by Kenneth A. Holt, Herrmann Ultrasonics, Inc.
Modern ultrasonic welding systems are capable of processing a wide array of thermoplastic parts and materials by using different welding modes. Determining which mode of operation can be intimidating to the user uninitiated with the strengths, applicability, and intricacies of each. Welding by time, energy output, peak power output, distance (either reference point or absolute), or even combinations thereof are all offered on advanced ultrasonic controllers with each having its own advantages given a specific welding operation. This article will explain each mode thoroughly while suggesting different modes and strategies for the optimization of different types of welding processes performed on various products and materials. In addition, the intricacies of interpreting the graphs of the ultrasonic welding graphs also will be explained. Further, this article will examine how these graphs can be used to optimize welding results, troubleshoot welding difficulties, and document the process for future use.
Introduction of Control Modes
Time. The time duration that ultrasonics is applied. Used for uncritical applications; easy to use; provides little repeatability.
Weld Travel Distance. Melt down distance (collapse), measured from the RPN = Reference Point Numeric (Trigger Point). Provides highly repeatable weld results, especially for rigid plastic parts with a designed weld travel.
Absolute Distance. Welding to a specified finished height regardless of trigger point. Used when the assembly height of the part is the criteria (e.g. battery case for laptop).
Weld by Energy. The weld energy expended during the time that ultrasonics is applied. Especially suitable when welding parts without a joint (extruded materials, films, textiles).
Peak Power. The maximum output in watts during the weld process. Used for various spot welding, staking, and swaging applications.
The visualization of the welding process also is a powerful tool for selecting the optimum parameters of the process. A quote from the Plastics and Composites Welding Handbook says that “prior to the application of DOE, it is necessary to perform screening experiments to determine the range for the various process parameters.” The visualization of the welding process, through graph interpretation, is a fast, efficient and easy way to do this. The affects of parameter levels can be seen instantly and related to their effects on other parameters. For instance, if booster ratios are being increased, it is easily seen when the maximum output of the generator is approached and to what degree the weld is completed at this time. Also, studies have shown that a constant downward velocity of the horn during the weld process produces more of a homogenous molecular structure (less notch effects), thus increasing weld strength, reducing standard deviation, and making for a more robust process. The ability to see this effect, and react to slower velocities with more force or amplitude in the process, is most easily done by graph interpretation. The results of the adjustments can be seen instantly.
Description of a Typical Weld Process
The beginning of a typical weld process entails the ultrasonic horn traveling downward toward the part (termed down stroke time/distance by some) at some set values. As the horn reaches the part to be welded, a selected pre-load or trigger force/pressure is attained which signals the system to begin the weld, introducing vibrations into the plastic parts. This is the origin of time and thus the point at which values for collapse distance (RPN- Reference Point Numeric), power outputs, energy, and force measurements of the weld are begun.
As the signal is increased in power (during the “softstart” or ramp up time), the vibrations introduced to the part by the horn begin to cause friction/heat and melting of the intended weld joint. This signal continues until a primary welding parameter value is met, e.g., weld time, absolute or relative distance, energy levels are attained, or a peak power output is attained. At the end of this portion of the weld, the hold cycle is instigated whereby the vibrations are ceased and the part is held under force to allow cooling and further compression of the melt. The end of this downward travel is called variously RPN after hold, maximum travel distance, or total stroke value. The horn then retracts and the weld cycle ends. Although there are variations on the previous description, it is a concise description of a typical weld process performed countless times a day the world over.
Machine Requirements
In order to have the capability to perform such tasks, the welding system must have a computer controller, both to provide the necessary controls and monitor the process. Most modern systems operate at a sampling rate of 1 ms, over many parameters, a rate that proves adequate. Such names as DIALOG, MPC, DPC, Ultra-Com, AE, etc. are used by various manufacturers to describe their systems. Certain devices are required to control and monitor each function: timers or clocks for time, linear encoders for distances, and power/energy modules to monitor outputs, and load cells/electronic pressure regulators to monitor forces.
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A typical welding graph showing force, power, and joining velocity (distance versus time). The diagram above shows the progression of the weld as the energy director (blue part) is forced against the red, melt is initialized, continues during the weld process, and finally covers the intended welding surface. Note the decrease in velocity at the end of this curve as the two parts come together. A peak power output (1.5kW) was achieved at 230 ms. |
Parameters Shown on the Graph
Time, on the abscissa, typically starts at the application of ultrasonic vibration, e.g., trigger point, the origin of the graph. All other variables are measured against this; their scaling is typically done automatically. The end of the time scale is typically when the ultrasonic power is no longer applied, although some systems allow the user to look at the hold cycle behavior also. This can be beneficial when trying to squeeze out the last, most demanding tolerances of the process. (Time also can start before the power is applied and can thus show the travel characteristics of the downward velocity of the horn. This is useful in setting up any hydraulic speed controls on the machine, for instance, in inserting or staking/swaging operations where the speed of engagement needs to be regulated.)
Distance, on the ordinate, is perhaps the most important variable in trying to understand a weld process. The slope of the distance versus time plot is of course the velocity of the ultrasonic horn during the welding, the steeper the line, the faster the melt with the opposite being true. The welding of semi-crystalline materials, with their absolute melting points, will typically have higher slopes (progressing much faster) than the welding of amorphous materials with their gradual glass transition.
An increase in the slope of the line means velocity is increasing. For instance, if there is a high amount of amplitude at the end of the horn for the material, we will see an exponential or hyperbolic type shape to this graph as the melt is generated, and the welding collapse occurs very quickly. If the opposite is true and there is not enough amplitude, we may see the graph remain flat, with slopes closer to zero until a point at which the material finally melts and the joint closes. By this time, the part in contact with the horn may have been damaged due to this long weld time.
Abrupt increases in slope also may indicate that the joint material has been overcome, or pushed out of the way, and the parts are unintentionally sliding by one another (cold forming). This also would be seen as a decrease in power as the dampening of the horn vibrations has gone away and power output lessens. Improperly supported side walls in shear joint welds are the most common example.
A decrease in slope means that velocity is decreasing, usually due to the process having used up all the joint detail (or easily melted material) and further energy is being used to melt areas outside of the intended weld joint area. A decrease of velocity accompanied by a “spike” in power may indicate that the power output of the generator may not be sufficient for the intended weld process.
Power, also on the ordinate, is the time rate at which work is done and is defined by the watt (one watt = one joule per second). Power is typically measured as an instantaneous peak output of the ultrasonic generator at a given time and these values are recorded throughout the process. Resultant weld data gives the highest value achieved during the process but does not show when or where this peak occurred. The graph will show this exactly and oftentimes shows two peaks.
An ultrasonic converter is a constant voltage device and as the vibrations of the horn are dampened, power output increases. Several factors influencing power output include higher weld forces, higher booster ratio (increasing amplitude), or an increase in the dampening as the weld joint material is melted. This is typically quite a dynamic graph as power output fluctuates due to varying amounts of dampening occurring within the cycle.
A “spike” seen in this graph, after full start up, indicates that increased dampening of the horn vibrations has occurred and that the generator output increased to overcome this. The generator will limit its output if this rate of rise, or slope, is too great, as well as if the maximum power output is achieved. If a spike is followed by a flattening of the distance versus time graph, the weld cycle was not completed as the generator shut down completely. An “overload” condition also can be caused by too much welding force, too long a welding cycle compared to secondary controls, or by too much amplitude being used or component failure.
All systems will exhibit an increasing value of power at the beginning of the cycle, i.e., “ramp up” as the inertia of the booster/horn mass is overcome and power is gradually supplied. This causes the initial increase in slope of the distance line.
Force, on the ordinate, in Newtons (N) or pounds force (lbsf) shows the output of the system for this variable. This is an input to the process and, as such, will remain close to set levels and observation can be used to verify settings and machine operation. Abrupt changes in force can follow such changes of the distance line and may indicate the cold forming or sliding of the two parts together.
Energy, on the ordinate, and measured in joules (2.778 x 10 -7 kWh) is the amount of power output during a given time and can be expressed as the summation of energy expended to particular time value. It is the amount of work done at any given time and can be thought of as how many vibrations at what amplitude have been introduced into the part. It is, by definition, the area under the power curve. As such, it relies on power and time inputs and the shape oftentimes reflects the power curve.
Summary
The interpretation of the line shapes and their interactions with one another can show what is occurring in this short duration process. This allows the user a “magnifying glass” to use on the process and, properly interpreted, can further the scientific study of this process and allow the “black magic” image of the process to be minimized.
Kenneth Holt has been involved in the plastics industry for nearly 20 years, 11 of which have been in the ultrasonic welding field. His involvement in all the aspects of application development and de-bugging hundreds of such projects gives him unique insight into the real problems experienced most frequently in the process. Acknowledgement for this article also goes to Thomas Herrmann, Gunter Manigel, and Gunter Fischer, Herrmann Ultrasonics, Inc. For more information, contact Hermann Ultrasonics, Schaumburg, Ill., at (847) 985-7344 or visit www.herrmannultrasonics.com.
