Today, laser welding is fact, not fiction. Recent developments in high-power laser diode technology make it a cost-effective method for joining polymers and thermoplastics. Changing market conditions and more affordable welding equipment are prompting manufacturers to consider the advantages of joining plastic parts with light beams. Although laser welding has been in experimental use for the last 10 years, many manufacturing engineers are still not familiar with this technology. It is relatively new compared to traditional joining techniques, such as ultrasonic, linear and orbital vibration, spin, stir, electromagnetic, hot-plate and extrusion welding, but it offers numerous advantages. For instance, in laser welding, the energy input is highly localized, which creates less thermal and residual stresses than traditional welding techniques, resulting in a high quality weld. Lasers deliver high values of irradiance to selected areas on a plastic part; no other welding process can deliver as much energy in as small a location. This can produce rapid heating in a very small region. The localized nature of the heating and formation of the melt-pool makes lasers ideal for many plastic welding applications. Laser welding is an economical alternative for joining many different types of thermoplastics. Today’s high-power diode lasers are also compact and reliable. They offer high efficiency—30 percent to 60 percent—with power up to 3 kilowatts. These systems also have low maintenance expenses. Laser welding technology is ideal for rigid injection-molded thermoplastics, such as polyamides. It compares favorably to the most common plastic joining methods, such as friction, hot-tool and electromagnetic welding. As a result, laser welding is becoming an increasingly popular technique for assembling plastic auto parts, such as air intake manifolds and tailight assemblies. Plastic parts can be joined with either noncontact laser welding or through-transmission laser welding. Both methods have pros and cons related to heat generation, plastic composition, joint stiffness, part design, joint thickness and color.
Noncontact vs. Through Transmission
Noncontact laser welding is typically used for joining rigid thermoplastics, and is typically used for butt joints. The process is similar to contact or noncontact hot-plate welding. With hot-plate joining, a heated tool is placed on or near the parts to be joined. When the plastic at each weld site melts, the parts are brought together under pressure. The materials flow together and interlock. Noncontact laser welding works much the same way. The laser is directed through a mirror at the weld sites. The plastic absorbs the energy and melts. The parts are then brought together under pressure, and the weld forms. Materials for noncontact laser welding should absorb energy and melt at local areas for interface formation. The noncontact method may be applied for joining similar and dissimilar polymers and thermoplastics. However, only limited interdiffusion should be expected with dissimilar polymers. Because of their difference in thermal properties, one plastic may melt faster than the other. Sometimes, one of the materials may not even reach its melting point at the interface. Due to the temperature gradient across the width of the melt-pool, the flow of the joined plastics is not uniform. Through-transmission welding may be applied for joining rigid and flexible plastic parts and films. In “traditional” through-transmission welding, the thermoplastic parts to be joined are brought into direct contact prior to welding. The process requires two thermoplastic materials that transmit the laser energy at different degrees. The top part must be optically transparent at the laser wavelength. It is uncolored or colored with non-light-absorbing pigment. The bottom part is optically dense. It is typically filled with carbon black, or colored with light-absorbing pigments. The laser beam is totally absorbed within the surface layer of the bottom part. Direct contact between the parts ensures heating of the top part at the joint interface. Welding occurs upon melting and fusion of both materials at the interface. For clear-welding applications, both joined plastics are optically transparent. A nearly colorless eye, which absorbs infrared laser light and converts it into heat, is applied to one of the parts. The dye can also be applied as a thin film between the parts. The laser beam is transmitted through the top part with minimal losses and absorbed by the dye. The heat radiating from the dye is enough to melt both the upper and lower parts, developing the necessary melt-pool. In both traditional through-transmission and clear-welding, it is critically important to achieve sufficient and consistent heating of the plastic in the joint during the premelt and fusion phases. This condition will produce a consistent thickness for the weld interface and the desired mechanical performance of the weld. The requirements for weld formation between the two parts should be taken into account when selecting plastics and dyes for laser welding. Optical data on laser energy transmission and absorption is helpful to the selection process. There are four techniques for through-transmission laser welding. The laser can be scanned along the weld contour. The laser can generate heat simultaneously, along the contour of the weld. Or, the laser beam can be guided with deflection mirrors along the weld contour at the frequency of up to 50 hertz. Finally, if there are areas that should not be heated, a mask can be placed over the parts to block the laser. The welding technique, processing parameters and optical properties of the plastics will influence the thickness and geometry of the melt-pool. In general, the mechanical performance of a welded joint increases with the thickness of the melt-pool. The melt penetration depth may reach over 300 microns for large parts, and less than that for medium and small parts. Plastics that are made for laser welding may be used to optimize processing parameters and increase the mechanical performance of the weld.
Pros and Cons
Through-transmission laser welding has numerous advantages, including:
- The ability to weld preassembled plastic components in the same orientation and position as the final assembly.
- Absence of vibration during welding. This permits welding of sensitive electronic and medical components.
- The process produces high-quality seams without flash.
- Minimal limitations on the geometry and size of the plastic parts.
- Minimal limitation of the weld length and geometry. Straight, curved and 3D welds can be made.
- Accurate, noncontact, localized heat transfer and the ability to control welding temperature at the joint interface.
- Ability to control the shape and size of the laser beam.
- No visible damage or marking on external colored surfaces.
- Easy to automate.
- Short cycle time. Rapid welding speeds permit joining of long and wide parts with acceptable cycle times.
- Low-cost tooling and fixturing.
- High mechanical performance of welded joints. Through-transmission laser welding also has several limitations, although these may be reduced by choosing the correct material and welding method:
- The process is material dependent. It requires plastics with different light absorption characteristics.
- Varied response to some additives (fillers, impact modifiers and pigments).
- Intimate contact (preassembly) is required at the joint area.
- Possible development of residual stresses at the joint interface for highly rigid plastics, when the laser beam is scanning along of the weld contour.
- Increased cycle time for clear-weld technology, due to the additional time needed to place light-absorbing dye.
- Fast overall processing cycles and excellent release from the molding tool.
- Predictable mold and annealing shrinkage. There’s a small tendency for warpage following welding.
- High flow and toughness in thin sections, which make complicated shapes easy to mold.
- Sufficient knit and weld line strength.
- Good mechanical performance of molded and welded parts after several remolding or regrind cycles. Mechanical property losses are minimal.
Laser welding can be used to assemble numerous products, such as cell phone housings, medical devices and connectors. It is ideal for joining automotive parts, such as taillight assemblies, fuel line components, and electrical and electronic modules. When developing a new part for an automotive application, manufacturing engineers can choose from a variety of fiber reinforcements, fillers, impact modifiers and resins. Polyamides are commonly used in various under-the-hood components. Polyamides are high-performance semicrystalline thermoplastics with a number of attractive mechanical and technological properties for laser-welded automotive parts. Uncolored PA 6-based Capron is particularly suitable for laser welding, because of its high laser transmission properties. No matter what type of plastic is used, it’s essential that engineers design the parts for the laser welding process. This can be done without sacrificing requirements for material properties, processing, joining technology and end-use conditions. Some plastic part designers are alarmed by the high initial costs of a laser welding system, as well as the cost of specially colored laser-transparent thermoplastics. Thermoplastics for laser welding typically cost about 20 percent to 30 percent more than standard thermoplastics. However, through-transmission laser welding offers numerous advantages that can eventually lead to significant cost savings. Demand for colored plastics for welding applications is increasing dramatically. Colored polyamide is a high-performance semicrystalline thermoplastic with a number of attractive physical and mechanical properties. Coloring polyamides is a separate area of plastic development and manufacturing because the pigmentation process may vary from simple to complex. Both short-term and long-term properties of welded plastic, such as resistance to impact, fatigue, creep and heat aging may be affected by colorants. Colored fiberglass-reinforced or reinforced glass and mineral-filled polyamide has the following advantages for welded components:
Researchers at BASF Corp. (formerly Honeywell International Plastics) used three types of commercially available polyamide—PA 6, PA 66 and amorphous polyamide—to investigate the efficiency of through-transmission laser welding on the mechanical performance of joints. For the sake of comparison, we also used a T-shaped butt joint similar in size and shape to automotive parts such as fluid reservoirs, air intake manifolds and resonators. All tensile tests of welded specimens were conducted at 23 C room temperature. We examined the tensile strength of short fiberglass-reinforced PA 6-based plastics that were joined with linear vibration welding. Colored plastics show sufficient mechanical performance after linear vibration welding. Similar results were expected for through-transmission laser welding. However, we discovered that the mechanical performance of laser-welded PA 6 plastics is equal or close to the tensile strength of joints created by linear vibration, orbital vibration or hot-plate welding. Editor's note: This article was adapted from a presentation the author delivered at the 2002 SAE International Body Engineering Conference in Paris.