TTLW HAS BECOME A VIABLE MANUFACTURING APPROACH
SEAN T. FLOWERS
Laser welding of plastics has transitioned from laboratory studies to real applications relatively quickly as manufacturers became familiar with the technical advantages of a laser welding technology that provides advanced capabilities to produce superior weld joints in many applications.
The drop in equipment costs for laser welding systems accelerated acceptance around the world and many consider the through transmission laser welding (TTLW) process as a viable manufacturing welding approach. The ability to achieve the highest quality weld joints with the simplest joint designs makes this technology attractive. While there is a learning curve for implementing TTLW, and some cultural change is required, the implementation process is well developed and the benefits make those changes attractive.
Before TTLW, CO2 lasers were utilized to weld plastic components as their infrared radiation (10.6 μm) was absorbed by the natural absorption characteristics of plastics. A melt layer was generated through the top weld component to the weld interface to melt the bottom component. This was not ideal because additional time was required to generate sufficient melt layer, and surface degradation was difficult to avoid.
Most plastics are transparent to near-IR absorption. Initial through-transmission attempts took advantage of broadband infrared lamp sources to provide a significant near-IR radiation that could pass through to the interior. An internal absorber was used at the weld zone. While these sources produced near-infrared radiation, they still carried longer IR wavelengths within the absorbing range of thermoplastics. It was difficult to avoid volumetric absorption and surface degradation. However, high quality interior weld joints were now possible with this approach.
Over a few years, dedicated, single-wavelength near-infrared yttrium aluminum garnet (Nd:YAG) lasers quickly became the most common infrared source for TTLW, advancing the transition from the broadband welding approaches. This approach allowed for heat generation only at the weld interface and minimized the heat affected zone. Over the last 10 years, significant advances have been made in new laser welding specific absorbers, colorants, and laser systems.
The availability of high-power solid-state diode lasers led to the rapid growth in TTLW as low-cost, compact laser systems provided beam delivery systems for plastic laser welding. The cost competitiveness and superior weld quality offered by diode lasers encouraged significant growth of the TTLW market.
How it works
The basic welding process is shown in FIGURE 1. TTLW is possible because most natural, unfilled thermoplastics are transparent to near-infrared wavelengths. Fixturing holds the parts in intimate contact at the weld interface having an absorber. The top component transmits infrared energy, and heat generated at the interfacial absorber is conducted into the top and bottom components to form a melt layer. The parts are held in intimate contact until intermolecular diffusion occurs and the weld interface cools.
The infrared transmission properties must be well understood for both weld components. Fillers or reinforcements can influence the transmission of laser energy. There are different interfacial absorbers that will generate heat at the weld interface. Broadband infrared and near-infrared laser (800 to 1064 nm) sources can be used for this welding process.
TTLW reduces the possibility of part marking because laser-produced heat delivery is non-contact. Precise control of energy input and heat generation at the weld interface minimizes the heat affected zone, resulting in minimal residual stress, superior weld strength, and little or no flash. Heating time is minimized with simultaneous heating of the weld interface, making the weld cycle time comparable to ultrasonic welding, the fastest welding process. High quality welds can be achieved for a range of applications from very delicate applications requiring fine line welds to large applications with wide weld widths.
Over the last five years, equipment manufacturers have seen the laser welding landscape grow, fostering plastic-specific welding systems. Self contained, dedicated systems with small footprints are designed for low- to mid-volume production. FIGURE 2 shows a typical automated cell for welding automotive housings. They include motion control and beam delivery systems to weld small- to mid-sized applications. These units are now cost competitive with ultrasonic, vibration, and hot plate welding systems. The cost of Nd:YAG and diode lasers, under 100 W, has continued to drop over the past 10 years, which has encouraged the rapid growth of TTLW.
TTLW only requires simple pneumatic cylinders to provide the low weld force to hold the components horizontal and normal to the laser delivery system. The fixture must not block infrared energy to the weld interface. Glass, polycarbonate, or other infrared transparent pressure plates are typically used to apply uniform weld force.
Currently, lower cost solid-state diode lasers are the most popular laser sources for plastic welding, integrated into compact, high efficiency systems that require little maintenance. Recently, high precision applications with fine line weld requirements have taken advantage of the small spot sizes and efficiencies offered by fiber lasers.
A well designed laser welding station can be used to weld multiple applications. With a computer numerical controlled (CNC) table or galvanometric mirror beam delivery system, only a simple programming change is required to weld parts with different weld patterns. Other beam delivery approaches may require a mask change or optic swap. In less than a few minutes, the system can be changed to weld a different application. This versatility is one of the core advantages of TTLW as compared to ultrasonic, hot plate, or other welding processes.
TTLW eliminates the concern of damaging internal components that tended to fail during friction-based welding processes. In most applications, there is no weld collapse or flash and the weld components remain in their initial orientation. There are no competing welding processes that offer high quality weld joints without flash generation.
As mentioned above (FIGURE 1), the optical properties of the thermoplastic components must be understood to successfully optimize the TTLW process. The amount of energy reaching the weld interface is dependent on the absorption, refraction, and reflection that occur within the top component. The absorption properties of the bottom component will impact the depth of the melt layer and heat generated during infrared exposure. In general, dissimilar material combinations can only be welded if they are chemically compatible.
Most natural thermoplastics are transparent in the near-infrared wavelength range. It is recommended to use natural thermoplastic without additives as the top component because the addition of filler in the top weld component can increase scattering and influence volumetric optical properties. Most color combinations can be welded using this process if the colorants are infrared transparent.
Different interfacial absorbers are available; with the simplest, a small added percentage of dispersed carbon black is added to the bottom component. Thin, black plastic film can be used as the absorbing layer at the weld interface between two transmissive components. Liquid infrared absorbers can be dispensed onto a plastic part to enable welding two natural or transparent components. Additionally, infrared absorbers can be compounded into resin to create a substrate with uniform absorber throughout. These approaches require that at least one weld component transmits near-infrared energy, and that there be a technique for absorbing the energy at the interface. Some visibly-opaque colored plastics are transparent to infrared radiation. Additionally, black-to-black weld combinations are possible by using a deep red colorant in the top component that appears visually black, but transmits infrared energy. Keyless entry remotes (FIGURE 3) can be laser welded using these specialized pigment combinations. It is best to work with laser welding experts to experiment with pigment options.
There are several options for beam delivery and energy input to the weld components. Contour welding relies on tracing a single laser spot around the perimeter of the part in one pass. Robotics is often employed or the fixtured part can be moved under a stationary beam source using a motion table. Weld collapse is avoided by rapid cooling since the entire weld interface is not simultaneously melted. This is the most flexible beam delivery technique because it can be used on any sized part and is only limited by the reach of the robot arm or motion table.
In simultaneous welding, dedicated beam delivery optics are used to simultaneously expose the entire weld interface to infrared energy. With this approach, the weld interface is uniformly heated. This method allows for collapse and can form weld flash. Traditionally, this approach is used for dedicated applications and requires optics changes to weld other configurations.
In mask welding, a large spot size or curtain of light is used that allows the infrared to pass to desired locations (FIGURE 4). This approach works well when one laser system is used to weld multiple applications. A mask change is only required to weld different applications. This approach wastes excess infrared energy, but simplifies the motion control programming.
Quasi-simultaneous welding uses a galvanometric mirror system (FIGURE 5) to rapidly trace the weld interface of a part. Rapid tracing of the beam takes place without the need for dedicated optics. The entire weld interface is melted simultaneously, which allows for melt squeeze and collapse.
Because of the benefits mentioned above, TTLW is used in production for several applications. The largest users of the technology include medical, automotive, and consumer products. Most applications require hermetic seals with minimal particulate generation.
Medical applications include disposable tools, fluid carrier systems, and diagnostic tools. Certain products, such as those found in medical diagnostic test kits, require precision welds. Micro-fluidic devices are used for continuous, real-time testing. Plastic micro-fluidic devices require sealing a cover film over the substrate (FIGURE 6). The cover must seal absolutely to the very edge of the fluid channels, which have critical dimensions that must be maintained. EWI has worked on developing fine-line through-transmission laser welding to successfully provide strong seals between transparent thermoplastic cover films and substrates (FIGURE 7).
The most popular automotive applications that use TTLW include sensor housings, underhood components, instrument panels, and tail lights. Until recently, tail light assemblies were more commonly welded with hot plate welders. Automotive suppliers have taken advantage of TTLW to weld lens covers to the tail light housing. Flash and particulate generation inherent to hot plate welding is significantly reduced. TTLW also results in minimal residual stress which reduces the susceptibility to stress cracking.
The TTLW process experienced rapid growth over the last decade as it became more cost competitive with equipment suppliers offering complete, dedicated TTLW systems. The advance of laser weld specific additives has also enabled this growth. The rapid growth of the TTLW, experienced over the last five years, is predicted to continue with the backing of equipment and absorber makers. It is now possible to produce superior thin and wide weld joints with almost any possible color combination. The advantages over competing welding processes will encourage more manufacturers to consider TTLW technology.
Sean T. Flowers ([email protected]) is an applications engineer at EWI, Columbus, OH.