I'm trying to design a plastic spring for a consumer product and was wondering if someone could point out a good source of guidelines for plastic spring design?
I an always leery of plastic springs. Creep and relaxation are always a factor that tend to mess them up. With that said, I admit to using them in circumstances where they will not be under load for protracted periods, or where failure is not critical. Coil springs are not normally made, but other varieties that can be readily molded can be used. If I knew a bit more about what the spring is to accomplish i could probably help you.
I agree with insideman, you have to be very careful designing polymer spring especially where either a fixed deformation is placed on the spring (the spring load reduces over time due to stress relaxation) or a permanent load (spring deflection increases over time due creep).
However if the loading, environmental (most importantly temperature) and service requirements are known accurately then it is possible to design very good springs.
Generally the springs I come across are either cantilevers or beams supported at either end. These types of spring are generally easier to design as the material is unixial tension and compression enabling good predictions to be made using standard material test data.
Coil springs place the material in torsion and it is rare for material suppilers to test polymers for long term behaviour in this loading condition which makes it difficult to design for long term life.
Accurate spring design must be part of the material selection process for the part as a whole as changing material at a later stage will change the performance of the spring both in terms of initial spring constant and long term performance.
If the spring has no permanent load or deflection and just undergoes periodic deflection then the most common cause of failure is through fatigue and this should be taken into account during the design.
Some of my collegues have written articles on spring design which you can download from our US website (
have some good information on plastics design. Additionally most major material suppliers will be happy to send out design books based on their own polymers.
If you wish to contact me direct then please ring the Ticona UK Office the number of which can be found on our european website in the contacts section.
