The benefits of welding plastics with high-power diode lasers are particularly suited to the needs of the automotive industry
The use of plastic materials in the automotive industry-particularly in cosmetic, structural, and functional components-continues to grow. As such, integration of materials and component subassemblies means the joining of plastic materials is becoming a critical technology.
Established techniques such as ultrasonic welding, hot plate welding, and adhesion have their place, but each has limitations in terms of the process, the materials that can be joined, or the high-cost, dedicated tooling required. However, a joining technology is available that addresses many of the restrictions of these techniques. The commercial availability of high-power diode lasers and the development of transmission welding techniques have radically improved plastic welding capability-to such an extent that laser joining could become a key plastic processing tool.
Lasers have been used for joining applications for more than 30 years, but their use for welding plastics has not been widespread. Employing them with polymers-where they offer a clean, noncontact process that allows the controlled application of large amounts of energy within a small spot-is beneficial.
High-power diode lasers
High-power diode lasers are based on semiconductor technology and are manufactured by combining the outputs from a number of diode arrays. The commercial availability of high-power diode lasers, with outputs in the near-infrared range, raises the possibility of using these devices for materials processing. Because they are based on solid-state technology, such lasers are reliable, with a typical minimum design life of some 10,000 hours. They also offer further significant benefits over established laser types in terms of compactness, beam size, electrical efficiency, and capital and running costs.
For example, the laser head of a 250W device measures only 260 x 125 x 0125mm and weighs 5 kg. Ancillary equipment is limited to a small control panel and a chiller unit. Systems that combine the control system and chiller also are available. The compact nature of the laser head allows direct mounting to a robot, giving multi-axis capability. The beam also can be delivered via a fiber-optic cable should the situation demand.
The laser’s output spot is naturally rectangular; with typical size at focus of about 5 x 0.5mm. Optical methods can be used to vary the footprint or to provide a circular spot. Depending on the intensity or speed demanded by the particular process, the beam area can be increased by working away from focus, which allows wide weld seams and high-strength joints. The electrical efficiency of diode lasers is greater than 30 percent, compared to CO2 and Nd:YAG, which feature ~10 percent and ~4 percent, respectively.
High-power diode laser systems are available today with greater than 6 kW output power, which provides sufficientintensity to carry out processes such as cladding, welding, and surface heat treatment of metals. However, it is in joining polymers that these lasers have found their initial applications in the automotive industry.
Although thermoplastics readily absorb radiation in the far infrared (IR) region, they transmit the near-infrared wavelengths produced by diode lasers. Much of the energy is not absorbed by the polymers. However, plastics can be made to absorb the laser energy by the selective use of additives such as pigments, fillers, and reinforcements
Transmission laser welding seeks to exploit the differential absorption and transmission characteristics of polymers and the additives used with them by utilizing a joint configuration that employs an overlap. With careful selection of materials and additives, the top half of the joint can transmit the laser beam while the bottom half can be engineered to absorb sufficient energy to cause melting at the interface and thus allow welding. An additive such as carbon is ideal for use in this respect as it readily absorbs diode laser energy.
Transmission laser welding offers several advantages over the more established plastic joining techniques. Unlike vibration/ultrasonic welding, transmission laser welding is a noncontact process and is less likely to damage sensitive component assemblies, such as those containing electronics. Furthermore, it requires little dedicated tooling, unlike most vibration/ultrasonic and hot plate welding systems, which contain significant amounts of tooling specific to the component.
Critical factors in the laser process include additives to the polymer: pigments, reinforcements, processing aids, joint design, and access.
Warwick Laser Systems (Coventry, England) has developed and commercialized transmission laser welding of plastics. Evaluation of the suitability of a range of thermoplastics has resulted in welding conditions established for a number of materials, including color components, polypropylene, polyethylene, acrylics, polycarbonates, glass-filled materials, and film material.
As a noncontact technique, laser welding eliminates the problems of tool contamination and product location associated with other conventional techniques. The laser beam from a direct diode system is also highly controllable and the output is both stable and predictable, allowing consistent welds. The high power capability and the wide beam size permit the welding of large weld areas, with virtually no limit on component size or complexity. However, the beam’s controllability and the very precise nature of beam delivery ensure that very delicate welding operations can be carried out if required.
Applications that exploit the different benefits offered by transmission laser welding already developed include electro-mechanical devices, food packaging, medical, and scientific. In food, medical, and scientific applications, the noncontact nature of the process provides protection against contamination, as does the fact that, unlike adhesion, no additional material is used. With electro-mechanical devices, delicate electronics are not disturbed, as they would be with vibration welding. Engineering products take confidence from the large joint area, which results in high-strength welds and complete sealing. Although these applications make good use of the technology, the automotive industry has been the most enthusiastic adopter of the new process.
A recent application involves laser transmission welding at Birkby’s Plastics Ltd. (Liversedge, West Yorkshire, England) in the manufacture of a new electronic throttle control (ETC) pedal assembly for motor vehicles. The pedal shown in Figure 1, which is manufactured as a glass-filled nylon molding, is being fitted initially to only certain Ford models, but it has wide-ranging application in the automotive industry.
The new ETC pedal features an integrated rather than a bolt-on sensor, making it compact, economical, and tamper-proof compared with competitive systems. When a driver presses on the pedal, a demand signal is transmitted to the engine management system, which compares it with the ignition map, allowing the engine to be precisely fueled to optimize efficiency in terms of economy, performance, and emissions.
Integration of the sensor within the pedal housing relies on welding the electronics “pot” precisely in position. Because the pot needs to be exactly located and zeroed, it was impractical to achieve the weld between the two plastic components by vibration welding. Furthermore, because two different grades of glass-filled nylon, with two different melting points, are used for the pot and the pedal, it also was impossible to weld the components together using heating techniques and ultrasonic welding.
After successful trials at Warwick Laser Systems’ development center, a direct diode laser transmission welding system was incorporated within Birkby’s purpose-built, automated assembly, weld, and test line to weld the two glass-filled nylon components of the electronic control throttle pedal assembly (see Figure 2). The system produces a 3mm-wide weld and hermetically seals the pot within the pedal.
In this case, the laser was mounted on a Kuka (Augsburg, Germany) robot and requires no dedicated tooling. The major benefits of robotic manipulation for this component are realized primarily in the flexibility of the system. Access to both sides of the component assembly is required-a robot in this case is the most cost-effective solution.
Several component iterations require fixture and programming changes. The robot programs are obviously stored, thus minimizing this down time.
Periodically, the laser power is monitored using a power meter. This can be mounted in any convenient position, as the robot working area is a large envelope; a simple program allows this to occur after a specified number of components are produced.
Plans are underway to accommodate a material test station within the cell which can be positioned anywhere between 90° and 180° away from the production area. The advantages are that the laser, robot, and safety costs are already accounted for. Fixturing and robot programming are the only other considerations.
Another application of this technology is the joining of a sealed-for-life fuel filter housing for the Pall Automotive Division (Portsmouth, England). The housing is made from acetyl and has a wall thickness of 2 mm. The housing is effectively a cylindrical doughnut requiring a butt weld on the inner and outer edges (see Figure 3). The filter media is compressed in the housing prior to welding, which rules out vibration type joining techniques due to the damping effects of the filter media. A minimum operating burst pressure of 12 bar was required with a safe operating pressure of 6 bar.
Although the usual automotive test specifications were performed successfully, the burst pressure of the filters was seen as the definitive test for joint strength, as this was the ultimate failure mode of the welds. A manual hydraulic cylinder was used to pressurize the filters, and a transducer measured the pressure. The filter was primed with oil and sealed. The hydraulic cylinder was pumped until the filter operating pressure of 12 bar was reached. This pressure was maintained for several seconds; any pressure drop-off would signify a leak. The filter was then pumped until catastrophic failure of the filter occurred. Burst pressures of >30 bar were the norm. Figure 4 shows the failure mode at burst. Note that the material has failed at ~60° to the weld line, not along it.
The production machine at Pall consists of two fixed stations, each with a 250W direct diode laser for the inner and outer weld, a pick-and-place robot at each station assembling and transferring the filter, and an in-line pressure test station. The equipment is shown in Figure 5.
High-power diode laser transmission welding is fast becoming a viable alternative to conventional technologies as a means of joining and welding plastics. Its benefits are particularly suited to the needs of the automotive industry and it is often the only technology suitable for some joining applications.
Brian Bryden, [email protected], is a senior research fellow at the Warwick Manufacturing Group, University of Warwick, in association with Warwick Laser Systems, Coventry, England.