Welding plastics with near-IR lasers
Advances in laser performance and economics have brought this application to "mainstream" levels
Improvements in the performance and cost-effectiveness of lasers have led to their wider use for welding thermoplastics. Increasing use of bonded plastics in commercial and consumer products promises many years of sustained growth for this application.
Welding involves joining two parts by locally melting and fusing material at their common interface. As with metals, thermoplastics can be thermally softened or melted, however, there are a number of important differences between welding metals and thermoplastics. Metals absorb most common laser wavelengths but are excellent conductors of heat, requiring very high local temperatures before they will melt and weld. Conversely, many plastics are transparent at these same laser wavelengths and are notoriously poor conductors of heat. Moreover, thermoplastics soften or melt at relatively low temperatures. In addition, plastics are best welded through reaching a highly softened state rather than a true liquid melt.
These differences have an impact on the way laser welding is performed in plastics. The majority of plastics welding applications involve transmitting laser light through one of the parts to be joined. The second part then absorbs the light, producing intense local heating and softening only at the interface between the parts. Both butt welds and lap welds can be created in this way, using geometries outlined in Figure 1, although lap joints are usually easier for several practical reasons.
Figure 1. Butt welds and lap welds are made by transmitting laser light through one of the parts.
In order to develop a "through transmission" welding application, it is necessary to understand the transmission/absorption properties of the plastic(s) to be joined. Because cost is often a driving issue, most plastics welding is performed in the near infrared where there are economical lasers with the required power level—a few watts to tens of watts. The near-infrared (IR) absorption properties of the materials could be measured with a spectrophotometer, but it is usually better to just use a laser and power meter. Because absorption is not always linear as a function of power, it's important to measure the absorption at the intended power level. Also, most job shops and plants simply won't have a spectrophotometer on site.
Ideally, one material will be completely transparent at the laser wavelength and the other will exhibit very high absorption. In practice, most clear plastics exhibit less than 10 percent absorption through one inch of material. As a rule of thumb, absorption levels up to 20 percent per inch thickness can usually be tolerated. This amount of absorption can produce modest bulk heating but will not significantly affect the overall process.
Once the light has been transmitted to the welding interface, how do we achieve high local absorption, given that the two parts must be of similar composition in order to get a strong weld? By far the most common method is to dope the second part with carbon black filler, which yields very high absorption, without significantly affecting overall strength. More recently, several research groups have been developing special high-absorption films to allow welding of two nominally transparent parts. One example of this is Clearweld, which was jointly developed by Gentex Corp. (Carbondale, PA) and the TWI Advanced Materials and Laser Processes Group. In practice, a thin layer of one of these new materials is applied at the interface of the two parts to be welded. The film's high absorption results in rapid heating and produces an optically clear join with no particles or visible discoloration.
During welding, pressure must be applied to the joint to hold parts in position and restrict the heated polymer from expansion. Close contact also allows heat to flow from the absorbing material into the non-absorbing material, so that both are adequately softened. Typically, a clamping force between 40 and 60 psi is sufficient, which is fairly modest compared to other welding processes. As shown in Figure 2, this clamping pressure can be applied by use of a transparent block of material such as glass or polycarbonate, or by use of opaque tooling with a through hole for the laser beam. The weld is then created by physically moving the parts and/or scanning the laser beam.
Figure 2. Weld pressure can be applied via a clear block of material or by using mechanical tooling with a through hole for the laser beam.
Other practical considerations are part size and weld speed. With a lap joint, the laser typically passes through thin material and, in theory, there is no definite limit on part size. A modest focus is usually used (800-µm spot diameter) and extended weld areas are produced by simply translating the beam or the part. With butt welds however, the laser has to pass through the full width of the transparent part. Also, with a butt weld, the laser must weld the entire edge thickness of the part simultaneously. Moreover, the laser beam can easily pass through a few inches of polycarbonate, but with other plastics like PTFE, polyethylene and polypropylene, transmission is usually limited to part thicknesses of a fraction of an inch. Depending on laser power, weld width and material thickness, weld speeds can range as high as tens of meters per minute.
Although robotics and translation stages would allow larger parts to be welded, in practice laser welding is best suited to parts with maximum dimensions of a few inches. Furthermore, larger parts require higher power lasers to achieve acceptable weld times, but the cost of these lasers negates some of the advantages of laser welding.
Often the laser beam is delivered to the welding fixture via fiberoptics. In the case of volume production of identical parts, this offers an alternative to scanning the beam. Specifically the laser light is delivered through a bundle, which is shaped to directly create the outline of the intended weld. Circles, rectangles and even elaborate curved outlines can be produced in this way.
Figure 3. These colorless polycarbonate type parts were welded by a 30W diode laser system using Clearweld film.
Figure 3 shows two transparent polycarbonate type plastic plates (coated with Clearweld film) that were welded using a 30W, 810nm diode laser system (Spectra-Physics Integra). Figure 4 shows an acrylic business card holder that was sealed using the output of a 50W multimode Nd:YAG laser (Spectra-Physics Tornado). Here the top piece is clear and the other piece is black-filled.
Advantages of laser welding
Laser welding of plastics offers a number of advantages over other welding methods or even bonding agents. First, there is no physical contact between the heat source and the part, resulting in clean product with no debris or overheated material. Laser welding only softens the plastic and the welding region is completely enclosed by the two parts being joined. Consequently, the process produces no fumes, which can be a significant safety advantage. In contrast, many bonding agents produce toxic fumes during curing. Laser welding also offers the advantages of speed, flexibility and controllability—it is very straightforward to regulate the amount of energy that reaches the weld area.
Static small welds are usually accomplished in less than one second and welding speeds for films are as high as tens of meters per second; thus, laser welding is as fast or faster than any alternative process. Because this is an optical technique, it offers maximum design/redesign flexibility. In addition, the laser energy can be precisely directed to the weld site, with almost no peripheral thermal damage, making this technique an excellent choice for delicate parts.
Laser welding also offers flexibility in the types of weld that can be created. By using tightly focused, short focal length optics, the process can create welds as fine as 100 µm wide. Alternatively, with a higher power laser, the beam can be defocused to produce welds as wide as 10 mm.
Kevin Hartke is manager of sales and marketing at the Mound Laser & Photonics Center Inc. (Miamisburg, OH), a group that specializes in developing laser applications in the areas of laser marking, laser micromachining and laser welding. One of Hartke's particular areas of expertise is plastics welding. He notes several additional advantages of laser welding. "When customers consider whether or not to implement laser welding, they're often comparing the process to other welding methods such as vibration, ultrasonic or radiant heat (lamps). In our experience, the weld strength from a laser process is equal to or better than any of these alternatives. It's also cost competitive with ultrasonic and vibration welding machines, which range from $50,000 to $100,000 or more. Furthermore, the excellent reliability and high electrical efficiency of the latest solid-state lasers mean that the overall cost of ownership and process costs are both competitively attractive." Hartke also notes that laser welding of plastics is an easy process to integrate into a high-yield production application, with complete control of the laser, scanning optics and infrared cameras for process monitoring.
Choice of laser technologies/wavelengths
Various thermoplastics can be laser welded, including polyethylene, acrylics and polycarbonates. All of these materials exhibit low absorption throughout the near infrared spectral region. This means that they can be welded equally well with direct diodes at wavelengths of 810–950 nm, or with Nd:YAG and Nd:YVO4 lasers at 1064 nm. In both cases it is best to operate the laser in a CW mode rather than pulsed. This is because weld speed ultimately depends on average power and total energy, which are both higher with CW operation.
Figure 4. This acrylic business card holder was weld sealed using the output of a 50W multimode Nd:YAG laser.
As a rule of thumb, diode-pumped solid-state lasers at 1064 nm offer much higher beam quality than direct diode lasers. As a result, they are the best choice for "precision" applications where the beam is directly scanned with galvanometers. Here the beam can be scanned at up to 6 m/sec. This is faster than the thermal retention time of the plastic and thus allows shaped welds to be produced simultaneously—the plastic experiences a time average of the laser beam motion.
Direct diode systems offer a lower cost per watt than their diode-pumped counterparts. In addition, these lasers are generally available in a very small package, which is an advantage for integration. However, direct diode lasers deliver relatively poor beam quality from a multimode fiber output. This can then be re-imaged into the weld area using recollimating and focusing optics. With a 25mm focal length lens, this produces a typical spot size of about 600 µm in diameter. Alternatively, the fiber can be coupled into a shaped bundle as previously discussed.
In conclusion, there are numerous advantages to using lasers to weld plastics. As the cost per watt of solid-state infrared lasers continues to improve, we expect this to become an important application as both demand and user awareness increase.
Mingwei Li and Mark Keirstead are with Spectra-Physics Inc., Mountain View, CA. Email email@example.com, www.splasers.com.