How to Read an HDPE Datasheet: The Six Lines Buyers Actually Use

High-density polyethylene property sheets are read differently by a resin buyer, a process engineer, and a quality auditor. The six lines most often pulled from a supplier certificate of analysis are melt flow rate, density, tensile stress at yield, nominal tensile strain at break, flexural modulus, and notched Izod impact strength. These values are generated on standardized laboratory specimens, not on actual part geometry, and each line carries a method-dependent tolerance that is larger than typical batch-to-batch variation. The table below fixes the common methods and the processing signal associated with each line.

Can the Melt Flow Rate Line Screen a Pellet Lot?

Melt flow rate is measured at 190 °C with a 2.16 kg load according to ISO 1133-1:2022 or ASTM D1238-23. The standard die has a diameter of 2.095 mm and a length of 8.000 mm. For HDPE, the low-load MFR ranges from 0.2 g/10 min for large-part blow molding grades to 20 g/10 min for thin-wall injection molding grades. The single-point value does not indicate high-shear behavior. A high-load value at 21.6 kg should be requested and converted to a flow rate ratio by dividing the high-load MFR by the low-load MFR. A ratio below 18 generally indicates a narrow molecular weight distribution; a ratio above 25 indicates strong shear thinning.

On a 40 L/D grooved-feed extruder running a 25 mm annular die at 250 kg/h, an upward drift of 0.05 g/10 min in the 2.16 kg value can reduce head pressure by 4–6 bar and increase wall-thickness variation by 0.3 mm. If the resin contains recycled content, melt volume-flow rate is more reliable than mass-flow rate because density assumptions are removed. The melt volume-flow rate is reported in cm³/10 min under the same temperature and load conditions.

Density, Crystallinity, and the Hidden Shrinkage Window

Density is measured by ISO 1183-1:2019 or ASTM D792-20. The typical HDPE range is 0.945–0.965 g/cm³. A density change of 0.003 g/cm³ is small in absolute terms, but the two-phase crystallinity calculation shows why it matters. Using a crystalline phase density of 1.000 g/cm³ and an amorphous phase density of 0.855 g/cm³, the weight fraction crystallinity is obtained from the measured density. Moving from 0.949 g/cm³ to 0.958 g/cm³ raises crystallinity by approximately 6 percentage points. This shift increases flexural modulus by 150–250 MPa, lowers slow crack growth resistance, and increases post-mold shrinkage.

On a 24-cavity closure tool with a 38 mm neck finish, an increase of 0.005 g/cm³ can raise shrinkage by 0.2–0.5%, producing thread engagement values outside the torque-removal specification. Recycled-content HDPE should be tested on homogenized pellets because density alone cannot detect a low-molecular-weight tail that reduces environmental stress crack resistance. A gradient column or autopycnometer measurement is preferred over a single floating specimen when the material contains carbon black or mineral filler.

At the specimen preparation stage, a Type 1A tensile bar is injection molded according to ISO 20753 and conditioned for 40 h at 23 °C and 50% relative humidity. Tensile stress at yield per ISO 527-2:2012 is typically 22–30 MPa for HDPE at a test speed of 50 mm/min. If the crosshead speed is reduced to 1 mm/min, the measured yield stress falls by 2–5 MPa. A datasheet without the test speed cannot be used for competitive comparison. Nominal tensile strain at break commonly exceeds 600%, but the gauge length, specimen type, and strain rate move the result. A 2 mm thick annealed plaque may report 250% while a 4 mm injection-molded bar from the same lot reaches 800%. Flexural modulus per ISO 178:2019 is run at 2 mm/min and typically ranges from 800–1300 MPa. That modulus is a short-term, 23 °C value; it should not be substituted for creep modulus under ISO 899-1:2017.

When Notched Izod Hides a Slow Crack Growth Liability

Notched Izod impact strength is reported per ISO 180/A or ASTM D256 at 23 °C. HDPE values usually fall between 4 kJ/m² and 15 kJ/m². The test uses a 10 mm wide specimen with a 0.25 mm machined notch radius and a pendulum of 1–4 J capacity. Because the deformation rate is high, the room-temperature line often reports ductile hinges or partial breaks. At -30 °C, the same resin can drop below 3 kJ/m².

A high Izod value does not validate a pipe resin. For pressure piping, the relevant durability test is the full-notch creep test under 4 MPa at 80 °C in a 2% solution of nonylphenoxy poly(ethyleneoxy) ethanol according to ISO 16770:2004. A resin with 25 kJ/m² Izod at 23 °C can still produce a slow crack growth time below 100 h if the comonomer distribution is not uniform. For blow-molded containers, a notched Izod line is less informative than instrumented drop testing. Container drop impact is conducted according to ASTM D2463-15 on actual bottles, and failure height is a better predictor of distribution damage.

The Thermal Lines: HDT, Vicat, and the Missing Continuous-Use Limit

Heat deflection temperature per ISO 75-2:2013 method A at 1.80 MPa typically ranges from 45–55 °C for HDPE. Vicat softening temperature per ISO 306:2022 method A50 typically ranges from 120–130 °C. Vicat A50 uses a 1 mm² indenter and a 10 N load; method B50 uses 50 N. Neither value is a continuous-use temperature. HDT records the temperature at which a specimen deflects 0.34 mm under a defined flexural stress; it is a modulus threshold, not a melting or failure point.

In a 72-cavity hot runner mold fed at 200 °C nozzle set point, gate freeze time and mold temperature control the injection process more than HDT. A converter should not compare HDT values across polymer families; polypropylene appears stronger, yet HDPE can serve in pressurized water duty at 80 °C where long-term hydrostatic strength is verified by EN ISO 9080. The datasheet thermal lines provide a rough check for sorting incoming material but do not replace part-specific thermal testing.

Where HDPE is used in food-contact or potable water service, the datasheet should be supplemented by a compliance matrix. The standards listed below are frequently summarized on a certificate of compliance but require separate verification if the resin is a compound or contains recycled content.

On a blow molding line producing 5 L jerrycans at 120 kg/h, the six lines are not read as separate accept/reject gates. A shift from 0.35 to 0.28 g/10 min in MFR combined with a density rise from 0.953 to 0.956 g/cm³ and a flexural modulus increase of 100 MPa indicates a higher-viscosity, higher-crystallinity lot. The extruder head pressure rises, die swell increases, and the operator may raise barrel zone temperatures from 170 °C to 180 °C. That compensation can elevate organoleptic migration and reduce drop-impact height. Reading the six lines together identifies the compensation risk before production begins.