Today, we’ll talk about “creep” in plastic materials.
● Definition
Creep is the phenomenon where a material deforms gradually over time when subjected to a constant load. This is sometimes referred to as cold flow.
There are generally three patterns of creep:
- Recovery → When the load is removed, the material partially returns to its original shape depending on its properties.
- Permanent deformation → The shape does not return to its original form even after the load is removed.
- Fracture → Continuous creep eventually leads to material failure.
Think of it like this:
- Marshmallow: Deforms when pressed but returns to shape once the pressure is released.
- Clay: Changes shape under force and stays deformed even after releasing the pressure.
- Tofu: Deforms at first, but under continuous force eventually breaks apart.
- Note: Marshmallows and clay can also eventually break if the applied force is excessive.
Creep behavior depends on several factors: material type, load, temperature, and time.
Particularly under constant load, temperature plays a major role. With identical load and duration, higher temperatures increase creep. This occurs in both plastics and metals, though the degree varies.
Harder materials generally resist creep better, while softer materials are more prone to it (hardness and compressive strength are useful indicators for assessing creep resistance).
● Creep Resistance
The higher the creep resistance, the less prone the material is to deformation under long-term load.
Among plastics:
- Thermosetting resins (such as phenolic or epoxy) generally have better creep resistance than thermoplastics.
- Among thermoplastics, creep resistance typically follows this order (from highest to lowest): PEEK > PPS > POM > PET > PP > PE > PTFE
■ Creep and Plastic Ball Bearings
In plastic ball bearings, excessive load can cause creep deformation in the raceway grooves where the balls roll.
Unlike wear, this deformation creates uneven surfaces (small dents), which increase rolling resistance and can impair the bearing’s rotation.
This can lead to poor rotational performance and, in severe cases, bearing failure, negatively affecting machine operation.
● Our Approach
In collaboration with a university, we have conducted research showing that when pressure is applied to the material of bearing balls, some indentation occurs; beyond a certain point, the deformation tends to stabilize (the creep rate diminishes significantly) and does not continue to increase, unless the load is excessive and causes failure.
Based on this, we are studying the optimal groove geometry (R-profile; groove radius “R”) of the raceway to minimize creep effects on the rolling balls. Our research aims to define R-shapes tailored to specific materials for improved bearing performance.
For metal bearings, optimal groove geometry ratios relative to ball size are well established from decades of experience. We aim to develop similarly optimized geometries for plastic bearings.
