Questions You Should Know about Ultrasonic Welder Machine Supplier

by Ken Holt, Hermann Ultrasonics, Inc.

What effects and influences will a glass reinforcement in a plastic have upon the ultrasonic welding process?
This is an excellent question as these types of additives are common in semi-crystalline parts. Unfortunately, the question has two considerations that need to be contrasted and compared. Both positive and negative influences can be seen.

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Positive Influences. In ultrasonic welding, glass reinforcement stiffens the plastic part and thereby lessens the amount of amplitude or vibration lost within the part in contact with the horn/sonotrode. Why is this good? By getting as much vibration to the weld joint area as possible, the best weld is assured to occur between the parts to be welded. This effect is most notable in ‘high-loss’, softer semi-crystalline materials, particularly polypropylene (PP), polyethylene (PE), polyamides (Nylons or PA), and Acetal (POM). The welding of these types of lower modulus plastics benefits from the addition of glass reinforcement or even a talc or CaCO3 (calcium carbonate).

Percentages of 10 to 33 percent are most commonly seen and are proven ratios for Nylons and Acetal in ultrasonic welding, particularly in the automotive industry where good stiffness is added and tighter molding tolerances are allowed.

Negative Influences. Plastic ultrasonic welding cannot melt those materials commonly added to plastic materials, only the plastic matrix in which they occur. The reinforcements do detract from the amount of plastic available to weld at the weld joint area and steps must be taken in the molding process to either produce a ‘resin-rich’ surface (which also results in shinier parts) or a surface with no more than the base ratio of reinforcement, e.g., 33 percent. In other words, keep the glass levels at or below the specified percentage in the weld joint area. If temperature profiles in the molding machine are run at low levels, a resultant and undesired glass-rich surface will occur.

Another negative influence is the wear evoked on the face of the horn/sonotrode by these materials, so proper coatings or hardenings of the tooling should be undertaken.

There seem to be many process parameters involved in the welding process. How can the process be controlled most thoroughly and accurately?
Fortunately, process control techniques and mechanisms are numerous. These can be broken down into three main categories for clarification: electronic, mechanical, and management.

Electronic. A computer-controlled welding system has many options to allow process control by allowing multiple limits of outputs to be set. For example, if welding by energy or, in other words, energy mode, it can be specified that only parts within a certain overall height tolerance be accepted. Likewise, those with weld times outside a researched, good range can be rejected. Typically, modern units allow for the setting of limits in most all other weld parameters than those programmed as main weld parameters. Once again, a computer-controlled welder is required for these.

Mechanical. There are many mechanical aspects of the welding process, including tooling, mechanical settings of the press, and auxiliary equipment influences (part clamps, protective film usage, pick and place part movement, etc.). The most common problem I have experienced in this regard is the adjustment of accessible controls by well-intentioned individuals who do not have a full understanding of the cause and effect relationships of the changes they make. For example, to increase the throughput of a machine, someone may change the weld force to shorten the weld time needed for the weld, but by doing so, inadvertently produces an unwanted and adverse strain within the part. Settings that are easy to adjust should be documented and verified on a regular basis and kept at the welding station for easy reference and comparison.

Welding units that do not allow such easy-to-reach adjustments will be changed in software settings, and proper password protection in software is common. In this way, only those knowledgeable in the cause/effect relationships are allowed to make adjustments. Medical manufacturing, in particular, can benefit from such systems as changes can be tracked and assigned for FDA-compliance purposes.

All mechanical adjustments to tooling relationships, likewise, should be recorded and should be repeatable. Leveling height adjustments are a common example and heights of the corners of the fixture plates can be measured easily and recorded.

At the root of this question is not only what adjustments are to be made but also, and most important, why are these adjustments necessary? What input to the process has changed and how can that factor be minimized? Is a variation in part warpage causing problems? How can that be minimized? Is a rise in moisture content of the plastic part to blame? How about the material lots? Has the melt index of the resin been changed with an addition, regrind, or a reformulation of the base product? Is parting line flash now interfering with the fit of the part into the fixture? Take appropriate steps upstream to alleviate downstream adjustments.

Management. As with all quality assurance programs, managerial ‘buy in’ is tantamount to success. Proper training of operators on the equipment, including access and reading of manuals, and good communication paths between the secondary operations department and molding departments are both items that, in my experience, can help people de-bug processes.

I have heard the term ‘amplitude’ used frequently regarding ultrasonic welding but have never been given a good definition of it, or its influence on the process.
Amplitude is nothing more than the amount of vibration, or strain, seen at the end of the horn/sonotrode and is expressed in microns or 10-3 mm. All sonotrode faces move out and in from an ‘at rest’ position (reciprocal movement). Amplitude can be expressed as a peak-to-peak excursion measurement of the faces movement; or it can be expressed as simply the peak amount of movement forward from the ‘at rest’ position of the face. This is called peak amplitude. It is the amount of travel that the face of the horn/sonotrode experiences in a cycle of vibration. This vibration, whose frequency is dictated by the welder’s operating frequency, e.g., 20 kHz, results from the activation of the piezoelectric crystals within the converter or transducer. It is amplified typically by the component called the booster and also by the shape of the sonotrode. The mechanism of amplification is outside the scope of this writing but can be discerned easily by visits to manufacturers’ web sites.

I am a big fan of using analogies in trying to explain new terms to people in training. As basic as this may sound, the following is a true and good analogy: If you wish to melt butter in a pan faster, you increase the flame height or heat introduction rate. Likewise, if you wish to perform a weld faster, you analogously increase the amplitude. Amplitude is a huge factor in dictating how fast plastics melt or reach their glass transition temperature.

However, we cannot go ‘full throttle’ for a variety of reasons; each plastic resin has a unique value or range of amplitude in which it effectively melts yet is not degraded. Likewise, each plastic part design will have a subset range within the aforementioned, in which parts weld well. Similarly, values of amplitude ranges can be found in manufacturers’ literature or by contacting their application labs.

Tooling engineers at the manufacturers’ shops should provide tooling capable of running correct amplitudes given a certain booster ratio, as well as long-running, robust designs. Also worthy of consideration is that the horn/sonotrode will have a maximum amount of strain that can be expected repeatedly.

Amplitude is an excellent variable to use in the design of experiment study of a welding process. The effect of too low of an amplitude in a welding process is commonly seen as an incomplete weld (even with proper leveling of the fixture), or a ‘cold forming’ of the weld joint. This is where the joint design appears to have been physically deformed but not melted. (Note: silicone weld release present in the weld area will exhibit the same look.)

It is possible to have too much amplitude. This is seen in processes having very low weld times (for instance, less than 0.1 seconds for a small part) or a process that produces flash from the weld area even when it is not fully melted down (or even physical part damage such as cracking of the parts). Lower amplitudes within the previously discussed ranges will provide a more repeatable and robust process. Welding distance versus time graphing can provide important and useful information in the application of the correct amplitude.

Ken Holt has been in the plastics industry for over 20 years and has written for various publications, as well as conducted training and educational seminars. He is the applications lab manager at Herrmann Ultrasonics, Inc., in Bartlett, Ill., where he deals with all facets of the ultrasonic welding process from initial design reviews to final field testing of equipment. He can be reached by at [ protected].

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.

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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.

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