Most materials will expand when heated and shrink when cooled. Consider expansion joints on bridges – even concrete with rebar will move with temperature changes. The coefficient of lineal thermal expansion is used to calculate how much expansion or contraction will occur when a plastic part is heated or cooled. The lower the figure, the more dimensionally stable that material will be.
The most common method to generate coefficient of linear thermal expansion data is ASTM E-831, Thermal Mechanical Analysis. Simply, a small sample is placed in a chamber where the material is cooled to typically -40F. A sensor rod touches the material and measures the growth as temperature increases to ~300F.
The data is actually generated as a curve (see below), but from that is reported a single, averaged value (see chart). Domestically, this value is reported as 10-5 inch/inch/⁰F. For most applications, this information is sufficient for designing proper fit for parts that will see temperature swings.
Note the effect of fiber reinforcement on stability (raises it as it does with other properties).
So, what does all that mean to the designer?
You can use these values to calculate growth. Say you have a 4” long wear pad made from cast nylon, and the part has been stored @ 73F but will see 125F in service. You take the coefficient of linear thermal expansion of .0005” x the 52F increase – the part will grow .0025” @ 125F. The same part in UHMW would grow .0052”.
Fit – you must therefore allow for growth in whatever structure the part functions – if it’s a bushing pressed into a housing, growth will be all on the ID and extra clearance must be added; on very long parts, the length will grow noticeably.
Attachment – it is best practice in wear pads to allow for growth by designing slightly oversize holes; with materials like UHMW, slots may be needed (like vinyl siding has), or the part may buckle. With thermoplastic bushings, allow for “close in” as frictional heat is experience.
Thermoplastics are isotropic – they grow equally in all axes. However, phenolics (while generally more stable) are anisotropic – they grow slightly differently in length (X axis) and width (Y axis), so orientation of the raw material is critical in determining growth of a finished part.
Confusing? Don’t worry, the team at WS HAMPSHIRE can help you! You’re in the right place!