In this article, we’ll be looking at one of the key physical properties of plastic materials: thermal expansion.
● What is Thermal Expansion?
- It refers to the change in a material’s size due to temperature variation.
- The extent of this change varies depending on the material.
● What is the Coefficient of Thermal Expansion?
- This is the rate of change in the size of a material per 1°C of temperature change, expressed as a ratio.
Put simply, even in the same thermal environment, different materials will expand by different amounts. That’s the idea behind the coefficient of thermal expansion.
You may have experienced thermal expansion yourself:
A bicycle tire left out in the summer heat that suddenly bursts, or a sealed bag of bread exploding in the microwave—these are both caused by air expanding due to heat.
Here’s another example: the familiar “clack-clack” sound of trains running over the rails is partly caused by expansion joints—gaps left between rail segments. Steel expands in the heat and contracts in the cold, and those gaps are designed to accommodate this thermal movement and prevent warping.
● Reference Values
Below are some example coefficients of linear thermal expansion for metals (unit: ×10⁻⁶/°C):
- S45C: 11.7
- SS400: 10.2
- SUS304: 17.3
- Aluminium: 23.0
(Note: A detailed explanation of linear expansion is omitted for brevity. Feel free to contact us for more information.)
Now here are some plastic materials frequently used in our products, listed in order of higher thermal expansion coefficients. A larger value indicates greater dimensional change with temperature.
Plastics – Coefficients of Linear Thermal Expansion (unit: ×10⁻⁵/°C)
- UHMWPE: 19 – 10
- PTFE: 13.7 – 12.2
- PP: 10 – 5.8
- PPEK: 5.00
- PPS: 5.00
- Phenolic resin: 4.5 – 3.0
- Carbon: 0.35
(*Values can vary slightly depending on the temperature range. Please refer to each material supplier for details.)
Did you notice something interesting here?
- Metals: ×10⁻⁶/°C
- Plastics: ×10⁻⁵/°C
In decimal form:
- Metals = 0.000001
- Plastics = 0.00001
This means that plastics expand roughly 10 times more than metals due to heat. (Carbon is an exception.)
For example, if you’d like to use PTFE but are concerned about dimensional changes, you can opt for filled PTFE, which reduces expansion and increases strength. At Kashima Bearings, we offer PTFE ball bearings—products not commonly handled by other manufacturers.
Plastics Expand More Than Metals
As we’ve seen, thermal expansion varies greatly by material. And even for the same material, reported values may differ between manufacturers, so always double-check units and data.
Have you ever experienced trouble due to dimensional changes? For example, parts not fitting properly or not rotating as intended?
If you’re using a combination of plastic and metal in your current or future equipment designs, thermal expansion is a critical factor.
What Happens When a Plastic Bearing Expands?
When heated, a plastic bearing naturally tries to expand outward. But if it’s housed inside a metal casing, which expands less, the plastic can’t expand externally. As a result, the expansion is redirected inward, reducing the inner diameter.
If this reduction isn’t accounted for in the design, the clearance between the shaft and the bearing can disappear, and the shaft may stop rotating.
What About Ball Bearings?
Compared to metal ball bearings, plastic ball bearings have larger internal clearances or play. Why? For the same reasons mentioned above.
If you insert a plastic ball bearing into a metal housing and mount it on a shaft, the outer race will attempt to expand outward—but is constrained by the housing. This causes its inner diameter to shrink.
At the same time, the inner race expands outward, reducing the clearance between the balls and the races. Eventually, the balls can become jammed, stopping rotation altogether.
This is why plastic ball bearings must be designed with greater clearances than their metal counterparts.
Thermal Expansion During Machining
● Cutting Fluids
When machining, friction generates heat that causes thermal expansion. As a result, part dimensions immediately after machining can differ from their dimensions once cooled.
Using cutting fluids during machining helps not only with cutting performance but also with cooling, which reduces thermal deformation.
However, with water-absorbent plastics, cutting fluids can lead to dimensional changes. In these cases, dry cutting or air cooling (air blow) is often used instead.
● Practical Applications of Thermal Expansion
Thermal expansion is also used in press-fitting techniques such as shrink fitting and interference fitting:
- (1) Shrink Fitting: The housing (A) is heated, expanding its inner diameter. The shaft or ring (B) is inserted. As A cools, it contracts, creating a tight fit.
- (2) Interference Fitting by Cooling: The shaft or ring (B) is cooled, shrinking its outer diameter. It’s inserted into the housing (A). As it returns to room temperature, it expands, locking in place.
Both methods rely on thermal expansion. However, to ensure a secure fit after assembly, the expansion coefficients of A and B must be taken into account. If not, temperature changes in the operating environment could cause loosening. This is especially true when A and B are made from different materials with differing expansion characteristics.
Tolerances and Design Tips
We sometimes receive part specifications using metal tolerances. In such cases, we always advise:
“Plastics don’t hold tolerances as tightly as metals.”
Why? Because plastics undergo greater dimensional changes in response to environmental and temperature shifts.
That’s why we recommend larger tolerances for plastic parts compared to metal ones.
Thermal expansion is just one difference.
As discussed in a previous article, water absorption also significantly differs between metals and plastics.
Only by considering all relevant physical properties can you ensure optimal product performance.
