Magnesium AZ91D vs. Plastic PMMA

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

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Plastic PMMABulk polymer (molded or machined)

Mechanical
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Yield strength	150MPasourcetypical, 0.2% tensile yield, ambient temperature	not given (tensile used)
Ultimate tensile strength	230MPasourcetypical, ambient temperature	80MPasourcetypical, 23 °C
Compressive strength	not sourced	110MPasourcecompressive yield stress, typical
Flexural strength	not sourced	115MPasourcetypical
Elongation at break	3%sourcetypical, ambient temperature	5.5%sourcetypical
Young's modulus (stiffness)	45GPasourcetypical	3.3GPasourcetypical, short-term
Density	1,810kg/m³sourcetypical	1,190kg/m³sourcetypical1.52x lighter
Strength to weight	82.9kN·m/kg23% higher	67.2kN·m/kg
Stiffness to weight	24.9MN·m/kg8.97x higher	2.77MN·m/kg
Poisson's ratio	0.35sourcebase material: AZ91C casting (MIL-HDBK-5J handbook physical constant)	0.37sourcetypical
Shear modulus	16.7GPa13.8x stiffer in shear	1.2GPa
Bulk modulus	50GPa	4.23GPa
Speed of sound	4,986m/s	1,665m/s
Thermal
Thermal conductivity	72W/m·Ksourcetypical379x more heat through a fin	0.19W/m·Ksourcetypical
Thermal expansion	25µm/m·Ksourcetypical, temperature range not stated	70µm/m·Ksource0-50 °C
100 mm part over a 50 °C swing	125µm growth2.8x less	350µm growth
Specific heat	1,050J/kg·Ksourcetypical	1,470J/kg·Ksourcetypical
Heats up and cools (diffusivity)	37.9mm²/s349x faster	0.109mm²/s
Thermal shock resistance	6,240W/m151x more resistant	41.5W/m
Melting point	470–595°Csourcemelting range	not sourced
Max service temperature	not sourced	80°Csourcemax. permanent service temperature
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.	Röhm PLEXIGLAS GS 0F00 cast acrylic sheet, typical values at 23 °C / 50 % RH (Röhm Technical Information Ref. No. 211-1)

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 PMMA?

Magnesium AZ91D is stronger: its yield strength is 150 MPa against tensile strength 80 MPa for Plastic PMMA (1.88x).

Which is lighter, Magnesium AZ91D or Plastic PMMA?

Plastic PMMA is lighter: 1,190 kg/m³ against 1,810 kg/m³ for Magnesium AZ91D.

Which is stiffer, Magnesium AZ91D or Plastic PMMA?

Magnesium AZ91D is stiffer: Young's modulus 45 GPa against 3.3 GPa for Plastic PMMA, so the same part in Magnesium AZ91D deflects less under the same load.

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

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; Plastic PMMA is lighter for a strong panel or plate.

Part that must be	Magnesium AZ91D	Plastic PMMA
Stiff rod or tie (tension)	lighter	8.97x heavier
Stiff beam (bending)	lighter	2.43x heavier
Stiff panel or plate	lighter	1.57x heavier
Strong rod or tie	lighter	23% heavier
Strong beam	about equal	about equal
Strong panel or plate	11% heavier	lighter

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 PMMA?

Magnesium AZ91D conducts heat better: 72 W/m·K against 0.19 W/m·K for Plastic PMMA.

Which expands less with temperature?

Magnesium AZ91D expands less: 25 µm/m·K against 70 µm/m·K for Plastic PMMA.

Among 72 materials

Stiffness against density

Strength against density