Technolog
TL-7
WELDING l STAKING l INSERTION l SWAGING l FORMING l SPOT WELDING l DEGATING l CUTTING AND SEALING
Amplitude ProfilingTM and
Weld Strength
Amplitude ProfilingTM and
Weld Strength
Amplitude ProfilingTM provides many potential benefits for ultrasonic welding, including:
• Increased weld strength.
• Reduced part marking.
• Minimize or eliminate damage caused by diaphragming.
• Minimize or eliminate damage to delicate internal components.
• Less flash and particulate.
This TechnoLog discusses how Amplitude Profiling increases weld strength.
What Is Amplitude?
Amplitude is the peak-to-peak longitudinal
displacement at the face of the horn and is the
most critical parameter in ultrasonic welding
(refer to TechnoLog TL-2, Amplitude Reference
Guide, for more detail on amplitude).
As shown in Figure 1, the horn face amplitude
is the product of the gains of the horn,
booster, and converter.
What Is Amplitude Profiling?
In standard practice, the amplitude of the horn
face remains constant during the entire weld
cycle. Recent advances in technology have
made it possible to change the amplitude of
the horn face during the weld cycle.
This is known as Amplitude Profiling. Figure 2
illustrates a typical profile where the amplitude
is reduced during the cycle. This type of profile
can increase weld strength.
How Amplitude Profiling Increases
Weld Strength
During the ultrasonic welding process,
the thermoplastic material goes through
several phases:
1. Start of melting (solid to solid material
contact);
2. Buildup of melt (molten and solid layer);
3. Steady state melt (molten layer);
4. Solidification of weld (during hold time).
To start the weld (phases 1 and 2), the amplitude and booster should be appropriate for the
material and application, based on the guidelines in TL-2. This will ensure that the entire
welding surface is heated uniformly and quickly
to its softening point. Once the molten layer is
fully formed and the entire weld surface has
been wetted with the molten polymer (phase 3),
the amplitude should be reduced to initiate the
two mechanisms which increase weld strength:
1. Molecular entanglement
2. Reduced residual stresses
The first mechanism, increased molecular
entanglement, is the result of two basic physical
relationships:
A. A
mplitude controls the temperature at
the joint;
B. T
emperature at the joint controls melt
viscosity.
When the melt flow moves quickly across the weld
surfaces (high temperature due to high amplitudes), the molecules of the plastic tend to align
themselves with the flow. If the velocity of the
melt is moderate, the molecules tend to align
themselves in a more random orientation and
become entangled (see Figure 4). If the molecules
are entangled, the weld is stronger. In order to
break the weld, a fracture must form through or
around the molecular chains.
The effects of Amplitude Profiling on molecular
entanglements in the weld area have been
confirmed by FTIR (Fourier Transform Infrared)
microspectrometry.
The second mechanism that helps produce
stronger welds is the reduction of residual
stresses. Residual stresses are stresses remaining
from the welding process and are produced by
squeeze flow and temperature gradients. As previously detailed, squeeze flow is moderated by
Amplitude Profiling, and this effect alone may
reduce residual stresses. Additionally, there are
fewer temperature gradients (because of the
moderation of the weld area temperature),
thereby further reducing residual stress.
Converter
20 Microns
X
Silver Booster
Gain = 2
X
High Gain Horn
Gain = 3
120
= microns
Converter
20 Microns
X
Gold Booster
Gain = 1.5
X
Unity Gain Horn
Gain = 1
=
30
microns
Figure 1. Amplitude at the face of the horn – equation.
area when amplitud
of this, the energy r
higher and, therefo
100
80
Amplitude (µm)
As thermoplastic materials such as polycarbonate,
ABS, and polystyrene are heated, they gradually
soften, or more precisely, their viscosity
decreases. The higher the temperature, the easier
the melt flows. Therefore, controlling the amplitude enables the user to control the melt flow (B).
60
40
20
0
0
500
1000
1500
Time (ms)
Figure 3. Weld area temperatures at different amplitudes
Figure 2. Changes in amplitude – a typical cycle graph.
When the melt flow78µm
moves
quickly across
Total 88ms
78µm ➠ 20µm @ 250mS,
78µm (high temperature
the weld
dueTotal
to1700mS
high
800 surfaces
20µm Total 3000mS
700
Figure 5. Residual stre
amplitudes), the molecules of the plastic tend to
600
without Amp
78µm ➠ 20µm
align 500
themselves with the flow. If the velocity of
20µm
the melt
the molecules tend to
400 is moderate,
While the bene
align 300
themselves in a more random orientation
will
vary
for each a
200
and become
entangled
(see
Figure
4).
If
the
moldetermined
by testi
100
ecules are
entangled,
the
weld
is
stronger.
In
0
shown increases fro
10001500200025003000
order to0
break500
the weld,
a fracture must form
In general, a typica
Time (mS)
through or around the molecular chains.
30% in weld streng
Figure 3. Weld area temperatures at different amplitudes.
Profiling is used.
Temperature (C)
In terms of (A), the average heating rate of
the polymer is proportional to the amplitude
squared. It is possible, therefore, to control
the temperature (similar to controlling the
barrel temperature in an injection molding
machine) by controlling the amplitude. Actual
temperatures are seen below at different
amplitudes (Figure 3).
Part
Part
Figure 4. Orientation with and without Amplitude Profiling.
Figure 4. Orientation with and without Amplitude
Profiling.
The effects of Amplitude Profiling on
molecular entanglements in the weld area have
been confirmed by FTIR (Fourier Transform
Infrared) microspectometry.
Setting Up Ampli
While every ap
ing of the welding
tude profile, there a
be used to set up an
Amplitude Profilin
1) Choose the star
son’s Amplitude
2) Drop the ampli
tial amplitude a
weld cycle time
3) TL-7
Extend the weld
Material Suitabili
product
Technolog
name
TL-7
Ultrasonic Welding
As the graph in Figure 5 shows, there is a
34% decrease in the residual stresses in the
weld area when amplitude profiling is used.
Because of this, the energy required to break
a weld is higher and, therefore, the weld is
stronger.
Residual Stress Level (MPa)
20
While the benefits of Amplitude Profiling will
vary for each application and can only be
determined by testing, laboratory tests have
shown increases from a few percent to 100%.
In general, a typical application should gain
10-30% in weld strength when Amplitude
Profiling is used.
Setting Up Amplitude Profiling
While every application requires fine tuning
of the welding parameters and the amplitude
profile, there are three guidelines that can be
used to set up an application with Amplitude
Profiling:
1. Choose the starting amplitude using
Branson’s Amplitude Reference Guide
(TL-2).
2. Drop the amplitude by 30-70% of the initial
amplitude at a point 50-90% into the weld
cycle time.
3. Extend the weld cycle time by 20-50%.
Branson
Address Ultrasonics
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Address
Eagle Road,
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www.bransonultrasonics.com
e-mail: info@bransonultrasonics.com
15
10
5
0
W/ PROFILING
NO PROFILING
Figure 5. Residual stress in welds made with and
without Amplitude Profiling.
Material Suitability for Amplitude
Profiling Amplitude
Profiling is suitable for both amorphous and
semi-crystalline materials. The chart below
illustrates the typical percentage of increase in
weld strength when amplitude profiling is used.
Results will vary depending on the specific
application and equipment used.
Material
% Increased Weld Strength
Polycarbonate
10-65
ABS
20-85
Nylon
0-15
PBT
0-10
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