Magnesium AZ91D vs. Plastic Polycarbonate

Compare Magnesium AZ91D vs. Plastic Polycarbonate strength, stiffness, weight and thermal properties.

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Plastic PolycarbonateBulk polymer (molded or machined)

Mechanical
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Yield strength	150MPasourcetypical, 0.2% tensile yield, ambient temperature	65MPasourcetypical, tensile yield
Ultimate tensile strength	230MPasourcetypical, ambient temperature	65MPasourcetypical, stress at break
Flexural strength	not sourced	97MPasourcetypical, flexural strength
Elongation at break	3%sourcetypical, ambient temperature	125%sourcetypical, strain at break
Young's modulus (stiffness)	45GPasourcetypical18.8x stiffer	2.4GPasourcetypical
Density	1,810kg/m³sourcetypical	1,200kg/m³sourcetypical1.51x lighter
Strength to weight	82.9kN·m/kg1.53x higher	54.2kN·m/kg
Stiffness to weight	24.9MN·m/kg12.4x higher	2MN·m/kg
Poisson's ratio	0.35sourcebase material: AZ91C casting (MIL-HDBK-5J handbook physical constant)	0.38sourcetypical, polycarbonate family (Covestro Part and Mold Design guide, Table 3-1)
Shear modulus	16.7GPa19.2x stiffer in shear	0.87GPa
Bulk modulus	50GPa	3.33GPa
Speed of sound	4,986m/s	1,414m/s
Thermal
Thermal conductivity	72W/m·Ksourcetypical	0.2W/m·Ksourcetypical, through-plane, 23 C
Thermal expansion	25µm/m·Ksourcetypical, temperature range not stated	65µm/m·Ksourcetypical, 23-55 C, parallel and normal
100 mm part over a 50 °C swing	125µm growth2.6x less	325µm growth
Specific heat	1,050J/kg·Ksourcetypical	1,172J/kg·Ksourcetypical (guide value), Bayer ASTM datasheet for Makrolon 2405, ASTM D 2766
Heats up and cools (diffusivity)	37.9mm²/s266x faster	0.142mm²/s
Thermal shock resistance	6,240W/m121x more resistant	51.7W/m
Melting point	470–595°Csourcemelting range	not sourced
Glass transition	not sourced	144°Csourcetypical, DSC 10 C/min
Max service temperature	not sourced	125°CsourceUL 746B RTI, tensile strength, 1.5 mm
Values for	ELEKTRON AZ91D magnesium die-casting alloy, as cast; Magnesium Elektron datasheet 475 (7/01), typical properties; Poisson from MIL-HDBK-5J for AZ91C castings.	Covestro Makrolon 2405 general-purpose PC (MVR 19), Covestro technical datasheet version 2026-09-28, typical values

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 Magnesium AZ91D stronger than Plastic Polycarbonate?

Magnesium AZ91D is stronger: its yield strength is 150 MPa against 65 MPa for Plastic Polycarbonate (2.31x).

Which is lighter, Magnesium AZ91D or Plastic Polycarbonate?

Plastic Polycarbonate is lighter: 1,200 kg/m³ against 1,810 kg/m³ for Magnesium AZ91D.

Which is stiffer, Magnesium AZ91D or Plastic Polycarbonate?

Magnesium AZ91D is stiffer: Young's modulus 45 GPa against 2.4 GPa for Plastic Polycarbonate, so the same part in Magnesium AZ91D deflects less under the same load.

Which is lighter for the same job, Magnesium AZ91D or Plastic Polycarbonate?

For the same stiffness or strength: Magnesium AZ91D is lighter for a stiff rod or tie (tension), stiff beam (bending), stiff panel or plate, strong rod or tie, strong beam.

Part that must be	Magnesium AZ91D	Plastic Polycarbonate
Stiff rod or tie (tension)	lighter	12.4x heavier
Stiff beam (bending)	lighter	2.87x heavier
Stiff panel or plate	lighter	1.76x heavier
Strong rod or tie	lighter	1.53x heavier
Strong beam	lighter	16% heavier
Strong panel or plate	about equal	about equal

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, Magnesium AZ91D or Plastic Polycarbonate?

Magnesium AZ91D conducts heat better: 72 W/m·K against 0.2 W/m·K for Plastic Polycarbonate.

Which expands less with temperature?

Magnesium AZ91D expands less: 25 µm/m·K against 65 µm/m·K for Plastic Polycarbonate.

Among 72 materials

Stiffness against density

Strength against density