SUMMER 2023

156
THE DISTRIBUTOR’S LINK
LAURENCE CLAUS DIFFICULT FASTENING APPLICATIONS - PART 2: THIN-WALLED PLASTIC BOSSES from page 132
¤ Use state-of-the-art thread designs that have been
developed especially for these types of applications,
such as EJOT’s Delta PT® and EVO PT® screws (See
Figure 4). These threads have been designed especially
for use in thinner-walled plastic applications and employ
sharp 30° thread profiles and other special features that
optimize their performance in thin-walled bosses.
FIGURE 4: EXAMPLE HOW A 60° THREAD WILL SPLIT A THIN-WALLED
BOSS THAT IS UNDAMAGED BY A 30° THREAD
plastics. Therefore, managing the installation practice to
minimize the effective relaxation is often more important
than simply figuring out the best driving and stripping
torque values.
DESIGN TO PREVENT PROBLEMS
Designers should make efforts to strengthen bosses
and protect them from common problems. This means
that strengthening gussets, and ribs should be used to
provide additional columnar and radial strength. Figure
5 shows a tall, thin-walled boss strengthened by ribs.
To prevent sink marks, the base of the boss should be
designed so that the boss and wall interface does not
have a cross section exceeding the wall thickness. Use
other generally accepted boss design best practices.
¤ Try to limit the boss’s height to three times the
outer diameter.
¤ Add ribs or gussets around stand-alone bosses.
¤ Optimize the bottom of the boss and depth of
cored hole to prevent sink marks.
DEVELOP AN INSTALLATION PLAN
Even the right screw can be a problem if the
installation practice is poor. It is critically important
that either prelaunch experimentation or simulation is
conducted to determine proper installation methods
and values. It is important to develop an installation
torque value that will reliably seat the fastener without
stripping, generate clamp load, prevent boss collapse or
destruction, and minimize the resulting relaxation of the
plastic.
DO NOT IGNORE THE IMPACT OF RELAXATION
In general, the most overlooked limitation of plastic
joints is relaxation. Relaxation is anticipated because of
the viscoelastic behavior of plastics. Although traditional
bolted joint engineering often teaches us to maximize
clamping load, this is often not the best practice with
In Summary
FIGURE 5: EXAMPLE OF STRENGTHENING RIBS
Fastening into plastic can be a tricky proposition under
any circumstance, however, when thin-walled bosses are
introduced into the equation, it can get exponentially
trickier. For this reason, good distributors will understand
the limits and advantages of the products they sell and
assist their customers in making good, educated choices
of the proper screws for these challenging applications.
LAURENCE CLAUS