General Design Principles for DuPont Engineering Polymers - Module

Part Design Considerations
Part design is an important variable, frequently
overlooked until tooling has been completed and
attempts have been made to weld the first molded
parts.
a) Joint Design
Perhaps, the most critical facet of part design for
ultrasonic welding is joint design, particularly with
materials which have a crystalline structure and a
high melting point, such as DuPont engineering
plastics. It is less critical when welding amorphous
plastics. There are two basic types of joints, the
shear joint and butt type joint.
Shear Joint
The shear joint is the preferred joint for ultrasonic
welding. It was developed by engineers at DuPont’s
Plastics Technical Center in Geneva in 1967, and has
been used worldwide very successfully in many
applications since that time. The basic shear joint with
standard dimensions is shown in Figure 11.47 and
11.48 before, during and after welding.
Figure 11.47 Shear joint—dimensions
E
C
B
A
B
D
B
Dimension A 0.2 to 0.4 mm. External dimensions.
Dimension B This is the general wall thickness.
Dimension C 0.5 to 0.8 mm. This recess is to ensure
precise location of the lid.
Dimension D This recess is optional and is generally
recommended for ensuring good contact
with the welding horn.
Dimension E Depth of weld = 1.25 to 1.5 B for maximum
joint strength.
Figure 11.49 shows several variations of the basic
joint. Initial contact is limited to a small area which is
usually a recess or step in either one of the parts for
alignment. Welding is accomplished by first melting
the contacting surfaces; then, as the parts telescope
together, they continue to melt along the vertical
walls. The smearing action of the two melt surfaces
eliminates leaks and voids, making this the best joint
for strong, hermetic seals.
100
Figure 11.48 Shear joint—welding sequence
D
C
Before
welding
B
A
B
E
B
During
welding
Flash
Weld
Supporting
fixture
Figure 11.49 Shear joint—variations
After
welding
Flash
The shear joint has the lowest energy requirement and
the shortest welding time of all the joints. This is due
to the small initial contact area and the uniform
progression of the weld as the plastic melts and the
parts telescope together. Heat generated at the joint is
retained until vibrations cease because, during the
telescoping and smearing action, the melted plastic is
not exposed to air, which would cool it too rapidly.
Figure 11.50 is a graph which shows typical weld
results using the shear joint. It is a plot of weld time
vs. depth of weld and weld strength. Depth and
strength are directly proportional.
Figure 11.50 Shear joint—typical performance
Depth of weld, mm
Burst pressure, MPa
4
3
2
1
0
0 0.4 0.8
1.2 1.6
100
50
Weld time, sec
0
0 0.4 0.8
1.2 1.6
Weld time, sec