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Plastic Polycarbonate vs. Steel AISI 4340

Compare Plastic Polycarbonate vs. Steel AISI 4340 strength, stiffness, weight and thermal properties.

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Plastic PolycarbonateBulk polymer (molded or machined)
Steel AISI 4340Wrought
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Mechanical
Yield strength65MPasourcetypical, tensile yield1,035MPasourcetypical, transverse
Ultimate tensile strength65MPasourcetypical, stress at break1,140MPasourcetypical, transverse
Flexural strength97MPasourcetypical, flexural strengthnot sourced
Elongation at break125%sourcetypical, strain at break18%sourcetypical, transverse
Young's modulus (stiffness)2.4GPasourcetypical200GPasourceMIL-HDBK-5J design value (AISI 4340, quenched and tempered)
Density1,200kg/m³sourcetypical6.53x lighter7,840kg/m³sourcetypical
Strength to weight54.2kN·m/kg132kN·m/kg2.44x higher
Stiffness to weight2MN·m/kg25.5MN·m/kg12.7x higher
Poisson's ratio0.38sourcetypical, polycarbonate family (Covestro Part and Mold Design guide, Table 3-1)0.32sourceMIL-HDBK-5J design value (AISI 4340, quenched and tempered)
Shear modulus0.87GPa75.7GPa87.1x stiffer in shear
Bulk modulus3.33GPa185GPa
Speed of sound1,414m/s5,049m/s
Thermal
Thermal conductivity0.2W/m·Ksourcetypical, through-plane, 23 C37.5W/m·Ksourcetypical
Thermal expansion65µm/m·Ksourcetypical, 23-55 C, parallel and normal11.3µm/m·Ksourcemean, -18 to 93 °C
100 mm part over a 50 °C swing325µm growth56.5µm growth5.75x less
Specific heat1,172J/kg·Ksourcetypical (guide value), Bayer ASTM datasheet for Makrolon 2405, ASTM D 2766448J/kg·Ksourcetypical
Heats up and cools (diffusivity)0.142mm²/s10.7mm²/s75x faster
Thermal shock resistance51.7W/m11,678W/m226x more resistant
Glass transition144°Csourcetypical, DSC 10 C/minnot sourced
Max service temperature125°CsourceUL 746B RTI, tensile strength, 1.5 mmnot sourced
Values forCovestro Makrolon 2405 general-purpose PC (MVR 19), Covestro technical datasheet version 2026-09-28, typical valuesCarpenter (Latrobe) Lescalloy 4340 VAC-ARC datasheet; mechanical row for 1100 °F temper; physical properties grade-level

Every value links to the document that states it. "Not sourced" means no citable source states it (or only a specification minimum); figures computed from it are left out too.

Questions

Is Plastic Polycarbonate stronger than Steel AISI 4340?

Steel AISI 4340 is stronger: its yield strength is 1,035 MPa against 65 MPa for Plastic Polycarbonate (15.9x).

Which is lighter, Plastic Polycarbonate or Steel AISI 4340?

Plastic Polycarbonate is lighter: 1,200 kg/m³ against 7,840 kg/m³ for Steel AISI 4340.

Which is stiffer, Plastic Polycarbonate or Steel AISI 4340?

Steel AISI 4340 is stiffer: Young's modulus 200 GPa against 2.4 GPa for Plastic Polycarbonate, so the same part in Steel AISI 4340 deflects less under the same load.

Which is lighter for the same job, Plastic Polycarbonate or Steel AISI 4340?

For the same stiffness or strength: Plastic Polycarbonate is lighter for a stiff panel or plate, strong beam, strong panel or plate; Steel AISI 4340 is lighter for a stiff rod or tie (tension), stiff beam (bending), strong rod or tie.

Part that must bePlastic PolycarbonateSteel AISI 4340
Stiff rod or tie (tension)12.7x heavierlighter
Stiff beam (bending)40% heavierlighter
Stiff panel or platelighter50% heavier
Strong rod or tie2.44x heavierlighter
Strong beamlighter3% heavier
Strong panel or platelighter1.64x heavier

For the same stiffness or strength, mass scales with density over stiffness (or strength) raised to a power set by how the part is loaded: 1 for a rod in tension, 1/2 for a beam in bending, 1/3 for a panel (the standard material-selection indices).

Which conducts heat better, Plastic Polycarbonate or Steel AISI 4340?

Steel AISI 4340 conducts heat better: 37.5 W/m·K against 0.2 W/m·K for Plastic Polycarbonate.

Which expands less with temperature?

Steel AISI 4340 expands less: 11.3 µm/m·K against 65 µm/m·K for Plastic Polycarbonate.

Among 59 materials

1k10k0.1110100Density (kg/m³)Young's modulus (GPa)Plastic PLAPlastic PETGPlastic ABSPlastic TPUPlastic NylonAluminumSteelIronTitaniumSteel AISI 1018Stainless 304Stainless 316Aluminum 6061-T6Aluminum 7075-T6Brass C360Copper C110GlassSteel AISI 1045Steel A992Stainless 303Stainless 17-4PHAluminum 2024-T351Aluminum 5052-H32Aluminum 7050-T7451Titanium Ti-6Al-4VBrass C260Nickel Inconel 718Magnesium AZ31B-H24Zinc Zamak 3Plastic ABS (bulk/injection)Plastic POM/AcetalPlastic Nylon PA6Plastic Nylon PA66Plastic PEEKPlastic UHMWPEPlastic HDPEPlastic PolypropylenePlastic PMMAFDM ASAFDM PolycarbonateFDM PAHT-CFFDM HIPSSLA Resin Tough 2000SLS PA12 NylonDMLS AlSi10Mg AluminumDMLS Ti-6Al-4V ELI TitaniumDMLS 316L Stainless SteelDMLS 17-4PH Stainless SteelPlastic PolycarbonateSteel AISI 4340
Stiffness against density
1k10k101001kDensity (kg/m³)Yield strength (MPa)Plastic PLAPlastic PETGPlastic ABSPlastic TPUPlastic NylonRubberAluminumSteelIronTitaniumSteel AISI 1018Stainless 304Stainless 316Aluminum 6061-T6Aluminum 7075-T6Brass C360Copper C110GlassSteel AISI 1045Steel A992Stainless 303Stainless 17-4PHAluminum 2024-T351Aluminum 5052-H32Aluminum 7050-T7451Titanium Ti-6Al-4VBrass C260Nickel Inconel 718Magnesium AZ31B-H24Zinc Zamak 3Plastic ABS (bulk/injection)Plastic POM/AcetalPlastic Nylon PA6Plastic Nylon PA66Plastic PEEKPlastic PTFEPlastic UHMWPEPlastic HDPEPlastic PolypropylenePlastic PMMAFDM ASAFDM PolycarbonateFDM PAHT-CFFDM HIPSSLA Resin Tough 2000SLA Resin Elastic 50ASLS PA12 NylonDMLS AlSi10Mg AluminumDMLS Ti-6Al-4V ELI TitaniumDMLS 316L Stainless SteelDMLS 17-4PH Stainless SteelBrickPlastic PolycarbonateSteel AISI 4340
Strength against density

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