At Kashima Bearings, we specialise in plastic bearings. One challenge we face daily is dimensional change—the way plastics subtly, and sometimes dramatically, alter their size.
This article looks at why it happens and how we process plastics to maintain the highest accuracy.
Why Plastics Change Size
There are two main causes:
1. Thermal Expansion
Like most materials, plastics expand when heated and contract when cooled. This principle explains stuck jar lids in summer, cracked glass cups from boiling water, and even heat-related rail track expansion.
Some products, such as traditional mercury thermometers, were intentionally designed to take advantage of this property.
2. Water Absorption
Unlike metals, plastics can absorb water—sometimes a lot. Certain polymers are designed for high absorption, as seen in baby diapers or cooling neck scarves. But in engineering components, this property can mean measurable changes in size.
The Manufacturing Challenge
For our machining team, dimensional changes—especially in the outer and inner diameters of ball bearings—are a constant consideration.
We never cut components directly to their final dimensions. Instead, we:
- Run small test batches a few days in advance
- Measure size changes over time
- Adjust machining accordingly for the production run
Environmental conditions matter, too:
- Summer: parts tend to expand.
- Winter: they contract, sometimes even at room temperature. We store them in temperature-controlled chambers at around 25 °C to maintain stability.
- Spring/Autumn: fluctuating conditions make accuracy the most challenging.
How Big Are the Changes?
Take one of the most unstable plastics we work with:
- A 1000 mm rod at 5 °C can measure 2.7 mm longer at 35 °C.
- With a water absorption rate of 0.8%, that same rod can grow 8 mm after one day immersed in water.
- At maximum saturation (theoretical), it could expand by 60 mm.
These figures are based on calculations, not extreme in-house tests—but they highlight why dimensional control matters.
Our Approach
Working with challenging materials demands constant care. We monitor, store, and process every part to achieve the highest possible precision.
Somewhere, someone needs components with exacting tolerances. We’re ready to deliver them.
Kashima Bearings manufactures high-precision plastic bearings from a wide range of materials. With extensive experience and technical know-how, we’re equipped to solve even the toughest dimensional challenges. Contact us to discuss your requirements.
