GlossaryKind of term: Manufacturing termThermal expansion
What is thermal expansion?
Thermal expansion is the change in size of a material with temperature; plastics expand far more than steel, and drawing dimensions apply at 20 °C.
Thermal expansion is the change in size of a material when its temperature changes. For a length, approximately ΔL = α × L × ΔT, with α the linear expansion coefficient in 10⁻⁶ per kelvin. As a guide, steel is around 11 to 13, austenitic stainless around 16, aluminium around 23, and many engineering plastics between 50 and 200. For example, 100 mm of aluminium grows by about 0.023 mm when warmed by 10 K.
What does thermal expansion depend on?
Heat reaches the workpiece from cutting, from a machine warming up, from a shop whose temperature changes through the day and from the hand that holds the part. How fast a part settles depends on mass, material and surroundings: a thin-walled aluminium part in minutes, a large steel block in hours. The machine grows as well: a spindle and axes warming up shift the tool relative to the workpiece, separately from the part's own expansion. The gauge expands too. Measure steel with a steel gauge at the same temperature and the expansions largely cancel; measure aluminium or plastic with a steel gauge and they do not. In plastics, α also varies with temperature.
What does the standard say about thermal expansion?
ISO 1 fixes 20 °C as the reference temperature for geometrical product specification: the dimensions and tolerances on the drawing apply at 20 °C unless the drawing states another temperature. When you measure at another temperature, you correct with the expansion coefficients of part and gauge, or include the temperature deviation in the measurement uncertainty. A correction is only as good as the known α and the measured part temperature. For plastics that absorb moisture, the moisture condition belongs with the dimension as well.
How does thermal expansion work in practice?
On tight dimensions expansion costs time or facilities: letting parts cool before measuring, or keeping shop and inspection room at temperature. On long dimensions in aluminium or plastic, the temperature difference between shop floor and inspection room alone can take up a noticeable share of the tolerance. Without an agreement on measuring temperature and soak time, good parts can be rejected and bad ones passed.
What is thermal expansion often confused with?
Thermal expansion is not moisture absorption: temperature changes a dimension within minutes to hours, moisture over days to months. Nor is it distortion from residual stress: expansion is largely reversible, while distortion from released stress stays when the part is back at temperature.
Frequently asked questions
At what temperature does a drawing dimension apply?
At 20 °C, the reference temperature of ISO 1, unless the drawing states another. If you measure at a different temperature, correct with the expansion coefficients of part and gauge, or wait until the part has reached temperature.
Why is a dimension right warm off the machine and wrong once cold?
Cutting heat expanded the part, and it shrinks as it cools. So measure once the part has reached the temperature of the inspection room, or correct for the difference.
Does plastic really expand so much more than steel?
Yes, often many times more, depending on grade and filler. On long plastic parts an ordinary temperature swing in the shop can take up a noticeable share of the tolerance.