How do the properties of different plastics affect fastening performance?

When it comes to determining the most appropriate and effective mechanical fixing approaches for plastics, it is crucial to consider how the material characteristics of different types of plastics will influence decisions. No two types of plastics will have identical compositions, but why does this matter where metal fasteners are concerned?

 

Flexural modulus or ‘stiffness’

Sometimes referred to as “bending modulus,” this is a ratio of stress to strain in flexural deformation which tells us a material’s tendency to bend. It is effectively a ratio to show how flexible the plastic is and that will determine how it responds to the application of threaded fasteners.

Why is flexural modulus important? Primarily because it influences composite selection in high-stress situations, and it helps to improve design quality for load-bearing applications. It is measured using a three-point bend test and the internationally recognised standard unit of flexural modulus is the pascal (Pa or N/m2 or m-1.kg.s-2).

More flexible plastics with a lower flexural modulus will allow the material to flow and enable thread forming. When selecting fasteners for thermoplastics which have a higher flexural modulus, consider their potential for thread-forming, particularly fasteners with a low helix angle to avoid excessive drive torque.

 

Innovative EJOT thread-forming solutions

The EJOT portfolio offers fasteners which are extremely effective at thread-forming in plastics with a higher level of flexural modulus. These include the EJOT EVO PT®, which features an innovative thread geometry and a unique lead-in thread to enable easy and straight positioning. The screw also centres automatically in the pilot hole during the installation to create a uniform load on the thread flanks when completely fastened.

Another EJOT fastener with sprecific thread-forming capabilities is the DELTA PT-P®, a plastic variant of the self-tapping PT screw. This is engineered for applications where the material being thread-formed may have considerably lower strength properties than a metal fastener, such as when using unreinforced plastics including PA6 (Nylon 6 or polycaprolactam), PP (Polypropylene) and ABS (Acrylonitrile Butadiene Styrene).

In such ‘brittle’ applications, it is sensible to equalise the two material strengths to achieve reliable clamp load retention under thermal stress. The EMS Grivory HTV 5H1 composition of the DELTA PT-P® enables this whilst also offering weight reduction of up to 85% compared to metal screws.

 

Fillers and reinforcements

Fillers and fiberglass reinforcements are commonly added to thermoplastics to improve their physical and mechanical properties, such as strength, stiffness and wear resistance. Whilst this can help manufacturers save production costs, as well as minimise raw material usage, it can have a knock-on effect in terms of fastening.

Changing the material properties can affect fastening performance because the benefits sought may result in decreased shrinkage and an increase in the material’s stiffness where it is not the primary objective. One example is where lubricants, such as silicone, have been added for moulding purposes. This can mean a reduced drive torque is required when inserting metal fasteners but it can have a negative effect on the clamp load.

Hence why it is important for the application to be tested with the proposed fastener early to assess the impact of any fillers or reinforcements on the thermoplastic’s properties.

 

Thermal expansion

Plastics are prone to changing shape with added heat, which can become problematic. If the two materials in contact – i.e. the metal fastener and the plastic – have dissimilar expansion rates, or if the temperature change is significant, we may see the plastic expanding quicker than steel under the same thermal loading.

As a result, gaps can form under the heads of the fasteners and the clamp load will be affected. Particular consideration, therefore, must be given to the potential for heat to affect the stress/strain curve for thermoplastics when selecting metal fasteners.

 

Creep rate

The extent to which a plastic deforms under constant stress – heat and load – over time is called the creep rate. There is no universal rate which applies as different plastics have different creep resistances. PTFE (polytetrafluoroethylene), for example, has a far lower creep rate than PS (polystyrene), and materials such as HDPE (high-density polyethylene) and nylon are more creep-resistant than LDPE (low-density polyethylene) and PS.

It is important to compensate for creep in joint design which can be achieved in a variety of ways. For example, stress may be reduced at the bearing surface by increasing the fastener head diameter, adding a flat washer, reducing the clearance hole diameter or lowering the initial clamp load at the assembly point.

Other ways to accommodate creep include adding a spring element, or it may even be sensible to increase the plastic’s flexural modulus – to make it stiffer – by adding a filler or changing the base resin.

 

Conclusion

Whilst the vast range of thermoplastics and thermoset plastics available for assemblies may provide the scope to achieve far-reaching benefits, particularly weight-reduction and resource savings, it can present challenges for reliable fastening.

As the issues highlighted here show, slight changes to the plastic’s composition or the potential for heat and load to impact the assembly in-situ mean best practice is to conduct a thorough assessment of the fastener at an early stage. The EJOT UK team can assist here, providing all the technical support required to achieve the optimum fastening solutions in thermoplastics.