Medical Assembly: Shedding Light on Plastics Assembly
For years now, laser welding of thermoplastics has been the focus of a lot of research and speculation. In spite of all the talk, though, there's been no getting around that 400-pound gorilla sitting in the corner-namely, expense.
Still, in the areas of medical and automotive assembly, the potential benefits are great enough that a number of companies have persisted in exploring new technologies and processes in the hopes of finding a market. And it looks like success, or at least an increasing acceptance of the technology, may be right around the corner.
Ironically, despite their differences, the medical and automotive sectors are attracted to lasers for the same reason: clean, hermetically sealed welds that are free of particulate matter. Whether it's a brake fluid reservoir or polyester medical filter, you don't want bits of plastic floating around gumming up the works. Laser welds are also very clean and aesthetically appealing, an important consideration in taillight assemblies and the transparent plastic components that are often used in hospitals.
Beyond that, laser welding offers a host of other features that make it attractive to the medical sector-if you can justify the costs. For example, it can be used to join parts made of different thermoplastics and is very gentle on the parts being welded. In contrast to ultrasonic and vibration welding, there are no violent motions that are liable to shake more delicate structures to pieces. All you need is some gentle clamping pressure while the laser creates the weld.
It is also very clean in that it does not create fumes that may require venting, as is the case with hot plate or solvent welding. This makes laser welding perfect for use in a clean room environment.
Finally, because there is no movement or vibrating of the parts, as well as an almost complete absence of flash, laser welders are extremely precise. This makes them ideal for use with smaller assemblies, like the tiny microfluidic devices that are increasingly being manufactured for everything from diagnostics to drug research. These "labs on a chip" can measure 1 centimeter in length and a couple of millimeters thick, and contain a myriad of channels measuring less than 100 micrometers across. Trying to assemble these chips using adhesives, or ultrasonic or hot gas welding can be problematic at best.
The result has been that lasers are currently being developed to assemble a wide variety of medical components, including filters, ostomy bags, tube fitments to end caps, reservoirs and tube-to-tube assemblies. Laser welders are also being configured to manufacture micro-titer plates, small assemblies containing dozens of fine holes that serve as tiny petri dishes.
Thus far, many of these applications are in the development stage-again, lasers aren't cheap; even a basic system will set a manufacturer back a good $100,000 to $150,000. In addition, many medical manufacturers already have processes in place that can do the job, if only imperfectly. Still, as new products are brought to market, laser welding equipment manufacturers are predicting that their systems will become increasing commonplace, because of what they have to offer.
"No doubt about it. [Laser welding] is an expensive proposition. You've really got to have a reason to go with it," says Dukane Corp. (St. Charles, IL) product manager Michael Johnston. "Still, I think you will see lot more of it in the next 2 to 5 years. In 5 years it will be a common assembly method."
Paul Rooney, product line manager for laser welding at Branson Ultrasonics Corp. (Danbury, CT), agrees.
"Three years ago there was lot of hype and excitement, but it has taken a while for laser welding to be accepted," Rooney says. "Now people are understanding both its advantages and limitations. It will definitely be used more in the next few years."
Another way in which laser welding stands apart is that it can be applied in a number of very distinct ways. Simultaneous welding, contour welding, mask welding, quasi-simultaneous welding, scan and Globo welding-even after making the decision to go laser, plenty of choices remain.
Still, all these techniques are similar in that they are based on the practice of near infrared (NIR) transmission welding. With this process, laser light is transmitted through a top, non-laser-absorbing layer to a laser-absorbing layer, which heats up to melt the surrounding plastic and create the weld. Cycle times can be as short as a second, and relatively light clamping pressure is required-as little as 100 pounds-just enough to keep the parts stationary and ensure there is no gapping.
Historically, the bottom layer has been opaque, incorporating small amounts of NIR-absorbent carbon black. However, in recent years, BASF Group (Florham Park, NJ) and the Clearweld division of Gentex Corp. (Carbondale, PA) have each created transparent NIR-absorbent pigments, which open up a range of new options. Pigments have also been developed that are opaque to the human eye, but non-NIR-absorbent, making it possible to laser weld entirely opaque assemblies, like those used for under-the-hood components in the automotive sector.
According to Michelle Burrell, development manager for materials joining technology at Clearweld, her company's additives are available in two forms: either as an additive that can be incorporated into the resin used to mold the actual part, or as a coating that is applied to the joint interface prior to welding. The latter type is available as a low-viscosity liquid that is applied via needle tip or microsolenoid dispenser. The coatings can also be sprayed for covering a wider area.
According to Burrell, needle tip dispensing is the most common method, because it is the least expensive. Microsolenoid dispensing is typically used for more precise applications.
When incorporated into the parts themselves, the additives impart a slight coloration or tint to the plastic. But according to Burrell, these can be easily compensated for with other additives to create the blue tint that is commonly found in medical products. When a coating is used, the additive is actually consumed in the course of the welding process, leaving a joint that is as clear as the plastic from which it is made.
In terms of the type of lasers being employed, the current trend is toward semiconductor, or diode, lasers transmitting light in the 810- to 980-nanometer wavelength range. The advantage to diode lasers-the type found in CD and DVD players-is that they are compact and relatively inexpensive. Diode lasers can be found in Branson's IRAM line of laser welders, Dukane's welding systems and the Novolas line from Leister Technologies LLC (Itasca, IL).
Nd:YAG lasers, which generate light in the 1064-nanometer area, offer a wider range of power levels and are sometimes used for laser welding larger parts. On the downside, they are initially more expensive. However, Burrell notes that they may offer long-term cost benefits, because maintenance and replacement costs are lower than for their diode counterparts.
CO2 lasers offer even greater power, making them excellent in cutting and etching applications. However, the light created by CO2 lasers is readily absorbed by glass and clear thermoplastics. This means it can't be used for transmission welding. The only real "joining" that can be done with CO2 lasers is in the context of cutting and sealing applications.
Pick A Style
Of all the methods for laser welding of plastics, the most flexible is spot, or contour, welding, in which the laser is focused onto a single point that is then traced along the length of the weld. This spot can be anywhere from 0.6 to 5 millimeters across, although 1- to 2-millimeter sizes are most common. It can be moved along the weld line either by fixturing the parts to an X-Y table, by attaching the laser to a robotic arm, or a combination of the two.
Using a six-axis robot to create the weld opens up the possibility of a wide range of weld contours, in contrast to ultrasonic, spin and vibration welding, in which the welded surface needs to be in or close to a single plane. It also makes for a flexible assembly system-once the weld parameters have been entered into the robotic controller, changing from one configuration to another can be done with the push of a button.
Of course, having to trace out the entire length of a weld with a single point of light takes time. So equipment manufacturers have also developed the simultaneous welding process, in which the laser light is collimated so that it shines along the entire length of the weld at the same time. (Branson uses fiber optic bundles and waveguides to deliver the energy to the weld area.) This method provides cycle times of as little as a second and permits a degree of material collapse that results in an easy-to-achieve hermetically sealed weld. Because a contour weld is performed at only one point at any given instant, it does not allow this same collapse. As a result, the prewelded joint needs to maintain tight tolerances if the weld is to create a hermetic seal. Any imperfections in the parts can result in leaks after they've been assembled.
In the past, collimating lens design was such that the technique was limited to welding square or rectangular geometries. However, lenses are now available for creating circular welds, creating the possibility for a wider range of parts geometries. The Branson system also offers complete flexibility in terms of weld geometries. As is the case with contour welding, weld widths should ideally be 1 or 2 millimeters. Of course, because a collimating lens is set in terms of the laser line it creates, changing welds means changing the tooling in the welder.
Similar to line welding is quasi-simultaneous welding, in which a set of servo-driven mirrors directs a single point of laser light along the weld path at a rate of 40 circuits per second. Although not as flexible as contour welding-it is limited to flat or slightly contoured joints-it generally offers faster cycle times. In fact, as long as the weld is a small one, cycles times are comparable to that of simultaneous welding. Because servomotors are used to trace out weld geometries, a single laser head can be used for multiple welds.
Also similar to line welding is scan, or curtain, welding, in which a line-focused laser is scanned over the parts to be joined, creating welds wherever it encounters any NIR-absorbent material. As is the case with quasi-simultaneous laser welding, this method offers flexibility in that the same welding head can be used to weld a variety of different parts. NIR-absorbent coatings like those manufacturing by Clearweld readily lend themselves to this kind of assembly, because they can be used to trace out an infinite variety of weld geometries, simply by changing the programming of an automated dispenser.
For creating small, extremely precise assemblies, the technique of choice is often a method developed by Leister Technologies known as mask welding. Using this approach, a line-focused laser is scanned over the parts being assembled, with a stencil, or mask, positioned below the laser so that only the actual weld areas are left exposed. Produced via photolithographic removal of portions of a metallic glass, the masks can be configured into an infinite number of shapes to make weld lines as narrow as 100 micrometers. As a further advantage, the masks can create weld lines of various widths and shapes, as opposed to the fixed weld thicknesses required via the other types of welds.
Finally, there is the Globo welding process, unveiled by Leister Technologies in late 2004. A variation on traditional contour welding, the Globo system uses a welding head that looks a little like a ballpoint pen, with a rotating glass sphere that both focuses the laser beam and presses against the parts being welded.
Manipulated via a robotic arm, Globo welding offers flexibility and the ability to weld complex joint geometries. The glass sphere also facilitates the optimal synchronization of clamping pressure and energy application.
Due to the potential danger posed by laser light, any laser assembly system needs to be shielded-in a situation that is somewhat analogous to vibration welding (although the latter needs to be shielded for sound). However, the enclosures offer little hindrance, and laser welding easily lends itself to automated or semiautomated assembly.
In the case of a semiautomated line, an operator can install the parts in a fixture, after which the welder joins the parts. Light enclosures are also available with doors on two sides so parts enter through one side and then exit from the other after assembly in an automated system.
Branson also manufactures laser welders in which the welding head itself has shielding that contains the parts being joined, obviating the need for a larger enclosure. The result is a welder that looks much like a conventional ultrasonic welder and can be easily incorporated into an automated line.