Laser Welding Gains Traction in Plastics Joining

Several factors point to a major acceleration of laser welding as a method of joining plastics, particularly for electronics enclosures:

* Major resin companies are expanding materials choices for laser welding of black and other colors;
* Comfort level with the technology among design engineers is growing, slowly but surely;
* Litigation over patent issues is giving way to royalty agreements, particularly in Europe, where the use of laser welding is more advanced; and
* New equipment technology is expanding joining options, particularly for complex geometries.

High equipment and materials costs, however, remain a concern, and will ensure growth is focused, at least for the near term, on areas that benefit the most. “Laser welding is growing because it a clean, non-contact process. Everything is aligned through clamping before the weld,” says Mark St. John, senior engineer, plastics welding at the Edison Welding Institute in Columbus, OH. Unlike competing processes, such as vibration welding, there is no movement of the parts. Unlike hot plate or solvent welding, laser welding does not generate fumes. And unlike adhesive bonding, laser welding requires procurement of no additional material.

Most thermoplastics possess a high optical transmittance of radiation in the visual and near infrared wavelengths. Laser radiation can be transmitted through a top layer into a welding zone, where it reaches a joining plastic that absorbs the radiation. Heat is generated back into the transmissive top layer causing it to melt.

Laser welding is most frequently used in automotive applications, particularly for under-the-hood electronic enclosures, but is also used in medical assembly for tube-to-tube assemblies, filters and ostomy bags.

One of the major applications of laser welding is attachment of a high-speed electrically activated Acoustic Control Induction System (ACIS) to a plastic surge tank on the 2006 Lexus RX350 luxury sports utility vehicle. Both parts are nylon 6. The new system “varies air induction by automatically adjusting the intake pipe length according to engine rpm, optimizing the fuel/air mixture to further boost torque across the full rev range,” says Yoshihiko Matsuda, chief engineer, Lexus Rx 350. The ACIS uses the intake air control valve to divide the intake manifold into two stages. The result is more power output.

Vibration welding, widely used for engine intake modules, leaves a rough burr of polymer at the mating surfaces. Lasers produce clean smooth joints and avoid the possibility of turbulent fluid flow in parts that carry liquids or air.

Nearly all thermoplastics and thermoplastic elastomers can be welded with laser radiation even when they are glass reinforced (up to 33 percent). The joint strength is comparable with that of the base material. Laser welding does not change the chemical or physical properties of the plastic, unlike adhesive bonds, whose aging or embrittlement can create design uncertainty. “You also don't have environmental issues, like you do with adhesives,” says Dan Jones, North American assembly technology leader for DuPont.

The prerequisite for a safe bonding of thermoplastics is a minimum chemical compatibility of the polymers. Most engineering plastics can be laser welded in their natural form: nylon 6 and 6/6, PBT-type polyester, polycarbonate, ABS, polyacetal, as well as non-engineering grades such as polypropylene and polyethylene. Different plastics may also be welded to each other, such as PBT to polycarbonate and ABS to acrylic. Check miscibility charts at your resin supplier for laser weldability.

One of the most common ways to make a polymer laser absorbing is the addition of carbon black, typically with a loading of 0.05 - 0.5 percent That's fine if cosmetics don't matter. Assembly of a natural PBT to a black PBT is an ideal candidate for laser welding. Many automotive customers, however, want under-the-hood enclosures that are all black.

BASF developed an additive called Lumogen that produces parts that appear black but allow laser transmission. They are described as thermally stable, highly transparent absorbers of near infrared energy. They contain a small amount of color in the visible range and provide absorption of emission wavelengths of 808 nm. They can be used in combination with other pigment systems, allowing laser welding of light-colored or even transparent parts. One of the major producers of Lumogen additive systems is a European color compounder called Treffert, which showed new applications in medical and automotive at last year's National Plastics Exposition.

A European patent holder (Marquardt) is requiring licensing fees from users of laser welding involving black polymers. There are several reports that major processors in Europe have reached settlements, and are now paying royalties. Other processors have found ways to work around the patent, such as using very dark gray pigments.

Toyota has been laser welding plastic components for more than 20 years, but the process really entered the industrial stage about seven years ago.

As a result, there is still a lack of familiarity with the process, how it fits, where it should be used and what are the basic design issues.

“Laser welding Is not here to rule out other types of welding processes,” says Jim Greene, vice president of sales and marketing for LPKF Laser & Electronics in Wilsonville, OR. “It doesn't make sense to invest in laser welding if you can use a less expensive process like vibration or ultrasonic welding. What laser welding does is come in where the other types of welding processes leave off. For that reason, we see electronics as one of the main application areas.”

LPKF sells laser welding equipment developed by a sister company in Germany called Laserquipment, which is a spin-off from the Bavarian Welding Institute.

Familiarity with the process is one factor holding back laser welding. Another is lack of attention to joining technologies. “Some design engineers tend not to look into the joining issue very much before they design the part,” says St. John of the Edison Welding Institute. The fact is, however, that attention to the design is important to avoid high tooling costs and poor joints.

“The design for laser welding is very important, unlike vibration welding,” says Chul Lee, applications technology leader for BASF. “We have found that the thickness of the transmissive layer and the absorbing layer must be determined first.” For starters, different plastics have different levels of transmissivity. A general range for the transmissive layer is 0.8 to 3 mm. Check your materials supplier for specific information. Glass loadings change the transmissiveness of the polymer, and must be taken into consideration.

It's also very important the mating part nests very tightly with the transmissive layer. Warpage, of course, is a problem for many injection molded parts. BASF's Lee recommends the engineer first ensure his processor uses a robust process that is as repeatable as possible. Still, you must always plan for warpage on certain parts. “You measure the warpage in the part, and then you go back to the tool and you compensate,” says Scott Schlicker, advanced development group mange at BASF. “Before you finalize the tool, you go to the mold maker, and you say this is an area I am going to sequester for the laser.”

Gate location is also important. Glass fiber can agglomerate in the area near the injection point. So, it's important to locate the gate away from the weld plane. It's also important to make sure the melt cools according to plan. Otherwise the part will have a higher degree of crystallinity. Like glass fibers, crystalline domains can scatter the laser radiation.

Finally, equipment selection is important. Consider Nd:YAG lasers for welding seam widths below 1 mm and for plane welding geometries with scanning head applications. Diode lasers are recommended for wider welding seams, circular seams and simple spot welds. In most cases the required power ranges from 30 to 150W.

One of the processes most commonly used are contour welding, in which the laser beam follows the welding seam, much like the welding of metal. The gap width that can be tolerated (approximately 100 microns) is often a determining factor for contour welding. In quasi-simultaneous welding, the laser beam rapidly passes over the entire welding area several times. The entire welding area melts simultaneously. This approach requires more power. Other systems include mask, radial and Globo, a concept developed by Leister that facilitates two and three-dimensional laser welding.

The number of equipment options continues to grow. Many suppliers are now offering lower-cost machines. But they also come with lower capabilities. Among new models introduced at last year's plastics show are 1) the Novolas Basic AT from Leister Technologies, described as an entry-level system that is aimed at integration into automated systems, 2) Branson Ultrasonics now has two infrared models. The IRAM 200 is a modular system and the IRAM 300 operates at 600W at 980 nm 3) LPKF Laser & Electronics introduced the LW-Power RT, a stand-alone unit with two-station rotary table and 4) Forward Technology displayed the VHIR-1445, a vertical hydraulic motion-controlled infrared welder that can handle parts up to 14 by 45 inches or multiple smaller parts.

Other important technology players include the Dukane Intelligent Assembly Solutions Div., Bielomatik and Gentex Corp. which offrs coatings that are designed to absorb laser energy in the 940 to 1,064 nm range.