Tuesday, March 16, 2010

What Happened to Your Ultrasonic Weld Quality?

By Miranda Marcus, Applications Engineer, Dukane IAS

The most important factor in troubleshooting problems in ultrasonic welding is understanding the fundamentals of the process. With this basic knowledge, most problems can be easily diagnosed and resolved. Even so, sometimes your time-tested weld recipe may suddenly fail for no discernible reason.

Perhaps your ultrasonic welder has been running the same application for months, maybe years, with no problems. Abruptly, this cheery continuity is disrupted. Has your weld strength decreased? Are you seeing excessive flash? Does your welder overload as soon as the cycle starts? Here, we will discuss a few unseen factors that can cause sudden changes in your ultrasonic weld quality and how to prevent and correct them.

KNOW THE BASICS
Ultrasonic welding works by applying a vibration at a frequency of 15 to 70 kHz to a plastic part. This vibration is generated through the use of piezoelectric ceramics in the transducer, that convert an electrical signal into mechanical motion. The transducer creates a vertical vibration that is then translated through the booster, and subsequently, the ultrasonic horn. The ultrasonic horn is typically designed to contact the part directly above the weld area so that the vibrations can travel though the upper part to the weld area.



The ultrasonic vibrations create cyclical strain at the weld area, which generates heat that melts the plastic in a restricted area and welds the two parts. Because the ultrasonic vibration acts on the entire weld surface, an energy director is often added to control the melting and reduce the amplitude necessary to achieve a weld.

It is important to prevent metal-to-metal contact on your ultrasonic horn to increase its longevity. Because the horn is a tool with acoustical properties, users should be careful to preserve its structural integrity. Any nicks or gouges in the surface of the horn act as stress concentrators that can rapidly lead to cracks when the horn is in use.

WARNING SIGNS
Many signs can indicate a change in your ultrasonic welding process. Some indications of a problem with your part include decreased weld strength, increased flash, and the appearance of cosmetic damage. Some things that signify a problem with the welder or ultrasonic horn are an increased wattage draw, a change in the sound of your weld (typically apparent on lower-frequency welders), and overloading.

The first step in eliminating unseen problems is to record your welding setup. Make a “Weld Process” sheet that includes information such as your weld parameters (weld time, hold time, trigger mode, amplitude); manual settings (thruster height, pressure); and the critical dimensions of your part (diameter and energy-director/shear-joint size). Also include photos of the welder, showing the alignment and design of the horn and fixture. Refer to this document when problems arise—it may save you a lot of time and trouble.

There are many not-so obvious factors that can negatively impact your ultrasonic weld quality. One of the most frequent causes of problems in a long-running process is wear on the mold that produces the parts to be joined. This is a slow, but sure, event in any molding process. Because most joint designs are relatively small compared with the size of the overall part, changes in their size or shape may go largely unnoticed. For many applications, a change in shear width from 0.016 in. to 0.020 in. can make a huge difference in weld quality. Such changes can be caused by just 0.002 in. of mold wear on each part.

Another important factor is environmental changes such as ambient heat, cold, or humidity. Humidity is a particular concern if you are using a hydrophilic material such as nylon, polycarbonate, or polysulfone. Very cold temperatures can cause polymers to become brittle, which may cause them to crack rather than weld at a normal welding pressure. High heat can lead to longer solidification times, causing problems if you are working with short hold times.
Some materials are less sensitive to process changes. Try switching to an easily welded material, like ABS, to achieve greater consistency in your process.

Probably one of the most overlooked factors contributing to ultrasonic welding problems is changes in the time from molding the part to welding the part. Proper ultrasonic welding setup can be drastically different when welding “cold” parts as opposed to welding “hot” parts. It is generally not a good idea to weld “cold” parts to “hot” parts.

If at all possible, leave plenty of time for the part to cool after molding before welding. “Hot” parts are more difficult to control and can cause inconsistency in your weld process. Also, try to perform the welding operation in a climate-controlled environment to eliminate seasonal effects on your process. This is especially important in humid regions.

MORE FACTORS TO CHECK
Sometimes poor ultrasonic welds can be traced back to the injection molding process. Injection mold wear can lead to a rounded energy director in the part (upper right), which produces a weak weld (lower right). A well-maintained mold produces a sharply pointed energy director (upper left), which produces a stronger weld (lower left) with lower welding amplitude and less flash.

If you know it is not your parts causing the problems, it could be your ultrasonic tooling. Occasionally a horn will develop a crack. While most horns will not run at all after forming a crack, some do. Those will often emit a high-pitched ringing sound or run at a higher wattage than normal. It is very important to discontinue use of a cracked horn because it tends to put excess stress on the transducer and can lead to broken piezoelectric ceramics.

Probably the easiest diagnostic test is to mix-and-match your ultrasonic stack if you have multiple welders of the same frequency. Try the horn with a transducer and booster that have been working well. If all is good after this switch, then you know the horn is not the problem. Likewise, you can put a working stack in a questionable welding machine. This is a quick and easy way to locate the trouble spot in your machine without any special equipment.

If you find that the problem is your horn, check it for cracks. To locate cracks in a horn, spray it with a foaming cleaner. Then use the test feature on your welder to introduce short bursts of ultrasonic energy into the horn. The cleaner will collect in the crack and turn a blackish color. WD-40 oil can be used if a foaming cleaner is not available.

Finally, the welding fixture has a significant effect on the accuracy and precision of your welds. Make sure the fixture is providing support to the entire joint area, and that there is no room for misalignment of parts during loading. When welding softer materials such as polyethylene and polypropylene, be sure that there is support around the joint area in both lateral and vertical directions. Soft materials tend to deform outwards, which will hinder or prevent proper welding.

Thursday, March 11, 2010

How does your Ultrasonic Probe/Stack work?

Ultrasonic Probe
Plastic welding is the most common application of ultrasonic assembly. To perform ultrasonic plastic welding, a vibrating tip is brought into contact with one of the work pieces as shown in Figure 1–1. Pressure is applied and ultrasonic energy travels through the material, increasing the kinetic energy (or heat) at the contact point of the two parts. The heat melts a molded ridge
of plastic on one of the pieces and the molten material flows between the two surfaces. When the vibration stops, the material solidifies forming a permanent bond.



Ultrasonic Probe Configuration
A basic ultrasonic probe consists of:
  1. A housing which contains the transducer which converts electrical energy into mechanical vibrations.
  2. A horn to transfer the mechanical vibrations from the transducer to the parts to be welded. A basic ultrasonic probe is shown in Figure 1–2. As indicated, the horn is secured to the transducer with a threaded stud. The transducer housing also has a connector for attaching the high voltage coaxial cable which delivers the high–frequency electrical signal for exciting the transducer. This signal is supplied by a separate ultrasonic generator.
ultrasonic probe

Transducer
The transducer supplies the ultrasonic vibrations by means of piezoelectric converters which transform electrical energy into mechanical movement. Power applied to the transducer at 20kHz can range from less than 50 Watts up to 3000 Watts.

transducer

The transducer is made from a number of polycrystalline ceramic elements separated by thin metal plates, clamped together under high pressure. When an alternating voltage (dV/dt) is applied to the converters (or ceramics), a corresponding electric field (dE/dt) is produced which results in a variation in thickness (dL/dt) of the ceramic elements. This variation in thickness induces a pressure wave (dP/dt). Because the molecules or atoms of a solid are elastically bound to one another, the pressure wave results in a wave propagating through the material which is reflected by the ends the metal mass of the converter.

See Figure 1–8 for a graphical representation of this. When the length of the assembly is tuned to its frequency of excitation, it resonates and becomes a source of standing waves. A typical transducer without its housing is shown in Figure 1–3. The output amplitude from a 20kHz transducer is only about 20 microns (0.0008 inches), so this amplitude needs to be amplified by the horn (and possibly a booster) to do useful work.

Ultrasonic Horn
The horn acts as an acoustic waveguide or transformer to amplify and focus the ultrasonic vibrations to the work piece. The ultrasonic horn has three primary functions:
  1. It transfers the ultrasonic mechanical vibrational energy (originating at the transducer) to the plastic parts through direct physical contact, and localizes the energy in the area where the melt is to occur.
  2. It amplifies the vibrational amplitude to provide the desired tip amplitude for the thermoplastic and weld process requirements.
  3. It applies the pressure necessary to form the weld once the joint surfaces are melted.



The ultrasonic horn is precision machined and designed to vibrate at either 15kHz, 20kHz, 30kHz, 40kHz, 50kHz or 70kHz. Figure 1–4 shows five aluminum alloy horns ranging from 15kHz to 50kHz. The higher the frequency, the shorter the acoustic wavelength, and consequently the smaller the horn. Notice that the 30Khz horn is only half the length of the 15kHz horn. The tuning of an ultrasonic horn is accomplished using electronic frequency measurement. Inherent variations in material composition prevent tuning by dimensional machining alone. Horns are usually manufactured from high–strength aluminum alloys or titanium. Both metals have excellent acoustical properties to transmit the ultrasonic energy with very little attenuation.

There are many different ultrasonic horn shapes and styles depending upon the process requirements. Factors which influence the horn design are the materials to be welded and the method of assembly. The gain of the ultrasonic horn is determined by its profile. Figure 1–5 shows four different gain profiles. The input vibration amplitude to a horn from a 20kHz transducer is only about 20 microns. This is not enough to generate enough friction achieve a melt temperature for most thermoplastics. Therefore the horn must amplify the mechanical vibration so that the amplitude is sufficient to melt the thermoplastic. The amplitude at the tip of the horn typically ranges from 30 to 125 microns (1.2 to 5.0 thousandths of an inch) at 20kHz.




An optional threaded tip can also be used when the application calls for staking, a swagging profile or a pointed spot weld. In Figure 1–1, one of the plastic parts had a small ridge used to initiate the melt process. Here in Figure 1–6, the tip provides the energy director since there is only one piece to be melted in a staking operation. Replaceable tips are not commonly used in high–volume production environments. For long–term or high–wear production, a horn with a custom machined tip coated with chrome, carbide or titanium nitride will provide excellent wear resistance.



As the frequency increases, vibration amplitude typically decreases. Higher frequencies are used for seaming of thin materials and delicate parts that do not require a lot of amplitude. Since the horns become smaller at higher frequencies, closer spacing can also be achieved. Some factors to consider for high–frequency (e.g. 40kHz) ultrasonic welding versus lower–frequency (e.g. 20kHz) ultrasonic welding are listed here.
  1. For a given amplitude, stress in the horn increases at higher
    frequencies.
  2. Wear on the horn is greater at high frequencies.
  3. Clean and flat mating surfaces between the horn, booster and transducer are more critical as the frequency increases. At 40kHz, surface flatness specifications are between 0.0005" and 0.001" (13 to 25 microns).
Booster
The primary function of a booster is to alter the gain (i.e. output amplitude) of the probe. A booster is amplifying if its gain is greater than one and reducing if its gain is less than one. Gains at 20kHz typically range from less than one–half to about three. A booster designed to be mounted in a fixture between the transducer and horn is shown in Figure 1–7. This is commonly referred to as a probe stack. Since the horn cannot be clamped, only the transducer and booster can be secured. Therefore a secondary function (and sometimes sole purpose) of a booster is to provide an additional mounting location without altering the gain when the probe stack is secured in a press. The neutral (1:1) or coupling booster is added between the transducer and horn and mounted in the press by its mounting ring which is placed at the nodal point (where the standing wave has minimal amplitude). See Figure 1–8 for a graphical representation. Note that the maximum stress occurs at the nodal points.



Wednesday, March 10, 2010

What are the effects of Ultrasonics on your Health?

Introduction
Users of ultrasonic welders and ultrasonic cleaners inquire, from time to time about the effect of ultrasonic energy on the health of the operator. This paper is a brief summary of the nature of ultrasonics and its potential effects on health.

The Nature of Ultrasonic Energy
Ultrasonic energy is mechanical energy as contrasted with other forms of energy such as nuclear particle radiation (x-rays, beta rays), electromagnetic energy (radio frequency waves, diathermy, microwave radiation), or invisible light (infrared, ultraviolet light waves). The source of ultrasonic energy is the ultrasonic transducer: a linear motor that converts
electrical energy to reciprocating mechanical motion similar to a high-speed hammer. The motion occurs at a rate of 20,000 strokes or more per second, and is above the hearing range of the average person.

The ultrasonic mechanical motion, when propagated in air, is severely attenuated since air is an extremely poor sound transmission medium as compared to the metal and plastics in which it is intended to travel. Further, when the energy is propagated into a three dimensional space from a point source, it diminishes at a cubic rate each time the distance is doubled (i.e., every time the distance is doubled, 1/8 of the energy remains). The ultrasonic airborne energy is also absorbed into soft, non-reflecting materials (e.g., worker’s clothing or noise abatement foam materials). The remaining airborne ultrasonic energy is therefore diminished thousands of times as compared to the source. Equipment operating at ultrasonic frequencies may produce noise in the audible frequency range due to the workpiece vibrating at an audible subharmonic of the ultrasonic operating frequency.

Direct Contact with Ultrasonic Energy
As with all high-speed rotating or reciprocating machinery, direct contact with ultrasonic energy must be avoided at all times. All Dukane welding equipment carries appropriate warning signs against contact with components developing high levels of ultrasonic energy.

Exposure to Airborne Ultrasonic Energy
Ultrasonic welding equipment has been in use for more than 40 years. Medical and scientific literature reports no documented health hazard from airborne industrial ultrasonic energy reaching an operator.

Cardiac Pacemakers and Ultrasonic Energy
Pacemakers are not affected by airborne ultrasonic energy, but may be affected by electromagnetic energy. All equipment capable of generating ultrasonic energy also produces electromagnetic energy (usually in the radio frequency range). All Dukane ultrasonic equipment must comply with Federal Communications Commission regulations specifying limits on the conducted and radiated energy which may emanate from the equipment. There are many types and kinds of pacemakers. It is not known to what extent the different types are sensitive to various levels of electromagnetic energy. Until more is known about pacemaker reaction to R.F. emission, it would be prudent not to place workers with pacemakers near ultrasonic equipment. A report discussing R.F. emission is: “The Biological Significance of Radio Frequency Radiation Emission on Cardiac Pacemakers,” Report SAM-TR-76-4; USAF School of Aerospace Medicine, Brooks Air Force Base, Texas 78235.

Ultrasonic Energy & Audible Noise
The human ear cannot respond mechanically to airborne ultrasonic energy; it therefore is inaudible. The associated audible noise and lower frequency subharmonics can in extreme cases, be disturbing, causing hearing discomfort, occasionally nausea, and sometimes a temporary shift in the threshold of hearing (sound pressure level, or loudness, that can be heard). Many countries control the amount of audible noise that a worker can receive. In the United States 90 dBA (“A”= international “A” scale) noise level can be maintained continuously for 8 hours. Higher noise levels are permissible for shorter periods of time, typically:



Ultrasonic welding uses intermittent energy. Only the noise generated during the few seconds of each cycle when the welding equipment is energized causes exposure to noise. The individual energy cycles are accumulated to equal the duration of exposure

Workers subjected over many years to excessive noise (e.g., textile mills, saw mills, coal mines) suffer a permanent hearing loss, such loss being greater than the normal loss associated with aging. It is desirable to limit the audible noise to permissible levels, by constructing sound enclosures, rotating employees at the workstation, or requiring the use of hearing protectors. A list of personal hearing protectors and attenuation data is available in HEW Pub. #76-120, 1975,
NTIS-PB267461, and can be obtained from National Technical Information Service, Port Royal Road, Springfield, VA 22161.

Noise Measurement with Sound Level Meters
Sound level meters are available which measure sound levels at the workplace. The sound must be measured at ear level where the worker stands while working. The noise exposure is measured in “dBA, slow response.” The calibration of such instruments is done at frequencies below 10 kHz, rendering them potentially very inaccurate at higher frequencies.
Standard noise level meters may be used only to accurately determine sound levels within their calibration range, and may provide erroneous readings above 10 kHz. Only instruments designed for use with special high frequency microphones will provide accurate results above 10 kHz. Pub. # S1-4-1971 (R1976) “Specifications For Sound Level
Meters” is available from American National Standards Institute Inc., 1430 Broadway, New York, NY. 10018.