Snap-Fit Joints for Plastics - A Design Guide - MIT

Annular Snap Joints D
Permissible undercut
The annular snap joint is a con-venient form
of joint between two rotationally symmetric
parts. Here, too, a largely stress-free, positive
joint is normally ob-tained. The joint can be
either detachable (Figs. 22a, 23), difficult to
disassemble or inseparable (Fig. 22b)
depending on the di-mension of the bead and
the re-turn angle. Inseparable designs should
be avoided in view of the complex tooling
required (split cavity mold).
The allowable deformation should not be
exceeded either during the ejection of the
part from the mold or during the joining
operation.
The permissible undercut as shown in Fig. 24
is limited by the maxi-mum permissible
strain
Y pm = � pm .d
Note: � pm is absolute value.
This is based on the assumption that one of
the mating parts re-mains rigid. If this is not
the case, then the actual load on the material
is correspondingly smaller. (With components
of equal flexibility, the strain is halved,
i.e., the undercut can be twice as large.)
W = mating force
y = undercut
� = lead angle
�' = return angle
t = wall thickness
d = diameter at the joint
Fig. 22: Annular snap joint
Fig. 23: Annular snap joint on a lamp housing
Fig. 24: Annular snap joint—symbols used
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