High-density polyethylene is defined in ASTM D883 as a polyethylene with a density at or above 0.941 g/cm³. In a practical grade chart, density and melt flow index are the primary sorting variables because they indirectly capture crystallinity, molecular weight, and processability. Commercial HDPE density ranges from 0.941 g/cm³ to 0.965 g/cm³ when measured by ISO 1183-1 or ASTM D1505, while the melt flow index spans from 0.02 g/10 min to above 80 g/10 min at 190°C/2.16 kg per ASTM D1238 or ISO 1133-1. A table that only lists density and MFI is insufficient because two grades with identical density and MFI can differ in molar mass distribution, comonomer type, additive package, and shear viscosity. Grade selection therefore requires matching the polymer to the forming process, tooling geometry, part wall thickness, regulatory standard and end-use mechanical load. The following sections form A Practical HDPE Grade Chart: Density, MFI, Process and Typical Part and identify the processing boundaries that cannot be inferred from the numerical entries alone.

Density is adjusted by copolymerizing ethylene with 1-butene, 1-hexene or 1-octene. Short-chain branching reduces crystallinity; longer comonomers provide more effective tie-molecule formation, which improves ESCR at equivalent density. Hexene-copolymer HDPE pipe grades frequently show slower crack propagation than butene-copolymer grades at the same 0.955 g/cm³ density. For high-molecular-weight grades, the 2.16 kg load may produce such a low MFI that measurement precision is poor. The high-load melt index condition 190°C/21.6 kg is reported as HLMI; the ratio of HLMI to MFI, often called flow ratio, provides a rough indication of shear sensitivity. A flow ratio above 100 indicates significant shear thinning, which is useful for blow moulding and pipe extrusion but not for injection moulding where short residence time and high shear are already present.

What separates a high-flow injection grade from a high-toughness pipe grade?

Density controls short-term stiffness and barrier performance. A grade at 0.940–0.950 g/cm³ has lower crystallinity and typical tensile yield from 18–24 MPa per ASTM D638 or ISO 527-2, while a density of 0.955–0.965 g/cm³ raises tensile yield to 26–31 MPa and flexural modulus to 1,200–1,600 MPa per ISO 178 or ASTM D790. Melt flow index, measured at 190°C/2.16 kg, reflects viscosity and molecular weight. A pipe grade with MFI 0.2–0.5 g/10 min at 190°C/5 kg has an ESCR value typically above 1,000 h in 100% Igepal CO-630 according to ASTM D1693. A thin-wall injection grade with MFI 20–80 g/10 min has far lower ESCR, often below 50 h, and lower tensile impact strength. The trade-off is not linear: narrow molecular weight distribution can increase stiffness but reduce ESCR, while broad molar mass distribution improves ESCR at the same MFI.

For load-bearing parts, the 0.455 MPa heat deflection temperature of HDPE typically falls between 65°C and 90°C for densities 0.950–0.965 g/cm³, measured by ISO 75-2 or ASTM D648. This is lower than the melting point; continuous service under stress is therefore limited by creep and slow crack growth rather than short-term heat deflection. The grade chart should be used with the end-use stress state, not only with the processing condition.

In thin-wall injection moulding of HDPE closures and pails, the grade chart is filtered first by MFI at 190°C/2.16 kg. Grades in the 4–40 g/10 min range are processed on servo-hydraulic machines with screw diameters from 25 mm to 80 mm and L/D ratios of 20:1–25:1. Typical melt temperatures for injection moulding are 200–260°C, with mould temperatures between 10°C and 40°C to preserve cycle time. High-flow grades with MFI 20–50 g/10 min are used for caps and closures with fill length-to-thickness ratios above 150:1; lower-flow grades with MFI 4–12 g/10 min are used for crates, pails and pallets where impact strength matters more than flow length. On production-scale machines, batch-to-batch MFI variation of ±0.1 g/10 min can shift injection peak pressure by 5–10% in a cold-runner tool; in-mould pressure sensors are used to adjust switch-over position and holding pressure.

Processing at melt temperatures above 280°C accelerates oxidation; stabilizer consumption measured by ASTM D3895 oxygen induction time can drop below 20 min in some grades. At melt temperatures below 160°C, unmelts and weld-line weakness appear. The practical process window is therefore narrower than the nominal melting range when long residence times or thin walls are involved. For injection moulding, a capillary rheometer per ISO 11443 is more diagnostic than a single MFI point. Viscosity at shear rates of 1,000–10,000 s⁻¹ correlates with thin-wall fill better than MFI. A grade with MFI 8 g/10 min and narrow molar mass distribution can have lower high-shear viscosity than a grade with MFI 12 g/10 min and broad distribution, depending on long-chain branching.

Pipe extrusion and injection moulding sit at opposite ends of the melt flow axis

Pressure pipe grades, such as PE100, are specified under ISO 9080 and ISO 4427. These grades have a required minimum long-term strength of 10 MPa at 20°C for 50 years. Their melt flow index is typically 0.2–0.5 g/10 min at 190°C/5 kg, not at 2.16 kg. Converting between the two conditions without the grade-specific rheology is unreliable; a pipe grade that appears to have almost no flow at 2.16 kg may still be extrudable on a grooved-barrel single-screw extruder with an L/D of 30:1–36:1. Processing temperatures are usually 180–220°C at the die head, with vacuum calibration tanks holding water at 15–25°C. The low MFI is required for slow crack growth resistance; ESCR values above 1,000 h in ASTM D1693 are common for PE100.

Pipe-grade lot qualification includes hydrostatic testing under ISO 1167 and ASTM D1598, where failures are classified as ductile or brittle. The transition from ductile to brittle slow crack growth is sensitive to density, molar mass and extrusion-induced residual stress; a drop in MFI from 0.4 g/10 min to 0.2 g/10 min at 5 kg may improve slow crack growth resistance but increases extrusion backpressure and can reduce line speed on a fixed grooved-barrel extruder.

Injection moulding, by contrast, uses MFI values that are one to two orders of magnitude higher because the melt must fill a closed cavity under high shear. A grade with MFI 0.3 g/10 min can be injection moulded only on large-diameter screws with low shear and high energy input; screw recovery time often exceeds the cooling time, making the part cost uncompetitive. The process conflict is that raising MFI to improve mouldability reduces ESCR and impact performance. For applications such as frozen-food containers, the compromise is a grade with MFI 6–12 g/10 min; for caps, MFI 20–50 g/10 min is accepted because the part is thin and the end-use load is lower.

When HDPE film is downgauged below 10 µm, a grade with density 0.948–0.957 g/cm³ and MFI 0.5–1.2 g/10 min at 190°C/2.16 kg is typically selected. Blown film lines use spiral mandrel dies with die gaps of 1.2–2.0 mm and blow-up ratios between 2.5:1 and 4:1. The density within this window controls stiffness, tear strength and moisture vapour transmission. Film tensile properties are measured by ASTM D882 or ISO 527-3; a typical high-density film grade may show machine-direction tensile yield between 20 MPa and 30 MPa and transverse-direction elongation above 400%. For heavy-duty sacks, a lower MFI of 0.3–0.7 g/10 min is used to maintain impact resistance; for thin liners, a higher MFI of 1.0–1.5 g/10 min is used to reduce extrusion backpressure. Surface moisture from outdoor storage at relative humidity above 80% can produce splay and should be removed with a dry-air hopper at 60–80°C for 2–4 h if visible condensation is present.

Film converters also evaluate blocking force and coefficient of friction. Slip and antiblock additives are migration-dependent, and their concentration is restricted in food-contact applications. A grade chart rarely lists additive content, so the converter must request a migration compliance letter under EU Regulation (EU) No 10/2011 or FDA 21 CFR 177.1520 before changing slip packages in a qualified structure.

When particle shape overrides melt flow rate in rotomoulding grade selection

Rotational moulding grades use density 0.940–0.946 g/cm³ and MFI 2–6 g/10 min at 190°C/2.16 kg. The resin is pulverized to a mesh size between 35 mesh and 60 mesh; particle size distribution influences wall thickness uniformity and bubble formation in the oven. Oven temperatures are typically 260–300°C with air speeds of 2–5 m/s. The lower density of rotomoulding grades reduces warpage and improves impact toughness after slow cooling. Tensile yield is usually 18–22 MPa per ISO 527-2. Notched impact data at -20°C reported per ISO 180 varies significantly with particle size and cooling rate; published data for this specific configuration is limited, and moulding trials are required. Typical parts include water tanks, kayaks, fuel tanks and playground enclosures. The grade chart entry for rotomoulding is misleading if the powder particle size distribution is not controlled; two resins with identical MFI can produce different wall thickness on vertical mould surfaces if the mesh fraction differs.

A Practical HDPE Grade Chart: Density, MFI, Process and Typical Part

The table below is a consolidation of commercial grade categories; the numerical ranges are representative of datasheet values and are not a specification. The melt flow index values are at 190°C/2.16 kg unless noted. Density is measured by ISO 1183-1 or ASTM D1505.

Melt flow index values are reported at 190°C/2.16 kg unless the row states 5 kg. The higher load is used for pipe grades to obtain a measurable flow value outside the low-resolution end of the standard condition.

The entries are not interchangeable. A blow-moulding grade at 0.957 g/cm³ and MFI 0.5 g/10 min may not injection-mould a thin-wall cap because molar mass distribution and viscosity at high shear are different. The grade chart filters candidate resins, but process validation with actual tooling remains necessary.

Verification standards applied to HDPE grade qualification

Regulatory and mechanical qualification for HDPE depends on the end-use. A grade chart should be paired with the following standards matrix.

Processing limits for HDPE are not defined solely by the grade chart. Continuous service at 60°C or above under constant stress requires creep modulus and slow crack growth data rather than short-term tensile values. HDPE is not recommended for continuous exposure to strong oxidizing acids, aromatic hydrocarbons, or chlorinated solvents at elevated temperature; stress cracking resistance should be verified with the specific fluid under ASTM D1693 or ISO 22088-2. Pre-drying is not routinely required, but resin stored at relative humidity above 60% should be checked for surface moisture; if condensation is present, a dry-air hopper at 60–80°C for 2–4 h prevents splay and weld-line porosity.