Ultrasonic Plastics Assembly Blog

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2 June 2011 -- Beginning in about 1985, ultrasonic welders began to be equipped with linear encoders measuring weld distance. It quickly became apparent that the two main methods would be absolute distance, giving the same functionality as an end-of-weld limit switch but with much greater accuracy and repeatability, and collapse distance, measuring the amount of carriage travel during the application of ultrasound. Of the two, it is weld by collapse distance that truly revolutionized welding of injection molded parts using an energy director or shear joint. The computerized controller would simply remember the encoder position when ultrasound was turned on via force trigger, and run ultrasound until a certain amount of distance had been traveled. This method made up for a significant amount of variation in part fit-up, plastic density or resin quality variations, and other things that had given users great difficulty before this method was introduced. In a matter of less than four years, all major manufacturers worldwide had introduced a machine with a linear encoder and weld by collapse distance welding mode. Such machines could now be equipped to monitor all major energy, power, time, and distance process inputs for a significant improvement in process stability and visibility.

12 March 2011 -- In the 1980s, pneumatic-clamped ultrasonic welders evolved from controlling only trigger force, clamp force, and exposure time (and amplitude by way of boosters) to a variety of strategies. The first of these was welding by energy. From a machine design standpoint, this is relatively easy to do, so when microprocessors first started making their way into ultrasonic machines, many machines were marketed with this feature as more advanced machines. Energy is the amount of work expended doing something, while power is the rate at which energy is expended. So what those units do is sample watts at regular intervals, from, in early systems 20 milliseconds, down to the current indsutry best of 0.5 milliseconds (shorter sample time is generally better). A running total is kept of these wattage readings, and when the total of all readings divided by the sample rate equals a certain predetermined amount of energy, ultrasound stops and the machine goes on to the hold part of the cycle. If these machines were put in weld by energy mode, they also could test the actual weld time against limits to see if a typical cycle was run. The converse was also available, welding by time and testing the actual energy against preset limits. The theory here is that if you know how much energy you put into a weld you know how much plastic you have actually melted. In practice, for most applications the best control mode remained weld by time, but energy data was collected, used to identify suspect parts, and the science of the process was advanced.

31 July 2010 -- Many sizes and shapes of parts can be ultrasonically welded, but it is important to follow a few rules. First, energy transfer from the horn/sonotrode to the joint area, as well as support from the fixture, are critical to obtaining consistent results. Horn/sonotrode contact is ideally in a plane parallel to and as close as possible to the joint, which will also ideally be planar. The parts ideally will have sufficient stiffness and structural integrity that deflection is minimal when the clamp load required to obtain a weld is applied. Delicate details in the immediate area of horn contact, along the energy transfer path, and in the joint area should be minimized so as to avoid destruction by the high-amplitude ultrasound. Horn contact should be on a semi-glossy relatively large surface so as to enhance coupling and minimize marking. Highly stressed areas such as gates should be located away from the horn contact or weld areas. Smooth engagement of the two parts enhances weldability, but any kind of interference fit degrades it.

1 June 2010 -- A shear joint by its very nature creates side load, and so it is best to include some kind of guide wall to prevent parts from distorting during the welding process. Energy directors also benefit from use of guide walls. Shear joints are often incorporated into pin-and-socket configurations or weld tabs. Here are some examples of energy director designs: 1 2 3 4. And here are some examples of shear joint designs: 1 2 3 4. To make a ridiculously broad generalization, the more complex joints will require more space in part designs but will often produce stronger results. The double shear in particular should be used with caution. Hot gases and molten material can be trapped under the tongue and produce leaks and wild variations in joint strength unless that area is vented by making one side of the double shear discontinuous around the perimeter of the part.

9 April 2010 -- Energy directors are effective with most thermoplastics, but some less so than others. Amorphous materials are generally easy to weld and once softened will generally retain enough heat while traveling away from the energy director to bond when clamp force is applied. Semi-crystalline materials are considerably less well behaved. Once away from the immediate area of the energy director recrystalization begins and inhibits bonding. The shear joint was developed to specifically address these issues. The telescoping nature of the joint during welding ensures the greatest amount of joint area is involved in process until final clamp force is applied. Shear joints are also effective with amorphous materials, and generally provide greater assurance of leak-free seals where used.

21 February 2010 - Ultrasonic welding is facilitated by providing an acoustic weak spot in the joint of the parts called an energy director. This is analogous to a fuse in an electrical circuit. From an acoustic standpoint, what is desired is point contact; ideally the apex of a triangular ridge running around the joint butted up against a flat surface on the mating part is the only contact between the two parts. Relatively large horn (sonotrode) contact area assures that the part couples to the horn and vibrates sypathetically with it. Large fixture contact area on the other part assures that it does not vibrate, and the relative motion of the two parts during application of ultrasound occurs only in the joint area. The apex of the energy director and the material very near it are then put in a state of rapid stress loading and unloading, causing repetitive deflection of the material that causes the molecules in the material to rub against one another and produce the heat necessary to promote melting. The energy director shape is generally accepted to have a 60-degree included angle across the point for semi-crystalline materials and a 90-degree included angle across the point for amorphous materials. Cones, pyramids, cross-hatch patterns, rounded ridges, and other variations are used in certain circumstances. Energy director height is normally in the range of 0.1 to 0.6 mm, but larger or smaller energy directors have been used in certain circumstances.

1 December 2009 -- Materials that are too soft to ultrasonically weld can be made stiffer by the addition of mineral (such as talc) or reinforcement (such as glass). Ten percent seems to be a good starting point for experimentation if this seems to be a possible solution to a problem. Generally, up to about twenty-five to thirty percent reinforcement or filler will imrpove welding results by increasing the stiffness of the material and therefore its sound transmission capability, as well as improving its dimensional stability part-to-part. Beyond thirty percent, the filler or reinforcement will be displacing weldable polymer and probably interfere with welding to a greater degree. Often the surfaces will be enriched, that is to say they will have a greater concentration of filler or reinforcement right where welding needs to occur, creating poorer than expected joint strength. Reinforcement tends to orient parallel to mold steel, and therefore parallel to the joint line in molded or extruded parts. There is almost no hope of getting any of this reinforcement to cross over the joint line and improve the strength of the joint, so in the most perfect of situations the best one could hope for is that the joint strength approaches that of unreinforced base resin. Since manufacturing generally does not occur in the best of all possible situations, the rule of thumb is to expect stregth to be eighty percent or less of the strength of the unreinforced base resin.

20 November 2009 -- Polymer materials consist of long-chain molecules. These molecules are many hundreds if not thousands of times longer than their width. They can be visualized as human hairs, and are usually illustrated by lines. Thermoplatic materials come in two general groups: amorphous or semi-crystalline. Amorphous materials have the same molecular structure when solid as when they are liquid, that is, no particular structure at all. The molecules randomly orient, flow easily, and may or may not have a predominant orientation depending on flow conditions and speed of cooling. The most common non-polymer amorphous material is glass. When glass is blown, the artisan heats parts of the object and applies force internally through air pressure and externally using tools to produce the particular shape desired. The glass flows and is formed but whether in the "liquid" or "solid" state it retains clarity. This is because there is no internal structure to inhibit the passage of light (or sound). Semi-crystalline materials behave (and appear) more like candle wax. They are brittle when frozen, that is, below the glass transition temperature, increase in toughness as temperature rises to the melt range, at which point they become semi-flowable, though they retain shape memory of their crystal structure when last frozen, and can tend to return to their frozen form if vibrated or heated. They flow freely when truly melted. An amorphous material has no true melt temperature, and as such has no true "solid" state, it is simply less and less flowable as temperature drops until it is experienced as a solid. Semi-crystalline materials have portions of almost all of the molecules locked into crystal structures in an overall amorphous matrix, giving them a true melt temperature, with a corresponding phase change energy spike in specific heat. A semi-crystalline material will run like water when hot enough (and appear clear), like the pool of liquid wax around the burning wick of a candle, mold like putty when not fully melted but near melt temperature, like the wax surrounding the molten pool, and exhibit true solid behavior when cooler, like the body of the candle not near the flame. Just like candle wax dripped on a cool candle, melted material up against cool solidified material will not intermingle to produce a homogeneous weld area, so semi-crystalline materials always require near-identical resin chemistry and melt characteristics to join well. Amorphous materials, since they will flow and intermingle over a relatively broad range of temperature, are much more tolerant of resin chemistry or melt charaterstic mismatches or variations.

13 November 2009 -- In order to weld a thermoplastic part using ultrasonics, four factors must come together. First, the material must have a sufficient loss modulus. This is a very fancy way of saying that material must be able to be heated by repeated rapid application of the compression/tension cycle that the pounding motion of the horn/sonotrode creates. Some materials are too flexible to heat sufficiently, some have too much internal lubricity. Second, the material to be joined must have flow characteristics and chemical compatibility consistent with intermingling of the materials such that the result is a weld and not merely surface adhesion (though in some applications surface adhesion may be enough). Third, sufficient amplitude must be available that the thermoplastic material gets hot enough to flow before the stresses of the clamp force and amplitude destroy the joint. Fourth, point contact or line contact must exist such that the heat created by amplitude and clamp force is localized in the joint area. This last can be accomplished by the use of energy director or shear joint designs.

7 September, 2009 -- Boosters, as discussed earlier, are typically of standard design, whereas a horn (sonotrode), the vibrating component actually contacting the work, can be any of a wide variety of designs. These can be as as simple as cylinders or rectangles, possibly stepped to increase amplitude, hollow round horns, etc. Basic rules of horn design are simple. The horn (sonotrode) must vibrate most strongly at a frequency near the fundamental frequency of the machine, in an essentially linear fashion. It must contact the work with an acceptable level of marring of the surface (often this acceptable level is zero) and transmit a proper amount of amplitude to the workpiece. It must not wear excessively, it must not break too soon. It must not violate clean room rules or be subject to rapid corrosion in the work environment. Its manufacture must be at an acceptable cost. There are probably other rules but those are the main ones. There are three common materials for horn construction. Aluminum is relatively inexpensive on a unit volume basis, is easy to machine, responds acoustically very well, and can be plated or coated in many ways. Of the three common materials it is lowest in fatigue strength, wears easily if a protective coating cannot be used or is breached, and in applications where frequent tool changes are made is subject to wear and stresses at the interfaces and connecting threads. Common tool steel had been used for many years, but is rapidly giving way to sintered steel alloys. These newer steels have very uniform grain structure, can be made relatively hard to resist wear, and allow nearly as much freedom of design as aluminum or titanium. Because of the hardening process, they can be expensive to manufacture, and compared to tools of aluminum or titanium, they are amplitude limited. Titanium alloy tools have the highest fatigue strength of any common ultrasonic tools and so are durable and capable of high amplitude operation. They are about midway between aluminum and steel alloys in hardness, and are approved for food contact or contact with implantable medical devices. Titanium is somewhat freer machining than tool steel although special technique is required to avoid work hardening the surface or setting chips afire (burns in a similar fashion as magnesium). Large horns are not often made of titanium because of material expense nor of steel because the relatively high density would result in very heavy tools. Machinable titanium stock is often not available in very large sections. One common question is why larger horns have slots parallel to the direction of sound transmission: there is a rule in solid waveguide design that as a secion approaches 1/3 wavelength in the transverse dimension, the efficiency of the waveguide drops precipitously. In order to allow larger horn designs, the slots are added to break up the transverse dimensions such that no section has a transverse dimension approaching or exceeding 1/3 wavelength. A consequence of this, however, is that generally the section directly driven by the booster will have the highest amplitude, followed by the sections nearest outboard of it, and so on. In advanced horn design, various means are used to more closely balance the amplitude, de-stressing the tool and allowing for higher overall amplitude and more even welding. Generally, an even number of slots are used in each driection slotted, so as to avoid putting a slot directly under the stud hole and creating a weakened area where premature stress cracks can develop.

22 May 2009 -- Amplitude and clamp force during an ultrasonic weld can be held steady, which is most common, or varied, which is less common. When varying amplitude and clamp force, studies have shown an increase in pure strength when these factors are decreased as the weld progresses, which slows the heating rate and allows heat to penetrate more deeply into surrounding material. This results in less molecular orientation and residual stress. David Grewell has done a lot of great work on this topic. Generally, amorphous materials respond better to varying amplitude while semi-crystalline materials tend to respond better to varying clamp force. The increase in time to complete the weld using an approach reducing amplitude or force as the weld progresses, however, often makes it necessary to compromise a bit on optimizing for strength alone. Increasing clamp force near the end of weld time seems to produce an altogether different effect, which, while theoretically resulting in less strength, often produces tighter joints offering a greater likelihood of a complete seal and uniform gap closure with a shorter weld cycle. Which is the best approach, or whether to simply use constant amplitude and force during the weld, are very application dependent. Some ultrasonic machines can exert a higher clamp force during hold time which has also been shown to increase the probability of closing gaps and creating leak-proof seals in many applications, particularly with challenging materials such as high melt temperature resins or compounds having high proportions of glass or other reinforcement.

February 14, 2009 -- The heating rate in ultrasonic welding is the product of the loss modulus of the material, frequency, clamp force, and the square of amplitude. Loss modulus is the ability of the material to convert repetitive variation in compressive load into heat. In other words, how easy it is to heat the material using ultrasound. Think of this as being inversely proportional to intermolecular lubricity. Slippery materials are harder to heat up than less slippery materials. This is an oversimplification, but it will work for now. Loss modulus "is what it is" when you go to weld a part, but knowing it is a factor can be useful in rectifying a troublesome application. Frequency is determined by the equipment and tooling and there is not much you can do about it if you are standing next to the machine with the tooling installed, but careful thought should go into selection of the right frequency for the job. More on that later. Amplitude, as we have seen, is determined electrically and acoustically. Clamp force is generally provided by an air cylinder and is adjusted by adjusting the pressure in the cylinder. The ultimate temperature of the joint is determined by the heating rate and the exposure time. In the simplest terms, for any given weld, one can increase the heating rate by increasing clamp force or amplitude and decreasing exposure time, or decrease the heating rate by decreasing ampltiude or clamp force and increasing exposure time. When changing the heating rate, it is important to remember that the heating rate is much more greatly affected by changes in amplitude than it is by changes in clamp force.

7 February 2009 -- Amplitude can also be controlled by varying the shape of the acoustical tooling components. The basic unit of ultrasonic tooling, the horn, sonotrode, or coupling bar, is in its simplest form when it is a cylinder of metal, typically aluminum, titanium, or hardened tool steel, that is a half wave long and less than 1/3 wave in diameter. Wavelength drives tooling proportions, so the size of a tool with comparable running characteristics will vary in inverse proportion to wavelength-- low frequency tools will be larger than high frequency tools. A straight cylinder will simply transmit amplitude unchanged through its length, but not by moving in piston fashion. The vibration enters one flat face of the cylinder which is driven in reciprocal fashion by the working face of the transducer or converter. This reciprocal motion drives the cylinder in to half-wave resonance. The simmplest way to describe this is to say the two flat faces of the cylinder move in opposite directions while the diameter at the midpoint of the length of the cylinder gets larger and smaller. This elastic motion is easy to visualize if you consider the cylinder as having two masses at each end joined by a spring in the middle. If the frequency of the motion exactyl matches the spring rate, the two masses will move in opposite directions in repeating fashion, in other words, the item will resonate. The reason we refer to this as a half wave is that the two ends of the device move in opposite directions, and the midpoint stands still. This point where there is no motion is referred to as a node, or nodal point. Now, if the cylinder is not a simple cylinder but has different diameters on each side of the nodal plane, the ampltude will be different at either end of the device. According to the law of conservation of momentum, the end with lower mass will move at higher amplitude than the end with higher mass. Thus, a booster is a half-wave component that either increases or decreases ampltiude according to its shape (or it may be a device which does not change amplitude at all!). If there is more mass on the input end than the output end, the booster will increase amplitude and vice versa. Any tool, even one not specifically called a booster will exhibit this characteristic. The difference in ampltiude between the ends is referred to as gain. Gain is expressed as a ratio, and common practice, at least in the US, is to refer to a device than increases amplitude in terms of its output amplitude compared to its input amplitude. Thus a device which doubles amplitude is referred to as having a gain ratio of 2:1, pronounced "two to one." This means of expressing gain is often reversed in Europe, so the same device would be said to have a gain ratio of 1:2, or "one to two." The reverse of this device would by common parlance be referred to as having gain of 0.5:1 or 1:0.5, so this difference in usage is less confusing than one might otherwise expect. Some manufacturers have dropped the "...to one" part of the ratio, and simply refer to gain as being 2.0 if the device doubles amplitude or 0.5 if it halves it. Bear in mind that a booster is designed for operation with a specific input end and output end, and may not be used as a booster with opposite gain ratio by simply swapping ends, as tuning will be compromised if this is done.

12 January 2009 -- Some form of amplitude control was available fairly early in the history of ultrasonic plastics assembly, even before ultrasonic welders had any other form of process control. Amplitude can be controlled either electronically or acoustically. Amplitude is typically defined as the peak-to-peak distance traveled by the work face of a transducer or tool. Sometimes amplitude is defined from rest to peak, and in fact amplitude can be measured at various places on a vibrating body in various directions, but it is essentially only the linear amplitude which is of any use in plastics welding, so we tend to limit our discussion to that. In most ultrasonic system design, voltage determines amplitude, so amplitude can generally be adjusted electrically by changing the output voltage to the transducer or converter. In many designs that predate the late 1980s (some of which are still on the market today), output voltage from the generator or power supply would be proportional to input voltage, which could cause process variations if line voltage fluctuations or variations occurred. The transducer or converter can be thought of as a reciprocating electric motor. The generator or power supply puts out a certain voltage, and then as the transducer is mechanically loaded (as resistance to motion is felt by the device), it will draw more amperage from the generator or power supply in an attempt to support the amplitude. Unless the generator is equipped to provide a compensatory increase in voltage, amplitude will actually sag under load, much as the speed of the electric motor on a power saw sags as the saw encounters resistance when cutting wood. Amplitude could also be controlled through manipulating the shape of the tooling components, which led to the creation of the booster.

22 November 2008 -- The reader may notice I took a long deep breath before launching into the next topic, ultrasonic welding. It is perhaps the most complex topic to address in the blog, so it deserves a pause to consider. Ultrasonic welding allegedly originally came about as an accidental discovery in a lab where an ultrasonic cell disruptor was being used in the presence of polystyrene sample containers. Apparently one of the containers was excited accidentally by the disruptor tip and bonded to another container. Thus was born one of the most common and least understood of the thermoplastics assembly methods. The basic concepts of ultrasonic welding were reasonably well fleshed out by the early 1960s. To look at some of the early equipment now reminds us of how simple the early applications were compared to what is commonly being done now, and the crudeness of the early equipment causes one to wonder that the process ever came into widespread use. The early sonotrodes, or horns, were usually conical, cylindrical, or rectangular, staying well within the 1/3 wavelength limit for transverse dimensions, the equipment was relatively low powered by today's standards, and timing and clamp force were up to the devices of a skilled operator. The early presses (assembly stands) were little more than manual arbor presses with a vibrating probe installed, often one manufactured for cell disruption, with timing of ultrasound controlled by a foot pedal manipulated by the operator. Whatever parts could be manufactured with any consistency whatsoever would have had to be made of the most favorable of materials, polystyrene or ABS, and of very simple design. Such parts would also be severely size-limited in order to be welded with such equipment. Round or rectangular parts of perhaps not more than 30mm transverse dimension could have been welded, but only once the operator had developed an instinctual feel for the process.

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