This time, we’ll be taking a closer look at the different types of plastics, focusing on their tensile, compressive, and flexural strength.
How do you choose the right plastic material for a given application?
Since plastic selection depends heavily on the conditions of use, it can be quite complex. At Kashima Bearings, we consider a variety of factors such as:
- Wear resistance
- Mechanical strength
- Heat resistance
- Cost
This article focuses specifically on “mechanical strength.”
There are various forms of strength, but the most representative ones are:
- Tensile strength
- Compressive strength
- Flexural strength
Let’s explore what each of these means.
● Tensile Strength
Tensile strength refers to the force required to stretch a plastic material in one direction until it begins to deform or break. When you hear “stretch,” you may think of chewing gum or rubber, but even plastics stretch under tension—up to a point.
Within the material, two opposing forces arise: one that resists stretching, and one that accommodates it. The resisting force is called “stress,” and when the material reaches its limit, it can no longer resist and begins to deform or ultimately fracture. That limit is defined as its tensile strength.
● Compressive Strength
This refers to the force required to crush or deform a material under compression. Most plastics tend to have higher compressive strength than tensile strength. This is especially important for materials that are likely to fail under compression—such as thermosetting plastics or reinforced composites like phenolic or epoxy resins, or carbon-based plastics.
Think of how a concrete block breaks: it doesn’t deform much—it just crumbles under heavy load. The same principle applies to certain hard plastics.
However, all materials experience some deformation before failure. In the case of tough materials like PTFE (Teflon) or UHMWPE, failure might not even occur in the traditional sense. Instead, compressive strength is defined by the load at which permanent deformation (e.g., 5%) begins.
Therefore, it’s important to check each manufacturer’s definition of compressive strength—some may use the yield point or deformation threshold rather than actual fracture.
● Flexural Strength
This is the force required to cause cracks or failure under bending stress. Flexural strength involves both tension and compression forces acting at once. Therefore, it’s different from “flexural stress,” which refers to the internal stress distribution and is more complex. For simplicity, we’ll focus here on flexural strength and leave the more advanced topic of stress distribution for a future article.
As these definitions suggest, strength refers to the force required to deform or break a material. So, the listed values are not safe operating loads, but failure thresholds.
● Comparing Strengths of Common Plastics
Let’s look at a few examples of tensile, compressive, and flexural strengths:
| Material | Tensile Strength (MPa) | Elongation (%) | Compressive Strength (MPa) | Flexural Strength (MPa) |
|---|---|---|---|---|
| PTFE | 20–34 | 400 | 10–15 (at 23°C) | – |
| UHMWPE | 40 | 300 | 20 (5% deformation) | 22 |
| PEEK | 98 | 20 | 119 (5% deformation) | 170 |
| Phenolic Resin | 68–108 | – | 127–167 | 137–196 |
| Epoxy Resin | 245–343 | – | 147–245 | 294–392 |
All values are in MPa (megapascals).
● Interpreting the Data
Looking at this data, epoxy and phenolic resins seem incredibly strong. However, they don’t stretch—they break suddenly. This means they don’t offer much elastic recovery, but they can bear heavy loads without deforming.
PTFE has no listed flexural strength because it doesn’t break—it just bends. Its tensile strength is quite low, but it stretches up to 400%, meaning it can elongate significantly under small forces. The same applies to UHMWPE. These materials are not ideal under high load conditions and must be used with care.
PEEK is somewhere in the middle—less brittle than epoxies, but not as flexible as PTFE. Plastics like PPS or POM also fall in this “middle” category.
● Real-World Application Notes
At Kashima Bearings, we specialise in sliding components like bearings and gears. We consider not just strength, but also wear resistance and heat resistance.
In one case, a customer was using UHMWPE in a high-load application. The machine developed issues, and they asked us to take a look. At first, the user assumed the component wasn’t the problem, but upon inspection, we found it had deformed considerably under pressure and heat. This deformation misaligned the shaft and caused performance failure—even though the part hadn’t fractured.
We solved the issue by selecting a different material. However, material changes affect more than just strength—they also influence wear, temperature tolerance, and other properties. So holistic evaluation is always required.
Unlike metals, where hardness and strength often go hand in hand, plastics are viscoelastic—they deform more easily and behave differently under load. While epoxies and phenolics have higher strength values than PTFE or UHMWPE, the latter often last longer in actual bearing applications due to better wear performance and elasticity.
In conclusion, numbers alone don’t tell the whole story. Always consider the full context of use when selecting plastic materials.
