Design Guide - Solvay Plastics

Figure 78: Cantilever Type Snap Fit
Figure 79: Cantilever Beam - Concentrated Load
at Free End
F
b
d
L
Y
L
Y
Molded-In Threads
One of the benefits of using plastic materials instead
of metals is the ability to mold thread forms directly
into the part. This eliminates the secondary machining
operations needed with metal parts to form the threads.
molded-in threads can be either external or internal. In
the case of internal threads, some type of unscrewing
or collapsible core is required. External threads can
be formed more easily if the parting line of the mold is
perpendicular to the thread.
Designing with Snap Fits
The use of snap fits in plastics is very prevalent. All snap
fit designs require the plastic to flex like a cantilever
spring as it moves past an interference that is designed
on the mating part. Once the flexible arm moves past
the interference, it returns to its normal unflexed,
unstressed position. Usually a step or protrusion has
been designed on the cantilever that engages and locks
into the mating part, creating a simple assembly method
without additional parts. This is shown in Figure 78.
Each cantilever arm must deflect a distance “Y” in order
to be inserted. The key to proper snap design is to not
exceed the strain/stress limits of the material being
used. A snap fit design that has been used for a ductile,
low modulus plastic will probably not be suitable for a
highly reinforced, very rigid plastic.
For rigid materials, the length of the cantilever may be
increased or the interference deflection "Y" reduced.
Adding a "stop" can prevent over deflection of the
cantilever during assembly.
Straight Cantilever Beam Equation
The relationship between maximum deflection and
strain for a straight cantilevered beam was calculated as
follows: From Table 46, the cantilever beam was chosen
and the drawing is repeated as Figure 79. The maximum
stress, is given by the following:
σ = FL
Z
Since this beam has a rectangular cross section,
Z =
I =
Therefore:
bd 2
6
bd 3
12
σ = FLd
2I
and
The deflection of the beam, Y, is given by:
Y =
FL 3
3EI
76 Amodel ® PPA Design Guide
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