Plastic Thermal Expansion Calculator and CTE Chart
Plastics grow and shrink with temperature far more than most designers expect, and on a close-tolerance part that movement can be bigger than the tolerance. Pick a material, enter the part length and two temperatures, and the calculator gives the change in length. The chart below lists the figures for the 12 plastics we machine and fabricate.
Thermal expansion calculator
Change in length = coefficient of linear thermal expansion × length × change in temperature. A negative answer means the part shrinks.
Change in length (grows)
+0.0354 in
Length in service: 10.0354 in
Uses the typical linear expansion figure from the chart below, which is an average over the datasheet’s test range. An estimate for design, not a guarantee for a particular part.
Coefficient of thermal expansion of plastics
Typical values, lowest first, not a specification. Each figure is the one on that material’s own page, and its datasheet and test method are linked in the row. A CTE is an average over the test’s temperature range and varies with grade, filler and direction, so confirm against the datasheet for the grade you are designing with.
* Laminate figure measured in the plane of the sheet. Laminates expand several times more through their thickness, and many datasheets do not list that direction; ask the laminate maker if it matters to your part.
Why thermal expansion matters on a machined plastic part
Among the unfilled plastics on this chart, UHMW expands about 4 times as much as PEEK. For a close tolerance the numbers get small quickly. A 10″ Delrin part grows 0.001″, one side of a ±0.001″ tolerance, with a temperature change of about 1.5 °F. A 24″ UHMW wear strip measured at 68 °F and run at 100 °F grows by about 0.084″.
So a tight dimension on a plastic part only means something at a stated temperature. ISO 1 sets 20 °C (68 °F) as the standard reference temperature for dimensional measurement; if the part will run hot or cold, say so on the drawing and design the fits and clearances for the service temperature, not the inspection room.
Temperature is not the only thing that moves a plastic part. Nylon also grows as it absorbs moisture, and PTFE has crystalline transitions near room temperature, at about 66 °F (19 °C) and 86 °F (30 °C), where its dimensions change more sharply than the average figure suggests. The material comparison chart and the plastic density chart cover the other properties.
Need a close-tolerance part machined from plastic?
Send us your drawing and the temperature the part will run at, and we will quote it.