KEVOS® MAKEOS · Materials
Thermal Stress and Movement in Plastic–Metal Assemblies
What stress builds up in a plastic part restrained by metal when the temperature changes, and how much do they move relative to each other?
Thermal stress in a plastics part fully restrained by a metal part, σ = (α_m − α_p) E_p ΔT, and the relative movement between unrestrained plastic and metal parts, from their expansion coefficients, the plastic’s modulus at temperature (or an apparent creep modulus from a creep curve) and the temperature change.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Expansion coefficient of the metal | µm/m·K | Linear expansion coefficient of the metal part. |
| Expansion coefficient of the plastic | µm/m·K | Linear expansion coefficient of the plastic part. |
| Plastic modulus | choice | Use the modulus at temperature, or derive an apparent modulus from a creep curve for long-held loads. |
| Tensile modulus of the plastic | MPa | Tensile modulus of the plastic at the temperature involved. |
| Stress on the creep curve | MPa | Stress level read on the creep curve. |
| Strain on the creep curve at the time of interest | % | Strain at that stress after the time of interest. |
| Temperature change | K | Temperature change from assembly (negative for cooling). |
| Length between fixings | mm | Length between fixing points (or a hole diameter). |
| Allowable stress for the plastic | MPa | Allowable stress for the plastic in tension or compression. |
Method and formulas
- Fully restrained plastic: σ = (α_m − α_p) E_p ΔT (handbook formula 29).
- Unrestrained: relative movement ΔL_rel = (α_p − α_m) L ΔT (formula 30).
- Apparent creep modulus E_app = σ ÷ ε_c from the creep curve.
- Example: a plastic part restrained by steel over a 40 K rise carries about 5.464 MPa; unrestrained it moves 0.2732 mm relative to the metal over 100 mm.
Limitations
- Full restraint is an upper bound; fixings that slip or flex relieve stress.
- Plastics relax under sustained stress; the creep modulus gives the longer-term value.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a plastic housing screwed or bonded to a metal frame
- Size slots or clearances for a plastic cover on metal
- Estimate long-term stress using creep modulus
Who it is for
- Design engineers
- Product engineers
- Materials engineers
What you get
- Thermal stress in the restrained plastic
- Relative movement if unrestrained
- Apparent modulus used
Interpreting the result
- Cooling usually puts the plastic in tension, which is the cracking risk; allow slotted holes or flexible fixings.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 578–580. Topic reference; no source tables or recommended settings are reproduced.
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