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:
- Stabilizers
- Plasticizers
- Fillers
While these may be added to improve performance, they’re sometimes used simply to reduce cost.
Stabilizers are added to prevent thermal deformation or UV degradation.
Plasticizers are used to add flexibility, improving thermal and mechanical performance.
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’re 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 fibers)
- Graphite
- Glass (beads or fibers)
- Molybdenum disulfide (MoS₂)
- Mica
- Iron oxide
- Copper
- Ceramics
- Wood
- Other plastics
So, why are fillers added to plastic bearing materials?
The most common reasons are:
- To improve heat resistance
- To increase strength
Other benefits include:
- Enhanced wear resistance
- Improved dimensional stability
Glass fibers 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 disulfide (MoS₂) is used due to its layered crystalline structure, which easily slides under friction.
Copper is sometimes used in PTFE-based materials to take advantage of its excellent thermal conductivity, helping dissipate heat and improve wear resistance.
The primary weaknesses of plastic bearings are heat and pressure.
To combat heat generated by friction, low-friction fillers are added to reduce the coefficient of friction. This minimises heat buildup, which in turn helps reduce wear. That’s why carbon, glass, and MoS₂ are commonly used.
On the other hand, to improve strength and resistance to pressure, fillers are 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 a material that can withstand high pressure.
But does adding more filler always improve performance?
The answer is: No.
Plastic materials rely on molecular bonding to maintain their form. If you add too much filler, those bonds are disrupted and the material can no longer hold together. In fact, over-filling can lead to reduced strength.
Take PTFE for example:
Comparing 15% vs. 25% filler content, the 25% version will be harder and more resistant to indentation. However, it may also become less ductile, meaning it loses its ability to absorb impact or stress.
In some cases, tensile strength decreases and the material becomes more brittle.
Ultimately, the optimal filler amount depends on both the plastic material and its application.
At Kashima Bearings, we specialise in:
- Choosing the right filler
- Inserting the right amount
- Into the right plastic bearing material
Feel free to contact us anytime for advice or questions regarding material selection. We’re here to help.
