Technolog

WELDING l STAKING l INSERTION l SWAGING l FORMING l SPOT WELDING l DEGATING l CUTTING AND SEALING

Amplitude ProfilingTM and Weld Strength

TL-7

Amplitude ProfilingTM and Weld StrengthAmplitude 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 ampli-tude and booster should be appropriate for the material and application, based on the guide-lines 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. Amplitude controls the temperature at the joint;

B. Temperature at the joint controls melt viscosity.

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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).

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 ampli-tude enables the user to control the melt flow (B).

When the melt flow moves quickly across the weld surfaces (high temperature due to high ampli-tudes), 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 previ-ously 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.

Figure 1. Amplitude at the face of the horn – equation.

TL-7© Branson UltrasonicsCorporation, 1996Reprinted in U.S.A. 4/04

When the melt flow moves quickly acrossthe weld surfaces (high temperature due to highamplitudes), the molecules of the plastic tend toalign themselves with the flow. If the velocity ofthe melt is moderate, the molecules tend toalign themselves in a more random orientationand become entangled (see Figure 4). If the mol-ecules are entangled, the weld is stronger. Inorder to break the weld, a fracture must formthrough or around the molecular chains.

The effects of Amplitude Profiling onmolecular entanglements in the weld area havebeen confirmed by FTIR (Fourier TransformInfrared) microspectometry.

The second mechanism that helps producestronger welds is the reduction of residual stress-es. Residual stresses are stresses remaining fromthe welding process and are produced by squeezeflow and temperature gradients. As previouslydetailed, squeeze flow is moderated by Ampli-tude Profiling, and this effect alone may reduceresidual stresses. Additionally, there are fewertemperature gradients (because of the modera-tion of the weld area temperature), thereby fur-ther reducing residual stress.

As the graph in Figure 5 shows, there is a34% decrease in the residual stresses in the weld

area when amplitude profiling is used. Becauseof this, the energy required to break a weld ishigher and, therefore, the weld is stronger.

While the benefits of Amplitude Profilingwill vary for each application and can only bedetermined by testing, laboratory tests haveshown increases from a few percent to 100%.In general, a typical application should gain 10-30% in weld strength when AmplitudeProfiling is used.

Setting Up Amplitude ProfilingWhile every application requires fine tun-

ing of the welding parameters and the ampli-tude profile, there are three guidelines that canbe used to set up an application withAmplitude Profiling:

1) Choose the starting amplitude using Bran-son’s Amplitude Reference Guide (TL-2);

2) Drop the amplitude by 30-70% of the ini-tial amplitude at a point 50-90% into theweld cycle time;

3) Extend the weld cycle time by 20-50%.

Material Suitability for Amplitude ProfilingAmplitude Profiling is suitable for both

amorphous and semi-crystalline materials. Thechart below illustrates the typical percentage ofincrease in weld strength when amplitude pro-filing is used. Results will vary depending onthe specific application and equipment used.

www.Branson-PlasticsJoin.com

Figure 3. Weld area temperatures at different amplitudes

Figure 4. Orientation with and without AmplitudeProfiling.

Figure 5. Residual stress in welds made with andwithout Amplitude Profiling.

% IncreasedMaterial Weld StrengthPolycarbonate 10-65

ABS 20-85

Nylon 0-15

PBT 0-10

Figure 4. Orientation with and without Amplitude Profiling.

Converter Silver Booster High Gain Horn 20 Microns Gain = 2 Gain = 3

120microns=

Converter Gold Booster Unity Gain Horn 20 Microns Gain = 1.5 Gain = 1

30microns=X X

X X

Figure 2. Changes in amplitude – a typical cycle graph.

Time (ms)0 500 1000 1500

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Figure 3. Weld area temperatures at different amplitudes.

78µm Total 88ms78µm ➠ 20µm @ 250mS, Total 1700mS20µm Total 3000mS

Time (mS)

78µm

20µm

78µm ➠ 20µm

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product name

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ISO CertificationsTL-7 © Branson Ultrasonics Corporation 2012The Emerson logo is a trademark and service mark of Emerson Electric Co. Revised and printed in the U.S.A. 8/28

Branson Ultrasonics Corporation41 Eagle Road, Danbury, CT 06813-1961(203) 796-0400 • Fax: (203) 796-9838www.bransonultrasonics.come-mail: info@bransonultrasonics.com

Technolog

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.

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%.

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.

W/ PROFILING NO PROFILING0

5

10

15

20

Res

idua

l Str

ess

Leve

l (M

Pa)

Figure 5. Residual stress in welds made with and without Amplitude Profiling.

Material % Increased Weld Strength

Polycarbonate 10-65

ABS 20-85

Nylon 0-15

PBT 0-10

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