Laser Vibrometers Help Optimize Production of Ultrasonic Welding Tools for Joining Thermoplastic Materials

In the plastics processing industry, ultrasonic welding is highly respected. This technology enables high process speeds with consistent and reproducible weld quality along with low energy requirements.

This technique offers benefits for high-volume production applications in the textile, semiconductor, packaging, medical, automobile, and electrical industries. Polytec single point and scanning vibrometers assist in the development of optimal ultrasonic welding tools.

Introduction

In contrast to traditional technologies, for example, thermal welding or gluing, ultrasonic welding does not affect the properties of a material. Additionally, ultrasonic technology provides multilayer laminating or welding and multiple process steps can be performed in a single step, such as cutting, perforating, and welding.

Based in Karlsbad, Germany, Herrmann Ultrasonics is a specialist in joining thermoplastic materials using ultrasound. Within the Plastics, Packaging and Non-wovens business units, solutions specific to each customer are discovered for a variety of ultrasonic welding applications.

How it Works

Mechanical vibrations are transferred to the plastic parts under pressure during ultrasonic welding. Molecular and interfacial friction generates warmth, which enhances the attenuation coefficient of the material.

The plastic starts to soften locally. This reaction is self-accelerating because of the increase in the attenuation of the plasticized material, and a major share of the vibration energy is transformed into heat. While maintaining joint pressure and after welding, a brief cool-off phase is required to homogeneously solidify the previously plasticized material.

Next, the rolls or parts of material now joined together using ultrasonic energy can be processed further. The ultrasonic welding method is initiated with a stack. The stack consists of a piezoelectric converter A, the amplitude transformation piece B and the actual sonotrode C (Figure 1).

The design of the stack. The vibration amplitude (red) is being accelerated on its route from the converter (A) to the sonotrode (C).

Figure 1. The design of the stack. The vibration amplitude (red) is being accelerated on its route from the converter (A) to the sonotrode (C).

High-Quality Requirements

A requirement for effective welding results in terms of density, optical quality, and stability of joints is joint tools that are engineered to suit both the material and the process.

The vibrational characteristics of each component are of significant importance in this respect, particularly regarding the vibration amplitudes. Every component of the ultrasonic stack is tested as an individual unit at Herrmann Ultrasonics. The amplitude measurement is of the greatest importance.

Measuring the Vibration Amplitudes

The vibration amplitudes are verified utilizing a Polytec single point or a Scanning Vibrometer depending on which individual component is at issue. Transformation and converter pieces are commercially available. These are standard components with strictly specified and fixed output vibration amplitudes.

The vibration amplitudes are verified by performing measurements with a Polytec CLV Compact Laser Vibrometer, therefore ensuring that they are within the correct range. Sonotrodes are distinct components customized to match the welded workpiece.

These components must geometrically fit the workpiece and generate important ultrasonic amplitude levels sufficiently high enough to create good welds. A 3-D CAD model (demonstrated in Figure 2) is employed to design and develop the sonotrodes by prototyping them on the workpiece.

The vibrational characteristics are then optimized using Finite Element Model (FEM) analysis until they achieve the required parameters. Only then does the production of a sonotrode occur. The features of the completed sonotrode are analyzed with the aid of a PSV-400 Scanning Vibrometer.

Figure 3 shows the measurement layout; on the left is the PSV-400 Sensor Head, on the right is the sonotrode mounted onto a fixture and in the middle there is the measurement screen showing the video image of the sonotrode surface in the PSV Software.

The amplitudes occur at a particular frequency and are evaluated by applying the PSV-400 Scanning Vibrometer to specific points of the sonotrode surface (Figure 4). For this measurement, specially configured software reduces time spent, enables safe operation, and provides user documentation.

The measured amplitude distribution is compared with the value taken from the FE model. The sonotrode can then be optimized further if necessary.

To gain further information, please contact a local Polytec sales/application engineer or visit Polytec’s web page www.polytec.com/industrial.

Modeling the dynamic behavior of a sonotrode.

Figure 2. Modeling the dynamic behavior of a sonotrode.

Measurement layout to characterize the sonotrodes.

Figure 3. Measurement layout to characterize the sonotrodes.

Amplitude distribution on the surface of the sonotrode.

Figure 4. Amplitude distribution on the surface of the sonotrode.

This information has been sourced, reviewed and adapted from materials provided by Polytec.

For more information on this source, please visit Polytec.

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