Designing of Plastic Products for Injection Moulding
Snap-fits are designed using classical beam theory and stress and strain during assembly of snap beam are calculated using standard beam equations. There are also equations to find out the deflection in the beam, length of straight snap beam, thickness of a straight snap beam.
Snap-fits for detachable joints have angled undercuts. It is mostly preferred to have a retaining angle between 30 degrees to 45 degrees to have a smooth snapping action.
A special class of snap-fit having 90 degree retaining angle is used for detachable assemblies. There is a hole or a window in the recess area through which a pin can be inserted. This pin pushes the cantilever and disengages the assembly.
U-shaped cantilever snap-fits are commonly used in battery cases. Due to the U-shape of cantilever plastic does not experience a much strain, so multiple flexes are possible without damaging the beam.