General Design Principles for DuPont Engineering Polymers - Module

Weld strength is therefore determined by the depth of
the telescoped section, which is a function of the weld
time and part design. Joints can be made stronger than
the adjacent walls by designing the depth of telescoping
1.25 to 1.5 times the wall thickness to accommodate
minor variations in the molded parts (see E on
Figure 11.47).
Several important aspects of the shear joint must be
considered; the top part should be as shallow as
possible, in effect, just a lid. The walls of the bottom
section must be supported at the joint by a holding
fixture which conforms closely to the outside configuration
of the part in order to avoid expansion under the
welding pressure.
Non continuous or inferior welds result if the upper
part slips to one side or off the lower part, or if the
stepped contact area is too small. Therefore, the fit
between the two parts should be as close as possible
before welding, but not tight. Modifications to the
joint, such as those shown in Figure 11.51, should be
considered for large parts because of dimensional
variations, or for parts where the top piece is deep and
flexible. The horn must contact the joint at the flange
(nearfield weld).
Figure 11.51 Shear joint—modification for large parts
0.3 mm
Support
Allowance should be made in the design of the joint
for the flow of molten material displaced during
welding. When flash cannot be tolerated for aesthetic
or functional reasons, a trap similar to the ones shown
in Figure 11.52 can be designed into the joint.
101
Figure 11.52 Shear joint—flash traps
Butt Joint
The second basic type of joint is the butt joint which
is shown in Figure 11.53, 11.54 and 11.55, with
variations. Of these, the tongue-in-groove provides the
highest mechanical strength. Although the butt joint is
quite simple to design, it is extremely difficult to
produce strong joints or hermetic seals in the crystalline
resins.
Figure 11.53 Butt joint with energy director
A
B
B
1.4 B
0.6 B
Dimension A 0.4 mm for B dimensions from 1.5 to 3 mm
and proportionally larger or smaller for
other wall thicknesses
Dimension B General wall thickness
Dimension C Optional recess to ensure good contact
with welding horn
Dimension D Clearance per side 0.05 to 0.15 mm
C
B
D
90°