by Bill Heatherwick, Automotive Market Manager, Branson Ultrasonics Corporation
I often hear the following questions about vibration welding: “What is the best application for vibration welding?” and “What material is best suited for the vibration welding process?” Taken at face value, these questions appear to be simple to answer; however, they are not. In order to begin to address these questions, we must go back to the beginning of vibration welding.
Thirty Years Ago
The linear vibration welding process was developed as a collaborative effort between Branson and DuPont in the early 1970s. Branson produced the first commercial vibration welder for joining a cover to a canister for an automotive apparatus commonly referred to as a “carbon vapor canister.” The weld requirements were for a strong structural bond as well as a hermetic seal. The material for this application was a semi-crystalline thermoplastic (Nylon 6-6). Such commercially available joining methods as ultrasonic welding, hot plate welding, and mechanical fasteners could not produce a final assembly that would meet the application requirements. So a new process that generated heat via friction using a linear reciprocating motion – linear vibration welding – was introduced to the market.
Vibration welding is a plastic joining technology in which the required melt temperature is generated by friction in the bond area. Typically, this is done using a two-piece sandwich configuration consisting of an upper and lower part to be joined. In general, one of the two parts to be joined is kept stationary while the other is moved in a reciprocating motion at a predetermined amplitude, frequency, and force for a given period of time. Upon completion of the melt phase, the parts are re-aligned and allowed to solidify under clamp force, resulting in a finished weld.
The key parameters of this process are amplitude, frequency, and clamp force. Amplitude is the displacement through which one part moves relative to the other. Frequency is the number of vibration cycles per second that the two parts are moved during welding. Clamp force is the pressure applied to the parts being welded. The process is similar to rubbing your hands together to warm them.
Originally, vibration welding seemed to be ideally suited for two areas: 1) Automotive components that required large areas to be welded or those that were large in physical size such as bumpers or instrument panels, and 2) The joining of semi-crystalline thermoplastics materials that required more energy or heat to bond them together due to their higher melt temperatures. That required the development of two primary frequencies: 100Hz (low frequency) that would accommodate the welding of large parts, and 240Hz (high frequency) for smaller parts.
The first of these “large” applications to be put into production in the early 1980s was a plastic bumper in which the end user welded plastic attachment brackets to the bumper fascia. Since that time, vibration welding has been used to assemble a myriad of automotive plastic components ranging from intake manifolds, fluid reservoirs, tail lamps, instrument panels, instrument clusters, center consoles, spoilers, and exterior cladding. In addition, the technology has found applications in white goods, consumer electronics, home and garden, and building and construction. These parts all range in material, size, shape, and complexity.
Today you can find a wide variety of commercially available vibration welding machines. These machines range from small formats, suitable for parts that easily could fit in your hand, up to a large format that is capable of assembling a full-sized automobile instrument panel. You may wonder why so many machine models are required, but that would be like asking a molder why there are so many different sizes of molding machines.
The key criteria for sizing the machine revolve around the following:
1. The type of material being considered for the application
2. The physical size of the parts to be assembled
3. The weld area to be joined during the welding process
4. The weight of the upper (drive) tool
Most machines have a recommended minimum and maximum tool-weight range along with guidelines on physical part size and weld area, which generally is based on the sizes of upper and lower machine tooling platens. The improper selection or use of a machine that is not sized correctly may result in poor weld quality or premature equipment failure.
Optimal Welding Parameters
The number of commercially available thermoplastic materials has only grown since the early 1970s, along with the number of resin manufacturers and compounders. That has resulted in working closely with resin suppliers to conduct weld studies which, in turn, helped to develop welding parameters for the various thermoplastic materials. These weld studies have led to many published papers and documents that suggest there are optimal welding parameters for a given material. Some materials may produce acceptable results at relatively low welding amplitude/frequency. Other materials require greater amounts of welding power, frequency, or amplitude to optimize the resulting weld.
It also is important to note that not all resins are created equal. Changing materials or their formulations can have an effect on weld strength and may require weld parameter re-optimization for your application.
Today, vibration welding is a common joining technology used all over the world. It is not uncommon to walk through a manufacturing plant and hear the machines low humming noise and know that the company is using vibration welding technology. The scope and breadth of applications can be mind-numbing. The ever-increasing use of plastics and the continuous addition of new polymers and copolymers should bring new applications and industry uses to the vibration welding process. This process can be used to create both molecular (polymer to polymer) and some mechanical (polymer to non-polymer) bonds.
So when asked if there is a single “best” application for this process, I can say with certainty that there is not. When asked if there is a best material for this process I can tell you that there are materials commonly associated with this process; however, you really are only limited by the design creativity and compatibility of the materials chosen for a particular application. I encourage you to consider vibration welding technology because that’s how innovations are born – much like the vibration welding process itself some 30 years ago.
Bill Heatherwick is the automotive marketing manager at Emerson Industrial Automation, Branson Ultrasonics Corp., in Rochester Hills, Mich. He can be reached at (248) 299-0400, ext. 108, or e-mail [email protected]