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iv) Snap-Fit Design: Snap-fits involve pushing a projection on one part past an
obstruction on a mating part. They rely entirely for their effect on the elasticity of
plastics. Generally, one part is more or less rigid while the other part is flexible or
resilient. Depending on the design, the
joint can be permanent or releasable.
Both parts can be plastics - either the
same or different types, or one part can
be a foreign material such as a metal
shaft or a laminated circuit board. The
three principal types of snap-fit are:
cantilever snap-fit, also known as a
snap hook, catch spring, spur, or lug,
the cylindrical or ring snap-fit and the
spherical snap-fit
Press-Fit Assembly: When one object such as a shaft is assembled to another by
forcing it into a hole that is slightly too small, the operation is known as press fitting.
Press fits can be designed between
similar plastics, dissimilar plastics, or
more commonly between a plastic and a
metal.
Press fits are simple and inexpensive but
the degree of interference between the
shaft and the hole is critical. If it is too
small, the joint is insecure. If it is too
great, the joint is difficult to assemble and
the material will be over-stressed. Unlike
a snap fit, the press fit remains permanently stressed and it is the elastic
deformation of the plastics part that supplies the force to hold the joint together.
Since plastics creep (or stress relax), the designer must allow for a large reduction of
the initial press-fit clamp force. A good design will minimize the strain on the plastic;
take tolerance stack-up into account and estimate the final residual clamp force due
to plastic creep relaxation.
Hot Staking: An attractive alternative to thread-forming screws, press fits, and
snaps fits for thermoplastics is heat
staking. Simply put, this involves
thermally softening a stake that is an
integral part of the moulding, then
forming a rivet-like retaining head
under pressure from a cold tool.
Advantages are stake is a part of the
moulding; due to its small size and
simple form, it adds almost nothing to
part cost or mould cost nor its needed
to purchase and stock separate Referance source - Design Guides for Plastics, Clive Maier, Econology Ltd, Email: cmaier@btconnect.com
fasteners. This also simplifies recycling; there are no foreign materials to remove
and sort. However, many snap fits can be released and re-made but heat staking is
a permanent method of assembly.

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