Laser Welding Plastic Parts
To manufacturers, a laser is much more than a beam of light energy. It is the means to faster, better and more cost-effective assembly of plastic parts.
Such was not the case in the early 1990s. Back then, many engineers considered laser welding of plastic to be the stuff of science fiction. Thermoplastic polymers weren’t stable enough to conduct laser energy and enable welding.
But, other engineers, such as those at the Aachen, Germany-based Fraunhofer Institute for Laser Technology, took on the challenge of making the process work. Over several years, Fraunhofer performed extensive research in conjunction with various laser welding equipment suppliers.
By the start of this century, through-transmission laser welding of thermoplastic became commercially viable as multiple suppliers introduced their first systems. Automotive, medical device and consumer electronics manufacturers, in particular, have embraced this technology—using it to cost-effectively weld parts of all shapes and sizes.
In this process, laser energy is passed through one plastic component (the transmissive part) and absorbed by the second component (the absorptive part). The energy locally heats and melts the surfaces at the joint interface, and, with the application of a controlled clamp force, the parts are joined.
“In the 1990s, laser welding of polymers was considered an unconventional technology suitable for only niche applications,” explains Andreas Rosner, polymer technology specialist at Fraunhofer. “Nowadays, it is well-established [and a] serious challenger to traditional processes of joining polymers, such as adhesive bonding and ultrasonic welding.”
Several Ways to Weld
Through-transmission laser welding is typically done with a diode laser having a wavelength between 808 and 1,064 nanometers. Cycle time ranges from a few milliseconds to 8 seconds, depending on material and part geometry. The laser is brought to the part in one of five ways.
With contour, or spot, welding, the laser is focused to a single point, which is then directed along a preprogrammed path to create the weld. The ideal spot size for this kind of welding is 1 to 2 millimeters, although spot sizes can vary from 0.5 to 2.5 millimeters, depending on the application.
Contour welding’s main benefit is flexibility. Almost any welding path can be programmed into the welding machine, which can direct the beam using a moving stage, robotics or a system of mirrors and servomotors. After programs have been entered into the controller, changeovers from one assembly to another are possible with the push of a button.
Somewhat less flexible, but faster, is simultaneous line welding. With this process, laser light is directed along a straight line. Typical weld dimensions are 1 to 2 millimeters by 30 millimeters, with a cycle time of 1 to 2 seconds. Multiple lasers can be used to create square or rectangular contours. If necessary, optics can be used to create curved lines.
Similar to line welding is quasi-simultaneous welding. A set of servo-driven mirrors directs a single point of laser light along the weld path at a rate of 40 circuits per second.
On the plus side, this method offers cycle times faster than contour welding and comparable to that of simultaneous welding if the weld is small. Also, because servomotors are used to trace out weld geometries, a single laser head can be used for multiple welds.
Quasi-simultaneous welding’s one big negative is lack of flexibility. It is limited to flat or slightly contoured joints.
With mask welding—a proprietary technique developed by Leister Technologies—the laser line sweeps across the entire part, which has been masked so that only those portions left exposed will melt to create a weld. Manufacturers like that the method creates precise and complex weld patterns. Applications include sensors and microfluidic components in medical diagnostic devices with weld lines as narrow as 100 micrometers.
Finally, there is GLOBO welding, which is a variation of contour welding and was introduced by Leister in late 2004. This method uses a welding head that looks a little like an oversized ballpoint pen. Its rotating glass sphere focuses the laser beam and presses against the parts being welded. Manipulated via a robotic arm, GLOBO welding offers flexibility and the ability to weld complex joint geometries.
Parts and Challenges
Many types of plastic parts are laser welded. Automotive parts range from filter assemblies, intake manifolds and control modules, to sensitive components that are hermetically sealed in plastic. These include oil and temperature sensors, and remote entry systems (such as those for Mercedes-Benz S-Class vehicles). The auto industry is also very keen on laser welding parts with clear-to-solid color combinations, such as windows for radios, taillights and instrument clusters.
Dave Girardot, micro division sales manager for Rofin-Sinar Inc., says medical device manufacturers tend to laser weld high-volume disposable parts, such as filtering elements and those with noncomplex housings. Other laser welded medical parts include components of blood-analysis and microfluidic lab-on-a-chip devices.
Manufacturers of consumer products are slowly but steadily increasing their use of laser welding. In the electronics arena, cell and smart phone housings have been laser welded for many years, whereas the technology is just beginning to be used for parts in entertainment consoles and microphones.
“The laser is basically just a heat source, and it does not principally determine part weldability by itself,” claims Alex Savitski, Ph.D., chief engineer for advanced technologies for Dukane Corp. “Thus, if parts are weldable, they can be laser welded, given a proper joint design. Laser welding’s localized heat input and low mechanical stresses enable welding of sensitive assemblies in medical design, electronics and automotive components without damaging them due to heat or vibration.”
As for white goods, Hugh McNair, manager of laser applications for Branson Ultrasonics Corp., says some appliance manufacturers are beginning to laser weld pump fittings. Girardot notes that aerospace suppliers are laser welding various cockpit components.
Lack of awareness or knowledge of laser welding is a major reason why plastic part manufacturers in several industries have not implemented the technology. Production volume relative to budget is another factor.
“The investment cost to get started with laser welding is $150,000 to $500,000,” explains Dax Hamilton, North American sales manager for LPKF Laser & Electronics. “To justify that amount of money, we recommend that the company produce around 200,000 parts per year. For example, one of our medical device customers welds about 1 million parts per year, and they ended up installing a tabletop system that cost about $150,000.”
From Lasers to Workstations
For the past year, Magna Donnelly (Shanghai) Automotive Technology Co. Ltd. has used the Novolas WS-AT system to contour weld an LED light assembly (with internal electronics) that fits into a door handle. The light is strictly a design element. Annual production volume is 150,000 units.
Andrew Geiger, manager of laser systems for Leister Technologies, says Magna prefers laser to ultrasonic welding for this application because laser welding produces a watertight seal and no flash. To ensure alignment of the LED assembly’s top and bottom components, Leister built additional positioning points into the clamping mechanism.
TRUMPF’s TruLaser Station 5005 with a TruDiode laser is ideal for manufacturers just getting into laser welding and those that need to weld small and medium-sized parts. An operator controls the workstation via a touch screen from a sitting or standing position. The workstation is equipped with up to five axes, several focusing optics or scanner optics and a large work area for easy fixture integration.
A global Tier 1 auto supplier operates the TruLaser Station 5005 at separate plants in Austria, Canada and China, according to Rick Davis, key account manager for TRUMPF. The manufacturer welds plastic lumbar adjustment devices that go into seating assemblies.
Clearweld is a solvent-based coating that 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 the coating—via liquid dispenser—onto the interface prior to welding.
Frank Goroleski, president of Crysta-Lyn Chemical Co., says the coating 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 very little to no visible color. Goroleski says the coating can also be custom formulated.
In 2011, Fraunhofer researchers discovered a way to laser weld transparent plastics without using Clearweld or an absorptive part molded with absorptive-enhancing pigments or additives. Engineers there studied various transparent polymers to determine what wavelengths of laser radiation they absorbed. A wavelength of around 1,700 nanometers proved to be the most effective.
The engineers then developed a system that matched this wavelength to the plastics’ optical properties. The system’s lenses are specially designed to reach maximum density at the beam waist—the lowest point of the beam. By focusing the waist on the weld point, the highest possible temperature can be applied to a very precise location.
DILAS Compact fiber-coupled diode lasers are cooled with air or water. Air-cooled models come in 808- and 980-nanometer wavelengths and provide up to 100 watts of power. The water-cooled models come in 450-, 808- and 980-nanometer wavelengths and provide up to 500 watts.
Girardot says the air-cooled models are especially well suited for medical device and automotive manufacturing. Both types offer high efficiency and are enclosed in a compact housing (19 rack mount) that easily integrates into production lines.
The TwinWeld 3D system from LPKF uses hybrid technology to weld complex 3D parts. It combines a near-infrared laser (980 nanometers) with special halogen lamps, the latter applying energy to the welding zone through polychromatic radiation.
Hamilton says hybrid welding increases welding speed and reduces the internal stresses on the weld seam. It also improves process stability and gap bridging between parts.
Dukane has made custom systems for contour laser welding of plastic parts since 2003, according to Mike Leuhr, applications technology manager for Dukane. Current systems feature fiber lasers with direct beam or fiber delivery.
Some unique applications of Dukane systems include a very tall consumer product that’s welded from both ends, and a medical device company welding a number of components into a large-diameter part.
Branson’s laser welding systems consist of a Radiance 3G or 3I controller and one to four laser banks that, individually, deliver 125 watts of power. Each bank has five diode lasers, and each laser has 10 points that homogenize light before injecting it into the part during welding.
The 3G benchtop controller operates one or two laser banks and requires an external chiller for laser cooling. Model 3I is a free-standing controller that handles up to four laser banks and features an internal chiller. Both models interface with the company’s 2000X actuator.