Today, plastics are used in a wide variety of fields — from household goods and mechanical components to medical equipment and even spacecraft.
Plastic materials are sometimes used as-is, but in many cases additional substances are mixed in to enhance or modify their properties. These additives include:
- Stabilisers
- Plasticisers
- Fillers
While these may be added to improve performance, they are sometimes used simply to reduce cost.
Stabilisers are added to prevent thermal degradation or UV-induced ageing.
Plasticisers are used to increase flexibility (soften the resin), which can improve mechanical performance and processability.
This article focuses on the third type: fillers, especially as they relate to plastic bearings.
Fillers are used in different ways depending on the application. In some cases, they are used to add colour or reduce cost — but in the context of bearing materials, fillers serve more functional purposes.
Here are some examples of common fillers used in plastic bearings:
- Carbon (powder or fibres)
- Graphite
- Glass (beads or fibres)
- Molybdenum disulphide (MoS2)
- Mica
- Iron oxide
- Copper
- Ceramics
- Wood flour or fibres
- Other plastics (blend/fibre reinforcement)
Why are fillers added to plastic bearing materials?
The most common reasons are:
- To improve heat resistance
- To increase strength and stiffness
Other benefits include:
- Enhanced wear resistance
- Improved dimensional stability
- Lower coefficient of friction (depending on filler)
Glass fibres are often used to improve both heat resistance and strength. Carbon and graphite are typically added to improve wear resistance. To improve lubricity (make the surface more slippery), molybdenum disulphide (MoS2) is used due to its layered crystalline structure, which shears easily under friction.
Copper is sometimes used in PTFE-based materials to leverage its excellent thermal conductivity, helping dissipate heat and improve wear resistance.
The primary weaknesses of plastic bearings are heat and pressure. To mitigate heat generated by friction, low-friction fillers are added to reduce the coefficient of friction. This minimises heat build-up, which in turn helps reduce wear — hence the frequent use of carbon, glass, and MoS2.
To improve strength and resistance to pressure, fillers are also added to reinforce the inherent softness or brittleness of certain resins.
For example, phenolic resins are naturally brittle (like hard candy), so they are reinforced with fillers such as wood flour or cotton fabric to create materials that can withstand high pressure.
But does adding more filler always improve performance? The answer is: No.
Thermoplastics and thermosets rely on molecular networks to maintain integrity. If too much filler is added, polymer chain interactions are disrupted and the matrix can no longer transfer loads effectively. Overfilling can lead to reduced tensile strength and toughness.
Take PTFE as an example: comparing 15% vs. 25% filler content, the 25% grade will usually be harder and more indentation-resistant, but may become less ductile, losing impact absorption capability. In some cases, tensile strength decreases and the material becomes more brittle.
Ultimately, the optimal filler type and loading depend on both the base polymer and the application (load, speed, temperature, environment).
At Kashima Bearings, we specialise in:
- Selecting the right filler
- Specifying the right amount
- Combining it with the right plastic bearing material
Feel free to contact us anytime for advice or questions regarding material selection. We’re here to help.
