Mechanical load capacity of engineering plastics
How mechanically resilient are engineering plastics? Learn which materials are suitable for tensile, compressive and impact loads and what influences the selection.
Engineering plastics are used in numerous industries in which mechanical stresses play a central role. Whether in the automotive industry, in mechanical engineering, in food technology or in medical technology – mechanical load capacity is a decisive selection criterion. Plastic components are increasingly replacing metal components, provided they offer comparable or even better properties at significantly lower weight and often also lower production costs.
But what does mechanical load capacity actually mean, how do the materials differ in detail and what role do processing and environmental influences play? This article gives you a comprehensive overview.
What is meant by mechanical load capacity?
Mechanical load capacity describes how well a material – in this case a plastic – withstands external forces. Different forms of load are considered:
Tensile strength
Tensile strength describes the resistance that a material offers to an applied tensile force. It is particularly relevant for components that are subjected to tension – for example ropes, fastening elements or thin-walled supports. High tensile strength prevents premature tearing or plastic deformation.
Compressive strength
Compressive strength refers to the ability of a plastic to absorb compressive or pressing forces without failing. This value is decisive for components such as bushes, bearings or interlocking housings that are subject to compressive load.
Flexural strength
Flexural strength indicates how far a material can bend before fracture occurs. It is particularly important for elongated or thin-walled components, e.g. for brackets, rails or guides. Good flexural strength contributes to structural stability.
Notched impact strength
Notched impact strength measures the ability of a plastic to absorb sudden, impact-type loads – especially at weak points such as notches, indentations or holes. A high value indicates good suitability for safety-relevant applications or for components potentially subject to impact loads.
Fatigue strength
Fatigue strength describes how a material reacts to cyclic, recurring load. Particularly in rotating or oscillating applications – such as gears, levers or joints – high fatigue strength is required in order to prevent material fatigue and thus failures.
Depending on the application, individual properties are more important than others. For example, gears require high fatigue and flexural strength, while bumpers tend to be optimised for impact strength.
Comparison of mechanical properties of engineering plastics
Mechanical load capacity depends strongly on the plastic chosen. The following table shows some typical materials with their key mechanical values:
| Material | Tensile strength (MPa) | Flexural strength (MPa) | Notched impact strength (kJ/m²) | Remark |
|---|---|---|---|---|
| PA6 (polyamide) | 70–90 | 100–110 | 4–6 | High toughness, wear-resistant |
| POM (polyoxymethylene) | 60–80 | 90–110 | 5–8 | Good dimensional stability, low friction |
| PET (polyethylene terephthalate) | 50–70 | 100–110 | 2–4 | Hard and dimensionally stable, medium toughness |
| PC (polycarbonate) | 55–75 | 90–100 | >60 | Very high impact strength |
| PEEK (polyetheretherketone) | 90–110 | 160–180 | 7–10 | High-performance plastic, very robust |
These values are typical guide values and can vary depending on the manufacturer, fillers, processing and orientation within the component. With fibre reinforcement (e.g. with glass or carbon fibres), the strength values can increase significantly.
Factors influencing mechanical load capacity
In addition to the material itself, numerous factors influence the mechanical performance of a plastic part:
- Temperature: As temperature rises, many plastics lose stiffness and strength. PEEK and glass-fibre-reinforced materials offer very high temperature resistance here.
- Moisture absorption: Particularly hygroscopic plastics such as PA6 absorb water from the environment. This can lead to a reduction in strength and a change in dimensional stability.
- Fillers and reinforcements: Glass fibre, carbon fibre or mineral fillers specifically improve certain mechanical properties such as stiffness, while impact strength often decreases in the process.
- Processing methods: Manufacturing methods such as injection moulding, extrusion or milling influence crystallinity and fibre orientation and therefore also the strength of the end product. Process parameters such as mould temperature, pressure or holding pressure time also play a role.
Selection criteria for applications
The choice of the right material must be made individually and with reference to the application. For statically highly loaded constructions such as machine components or support systems, PET, POM or PEEK are suitable, as they have high stiffness and dimensional stability. Dynamically stressed components such as bearings, gears or joints place particular demands on wear resistance and fatigue strength – here PA6 (possibly modified with MoS₂) or POM-C are suitable materials.
If components are regularly subjected to impact loads, for example in the case of protective housings or covers, plastics with high impact strength such as PC or PEEK are to be preferred. In applications where both high mechanical and thermal requirements exist – for example in machine or vehicle construction – high-performance plastics such as PEEK or PPS score with their temperature and chemical resistance. For environments with aggressive media or cleaning processes, chemical resistance is additionally decisive, for which materials such as PVDF or PTFE are suitable.
Depending on the application, a combination of different materials can also be useful, e.g. through multi-component components or connections with metal inserts. from different materials can also be useful, e.g. through multi-component components or connections with metal inserts.
Conclusion
Mechanical load capacity is one of the central properties when selecting engineering plastics. Only through a precise understanding of the forces acting, the environmental conditions and the material properties can optimum results be achieved. The large number of available engineering plastics makes it possible to meet even complex mechanical requirements – provided the selection is made deliberately and with reference to the application.
A careful material selection taking into account all influencing factors, including processing, not only reduces the risk of component failure, but also promotes the economic efficiency and service life of the overall solution.
FAQ on the mechanical load capacity of engineering plastics
Which plastics are particularly impact-resistant? Polycarbonate (PC) offers one of the highest notched impact strengths among all thermoplastic plastics. PEEK also offers good toughness combined with high temperature resistance.
How does glass fibre reinforcement affect mechanical load capacity? Glass fibre reinforcement primarily increases the stiffness and tensile strength of a plastic. However, impact strength often decreases in the process, particularly with very short fibres.
Are engineering plastics suitable for permanent load? Yes, particularly materials such as POM, PA6 or PEEK are suitable for components under cyclic or permanent load. The prerequisite is a proper design and, if necessary, additive modification.
Does mechanical load capacity change under heat? Yes, almost all thermoplastic plastics lose mechanical performance as temperature rises. PEEK and PTFE are significantly more stable here than standard plastics such as PE or PP.
What role does processing play in load capacity? Influencing factors such as flow direction in injection moulding, holding pressure, cooling or mould temperature can influence the orientation of molecular chains or fibres and thereby significantly change the load capacity in the longitudinal or transverse direction.
