9708 is not immune to these effects. Parts ejected at too low elastic modulus can deform; therefore, mould surface temperatures in the 10 °C to 40 °C range should be controlled to avoid hot-part distortion during automated mould sweep or robot removal.

Thin-walled food storage and produce containers are formed from HDPE 9708 in multi-cavity tools where stack heights and cavitation increase output. In closure and cap moulding, the grade is used for articles that require stiffness and short cycle times; however, sealing performance is not governed by resin flow alone. Closure dimensions, liner compatibility, torque retention, and environmental stress crack resistance under liquid contact should be evaluated under end-use conditions. Published ESCR data for this exact grade is not uniformly reported across application environments; therefore, testing under ASTM D1693B using the intended chemical agent is recommended. HDPE 9708 is also reported in non-food housewares, storage totes, and industrial crates where high density provides load-bearing rigidity and the melt flow permits fast filling of ribs and bosses.

When 9708 Replaces Medium-Melt-Index HDPE in Multi-Cavity Tooling

Differences from other HDPE grades are most visible when a converter moves from a medium-melt-index injection moulding grade to HDPE 9708. The higher melt mass-flow rate of 8.0 g/10 min allows a reduction in melt temperature or injection pressure, but it also reduces low-shear melt viscosity and may shorten hold-pressure transmission through the gate. Compared with a nominal 2.0 g/10 min medium-flow HDPE of similar density, HDPE 9708 can fill thinner sections and may permit shorter cycle time; however, the high-flow polymer may exhibit lower notched impact strength and lower environmental stress crack resistance when all other factors are equal. These trade-offs should be confirmed by testing according to ASTM D256 and ASTM D1693B for the target part.

Comparative categories are derived from publicly available general HDPE processing classifications and do not represent a single named product. They are shown to illustrate the position of HDPE 9708; they are not contractual comparisons. The density difference is significant even when melt index is held constant. At 0.962 g/cm³, HDPE 9708 delivers higher flexural modulus than lower-density HDPE copolymers, but it also has lower optical clarity where thin sections are not pigmented. In applications requiring high environmental stress crack resistance, a lower-density or bimodal HDPE may be preferred. Converters should evaluate the actual chemical exposure because ESCR is not an intrinsic material constant; it depends on surface stress, quench rate, and active wetting agent.

The base resin is not inherently UV-stabilised and may contain processing antioxidant systems. The addition of colour masterbatch or nucleating agents modifies shrinkage, heat deflection response, and impact behaviour. A nucleating agent can refine crystalline structure and reduce shrinkage variability, but it can also lower impact resistance and change optical hazing. There is no published single-data set for HDPE 9708 with all additive packages; converters should not transfer shrinkage values from natural resin to pigmented parts without confirmation.

Compared with polypropylene injection moulding grades, HDPE 9708 has a lower melting point and often lower flexural modulus, but it may retain impact resistance at lower temperatures more effectively. The choice between HDPE and polypropylene is governed by end-use chemical resistance, hinging requirements, and thermal deflection. For parts requiring high stiffness and high heat deflection, polypropylene may be selected; for parts needing HDPE chemical resistance and low-temperature ductility, the HDPE grade is evaluated. Direct substitution should not be attempted without part testing under ISO 527-2 and ISO 178 or the equivalent ASTM methods.

For food-contact articles, compliance must be established on the finished part, not by resin selection alone. The base grade can be evaluated under FDA 21 CFR 177.1520(c) for olefin polymers and under Commission Regulation (EU) No 10/2011 for plastic food-contact materials where migration testing supports the end-use. The converter is responsible for verifying additive packages, colourants, and processing aids. The resin is not formulated for long-term outdoor UV exposure; long-term weathering requires a UV stabiliser masterbatch and validation under ASTM D2565 or ISO 4892-2. Contact with strong oxidizing acids, aromatic solvents, and some chlorinated hydrocarbons can promote swelling or stress cracking; qualification in the actual chemical environment is required before commercial use.