Avoiding stress cracks: How to treat engineering plastics correctly
Recognising and preventing stress cracks in engineering plastics: find out how internal stresses, chemicals and mechanical load lead to damage – and how to prevent this specifically through suitable material selection, processing and post-treatment.
Stress cracks are among the most common types of damage in engineering plastics – and at the same time among the hardest to predict. Although they often begin inconspicuously, they can massively impair the mechanical integrity of a component and, in the worst case, lead to total failure.
Particularly in industrial applications, where high demands are placed on safety, resistance and service life, the targeted avoidance of stress cracks is essential. The following article shows how stress cracks develop, which materials are particularly at risk – and above all, how long-term damage can be reliably prevented through the right choice of material, processing and post-treatment.
What are stress cracks?
Stress cracks (also: stress corrosion cracking or environmental stress cracking, ESC) are fine, usually surface-level cracks in a plastic component that arise from the interaction of mechanical load and external influences such as chemicals, temperature or UV radiation. These cracks often develop unnoticed, as they are initially microscopically small and have no immediate effect on function. Over time, however, they can grow further and spread through the entire material. What is particularly dangerous here is that stress cracks often lead to sudden and seemingly inexplicable fracture – without any prior visible deformation or warning signs.
Typical characteristics:
- fine, line-like cracks on the surface
- brittle, inelastic fracture behaviour
- failure of the component without visible advance warning
Causes of stress cracks
1. Internal stresses from processing
Internal stresses can arise in the plastic during injection moulding, milling or bending. These are often not visible, but they form the basis for later damage.
2. Chemical influences
Contact with cleaning agents, oils, solvents or even air humidity can make certain plastics sensitive. Polycarbonate (PC) and polystyrene (PS), for example, are particularly susceptible.
3. Continuous mechanical load
Constant or cyclic loads – for example from clamping, bolting or vibrations – promote crack growth.
4. Temperature changes
Rapid temperature changes or continuous temperature load can weaken the material structure and lead to crack formation.
Avoiding stress cracks: the most important measures
To specifically avoid stress cracks, a holistic approach is required that takes into account both the choice of material and the processing and later use. Even small optimisations in the design or the manufacturing process can help to significantly reduce the risk.
Suitable choice of material
Not every plastic is suitable for every environment. For applications involving chemicals or continuous load, stress-crack-resistant materials such as POM, PA12 or certain modified PMMA types should be chosen. Fluorinated polymers such as PTFE also show high resistance to chemical influences and internal stresses.
Low-stress processing
Controlled processing is crucial. Even during manufacturing, processes should be designed in such a way that internal stresses are minimised:
- Optimise temperature control during injection moulding or thermoforming
- Post-annealing to reduce internal stresses (especially with PMMA, PC)
- Avoid sharp edges – rounded edges reduce stress peaks and promote a more even distribution of mechanical forces
Reduce mechanical load
Design and assembly should be arranged in such a way that local overstressing is avoided:
- No excessive preload with bolted connections or clamping
- Plain bearings instead of interference fits for moving components
- Distribute assembly forces evenly, e.g. through larger contact surfaces or elastomer seals
Avoid or control contact with chemicals
Chemical media often act as a catalyst for stress cracks. Therefore:
- Use suitable cleaning agents, specifically for the respective material
- Check sealants and lubricants for material compatibility
- Protect contact surfaces (e.g. through coating or sealing), especially with transparent or pressure-loaded components
With these measures, the risk of stress cracks can be greatly reduced even in the early development phase.
Subsequent treatment to reduce stress
After machining (e.g. milling, sawing, bonding), stress-sensitive plastics should be post-treated by tempering. In this process, the material is heated over a longer period to a defined temperature below the glass transition temperature. This allows internal stresses to heal.
Example tempering conditions:
| Material | Temperature (°C) | Duration (h) |
|---|---|---|
| PMMA (acrylic glass) | 70–80 | 2–4 |
| PC (polycarbonate) | 120–130 | 2–4 |
| PSU (polysulfone) | 150–160 | 3–5 |
Summary
Stress cracks usually arise from a complex interaction of internal stress, chemical influences and mechanical load. Anyone who wants to use engineering plastics reliably in the long term must know these factors and take them specifically into account when choosing materials, processing and application. A low-stress design, chemical compatibility and, if necessary, post-treatment are essential in order to significantly extend the service life of technical components.
FAQ
Which plastics are particularly sensitive to stress cracks?
Polycarbonate (PC), polystyrene (PS), SAN and PMMA are particularly susceptible. ABS can also be sensitive if processed incorrectly.
What can be done if stress cracks are already visible?
As a rule, only replacement helps. In exceptional cases, careful tempering can stop cracks, provided they are surface-level and detected early.
How do I recognise internal stresses?
Polarising filters can make internal stresses visible. Alternatively, empirical values or stress tests with test liquids can help.
