Tara Meinck discusses the performance expectations for threaded inserts in plastic assemblies
The use of threaded inserts in plastic assemblies allows designers to replace machined and cast metal components with plastic to achieve significant cost and weight savings without any loss of joint strength.
Insert performance — resistance to rotational torque and tensile loading (also known as pull-out) — is a common concern in design, but answering how an insert will perform is rarely a simple ‘yes or no.’ Insert performance is only partially influenced by the insert itself. Other factors include:
• Plastic material and filler composition
• Design and quality of the plastic components including hole consistency
• Installation process and resultant quality
• Alignment of the mating component and how it supports the installed insert
With so many different combinations of insert materials, plastic types, and assembly configurations it’s impossible to determine what performance could be achieved in a specific application without understanding these factors. In fact, up to 75% of the insert’s performance is a direct result of how well it was installed.
To establish a benchmark for an application, it’s critical to assemble and test the joint under the application’s production environment. Since questions surrounding performance often arise earlier in the design process, it is important to consider which of the previously listed factors may be easily adjusted compared to those with a more significant impact if modified. An insert can be recommended to meet a specific application’s requirements in the design stage, and/or modifications can be proposed to an existing assembly to optimise performance based on prior experience and extensive performance data. In some cases, it is preferred to test the insert in the application to determine the installation criteria that will yield the best performance. Ultimately, acceptance of any threaded insert is subject to testing and evaluation to confirm performance in the actual application.
Once a benchmark has been established and the application moves into production, it is imperative to remember that variation to any of the aforementioned factors may affect the insert’s ability to resist rotational torque and pull-out. Limiting the number of suppliers involved with an assembly can also minimise variables that affect performance. Therefore, if possible, purchase both the threaded inserts and installation equipment from the same supplier.
Many insert manufacturers provide performance data in their product catalogue. This data should be used solely as a comparative guide for insert types and sizes. It should not be used to compare across manufacturers since testing methods and materials are not consistent, nor should it be used to estimate a performance benchmark.
Given the variety of plastic materials, fillers, and the complexity of component design, it is impossible to provide insert performance data which can be applied to specific applications. It is recommended that designers partner with application engineering experts who specialise in threaded inserts for plastics as early as possible in the design stage to determine the most appropriate insert for their assembly. Final approval of the threaded insert should always be based on testing and approval in the actual application.
Tara Meinck is product manager at SPIROL:
