04-02-2013, 11:30 AM
Joining Techniques
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SCREWS, BOLTS AND RIVETS
When using common mechanical methods
for securing parts, pay special attention
to the fastener’s head. Conical
heads, called flat heads, produce undesirable
tensile stress in the mating parts
and should be avoided (see figure 1).
Bolt or screw heads that have a flat
underside, called pan or round heads,
produce less harmful, compressive
stress. Use flat washers under both nut
and fastener heads, because these help
distribute the assembly force over larger
areas (see figure 2).
MOLDED-IN THREADS
When your application requires infrequent
assembly and disassembly, you
can use molded-in threads for mating
thermoplastic to thermoplastic parts
(see figure 3). For easier mold filling
and better part tolerances, avoid designing
parts with threads of 32 or finer
pitch. Do not use tapered threads, such
as pipe threads, unless you provide a
positive stop. Overtightening can cause
excessive stress following assembly that
can lead to part failure.
THREAD LOCKERS
Generally, thread lockers can be chemically
aggressive to plastics. If you are
using a thread-locking liquid to secure
metal fasteners, fully test the liquid for
chemical compatibility with the thermoplastic
material before production use.
Request a copy of Chemical
Compatibility Test for Unreinforced
Thermoplastic Resins for further
information.
RIVETS
Rivets provide a low-cost, simple installation
process that can be easily automated.
Use them to join thin sections of
plastics, plastic to sheet metal or plastics
to fabric. To minimize stresses, use
rivets with large heads — three times
the shank diameter is suggested — and
washers under the flared end. Never
use countersunk rivets (see figure 8).
Calibrate the rivet-setting tools to the
correct length to minimize compressive
stress and shear in the joint area.
SPRING-STEEL FASTENERS
Self-locking steel fasteners and pushon
spring-steel fasteners, such as
Tinnerman* nuts (see figure 9), offer
another option for assemblies subjected
to light loads. Usually pushed over
a molded stud, these fasteners are
frequently used in applications such as
circuit boards. The plastic stud should
have a minimum 0.015 inch (0.38 mm)
radius at its base.