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Blog Topic Index
Ultrasonic Plastics Assembly
Hot Plate Welding of Thermoplastics
Thermal Staking, Swaging, Insertion
Snap Fit, Press Fit, and Adhesives
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.
14 February 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 exactly 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.
23 July 2008 -- It seems that one could avoid some of plate-cleaning issues associated with hot plate welding by switching to non-contact hot plate welding. This technique has essentially the same cycle as hot plate welding using the same equipment, except that instead of pushing the parts to be joined onto the hot plate, they are simply held in close proximity-- about one mm typically-- to the hot plate. This is a lot harder than is sounds. First, part fit up is about ten times more critical in non-contact hot plate welding than the standard approach because the small gap between the plate and part needs to be essentially the same across the entire joint. The plate needs to be considerably hotter in order to drive the heat across the air gap, air being a reasonably good thermal insulator. Air also contains oxygen, which degrades most plastics when they melt in its presence, so the skinning effect is more pronounced than with contact hot plate welding. Degradation can be severe enough that heating time must be reduced, therefore heat is not driven as deeply into the parts, which in turn causes lower joint strength. Since the hot plate itself will often need to be 100 to 150 degrees C hotter in a non-contact process, and since the materials that are most difficult to weld in a contact process are typically those with higher melt temperatures anyway, the factor that most often makes this approach unfeasible is simply having to run the equipment at such extreme plate temperatures. While 200 to 250 dgress C as used in a contact process is one thing, 350 to 500 degrees C in an non-contact process is another thing entirely. It is very important to consider the bushings and cylinder seals and sensors and various tooling components that are used when temperatures run that high. So, while it often seems like a good idea, non-contact hot plate welding is actually a rare process.
21 July 2008 -- Hot plate welding works well with non-planar joints. That said, part fit-up is always important, and especially so when parts are relatively large as is often the case when the process is selected. A quick rule of thumb is to avoid having to hot plate weld joints that run more than 60 degress from the plane of the plate carrier, and try to hold it to 45 degrees maximum if possible.
3 July 2008 -- Success in hot plate welding depends on generation of flash. This seems a little counter-intuitive, but it makes sense if you think about it. The material is pressed againt the hot plate. It begins melting at the interface and if no force were applied to keep it against the surface of the plate, a gap would soon appear. To prevent this, clamp force is applied and the material allowed to flow naturally until the part has collapsed about 0.3 to 0.4 mm, typically controlled by tooling stops. The displaced material forms a flash dam around the melting plastic, shielding it from air and allowing the heat to penetrate into the plastic. This penetration of heat is important, because enough heat must be stored that the material in the joint area remains molten during the change over or open time. Some studies suggest that joint strength increases with heating time but cannot be improved with heating times beyond 13 seconds. Sometimes this heating time is as little as four to five seconds. Plate temperature can be the subject of some experimentation, but good results are generally obtained when the plate temperature is set at the material's highest recommended heater zone setting for molding or extruding. This will be somewhat below the degradation temperature but is usually well above the melt temperature. The change over or open time is meant to be a short as practical. What happens during this time is the parts are pulled off the hot plates, the plate gotten out of the way, then the parts brought into contact in final alignment and under clamp force. Fixtures must be designed to make sure parts are securely held because often it will take some force to get them to release from the plates. During change over time, the hot material is in contact with relatively cold air and will oxidize and skin over a bit. When the parts are brought back into contact, clamp force presses the parts together and causes flow of this damaged material out of the joint in the form of flash. During the join or weld time, the parts are allowed to collapse another 0.3 to 0.4 mm per side, for a total joint collapse of the entire assembly of about 1.5 mm, again, typically controlled by tooling stops. If heat has been allowed to soak deeply into the parts, resolidifcation will occur somewhat slowly. This is important for joint strength, because the flow of material perpendicular to the joint plane (causing flash) will cause molecular orientation to be parallel to the joint plane and therefore weaken the joint. The longer it takes the joint to cool, the less molecular orientation there will be, as the molecules are free to reorient randomly for a longer time. In hot plate welding, speed of process is always traded for joint strength. Proper flash formation indicates that the material was in fact hot enough to be melted, and that most of the material damaged by contact with cold air during changeover time has been expelled from the joint. Flash can be hidden or removed, but it is a necessary by-product of a well-fused joint. The good news is that hot plate welding does not produce particulate like the frictional processes do. The bad news is that hot plate welding is not recommended for materials that have a relatively small spread between melt temperature and degradation temperature.
5 June 2008 -- There has been a lot of discussion over the years about which materials or coatings to use for hot plate surfaces to minimize sticking and stringing of the plastic material. If one can minimize sticking, one can minimize plate cleaning activities. Every now and again someone will claim a miraculous coating that eliminates sticking and stringing, but quite often it proves to be less than completely miraculous and require regular recoating. Hot plate surfaces are usually made of copper or possibly aluminum. Using bare aluminum hot plates is generally not recommended, and some kind of coating is needed. Bare copper is a somewhat better situation for most materials, but if the material is filled or reinforced, bear in mind that fillers and reinforcements are quite abrasive and neither copper nor aluminum stand up well long term without a tough coating. Either copper or aluminum can be plated, but quite often at hot plate temperatures the plating will begin to crack and peel, often from differential thermal expansion rates. Be aware that some high-temperature thermoplastics will require high plate temperatures that may actually result in heat treating (annealing) of copper, aluminum, and possibly even steel structures in the plate assembly-- a good reason to avoid hot plate welding high temperature materials. Some have used fluoropolymer (i.e. Teflon TM) coatings to some success, but care must be exercised to avoid overheating the coatings, and they are generally only effective with polyolefin materials because almost every other thermoplastic requires plate temperatures that will quickly destroy fluoropolymer coatings. An approach that often yields success is to oil the hot plates repeatedly over several hours of heating to build up what could be termed a seasoned surface, much like what develops on an iron skillet after cooking with it several times (one usually tastes quite a lot of iron in the first few meals prepared with a brand-new skillet). Some of the lower temperature thermoplastics can be processed with a fluoropolymer-coated paper covering the hot plates, and often machines are built that have either manual or automatic paper-changing systems to ensure reasonably fresh surfaces are used at all times. This eliminates the problem of coating breakdown but adds a consumable to the operation. Finally, sooner or later the plates will have to be scrubbed, and this activity repeated many times will result in surface damage to the plates. For most operations, the plates are mounted on a plate carrier, the carrier being part of the machine itself, and the plates considered renewable to a point but ultimately consumable. Many times when coatings are used, multiple sets of plates will be made so that one set is in production, one is being recoated, and one is waiting to go into the machine. Ultimately, choice of plate material and coating is on a case-by-case basis and depth of experience along with some experimentation will determine the right direction to go.
21 May 2008 -- When contemplating use of hot plate welding, a critical consideration is the thermoplastic material to be joined. Generally, one would want to join materials with relatively low melting temperatures, low thermal conductivity, a good spread between the melting and the degradation temperatures, relatively high viscousity when melted, and resistance to interaction with oxygen when melted. If it sounds like this list eliminates a lot of materials, it probably does, but it also includes a lot of materials. Materials with reinforcements and fillers complicate matters and may be better suited to another process. The reason to avoid hot plate welding filled or reinforced materials is the potential for buildup of a residue of filler or reinforcement on the hot plates. This tends to become more of a problem the more parts are run and requires frequent scrubbing or scraping of the plates. This plate cleaning can be relatively easily automated if the plates are planar, but can be complicated and expensive if the plates have complex geometry. Materials that work well with hot plate welding include olefins like polyethylene and polypropylene, fluoropolymers, polystyrene, polymethyl-methacrylate, and acrilinitrile-butadiene-styrene. Materials to avoid hot plate welding include polyamid, polyvinylchloride, and almost all of the very high temperature thermoplastics. Depending on the application, polycarbonate can be hot plate welded but its relatively high melt temperature and especially its relatively high thermal conductivity can make it difficult to work with. High material lubricity is not an issue for hot plate welding and therefore recommends it over frictional methods. Hot plate welding also handles complex joint geometries pretty well, which also recommends it over frictional processes for these materials. Part warp issues must be minimized by fixturing and can be an issue with some assemblies, especially larger assemblies.
13 May 2008 -- Hot plate welding is the oldest of the plastics welding processes. Even today, some assemblies are put together by sliding a hot piece of iron between two parts until the plastic flows and then pressing the parts together by hand. While this may be acceptable practice for some assemblies, it is hardly practical for the vast majority. Still, all of the basic principles of hot plate welding are present in this simple process. First, plastic parts are fixtured, then pressed against a hot tool until the surfaces are melted, then the hot tool is removed and the parts are pressed together until they cool sufficiently to remain joined. The key variables in the process are the heat of the plate, the pressing time, pressure, and distance traveled when against the hot plate, the speed of the change-over, that is, the time the parts are separated but not against the hot plate, and the time, pressure, and distance traveled when the parts are pressed together to effect the weld. A key feature of traditional hot plate welding is actual contact between the plastic parts and the heated tool itself. Several attempts have been made to eliminate this contact over the years, some more successful then others, but traditional hot plate welding is still a common and steady process for manufacturing a wide variety of assemblies in a dizzying array of industries.
26 April 2008 -- A heat staking machine can be used for hot tool insertion as well as thermal staking or swaging. The most common inserts for hot tool insertion are brass female-threaded fasteners placed in molded holes to provide good purchase for bolts used to secure circuit boards or other components/subassemblies to a chassis or inside a case, or to secure the case halves. Brass inserts are preferred to steel for this operation because the copper content of the brass assures a relatively low thermal mass for the inserts and they will change temperature rapidly. The inserts are typically placed in the holes partially engaged. The press is actuated and the tips come into contact with the inserts. At this point, an insertion delay can be programmed to allow the inserts to warm up to a temperature that allows for melting of the thermoplastic material. After the delay, the press continues to press the inserts into the holes. Depth is usually controlled by a mechanical stop. After contacting the stop, the press can simply retract if the insertion is low precision, or the press can stop while compressed air is used to cool the inserts and tips to prevent the inserts floating back out of the hole on a cushion of expanding hot plastic. Getting the tip temperatures and delay times just right takes a bit of experimenting. Beware of the temptation to simply use press force to jam not-quite-hot-enough inserts into their holes, as holes almost always have suceptibility to cracking at the knit or weld lines when under hoop stress. The pull-out and torque strength of the set inserts can usually be improved by slowing the process down and allowing the heat to soak a little more deeply into the plastic during the process. As with thermal staking, inserts of various sizes can be set on multiple levels simultaneously.
10 April 2008 -- Should you call a thermal staking machine a heat staker? Most thermal staking presses actually have interchangeable tooling. Since they can be easily be converted from heat staking to thermal insertion, embossing, or degating, they should probably more properly be called thermal presses. It is possible to build a machine for the sole purpose of heat staking, in which case the term heat staker could be applied.
5 April 2008 -- Hot tool staking probably started with a screwdriver or some similar tool heated with a torch and then applied to a tab or post to capture another part. The process does not work much differently today, though the equipment and tooling has become much more sohpisticated. Most thermal staking machines today use electric coil heaters with embedded thermocoules and digital temperature controllers to maintain probe temperature. The tip, attached to the probe, is designed to have low thermal mass, that is to say it can change temperature easily. The tip is heated by the probe, which is mounted to a press that delivers the probe and tip assembly to the work and applies force to deflect the head(s) to be formed. When the tip contacts the work, it transfers heat to the work to melt the material and form the detail. If the press were simply retracted at this stage in the process, most materials would stick to the hot tip and either form strings or the head would actually be pulled off of the tab or post. To prevent this, pressurized (and sometimes cooled) air is delivered to the tip long enough to reduce the tip temperature enough to allow a clean release and leave a well-formed detail with sufficient strength to hold the assembly together. This action is usually called post-cooling. Following retraction, the tip is then reheated by the hot probe. If more than one probe assembly is installed on the machine, they can be individually controlled if there are a sufficient number of heater controllers on the machine, or they can be zoned together, with one thermocouple providing temperature feedback to the heater controller, and other heaters modulating in open-loop fashion. This works better then one might think when identical probe/tip assemblies doing the same work are on the same zone and heaters are closely matched. There is considerable freedom in tooling design for the heat staking process, and multiple size heads can be formed at multiple levels in the assembly. Probes cannot go into tight spaces in parts, though, as they may thermally deform nearby details. Likewise, not all plastics respond well to thermal staking, including matrials with sharply crystalline melting points such as polyamid (nylon), or narrow melt to degradation temperature ranges such as polyvinyl chloride.
3 April 2008 -- Press fits work on a somewhat different principle than snap fits. In a snap fit, the goal is to temporarily deflect a detail and have it return to rest after capturing a matching detail in another part. Press fits work by permanently deflecting a detail and using the surface friction and surface affinity of the plastic material to hold the assembly together. The classic press fit is a pin engaging a slightly smaller hole. The technique is not limited to round pins in round holes; a common variant is a round pin in a hexagonal hole. The interference fit between pin and hole and the wall thickness of the boss are very important in making this type of assembly work. Sloppy molding practice that embrittles the bosses or leaves a weak weld line in the boss can sabotage the best press fit design. The optimal interference dimensions and length of engagement also depend on material properties including stiffness and lubricity, and creep can be an issue as assemblies age. Some years ago a technology was introduced that executed the press fit at high velocity and claimed to actually weld the pin in the hole through heat produced by surface friction. As with all other techniques, success depends on good design and careful testing to get the details right. Press fit assemblies generally cannot be disassembled, which may be either an advantage or a disadvantage compared to snap fits. At one manufacturer, a war broke out between engineers who favored press fits and engineers who favored ultrasonic welding. Recollection is that good points were made on both sides, and it is possible that the conflict my still be unresolved more than a decade later.
26 March 2008 -- The words snap fit indicate exactly what is going on with this type of assembly. Some detail of one part is deflected from its rest position by some detail of the other part and it snaps back to rest position to lock the assembly together. A post with a hook detail going into a slot is the most easily imagined form. There are in reality a vast array of snap fit possibilities. It is impossible to relate more than general information in this format, but the imagination of designers has not yet been exhausted when it comes to the endless variety of configurations. What remains constant, however, is the need to deflect the material enough to get sufficient engagement to do the job without stressing the material to the point of weakening it. This is why it is generally advantageous to have many smaller details rather than one larger one. Another key consideration with snap fits is the need to incorporate the necessary undercuts without creating unnecessarily complex and expensive mold features. Often, snap fits can be designed so the part will snap out of the mold without requiring side action, other times, the part wall can be relieved so the undercut can be formed with a straight-draw detail. Snap fits can be designed so the part can be disassembled, but quite often a snap fit is a one-way street. Given the creep issues with plastic parts as they age, careful thought and attention needs to be paid to the dimensions of the details so that assemblies do not loosen up over time. Also, if the assembly requires a seal, the seal will have to be provided for mechanically and separately from the snap detail.
24 March 2008 -- Fasteners have been used to assemble thermoplastic components almost as long as there have been thermoplastic components. Of course, some of the earliest plastic parts were things like combs that did had no assembly requirement, but it wasn't long before someone drilled a hole in a part, put a wire loop in it, and hung it on a chain around his/her neck. Thus the realm of fastener use on plastic parts was born. Thermoplastics parts have been bolted, screwed, riveted, clipped, stapled, and held together with just about every fastener imaginable, but there are a few key points to consider when using fasteners on plastic parts. First, few plastics exhibit a really high degree of crystallinity, so are therefore amorphous to some degree or another when "solidified." Fully amorpous materials, which includes a great many thermoplastic materials, can never be said to be really "solid." They just flow so slowly at normal temperatures that we humans experience them as solids. What this means is that almost all thermoplastic materials exhibit more or less creep; creep being the tendency of a thermoplastic part to change dimension over time. Since most thermoplastic parts have some degree of internal tension, the parts generally get smaller over time. This is why the vinyl trim in an older car will pull away from openings and open up gaps. Creep is exacerbated by stress on the material in the form of temperature extremes, vibration, and such. Creep can be a major problem for any thermoplastic assembly but is particularly troublesome where screws may loosen as material shrinks, or where a lessening wall thickness will cause a rivet to become loose and start "working" or moving around in a hole. In some cases, where a fastener needs to be set to a certain torque to remain tight, like in an under-hood automotive application, it becomes necessary to insert metal sleeves into bolt holes to prevent crushing of the plastic material and looseneing of the bolt as the plastic shrinks. Where bolts need to be threaded into a thermoplastic component, especially when disassembly is a possible need, threaded inserts are strongly recommended. There are several great books on fastener use in thermoplastic components, so again we'll not belabor a point that is near the edge of our expertise here. Just be sure to have done your homework before incorporating fasteners in a new design to avoid unpleasant surprises later in product life.
21 March 2008 -- It's hard to say which thermoplastic joining process came first, hot plate welding, hot tool staking, solvent bonding, adhesives, snap or press fits, or fasteners. We know that ultrasonic welding came in the early 1960s, vibration welding in the late 1970s, and laser welding in the 1990s, but the others are really anybody's guess. I believe some of the earliest plastic materials were cellulosics, so it's hard to imagine anyone having much early success with heated tools, though it is possible. Another early material was polyamid (nylon), so again, hard to imagine a lot of success with heated tools. My guess is adhesives came first. But it's just a guess. Adhesives have been used in plastics assembly for a long, long time. Pure adhesives work totally on the basis of surface affinity; in the simplest terms, the materials stay together because chemically they simply want to. Some adhesives use a combination of surface affinity and solvent action and therefore are possibly more correctly called cements (I am open to correction on this). Solvent bonding is the action of chemically breaking down the surface of the joint by dissolving the materials, allowing flow, and then resolidification through the evaporation or diffusion of the solvent. Solvent bonding is not technically an adhesive process, but it is usually lumped into that category by those of us outside of that realm. The materials to be joined will greatly influence the choice of adhesives or solvents. Solvent action depends on participation of the molecules of the material, and solvents that will bond one plastic will quite often have no effect whatsoever on another. Surface affinity bonds are somewhat more generic, with the most basic rule that low surface tension plastics work best with high surface tension adhesives and vice-versa. Anyway, we're coming dangerously close to exhausting my knowledge on the subject (and we may already have crossed the line) so this discussion will end with the following admonition: If considering adhesive or solvent bonding, work very closely with the suppliers of the bonding agents, as reliable and durable bonds depend on getting the chemistry right.
19 March 2008 -- One of the more basic questions about plastics assembly is how to know which process to specify for the assembly. Often, the size and configuration of the parts and the material inolved will rule out various options. Since very few people do thermoplastic assembly for the pure joy of it, the economics of the project will definitely enter into the discussion as well. Often, a designer will have a preference for or familiarity with a particular process and drive their design toward what they perceive as the right combination of factors to make that process feasible. Sometimes, the joint strength requirement will drive the decision. Sometimes, it's more a matter of which equipment is already owned. Whatever drives the decision, the cost of the headaches of using the wrong process will almost always be greater than the cost of doing it right in the first place.
18 March 2008 -- When considering whether to even attempt to blog about assembly of thermoplastic components, the issue of whether I was enough of an expert to be holding forth on the topic kept entering my mind. I have written a lot of magazine articles and technical papers over the years, but they have always been peer reviewed or at least reviewed by an editor of some kind. Blogging is riskier because of the possibility of a remark becoming part of the permanent record even after it has been removed from the site in favor of a better second thought. But then, there are a lot of people who blog on about many things of which they know little, and that does not stop them from doing it, or from being viewed as experts by others who almost mystically assign importance to anything they read on the internet. The truth is that anyone with an opinion and ten bucks a month can have a web site about whatever they wish, without having any qualifications whatsoever. Negative opinions and statements are generally presumed to be true while positive ones are presumed false. For example, if I said a certain restaurant was very good, it would be presumed false because my motive would be suspect; if I said a friend of a friend said they served dog meat, that would be enough to drive them out of business. The more pervasive the internet becomes, the greater this effect. Anyway, my desire is that by blogging the knowledge and experience resulting from my long career in the industry I may help someone to find the answer to a manufacturing puzzle he or she is trying to solve, and to shed light on the various tricks of the trade learned over the years that may or may not be well known or thought of at the right moment in time. Oh, and just to remove any question about the commercial intent of this endeavor, if you are reading this I hope you will buy our stuff. It's good stuff. What appears in the blog is true to the best of my knowledge and I will call it like I see it. So, enjoy the blog. I hope it turns out to be something of value for you.
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