Snap Latches
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 Snap Latches
Snaps allow an easy method of assembly and disassembly of plastic parts. Snaps consist of a cantilever beam with a bump that deflects and snaps into a groove or a slot in the mating part.

Snaps can have a uniform cross-section or a tapered cross section (with decreasing section height). The tapered cross-section results in a smaller strain compared to the uniform cross-section. Here we consider the general case of a beam tapering in both directions.

When Rh=1 and Rb=1 , the above formula does not apply, L'Hospital's rule applies and the formula is simplified to the following:

The disassembly force is a function of the coefficient of friction, which ranges from 0.3 to 0.6 for most plastics. The coefficient of friction also varies with the surface roughness. The rougher the surface, the higher the coefficient of friction.

There is an angle at which the mating parts cannot be pulled apart. This is known as the self-locking angle. If the angle of the snap is less than this angle, then the assembly can be disassembled by a certain force given by the above formula.

The self-locking angle a = tan-1(1/µ)

where µ is the coefficient of friction which ranges from 0.3 to 0.6 for most plastics.

This computes to angles ranging from 73° for low coefficient of friction plastics to 59° for high coefficient of friction plastics.

If this angle is exceeded then the snaps will not pull apart unless the snap beam is deflected by some other means such as a release tool.

This property can be used to advantage depending on the objective of using the snaps. If the snaps are to be used in the factory for assembly only (never to be disassembled by the end user), then the ramp angle the self-locking angle should be exceeded. If the user is expected to disassemble (to change batteries in a toy for example), then the angle should not be exceeded.

Tooling for snaps is often expensive and long lead time due to

 - The iterations required achieving the proper fit in terms of over travel. The amount of over travel is a design issue. This will control how easy it is to assemble, and how much the mated parts can rattle in assembly. This rattle can be minimized by reducing the over travel or designing in a preload to use the plastic's elastic properties. However, plastics tend to creep under load, so preloading is to be avoided unless there is no other option. - Often, side action tooling (cam actuated) is required. This increases the mold costs and lead times.Cam actuated tooling can be avoided if bypass coring can be used that results in an opening in the part to allow the coring to form the step.

Some common problems of using snaps:

 - Too high a deflection causing plastic deformation (set) of the latch (the moving member). Care has to be taken that the latch does not take a set. Otherwise, the amount of latch engagement could reduce, reducing the force to disassemble. If the set is bad enough the engagement might even fail. - The moving arm could break at the pivot point due to too high a bending stress. This can be avoided by adhering to the design principles and not exceed the yield strength of the material-in fact it should be kept well below the yield strength depending on the safety factor used. - Too much over travel leads to a sloppy fit between mating parts resulting in loose assemblies that can rattle.

Good snap design practices

 - Design the latch taking into account the maximum strain encountered at maximum deflection. - In general, long latches are more forgiving of design errors than short latches for the same amount of deflection, because of the reduced bending strain. - Build mold tooling with "tool safe condition". By this we mean that the deflection or over travel, or length of engagement can be changed easily by machining away mold tooling, rather than add material to mold tooling, which is more expensive and not good mold practice. This "safe" condition allows for a couple of tooling iterations of the latch, until the snap action is considered acceptable.