Fundamentals Of Ultrasonic plastic welding

Donald R. Patten
Applications Manager,
Plastics Stapla Ultrasonics Corp.
Wilmington, Mass.

The last two decades have seen substantial improvements in ultrasonic plastic-welding machines. Bench-top models feature proportional-valve controls, linear encoders, line-voltage regulation, internal process-control functions, and more. These advances let OEMs glean better yields and efficiencies with welders that are more robust, easier to use, and feature highly integrated calibration and process controls.

It's useful to understand the function of the ultrasonic welding machine's components and how the system works. This helps streamline the process of determining the best welding method for each application.

MACHINE BASICS
At the heart of ultrasonic plastic-welding machines is the transducer assembly — often referred to as the "stack." The stack typically consists of three components: the converter, booster, and weld horn. These parts connect together at a specific torque value and function as a resonating or vibrating tool.

The converter is the vibration source. Inside the converter are piezoelectric ceramic disks sandwiched around metal plates. They are clamped under high force to a carefully designed titanium cylinder. A generator supplies the high-frequency electrical energy to the system power module. A cable feeds the power to the converter and the piezoelectric ceramics expand and contract in response to the sinusoidal, high-frequency electrical signal. The titanium cylinder transmits this mechanical vibration to the remaining components of the stack.

The vibrations generated by the converter are approximately 9mm measured zero to peak. This is not enough useful vibration to get a melt temperature for most thermoplastic resins. The booster amplifies the input vibration, amplifying the vibration according to a ratio defined by the booster design. For example, with a 1:2.0 booster, a 9m input would give a measurable vibration output of 18 m at the end of the booster segment.

The weld horn transfers the generated energy to the thermoplastic workpieces. The horn design matches the workpiece contours. It attenuates the vibration delivered from the booster to a specific value and applies the forces necessary for the process to work. The weld horn and booster are designed to have an axial mode of vibration at the frequency that the generator signal oscillates the converter.

The resulting mechanical vibration initiates melt at the interface of the two thermoplastic components. The term used to describe the amount of vibration in an ultrasonic stack is " amplitude." Each thermoplastic will have a specific amplitude range that will efficiently initiate melt. The right converter, booster, and properly designed weld horn are all key for delivering the amplitude a particular application needs. Amplitudes out of range can cause poor bonds or destroy the materials all together.

Trial and error is one way of determining a thermoplastic's most efficient amplitude. But most suppliers of ultrasonic welders have amplitude data available for a wide range of thermoplastics. For example, polycarbonate requires an 18 to 22- m (0 to peak) amplitude. However, other factors impact the needed amplitude. These can include the size of the plastic components, the offset from the end of the weld horn to the interface between the two plastic components, and additives in the plastic resin.

ADVANCED TECHNOLOGY
Plastic-welding machines have many settings that adjust amplitude, force, and exposure. These three factors interact and affect the success or failure of the plastic melt or welding process. Plastic melt occurs when the material at the part-to-part interface melts and the parts bond with the necessary strength and seal requirements of the assembly.

Amplitude is an expression of the amount of expansion at the end of the weld horn from a static position, measured in microns. It can be set as either a percentage value, as a process parameter, or as a relative position of a pot. It is measured externally as microns (0 to peak, or peak to peak), or as a decimal value of an inch. For most thermoplastic resins, empirical data suggests a range of amplitude that should be present at the end of the weld horn in order to achieve a "normal" welding process. For a 20-kHz ultrasonic-welding process, for example, polystyrene (PS) requires 15 to 20 m (0 to peak), and high-density polyethylene (HDPE) needs 45 to 50 m (0 to peak).

Force is the actual applied force pressing on the parts, measured in Newtons, or pounds of force (lbf). The amount of force required for a " normal" welding process is primarily influenced by the size of the parts. As a general rule, about 1 lbf gives 1 linear mm of energy directed at the part-to-part interface. For example, a 1.5-in.-diameter assembly needs a welding force of about 120 lbf. Depending on the equipment, the unit of measure for force could be psig, bar, lbf, or Newton (N). Last, exposure can be a time value, distance value in standard or metric units, or power as watts, Joules, or watt/sec.

Exposure is the duration of the application of vibration and force, measured in seconds.

Advances in ultrasonic plastic technology address each of these variables.

Line-voltage regulation advances in generators and power modules ensure the high-frequency signal applied to the converter produces output amplitudes that lie within tightly controlled specifications. This is critical to the welding process. Amplitude is the primary parameter that makes the welding process efficient. The booster and weld horn components are merely reactive segments of the compression waveform in the stack. Therefore the input amplitude from the converter must be correct to get the target amplitude at the end of the weld horn.

The amplitude of the system needs calibration to maintain weld quality and consistency. Users should note each converter is manufactured within an exacting specification. The amplitude output of any converter will be within specification regardless the generator it attaches to. This lets a validated process be established on one machine, perhaps in an R&D lab. The process can then transfer to another machine — in another manufacturing facility, for example — with a high degree of confidence in performance.

Proportional-valve assemblies control the pneumatic system of a plastic-welding machine. They thus dynamically control the forces throughout the plastic-welding process. Amplitude and force are interactive. Amplitude describes the compressive waveform applied to the workpiece. Force describes the method of transferring the vibration into the workpiece. The control voltages of the proportional valve are calibrated. This calibration lets a numeric value for the welding parameters of trigger force, weld force or hold force represent an actual applied force.

Additionally, a pressure transducer in the proportional-valve system provides closed-loop control of the pressure in the drive cylinder. This eliminates variations in the incoming air pressure. System calibration enables validated process parameters to be implemented on multiple machines. In fact, there can be a high level of confidence for process-validation protocols performed with ultrasonic plastic-welding machines that feature calibrated amplitude and force.

Exposure is set by the weld method and is the factor that turns off the power module. The four standard weld methods are: time, energy, relative distance, and absolute distance. Of course, various manufacturers of ultrasonic plastic-welding equipment may use slightly different terms for these methods; for example, one may use the term "collapse" for relative distance.

Plastic assemblies specifically designed for ultrasonic welding incorporate geometries at the interface between the two-or-more components that help initiate plastic melt in the weld joint area. These geometries are based on standard joint design criteria. They represent a volume of plastic to be melted. As the plastic melts and flows within the joint the welder can monitor a weld distance or collapse. This distance strongly correlates to the strength of the weld joint.

Some assemblies require tight tolerances on the overall length (OAL) of the finished part. Exposure schemes based on absolute distance will stop the welding process when they see the OAL value. Occasionally, production rate or weld station timing in a fully automated assembly line is more important than optimizing the strength of the weld joint. In that situation time is the preferred weld method.

Many systems also offer energy as a weld method. They use a specific energy value calculated by the power module. Note this energy value is the amount of power used to maintain the amplitude output of the converter. The dynamics of initiating and sustaining the plastic melt in the joint design area affects how much power the converter needs. But other process variables can also impact the power and energy values. Sometimes an energy value correlates to a good weld. However, it is common practice to base the decision about weld method on data from weld trials that the welding-equipment supplier performs.

MAKE CONTACT:
Stapla Ultra Sonics Corp.,
(978) 658-9400, www.staplaultrasonics.com