Reading a Polycarbonate Datasheet: Density, Melt Flow, HDT and Impact Explained

A polycarbonate datasheet is a standardized technical snapshot, and its utility depends on reading each property alongside the exact method, specimen geometry, conditioning history, and load condition that produced it. For unfilled polycarbonate (PC), the four most frequently compared entries—density, melt flow, heat deflection temperature, and notched impact—become misleading when extracted without their test designations. The same commodity PC can appear superior or inferior based solely on a change from ISO 75-2:2013 method A to ASTM D648 at a different specimen thickness. The following sections treat these datasheet fields as process-control variables rather than absolute rankings.

Density, as reported under ISO 1183-1:2019 by immersion at 23°C, is the least variable entry for unfilled polycarbonate. General-purpose unfilled grades concentrate at 1.20 g/cm³, with commercial tolerances typically spanning 1.19–1.21 g/cm³. A shift of 0.01 g/cm³ changes part mass by approximately 0.83% relative to 1.20 g/cm³, which becomes significant in a 500 cm³ molded housing. Density is not bulk density, and it does not replace dimensional shrinkage measurements under ISO 294-4. Glass-reinforced polycarbonate datasheets move density upward by filler volume. A 30% glass-filled PC typically reports 1.43 g/cm³, and that difference belongs in cost-per-part calculations because the same cavity yields a heavier component at the same volume. Conditioning before density measurement is also material-specific; PC absorbs water under ISO 62:2008, and moisture uptake of 0.15% after 24 h water immersion can shift density only slightly but can confound dimensional and mechanical comparisons.

Why Does a 10 g/10 min Melt Flow Rate Only Establish Processing Boundaries?

In grade-to-grade substitution, melt flow rate is the first screening value, but it is a single-point capillary measurement rather than a full viscosity curve. Polycarbonate is tested under ISO 1133-1:2022 at 300°C with a 1.2 kg load, and the reported grams per 10 min reflect dried granulate. The standard die has a length of 8.000 mm and a diameter of 2.095 mm, so the test imposes a defined wall shear stress at the die. A typical general-purpose injection grade reports 9–15 g/10 min, while extrusion and sheet grades fall near 5–8 g/10 min and high-flow optical thin-wall grades range from 18 g/10 min to 30 g/10 min at the same condition. Lower melt flow rate generally correlates with higher average molecular weight, but two grades with identical 10 g/10 min MFR can differ in molecular weight distribution, melt strength, and shear-thinning behavior. On a production line, a 20–30 g/10 min grade fills a 0.8 mm wall connector mold more readily, while a 6 g/10 min extrusion grade resists draw-down in sheet forming but requires higher melt temperature and screw torque. Melt flow rate does not provide a complete rheological signature; capillary rheometry at multiple shear rates or injection-molding simulation inputs are needed when the tool has hot-runner pressure limits.

Moisture is a critical hidden variable in melt flow testing and processing. Polycarbonate hydrolyzes at processing temperatures, and suppliers specify pre-drying at 120°C for 4 h in a desiccant dryer with a dew point at or below −40°C to reduce moisture to 0.02% or lower before molding or melt flow measurement. Granulate exposed to ambient air above 60% relative humidity can exceed that moisture limit and produce inflated melt flow readings because chain scission lowers melt viscosity. In a 40:1 L/D co-rotating twin-screw compounding line, wet PC also builds hydrolytic degradation at barrel temperatures above 300°C, causing splay, bubble formation, and reduced impact. Therefore a datasheet MFR value should be read as a dried-granulate value, not a guarantee for material left unsealed at the hopper.

When 0.45 MPa and 1.8 MPa Heat Deflection Readings Diverge

Heat deflection temperature is not a melting point and not a continuous-use temperature. Under ISO 75-2:2013, the test tracks the temperature at which a rectangular specimen reaches a specified flexural deflection under a constant outer-fibre stress. The specimen dimensions are 80 mm × 10 mm × 4 mm, the heating rate is 2 K/min, and the deflection trigger is 0.34 mm. Two stress levels dominate PC datasheets: 0.45 MPa and 1.8 MPa. Unfilled PC typically reports 125–130°C at 1.8 MPa and 137–142°C at 0.45 MPa. The gap between the two values reflects how modulus declines as temperature rises; under lower stress, the same deflection occurs later. A datasheet that lists only HDT/B at 0.45 MPa can overstate load-bearing thermal capability compared with HDT/A at 1.8 MPa. For glass-filled PC, HDT/A increases to approximately 135–145°C because the glass network resists bending deflection, even though impact may fall sharply.

Molded-in stress, annealing, and moisture conditioning also shift HDT. A part molded with high packing pressure and frozen orientation can show lower deflection temperature than an annealed specimen because residual stress relaxes during the test. Annealing can raise HDT by relieving those stresses, but it also changes dimensions. That is why HDT alone cannot qualify PC for load-bearing hot-water or steam environments. Polycarbonate in contact with water at 85°C can lose toughness over time through hydrolysis, despite an HDT/A above 125°C. For thermal screening, HDT should be read with ISO 306 Vicat softening temperature and with creep data where the part is under continuous load.

Notched Izod, Charpy, and the Thickness-Sensitive Ductile–Brittle Transition

Notched impact data on a polycarbonate datasheet require more scrutiny than any other single entry. The common standards are ASTM D256 notched Izod, ISO 180-1A notched Izod, and ISO 179-1/1eA notched Charpy. Specimen thickness, notch radius, and conditioning temperature govern the result. Unfilled polycarbonate at 23°C typically reports 600–850 J/m by ASTM D256 at 3.18 mm thickness and 20–35 kJ/m² by ISO 179-1/1eA notched Charpy. Those values are not convertible directly because the Izod test clamps the specimen as a cantilever and the Charpy test uses a three-point flexural impact configuration. A 30% glass-filled PC may fall to 100–160 J/m by notched Izod even though its density and HDT improve; the glass fibers create stress concentrations that reduce ductility. The datasheet comparison therefore cannot rank impact and stiffness as if they move together.

Thickness changes the fracture mode. Polycarbonate exhibits a ductile-to-brittle transition that depends on notch sharpness, specimen thickness, and temperature. At 3.18 mm, unfilled PC often deforms and yields before crack propagation, producing high energy absorption. At 6.4 mm or with a sharper notch radius, fracture can occur in a more brittle mode, and the same grade reports a lower impact value. Low-temperature data are equally important. A grade that shows 25 kJ/m² at 23°C may fall below 15 kJ/m² at −30°C under ISO 179-1/1eA. If a supplier datasheet omits low-temperature impact, published data for the specific grade and notch configuration should be obtained before using PC in cold-impact service.

When comparing an unfilled 10 g/10 min MFR grade with a 30 g/10 min high-flow grade for a 1.2 mm wall-thickness enclosure, the evaluator must align density, MFR condition, HDT stress level, and notched impact thickness. A high-flow grade may fill the tool at lower pressure but report lower notched impact and lower HDT/A. The datasheet is not a performance guarantee under all processing conditions; it is a matrix of values generated under specified standardized states. Without that alignment, a ranking between two polycarbonate grades carries no technical authority.