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Straight Talk About Laser Welding of Thermoplastics
by Tom Kirkland, Dukane Intelligent Assembly Solutions, St. Charles, Illinois, USA

Assembly Update

Laser welding of thermoplastics has been a hot topic for the past ten years, and many applications are now in production around the world. While there is a lot of interest in laser welding, many manufacturers are not sure whether they have laser welding applications or how to sort through the laser welding vocabulary and the various approaches to implementing lasers in plastics assembly.

How Does Plastics Assembly With Laser Light Work?
The lasers used in plastics assembly are typically diode lasers at 808 to 940 nanometers wavelength, in the near-infrared spectrum. CO2 lasers at 10,600 nanometers wavelength can be used for cutting and marking thermoplastics, but have limited applicability for welding. There are also some laser welding applications using excimer lasers in the ultra-violet (short wavelength) range, but are beyond the scope of this discussion. The range of visible light is from about 400 to 700 nanometers, so all plastics welding laser beams are invisible; laser beams are represented on manufacturers’ brochures for dramatic purposes only.

Most natural un-colored thermoplastic materials are non-absorbent of any infrared light, laser or otherwise. Amorphous materials generally transmit laser light very easily, while laser light is scattered to a greater or lesser degree by all semi-crystalline and liquid crystal polymers. Fillers and reinforcements, as well as some kinds of molding defects and surface finishes can reduce the ability of laser light to pass through a thermoplastic part.

The infrared laser light is used in through-transmission mode, meaning that the laser light is passed through one part of the assembly (the transmissive part) and is converted to heat at the surface of a second part (the absorbent part) that is colored so that it converts laser light to heat. When the laser light strikes the absorbent part, heat is transferred by direct conduction to the transmissive part, both parts melt locally, intermix, and when the light source is removed, re-solidify under clamp force. 

The colors of the two parts in the assembly are critical to making the process a success. While the easiest laser welding situation is a natural part over a carbon-black loaded one, there are many possibilities for part colors that can be made to be either transmissive or absorbent as required. Some of the colors appear to be very nearly the same to the human eye (i.e. both parts look blue) but react quite differently to the laser light. It is even possible to weld clear-to-clear parts using special dyes.

There are two general strategies for through-transmission infrared laser welding of plastics assemblies, collapse and non-collapse. The simplest way to think of these two approaches is that collapse welding will always fully illuminate an entire weld joint so that welding occurs at all points simultaneously and the parts “collapse,” that is to say one dimension of the assembly is reduced a small amount in the process. Contained welding illuminates a smaller area of the part, and not necessarily all at once, so there is no dimensional change in the parts assembled.

What Does It Cost?
System prices are dependent on concept, function, and speed. Some low-power laser diode component sets can be purchased for integration into systems for under US$50,000, while an integrated work cell with high total laser power would have a price tag similar to that of a comparable-sized machining center, perhaps US$250,000 or more. Dedicated tooling can cost as little as US$1,000 to as much as US$100,000 depending on the system concept, part size, and part complexity. All plastic welding laser systems will consist of a laser light source, a laser power supply, a cooling system, optics to deliver the beam, fixtures for holding the parts, a means of positioning the laser light and/or the parts, and a containment enclosure of some kind. The enclosure not only protects human eyes and skin from being damaged by laser light, but protects the laser optics from contaminants. Most industrial “dirt” is laser-absorbent and lenses and such can be scarred as dirt is burned off.

Why Use Lasers To Weld Plastic Parts?
Non-collapse contained laser welding completely eliminates the possibility of flash or particulate in an assembly, and there is no dimensional change in the parts during the process. It handles warped parts very well, but can be sensitive to surface imperfections in the joint area. Collapse laser welding presents very little possibility of particulate, and while it produces flash and dimensional change in the parts, it has greater ability to handle surface imperfections in the joint area. It does not handle warped parts quite as well as the contained process. Either process handles material compatibility issues better and produces joint strength much closer to ultimate resin strength than any other plastics welding process. Optically clear and/or aesthetically pleasing joints are easy to attain. Systems can be designed with a tremendous amount of flexibility and capability. Also, laser welding does not expose assemblies to vibration or heat like many other processes do.

What Types Of Systems Are Available?
Laser systems usually fall into one of four categories, depending on optical configuration and construction.

Contour Systems: Contour systems make laser welds in the same manner as a pen makes lines on a sheet of paper. Parts can be moved relative to a laser with fixed optics, or an optical fiber may deliver laser light to the end of a robot arm or other positioning system, or the entire laser module may be mounted on a robot arm or positioning mechanism. System concepts run the full breadth of human imagination, from rectangular coordinate systems in two and three dimensions, to six- and seven- axis robotic cells, to simple two-axis “rotisserie” systems for seaming container-like parts. Since these systems are usually quite simple in concept, they are also usually quite flexible, and changeover from job to job can be as simple as calling up a program and installing a simple fixture. Dedicated tooling costs are low, so these systems lend themselves well to low volume or job-shop production. The systems can be used only in non-collapse contained mode, but can be built as large as required. Cycle times are longer than for other approaches, but this can be overcome to a degree by either increasing laser power or mounting multiple lasers.

Scan-mask Systems: These systems typically use a bank of diode lasers to form a line of laser light. This line of light is then swept across a mask that has been prepared so only those areas of the assembly to be welded are exposed to laser light. These systems are quite flexible, and have relatively short cycle times. The masks are typically made of a special glass that has been nickel coated, and must be kept clean or they may become scarred as the laser burns dirt off the mask. This type of system typically works best with flat parts, and is usually limited to small-to moderate-sized parts. They can be used in non-collapse contained mode only. Despite these limitations, mask welding is capable of producing the finest weld lines in parts, and is therefore commonly considered for very delicate welding of medical diagnostic components, for example. Dedicated tooling costs are low to moderate, and changeover from job to job is relatively quick and simple.

Simultaneous Systems: A simultaneous system uses a bank of laser diode modules to illuminate the entire joint of a part simultaneously. The laser light can be delivered directly from the diodes by fixed optics, but is more commonly delivered by bundles of optical fibers. The individual fibers are often “scrambled” to ease the task of balancing light intensity from the various diodes. These systems deliver the fastest cycle times of any laser welding system. If the system uses fiber bundles, the dedicated tooling costs can be quite high, and changeover times from job to job can be long. This approach can be used in either collapse or non-collapse contained mode, and system size is more often limited economically than technically.

Quasi-simultaneous Systems: A self-contained servo-controlled mirror arrangement called a “galvo head” can be affixed to a laser to steer a beam in a particular pattern. This technology was developed for laser marking applications, but has been adapted for use in laser welding. Because the mirrors and the servos that move them are quite small, inertia is quite low and moves can be made extremely quickly. A galvo head can be used in “contour mode” for beam positioning during non-collapse contained welding, but because the mirrors can be positioned so quickly it can also be used in quasi-simultaneous mode. In this mode, the beam is swept over the entire welding pattern very rapidly multiple times, which results in increasing the temperature of each point in the weld joint in stairstep fashion, until all points on the weld joint reach the melt temperature at essentially the same instant, and melt collapse begins to occur. In actual practice, this technique is limited to parts under about 100 mm in diameter with planar joints and favorable transmission/absorption contrast. Relatively high laser power is required because of heat losses; each point on the joint is being heated for a comparatively small amount of the total cycle time, so heating rates can be slow. For this reason, most galvo heads are actually used as beam positioners in contour mode systems, with individual galvo heads able to cover a circular area over 250 mm in diameter. Still, for small parts that require a collapse mode weld, the quasi-simultaneous technique is a viable alternative to the hardware intensity of a true simultaneous system.

Do I Have To Change My Part Design?
If you designed the part with hot plate, ultrasonic, vibration, or spin welding in mind, depending on the path light would have to take through your part, you may not have to do much at all, or you might have to completely redesign the joint area. In some cases, you may need to rethink the way the assembly is sectioned into individual parts. In many cases, redesign is not required, or the changes are modest. 

Some of the greatest benefits are available when a part is designed from the ground up for laser welding. Laser welding brings new freedom to the part designer to put the joint where it is most useful or aesthetically pleasing, not necessarily where it will best receive transmitted sound or vibration.

The biggest factors in part design are transmission, absorption, reflection, and refraction. The two new concepts in this section are reflection and refraction, and have to do with the surface condition of the material and the angle at which the laser light must enter the part. For best results, light should enter the part perpendicular to the surface and as close to and directly in line with the weld joint as is possible.

Anything Else I Should Know?
Some people are surprised to find out that all laser systems have life-limited parts. Some of these parts last as little as 500 hours and others last as long as 10,000 hours. Other components (especially optics) are easily damaged, especially by carelessness, and are considered by the manufacturer to be consumable for warranty purposes. It is better to find out about the economics and availability of these life-limited and consumable components before system purchase than when you have a big order of parts to get out. 

A laser welding system is a little like a house. What is best for you is not necessarily what is best for somebody else, and vice-vice versa. Also, you are dependent on your laser system supplier not only for good advice prior to purchase and all along as you bring new assemblies on stream, but also for the aforementioned life-limited and consumable parts and probably for the dedicated tooling. Get all the facts up front, then carefully weigh all you have learned about the process. Laser welding can be successful in a wide variety of industries and applications, but like so many other technologies, it must be intelligently applied after some careful research and critical thought. n

Tom Kirkland is the Business Unit Manager for the Ultrasonic and Plastics Assembly Systems Division. He functions as product manager for all of Dukane’s non-ultrasonic plastics assembly products. He can be reached by calling 630-797-4922 or by email at [email protected].