Plastics Assembly: Welding With Light
Indeed, there’s a widespread misnomer that lasers will damage or destroy plastic parts. However, new technology and new applications are making laser welding a serious alternative to more traditional plastic joining techniques, such as hot-plate welding, ultrasonic welding and vibration welding.
Laser welding was introduced as a viable method for joining thermoplastics more than 10 years ago, but the technology has been slow to catch on due to high costs, safety concerns and reliability issues. Today, changing market conditions and more affordable laser modules are prompting manufacturers to review the pros and cons of joining plastic parts with light.
Many manufacturing engineers are still learning about the technology and how it can be applied. Experts predict it will gain more serious consideration for assembly applications as people become more familiar with this ultrafast joining process.
"We’re definitely seeing more interest in laser welding, especially within the auto industry," says Craig Norrey, a Swiss-based technical consultant for DuPont Engineering Polymers (Troy, MI). "It’s not a replacement for traditional technology, such as ultrasonic welding, but there is huge potential. We expect to see a lot of new developments during the next 12 months. There is some exciting research being done in Japan, Germany and the United States."
However, Norrey admits there’s a bit of a learning curve involved. "Whenever you use the word ‘laser,’ people become very timid," he points out. "Safety concerns, such as radiation, pop into their heads, in addition to the risk of melted material."
According to Norrey, material suppliers still have some hurdles to overcome, such as combating the "novelty factor" associated with the new technology. He also says some technical issues must be addressed, such as developing polymers that are both laser transparent and flame resistant.
"Cost is no longer a big factor," claims Norrey. "The costs of laser welding machines are very comparable to ultrasonic and vibration welders."
Many observers believe it’s only a matter of time before manufacturing engineers become confident in plastic laser welding. "It is a new method," says Jerry Zybko, general manager of Leister Technologies LLC (Schaumburg, IL). "With ultrasonics in the 1970s and 1980s, piezoelectric crystals and their actions were difficult to explain and something you couldn’t see," he points out. "It took time to develop confidence.
"The human eye can see colors and light up to the 750 nanometer wavelength. Laser welding operates between 840 and 1,060 nanometers and is invisible to the human eye. The learning curve of understanding the technology and having confidence in the output is growing fast."
Unique AdvantagesLaser welding offers many advantages to plastic assemblers, such as the ability to weld dissimilar plastics. Water- and gas-tight welds are possible, in addition to small, precise weld lines. Other advantages include:
- Speed. In many applications, weld Arial for static welds range from milliseconds to the 1- to 2-second range. However, researchers at TWI (Cambridge, UK) have demonstrated weld speeds of several hundred meters per minute with polyethylene films.
- No contact. The welding instrument does not contact the assembly. A smooth glass plate clamps the substrate leaving no witness or abrasion. It is ideal for very delicate or sensitive parts that require vibrationless and abrasion-free welding.
- Minimal heat-affected zone and thermal load allows for welding in close proximity to sensitive structures and eliminates the risk of part distortion or damage to internal components. It is ideal for assembling small, fragile parts and housings in which heat-sensitive components, such as electronic circuit boards, are contained.
- The parts to be welded are in contact before, during and after the weld. Unlike other processes, there is very little relative motion between the parts so high levels of accuracy are possible.
- Selectivity. Based on the materials involved and the desired weld location, assemblers can select the laser type to create a weld at a surface or deep in the parts, well under the surface.
- Materials that absorb different wavelengths of light can be positioned in the weld line to facilitate heating at that site. This can also be done with inks and transparent dyes. It is possible to make welds that are nearly invisible between clear materials.
- No part movement or stressing. Parts are clamped with a light force and remain in a fixed position throughout the weld process.
- Emission-free. The weld area is concise and encapsulated; therefore, no flash or particulate is generated. It is a very clean process in which there are no excess friction fines or molten plastic.
- By using a scanner, a laser beam can be directed to produce very complex weld patterns. Alternatively, wide areas of welding can be achieved in a similar way.
- The process is suitable for high-melting polymers.
- Power levels can be controlled directly or by defocusing the laser.
- The process is perfect for hard-to-reach weld seam geometries.
- Precise weld patterns. Weld widths can be as narrow as 100 microns (0.004 inch) with a mask system, or 600 microns (0.023 inch) with a contour system.
- Simple part geometry. No energy directors are required—only flat, clean surfaces.
- High weld strength. Weld strength rivals that of alternative methods such as vibration, ultrasonic or radiant heat.
Fast ArialLaser welding compares favorably to ultrasonic welding, vibration welding and other traditional plastic joining techniques. In fact, it is faster and more cosmetic. "Scans of 10 meters per second complete a weld in 2 to 3 seconds and beadless weld seams are possible," says Frank Buck, president of Bielomatik Inc. (New Hudson, MI). Weld time can be 0.5 to 1 second with simultaneous welding, a mask system and small contour patterns.
"The bonding area is more cosmetic, as there are no energy directors to melt down and flow," adds Leister’s Zybko. "The melt occurs between the layers and is encapsulated. There is no marring or damage to the top surface. Only a clean, clear piece of glass touches the top of the part. Light clamping force—20 to 100 pounds of force—is required to ensure the surfaces touch so heat transfer can occur from the bottom, absorbing layer to the top layer."
According to Zybko, there are things that lasers can do that other welding methods can’t, such as welding a clean line 0.03 inch wide, easily and quickly changing weld platforms, and joining different plastics.
Although lasers are still relatively expensive, costs are coming down. "Plastic laser welding technology is becoming competitive with other joining technologies, such as ultrasonic welding," says Robert Grimm, lead research engineer for microjoining and plastics at the Edison Welding Institute (EWI, Columbus, OH). "They are competitive at present and will become more so particularly as solid-state lasers become more developed."
Grimm points out that an ultrasonic welding machine equipped with state-of-the-art controls can cost up to $60,000, which puts it in the same ballpark as a high-powered laser. "However, if one only needs lower power for welding small parts or through-transparent polymers, costs are much lower," notes Grimm. "Vibration welding machines are in the range of $100,000 or more, so welding with light is less expensive. Hot-plate welding machines are less expensive, but again, controls are important in determining costs."
"The hardware is more expensive," admits Zybko, "but the life of the systems and components is longer because of the limited stresses applied. The payoff comes in producing consistent parts. The tooling is typically cheaper because flat-to-flat mating areas are all that is required. There is no marring of the surfaces; it’s noncontact. There is no excessive heating or vibrating, so there is limited part damage. Laser produces cleaner welds so secondary cleaning operations are not required.
"Until recently, only turnkey, standalone equipment was available," adds Zybko. "OEM versions for integration into existing production lines are now available. They have made the purchase more equitable."
Light SourcesLaser welding is typically done with carbon dioxide (CO2), neodynium-yttrium-aluminum-garnet (Nd:YAG) and diode lasers. A CO2 laser operates at 10.6 microns. A Nd:YAG laser operates at 1.06 microns. A diode laser is similar to the Nd:YAG laser, but the operating wavelength depends on the element used to construct the diode. The most common diode laser operates at 0.81 micron.
There are two different types of laser welding techniques: direct welding with a CO2 laser and through welding, with conventional solid-state laser sources or diode laser arrays, in which light passes through a natural material onto an opaque absorber. Jay Eastman, an EWI applications engineer for laser processing, says the process used most frequently for welding thermoplastics is through-transmission infrared using a Nd:YAG laser as the heat source.
Transmission laser welding joins thermoplastics that have different optical absorption properties at the laser wavelength. For example, a piece of clear plastic is placed on top of a piece of dark plastic. The top piece is then illuminated with a wavelength of light that travels through the clear plastic but is absorbed by the dark plastic. The laser beam passes through the top, transparent layer and produces the weld line at the mating surface of the lower absorbing layer.
"Heat and therefore a weld forms at the interface," says Eastman. "Through-transmission infrared welding can be done with infrared lamps and with Nd:YAG lasers because many plastics are basically transparent to the 1.06 micron wavelength of the Nd:YAG laser."
"Laser welding and infrared welding are very similar except that the higher power output and single wavelength from a laser makes it more suitable for mass production," adds Grimm. "Infrared welding is much less costly, however. In addition to through welding, direct heating of a black or other absorbing surface can be done directly by shining the beams from very low power diode lasers directly on the surface to create a melt surface, just as with hot-plate welding."
The laser energy necessary for welding can be presented to the parts in the form of either a spot or a line. Steven Kocheny, applications engineer for laser technology at Leister Technologies says 60 percent of applications use contour welding. "A focused laser beam scans along the contour of the weld line and the components are welded in sequence," he points out.
The absorption and thermal conductivity of the plastic parts determine the welding cycle. Process variables are generally the same as with other welding processes. "Temperature, pressure and time are the main variables," claims Grimm. "Temperature and time are governed by power settings and focus level. Pressure is something that is found by experiment."
Because it’s a noncontact method, laser welding gives assemblers flexibility for fixturing parts. Clamping is required to ensure that the parts to be welded are touching in the desired welding area.
"Pressure must be applied to the joint to both hold the part in position and restrict the heating polymer from expanding," notes Kevin Hartke, product manager at Spectra-Physics Inc. (Mountain View, CA). "This restriction of expansion will allow the polymer to cross-link across the joint interface and a weld to occur."
A basic fixture is required to hold the bottom piece. The top piece can be glass or clear, strong plastic because the light will be penetrating through the glass or plastic, through the top part and onto the bottom piece. According to Hartke, it is also possible to apply mechanical tooling very close to the joint interface but not directly over it.
"Pressure requirements are low compared to other welding processes," says Grimm. "Fixtures need to allow the light to pass so glasses, quartz or translucent or clear polymers can be used for fixture windows."
Material MattersA wide variety of thermoplastics are compatible with laser welding. As with other welding methods, generally any thermoplastic can be joined. Limitations depend on the transparency and absorbency of the material due to additives or coloration. "Glass fillers and impact modifiers will affect the welding conditions, but these substrates are weldable," says Grimm. "However, thermosetting polymers are not weldable by any process."
"With laser welding, a wider range of dissimilar materials are able to be joined," adds Zybko. "The reason is that you essentially dial in the temperature you want between the pieces. You control the temperature by laser power and movement of the part."
A polymer’s light transmitting and absorbing characteristics determine weldability. For instance, welding through opaque materials is impossible. Indeed, one material must be transparent to laser light and the other must be opaque or absorbing.
The best combination is two polymers that are chemically compatible, with one that absorbs and one that transmits. An example is the combination of clear polycarbonate and carbon black-filled polycarbonate.
"The clear polycarbonate does not attenuate the laser beam, so there is very little absorption," explains Hartke. "However, the carbon black-filled polycarbonate does attenuate the laser beam and shows a significant amount of absorption."
Carbon black is a common absorbing material used in polymers to absorb infrared radiation. It is an insoluble material that behaves as a pigment in polymers with high absorption over the visible spectral region. Welds containing it have little to no see-through transmission and are black in color.
"Material has to meet specific requirements regarding absorption of the laser radiation," adds Hans Herfurth, senior project manager at the Fraunhofer USA Inc. Center for Laser Technology (Plymouth, MI). He says researchers are developing "special, absorption-enhancing films to join plastic materials that typically do not absorb the laser radiation."
A recently unveiled product called Clearweld allows two clear parts to be joined without the use of opaque materials or the addition of unwanted color, such as carbon black. End users simply apply a special dye—via liquid dispenser or ink jet printer—onto the interface prior to assembly. A thin layer of material applied at the interface of two pieces of plastic absorbs light and acts as a focal point for the laser.
Localized heating of the substrates occurs resulting in an instant, optically clear joint with no particulates or visible color. Clearweld was developed jointly by Gentex Corp. (Carbondale, PA) and the TWI Advanced Materials and Laser Processes Group.
Plastic laser welding is different than welding metal parts. For instance, metal laser welding uses much higher power densities and highly focused beams. Since polymers melt at much lower temperatures, much lower powers are needed to achieve melting. In addition, metal laser welding often produces a keyhole through the molten metal. A keyhole can be produced with polymers, but the polymer is degraded extensively by doing so.
It is much more challenging to weld plastic with laser energy. For example, most plastics don’t behave like metals. When heated, they do not become liquid and flow. Rather they soften and contract. Metal welding typically uses CO2 gas lasers, which have power levels of 150 to 2,000 watts. With plastic welding, laser power is less than 100 watts.
"The difficulty comes in presenting the right amount of light, or energy, so as to soften the plastic material but not reach its melting or flash point," says Kocheny. "With CO2, you can easily present too much power and lead to instant flash or burning. With a well-controlled diode laser, presented in a consistent speed or duration, the precise outcome of both materials softening and bonding is achievable, repeatedly."
It is possible to join plastic parts to metal parts using laser welding. "But, as with any metal-to-polymer melt joint, the metal must reach the ‘melting’ temperature of the polymer to allow it to wet out on the surface of the metal," says EWI’s Grimm.
Joint DesignLaser welding is popular because it creates liquid- and gas-tight joints. Lap joints are the most common configuration. The upper part is transparent for the laser radiation and the lower part absorbs the energy at the surface. As a result, melting takes place between the parts right at the interface.
Butt joints are more difficult to weld because the energy has to be absorbed in the entire cross section at the same time. Absorption properties of the material must be well-adapted to the laser wavelength to allow the laser beam to penetrate into the material.
According to Leister’s Zybko, the ideal part size for laser welding is 6 by 6 by 1 inch or smaller. "With a robotic arm or large X-Y tables, the size is open but the clamping of the parts may prove challenging with larger parts," he points out.
"With a clear acrylic material, the top piece can be 3 to 4 inches thick or more with special optics," adds Zybko. "The bottom piece can be any thickness. With semicrystalline materials or other amorphous materials, the top part should be 1/8 inch to 1/2 inch thick."
"I don’t think there is an ideal size," counters Grimm. "This is an advantage of welding with light. With high power levels, welds between films of polyethylene have been made at speeds in excess of 600 meters per minute. With very low power levels, very fine welds—fractions of a millimeter in width—have been made.
"With defocused beams, we have made welds that are more than a centimeter wide. With a scanner, the beam can be scanned across an area or in a delicate pattern at frequencies of several kilohertz. The use of large fiber optic bundles of several square centimeters offers even more versatility.
"As for thickness, it depends on the polymer," adds Grimm. "With clear acrylic and polycarbonate, for example, we have made welds at a depth of more than 24 inches when the beam was passed through a rod of acrylic that was abutted to a black acrylic sheet. Welds were also made around corners by passing the beam into one end of a strip of acrylic or polycarbonate and using the strip as a wave guide to carry the energy to a remote weld site.
"For semicrystalline polymers, scattering and reflection from crystallites becomes important, but still, welds can be made through 10s of centimeters. It is mainly a matter of how much power one wants to use and the size and opacity of the polymer. Some of the work EWI has done enables the calculation of thickness that one can weld through."
Assembly ApplicationsAutomotive and medical device assembly are the two leading applications for plastic laser welding technology. "Many industries are investigating this process to replace current joining processes," claims Spectra-Physic’s Hartke. "It is an area in which many individuals are attempting to patent the process."
The auto industry is driving demand, with Toyota Motor Co. (Tokyo) leading the way. According to Bielomatik’s Buck, the auto industry is very keen on laser welding for assembling high-volume thermoplastic parts, especially on clear-to-solid color combinations such as light fixtures, windows for radios and instrument clusters. Other auto parts typically assembled with laser welding include sensitive components that are hermetically sealed in plastic, such as oil sensors, temperature sensors and keyless entry systems.
A prime example is the housing used for Mercedes-Benz remote entry keys and security systems, says Fraunhofer’s Herfurth. The Keyless-Go chip card system is available on many different models, such as the S-Class. Manufacturing engineers at Siemens Automotive Corp. (Auburn Hills, MI) are using laser welding equipment to seal the sensitive electronic components into a waterproof casing the size of a credit card without damaging the components.
Herfurth claims conventional welding processes are unsuitable due to the generally high mechanical loads that the electronic components are subjected to. With laser welding, the necessary process energy only enters a thin layer of approximately 100 microns within the joining zone.
"The energy needed to melt the thermoplastics is transmitted through the transparent section of the casing through to the join level," explains Herfurth. "Only when the laser radiation is absorbed by the lower half of the casing does the temperature rise enough to melt both parts. The electronic components are unaffected by heat or mechanical vibrations, which might otherwise lead to a high reject rate."
Plastic medical devices joined with lasers include biomedical sensors, filter housings, containers and syringes. Researchers are currently developing applications that will allow welding at great distances, welding through curved substrates, welding of microtubes into manifolds and welding by scattered light.