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Perhaps
the most critical facet of ultrasonic welding is joint design (the configuration
of two mating surfaces). It should be considered when the parts to be welded are
still in the design stage, and incorporated into the molded parts. There are a
variety of joint designs, each with specific features and advantages. Their selection
is determined by such factors as type of plastic, part geometry, weld requirements,
machining and molding capabilities, and cosmetic appearance.
The butt joint with energy director
is the most common joint design used in ultrasonic welding, and the easiest to
mold into a part. The main feature of this joint is a small 90" or 60"
triangular shaped ridge molded into one of the mating surfaces. This energy director
limits initial contact to a very small area, and focuses the ultrasonic energy
at the apex of the triangle. During the welding cycle, the concentrated ultrasonic
energy causes the ridge to melt and the plastic to flow throughout the joint area,
bonding the parts together.
For easy-to-weld resins (amorphous
polymers such as ABS, SAN, acrylic and polystyrene) the size of the energy director
is dependent on the area to be joined. Practical considerations suggest a minimum
height between .008 and .025 inch (.2 and .6 mm).
Crystalline
polymers, such as nylon, thermoplastic polyesters, ocetal, polyethylene, polypropylene,
and polyphenylene sulfide, as well as high melt temperature amorphous resins,
such as polycar-bonate and polysulfones are more difficult to weld. For these
resins, energy directors with a minimum height between .015 and ,020 inch (.4
and .5 mm) with a 60" included angle are generally recommended.
The
90" included angle energy director height should be at least 10% of the jioint
width, and the width of the energy director should be at least 20% of the joint
width. Image 1 (to the right) shows a butt joint with a 90" included
angle energy director. With thick-walled joints, two or more energy directors
should be used, and the sum of their heights should equal 10% of the joint width.
To
achieve hermetic seals when welding poly-carbonate components, it is recommended
that a 60" included angle energy director should be designed into the part.
The energy director width should be 25% to 30% of the wall thickness. Image
2 (to the right) shows a butt ioint with a 60" included angle energy
director. Image 3 (to the right) shows how the ports should be dimensioned
to allow for the flow of molten material from the energy director throughout the
ioint area.
With assemblies whose components are mode of identical
thermoplastics, the energy director can be designed into either half of the assembly.
However, when designing energy directors into assemblies consisting of a part
mode of copolymers or terpolymers, such as ABS, and another part made of a homopolymer
such as acrylic, the energy director should always be incorporated into the homopolymer
half of the assembly.
The step joint with energy directory is illustrated
in Image 4 (to the right). This joint molds readily, and provides a strong,
well aligned joint with a minimum of effort. This joint is usually stronger than
a butt joint due to the fact that material flows into the vertical clearance.
The step joint provides good strength in shear as well as tension, and is often
recommended where good cosmetic appearance is required. When working with crystalline
meterials a 60° included angle enegy director should be used instead of the
90° included angle energy director.
The tongue and groove
joint with energy director is illustrated in Image 6 (to the right).
This joint is used primarily for scan welding, self location of parts, and prevention
of flash both internally and externally. It provides the greatest bond strength
of the three joints discussed so far.
The shear joint of interference joint shown
in figure 7 is generally recommended for high-strenght hermetic seals of parts
with square corners or rectangular designs, especially with crystalline resins.
Initial
contact is limited to a small area which is usually a recess or step in either
of the parts. The contacting surfaces melt first. As the parts telescope together,
they continue to melt along the vertical walls. The smearing action of these two
melt surfaces eliminates leaks and voids, making this the best joint for strong
hermetic seals.
Several important aspects of the shear joint
should be considered 1) the top part should be as shallow
as possible, 2) the outer walls should be well supported by
a holding fixture, 3) the design should allow for a clearance
fit, and 4) a lead-in (A) should be incorporated.
Less than 0.75"
(19 mm) |
0.008" to 0.012"
(0.2 to 0.3 mm) |
.75" to 1.50"
(19 to 38 mm) |
0.012" to 0.016"
(0.3 to 0.4 mm) |
Greater than 1.50"
(38 mm) |
0.016" to 0.020"
(0.4 to 0.5 mm) |
The
shear joint requires weld times in the range of 3-4 times that of other joint
designs because larger amounts of resin are being welded. In addition, a certain
amount of flash will be visible on the surface after welding.
The scarf joint, illustrated in Image 11, is generally
recommended to high-strength hermetic seals on parts with circular or oval designs,
especially with crystalline resins.
Image: 11
The scarf joint requires
that the angles of the two parts be between 30' and 60' and be within one and
one half degrees. If the wall thickness is .025" (0.63mm) or less, an angle
of 60' should be used. If the wall thickness is .060" (1.52mm) or more, an
angle of 30' should be used. Intermediate angles are recommended for wall thickness
between .025" and .060" (.063 and 1.52mm).
A minimum
wall thickness of .030" (0.76mm) at the outer edge of the scarf is recommended
to prevent "blowout," or melting clear through the wall, during welding.
The
scarf joint is not commonly used due to the difficluties encountered in maintaining
component concentricity and dimensional tolerances. However, this joint is highly
recommended when limited wall thickness preclude the use of a shear or modified
shear joint.
A modified scarf joint is illustrated in Image
12.
As shown in Figure 13, a flash well can be incorporated
in the scarf joint to contain the excess molten material generated when the parts
are welded. The length of the well should be at least equal to the cross sectional
thickness of the part being welded.
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