ULTRASONIC PLASTIC WELDING EXPERTS PUT ALGOR FEA TO THE TEST

Ultrasonic plastic welding is used in the production of a vast array of plastic products. These custom designed horns transfer ultrasonic energy to the plastic material.

If you've ever examined a product made of plastic to see how it was constructed, you may have assumed that the various parts were glued together. In fact, there's a good chance that the pieces were joined using a process known as ultrasonic plastic welding. There's also a good chance that the equipment used to weld the pieces was designed using Algor Finite Element Analysis (FEA) software.

Ultrasonic welding is used in a wide variety of industries including automotive, toys, packaging, textiles, medical equipment and electronics. The process eliminates the need for costly and often dangerous adhesives and provides a bond that is strong and permanent.

The equipment that performs ultrasonic welding consists of several components. An ultrasonic generator converts standard 50-60 Hz electrical current into 20,000 or 40,000 Hz (20-40 kHz) current. The current passes through a transducer which uses piezoelectric ceramics to convert the electrical energy into ultrasonic mechanical motion. The motion is passed to a booster which regulates the amplitude of the vibration. These parts are similar in all ultrasonic welding equipment.

The ultrasonic energy is then passed to a horn which is machined from metals such as aluminum or titanium. This part must be custom designed for each specific application. The horn transfers the ultrasonic vibrations to the plastic causing it to melt at the point where two surfaces meet. The bond created by this controlled melting is as strong as the parent material.

Horns come in an almost endless variety of sizes and shapes and each one must be carefully optimized to assure that it resonates in a purely axial mode and that the vibration is constant across the area of the horn in contact with the plastic. In addition, it is essential that unwanted adjacent non-axial resonances be avoided because they can interfere with the welding process. Due to the high frequencies involved, all horns are subject to fatigue stress.

Two Experts Agree

	Kevin O'Shea, Research Engineer for Branson Ultrasonics Corporation.
	Donald R. Culp, President, Krell Engineering.

Recently, two design engineers presented research papers on the use of finite element analysis in the design of horns for ultrasonic welding. Donald R. Culp is president of Krell Engineering, a Baltimore, Maryland, consulting firm which specializes in the design of ultrasonic welding components. Kevin O'Shea is a research engineer for Branson Ultrasonics Corporation located in Danbury, Connecticut. Branson is the leading manufacturer of ultrasonic welding equipment.

FEA Versus Cut-and-Try

In both papers, the researchers compared the use of Algor FEA to the conventional "cut-and-try" method of horn design in which an engineer uses his experience to create an initial configuration, and then performs tests on several prototype horns to optimize the design. The researchers found that FEA can be used to reduce the need for these time consuming and expensive prototypes.

Many designs utilize slots which are machined through the horn. By varying the number, length, width and spacing of the slots, the designer can assure that the horn will resonate in a longitudinal mode. These adjustments also allow the engineer to "tune" the horn to achieve a more uniform level of vibration at the output surface, and provide as much separation as possible between the desired axial frequencies and unwanted transverse frequencies.

Comparing with Reality

Both studies compared finite element analysis results of resonant frequencies and frequency distribution to measurements using actual prototype horns. The results show that FEA is a viable alternative to the "cut-and-try" method for critical horn designs that otherwise would require construction and evaluation of a large number of prototypes.

Branson Ultrasonics Corporation

Kevin O'Shea (pictured above), research engineer for Branson Ultrasonics Corporation, has been a pioneer in the use of finite element analysis in the design of ultrasonic plastic welding components. He has compared the results of finite element analysis with real world tests for a large number of ultrasonic horn configurations.

Model shows modal analysis for resonant frequencies. Green lines are undeflected shape.

Above is a model of a 4.5" x 6" cross-section titanium horn designed to resonate axially at 20 kHz. In this case, Mr. O'Shea was optimizing the location, size and number of slots to be machined into the part. "Mode shapes and frequencies from 17-23 kHz were examined both experimentally and using FEA," says Mr. O'Shea. "An optimum configuration based on the results was prescribed. An excellent correlation between the analytical and experimental data was found."

The following table shows a comparison between FEA and prototype testing for frequency distribution analysis.

Krell Engineering

Donald R. Culp (pictured above), president of Krell Engineering in Baltimore, Maryland, has published several papers on the use of finite element analysis software in the design of ultrasonic plastic welding components.

In this example, Mr. Culp analyzed a 5" x 5" aluminum horn designed to operate at 20 kHz. In his words: "This horn is widely used for ultrasonic plastic assembly. It is prone to fatigue stress cracking at the ends of the slots. FEA is the only way to verify this high stress condition because it allows me to see the stress levels in the interior of the horn where they can't be measured."

Hidden element view of model shows highest stress levels are in horn interior.

The model shown above contains 2972 type 5 brick elements and has 3850 nodes. Through careful modeling, Mr. Culp was able to accurately represent the complex geometry where the perpendicular slots intersect in the horn's interior.

The above table illustrates a high correlation between FEA and prototype results for resonant frequencies.

Copyright © 1991 Algor, Inc. All rights reserved.