Design Guide - Solvay Plastics

Figure 76: Torque Developed During
Screw Installation
Stripping Torque
Figure 77: Boss Design for Ultrasonic Inserts
Insert Diameter
Boss diameter =
2 x insert diameter
Torque
Thread
Forming
Torque
Head makes contact
with captured material
Driving Torque
Clamp
Load
0.7 t
t
Penetration Depth
Tightening Torque
Figure 76 shows how torque changes as a function
of screw penetration. Tightening torque is the
recommended installation torque for a given application.
It must be high enough to fully engage the screw
threads and develop clamp load but lower than the
torque that would cause failure of the threads, known
as the stripping torque.
The optimum tightening torque value can be calculated
from the average driving torque and the average
stripping torque using the following equation.
where
1 3 1
T T
= T D
+ T S
2 2 2
T T
= Tightening torque
T D
= Average driving torque
T S
= Average stripping torque
Some self-tapping screws have been designed
specifically for use with plastics and these have the
advantage of having a greater difference between driving
and stripping torque than the typical screws designed
for metal. These special fasteners can provide an
additional safety factor for automated assembly.
Pull Out Force Calculation
The strength of a joint can be characterized
by the amount of force required to pull-out a screw.
The pull out force can be estimated by using the
following equation:
where
F = πSDL
F = Pull out force
S = Shear strength
D = Pitch diameter
L = Thread engagement length
When repeated assembly and disassembly are required
or expected, threaded metal inserts should be used
instead of self-tapping screws.
Threaded Inserts
Threaded metal inserts can be used to provide
permanent metal threads in a plastic part; a wide
variety of sizes and types are available. Inserts are
usually installed in molded bosses whose internal
diameter is designed for the insert. The most commonly
used metal inserts are either molded-in or ultrasonically
placed in the part as a secondary operation. In the case
of the molded-in insert, the insert is placed in the mold
and the plastic is injected around it. Stress will develop
when the plastic cools around the insert. To reduce this
stress, heat the inserts to the temperature of the mold.
The ultrasonic insert is pressed into the plastic by melting
the plastic with high frequency vibrations, generated by
an ultrasonic welding machine. The ultrasonic welding
melts material around the metal insert as it is being
installed, forming a bond between the insert and the
plastic that is usually strong and relatively free of stress.
Figure 77 depicts the recommended insert and boss
designs for use with Amodel ® PPA resin.
Design Information
Amodel ® PPA Design Guide
75