Thermal Conductivity vs Thermal Expansion in Part Design

Thermal conductivity and thermal expansion are the two properties that decide how a part behaves when it heats up — and they are routinely confused, even though they measure completely different things. Thermal conductivity (k, W/m·K) is how fast heat moves through a material; thermal expansion (coefficient of thermal expansion, CTE, in µm/m·°C) is how much a material's dimensions change with temperature. A material can conduct heat superbly and expand a lot (aluminum), or insulate and stay dimensionally stable (many ceramics). Both matter in design — in different places.

The numbers that frame the decision: aluminum 6061 has a thermal conductivity of about 167 W/m·K and a CTE of 23.6 µm/m·°C; steel (mild) conducts at about 50 W/m·K with a CTE of 12 µm/m·°C; copper conducts at 400 W/m·K with a CTE of 16.5 µm/m·°C; stainless 304 conducts poorly (16 W/m·K) and expands at 17 µm/m·°C; engineering plastics like ABS sit at 0.17-0.25 W/m·K and expand at 60-110 µm/m·°C — three to five times more than metals. That last comparison is why plastic-metal assemblies need expansion management.

Where Conductivity Matters in Design

Conductivity decides heat flow: heat sinks, LED mounts, motor housings and electronic enclosures need high conductivity (aluminum, copper) to move heat away from the source. When the thermal path matters, the material choice and the contact design are both critical — a heat sink in aluminum with a machined surface beats one in stainless by 10× in heat transfer, and the interface (thermal paste, gap pad) often dominates the resistance. In molding, conductivity matters inside the tool: mold steel conducts heat away from the molten plastic, which sets the cycle time — which is why mold cooling channel design is a core DFM activity, and why aluminum rapid tooling cycles faster than steel (but wears faster).

When a part must insulate — handles, housings around hot components, electrical isolation — low conductivity is the feature: plastics, ceramics and stainless are the choices. The design question is always the same: where is the heat going, and what is the thermal path?

Where Expansion Matters: Tolerances and Assemblies

Expansion decides fits at temperature. A steel shaft (CTE 12) in an aluminum housing (CTE 23.6) that fits at 20°C will change its fit by about 12 µm per 100 mm per 50°C of temperature rise — a clearance fit can become an interference fit, and vice versa. Plastic over metal is the classic trap: a plastic gear on a steel shaft at 80°C expands several times more than the shaft, so the bore loosens or the gear stresses itself. The design fixes: choose the material pair with matched CTE, design the fit at the operating temperature (not the assembly temperature), or use compliant elements (O-rings, springs) that absorb the differential.

Expansion also sets tolerance strategy: a molded plastic part measured at 20°C will be 0.1-0.5% different if measured at 40°C or at 5°C, depending on the material — which is why dimensional inspection happens at controlled temperature, and why plastic part tolerances are rarely better than ±0.05 mm without environmental control. Glass-filled grades reduce expansion (30% glass in nylon cuts the CTE roughly in half) and improve dimensional stability — the reason structural plastic parts are so often specified filled. Our strength properties article covers the mechanical side of the same material selection.

If your assembly is failing at temperature — seized fits, loose joints, thermal cracking — send us the materials, the temperature range and the assembly drawing. We will model the thermal behavior and recommend the fix. Contact us to start.

In molded parts, expansion interacts with the molding process itself: the part shrinks as it cools from melt temperature, and the shrinkage is not isotropic — flow direction, glass-fiber orientation and wall thickness all affect how much and in which direction the part contracts. That is why a molded plastic part's dimensional tolerance is rarely better than ±0.1% of the dimension without careful process control, and why glass-filled grades are specified when stability matters. When your assembly mates a molded plastic part with a machined metal part, the expansion mismatch and the molding shrinkage stack up — the fit should be designed at the operating temperature, with the molding shrinkage allowance from the supplier's process data, not from the nominal data sheet.

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