VIEW THE ONLINE EDITION
High-Tech Contract Decorating at Emerald Corporation
2012 SGIA Expo Show Preview
Ask the Expert
Plastics Surface Energy Wetting Test Methods
Scratch-resistant in One Step
Letter from the Chair
TopCon Rolls Through Indy
Digital Decorating Webinar Scheduled for August 28
Ultrasonic Welding: The Need for Speed Control
How to Close Sales that are Over the Budget
PLASTEC West, Anaheim Convention Center, Anaheim, CA, www.plastecwest.com
Plastics Crossroads Summit,
Sheraton Hotel, Anaheim, CA, www.rjginc.com/plasticscrossroads
PLASTEC South, Orange County Convention Center, Orlando, FL, www.plastecsouth.com
AWA DecTec USA, Orange County Convention Center, Orlando, FL, www.awa-bv.com
SPE ANTEC® 2013, Duke Energy Convention Center, Cincinnati, OH, www.antec.ws
HBA, June 18-20, Jacob K. Javits Convention Center, New York City, NY, www.hbaexpo.com
PLASTEC East, , Pennsylvania Convention Center, Philadelphia, PA, www.plasteceast.com
Copyright 2010 Peterson Publications, Inc.
Plastics Decorating Magazine
2150 SW Westport Dr., Suite 101
Topeka, KS 66614
(785) 271-5801Â Fax (785) 271-6404
Ultrasonic Plastic Welding: Weld Processing Modes, their Descriptions, Functions, and Applications
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
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.
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
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.
This graph indicates an optimized and robust welding process that
provides a wide processing window capable of withstanding slight
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
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
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
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
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
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.
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 twenty
years, eleven 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