I'm designing a plastic holder that will secure a metal part with snaps. Which plastic will be able to take multiple removals and re-insertions without getting weaker (bent out of shape) over time?
The following is my progression as I attempt to get the answer. Some of it is me thinking out loud.
I've referenced these two documents and only found the following information:
BASF Snap Fit Design Manual
(Page I1) Although snap-fits can be designed with many materials, the ideal material is thermoplastic because of its high flexibility and its ability to be easily and inexpensively molded into complex geometries. Other advantages include its relatively high elongation, low coefficient of friction, and sufficient strength and rigidity to meet the requirements of most applications.
So I'm aware that thermosplastics are better. But which ones?
(Page III-1) Rigidity can be increased either by using a higher modulus material (E) or by increasing the cross sectional moment of inertia (I) of the beam.
Ok so I need to find a list of plastic materials with higher modulus to make it stiffer. But does that correlate with repetitive loading?
(Page III-2) However, as the beam deflection increases, the beam stress also increases. This will result in a failure if the beam stress is above the yield strength of the material... The calculated stress or strain value should be less than the yield strength or the yield strain of the material in order to prevent failure.
So the ideal plastic needs a high yield strength.
(Page VI-2) Fatigue, or repetitive loading, is the third major cause of failure. Fatigue concerns primarily apply if hundreds or thousands of cycles are anticipated. While the design stress level might be well within the strength of the material, the repeated application of this stress can result in fatigue failure at some point in the future. Some polymers perform better than others in this regard, making them ideal candidates for snap-fits or living hinges that must flex repeatedly. The first way to avoid a fatigue failure is to choose a material known to perform well in fatigue.
What are these ideal candidates? It continues with the following:
This can be done by comparing the so-called S-N curves of the materials, which show the expected number of cycles to failure at various stress levels and at different temperatures of exposure. The second way, still using the S-N curves, is to choose a design stress level, at the correct temperature, that results in the required number of load applications prior to failure. This method will usually be conservative since S-N curves are typically generated at much higher frequencies than would be anticipated for repeated application of a snap-fit assembly.
Ok so I need to find these curves and compare.
Page IV-4 also contains this table showing allowable strain value.
This table is unclear to me as it appears to mix 70% and 100% yield strain values. If I'm reading it correctly, if the PEI value is 70%, it would be the best. If the PEI value is 100%, Acetal would be the best. Either way I think the list of compared materials could be longer.
Bayer Material Science Snap Fit Joints for Plastics
(Page 4) In view of their high level of flexibility, plastics are usually very suitable materials for this joining technique.
(Page 11) The permissible deflection y (permissible undercut) depends not only on the shape but also on the permissible strain E for the material used. In general, during a single, brief snap-fitting operation, partially crystalline materials may be stressed almost to the yield point, amorphous ones up to about 70% of the yield strain.
So assuming a partially crystalline and amorphous plastic have the same yield point, the partially crystalline one would be better because it can flex further.