Sealing Plastic Seams with Laser Welding

LASER WELDING

Direct welding of plastics is often preferred to adhesives and other bonding methods for joining or sealing plastic medical devices, particularly disposables. Laser welding can be used for products such as catheters, fluid containers, syringe seals, cardiac devices, and small pumps. The process offers many performance advantages over other plastics-welding techniques, such as hot plate and ultrasonics. The availability of low-cost, reliable lasers with integral fiber-optic output means that laser welding can be a flexible, cost-effective tool. It can be used on many types of plastics such as polyethylene, polypropylene, acrylic, nylon, and even Teflon. This article presents an overview of this increasingly popular method.

Advantages of Laser Welding

There are many advantages to the use of plastics welding for device OEMs. First, it is a noncontact method, which means it provides short- and long-term consistency because there is no tool wear to consider. It's also a clean technique with no consumables or hazardous chemicals that could require special handling and disposal. There are no glue fumes or solvents to deal with, and laser welding generates no particulates or other debris that would need careful cleaning after bonding. Plus, there are no glue nozzles to block or replace. This lack of hazardous chemicals for processing or postprocess cleaning contributes to the growing trend to implement green manufacturing whenever possible.

Laser welding is capable of submillimeter precision when required. Once the process parameters have been optimized, such as laser power and weld speed, only the precise, localized area illuminated by the laser is melted, with minimal heat flow into the surrounding plastic. The seams created are both fully sealed and mechanically strong.

Flexibility and use in a variety of applications are other important benefits that increase the return on investment for medical device manufacturers. The same laser welding station can be used to seal fluid bags, join catheters, and weld large bottles (see Figure 1). Dimensions and geometries are accommodated in software. Another advantage is speed; weld speeds beyond 2000 mm/min are possible,
depending on the material.

Another lesser advantage is cosmetic appearance. Even though function often supersedes form when it comes to medical devices, products that have rough or charred seams can be perceived to be defective and unacceptable. Laser welding produces cleaner, finer, and smoother seams than other methods, and with no discoloration.

Laser Welding: The Basics

The goal of welding is to join two pieces of material by quickly melting both of them at their mutual interface, followed by rapid resolidification. Welding of metals is a long-established process using a variety of different edge-joining geometries, with names such as butt welding and lap welding. Laser welding of plastics usually relies on the so-called lap weld, in which two sheets of material are overlapped. This type of weld creates a large-area surface-to-surface join, rather than an edge-to-edge join that is produced in a butt weld. The lap-weld geometry not only creates a strong joint, but more importantly, it enables the laser to create this joint in a process known as transmission welding (see Figure 2).

In laser transmission welding, the two layers to be joined are held in close contact. A laser beam is then directed through the first material, which must be transparent at the laser wavelength, and is then strongly absorbed by the second plastic. The absorption of laser light by any material is governed by Beer's law, which essentially states that at any given depth, the material absorbs a fixed proportion of the light reaching that depth. This steady absorption ratio causes the laser intensity—and hence the absorbed energy—to decrease exponentially below the surface. At the appropriate laser power level, this absorbed energy causes surface melting in the lower material. As a result, heat is rapidly transferred to the second material, again causing surface-layer melting. A continuous weld seam is created by scanning the laser beam or by moving the parts to be welded in relation to a fixed laser beam.

What are the process requirements for successful laser welding? First, the materials must be weldable—i.e., chemically similar enough to coalesce and form a strong joint. This is true for any welding method. Just as important, laser welding is an optical method and, as such, relies on the optical properties of the materials being joined. These optical requirements are reasonably relaxed.

The transparent material must absorb no more than 10% of the total laser energy across its entire thickness. This ensures that there is no thermal damage or even melting in the bulk of this material. The opaque (absorbing) layer must absorb more than 90% of the incident laser light within a thickness equivalent to the desired weld depth.

It is important to note that light absorption by plastics is a varying function of wavelength. Most laser welding of plastics is performed with near-infrared lasers. Materials that are transparent to human vision can sometimes have sufficient near-infrared absorption to enable their use as the second (absorbing) layer in transmission welding. But even plastics such as PTFE, with low absorption at the laser wavelength, can be used as the second layer by one of two methods. The first is to add a pigment to the resin that does not affect the critical mechanical properties of the plastic. Pigmented plastic sheets, tubes, and other substrates are available for this purpose. An alternative is to coat the contacting surface of one of the two materials with an absorbing coating. This method has been pioneered by Gentex Corp. and goes by the trade name Clearweld. The name arises from the fact that the materials and coating are all clear (transparent) in the visible wavelength.

There is less flexibility with the transmission layer because there is generally not much that can be done to reduce absorption. Therefore, this layer must be a plastic that is naturally transparent at the laser wavelength. Table I shows common combinations of plastics that can be welded using a near-infrared laser system.

Laser power is another key consideration for welding applications. The laser must provide sufficient power over the required area at adequate (i.e., economically enabling) throughput speeds. These factors are application-specific and dependent on the joint configuration, the particular combination of materials, and the desired weld strength. However, it's important to understand that weld speed, weld strength, and laser power are all interrelated. For instance, to achieve a specific weld strength, there is a direct and linear relationship between laser power and maximum weld speed. Figure 3 shows this relationship in tests on one particular joint configuration: a 10-mm-wide lap weld of two 3-mm-thick sheets of polypropylene. This homopolymer combination was chosen because it is widely used and is also generally considered a difficult joint to create. For a constant weld strength, the maximum attainable weld speed increases almost linearly from 300 mm/min to more than 2000 mm/min as the average laser power is linearly increased from 40 to 120 W.

Another factor that determines how much laser power is required for a particular application is the target weld strength. For a given dwell time (how long the laser is applied), the weld strength increases as the laser power increases. This relationship is shown in Figure 4 for a spot weld on the same polypropylene/polypropylene samples and a dwell time of 0.48 seconds.

Finally, there is also the question of laser cost and cost of ownership. Higher-power lasers cost more, so it's important to not overspecify the laser. Most reputable laser manufacturers and welding system builders have a database of different plastic systems and can often make a good first estimate on a buyer's optimum laser power. Nonetheless, each application should be fully evaluated in a controlled setting, such as an applications laboratory, before purchasing a laser welding system. The cost of ownership is determined by the lifetime of the laser. Thanks to advances such as aluminum-free active-area technology, diode laser technology has matured. The result is that these lasers are capable of providing more than 20,000 hours of operation before any maintenance is required. This long life results in the lowest possible cost of ownership.

Diode Lasers and Optical Fibers

A major reason for the growth in laser plastics welding has been the development of high-power diode lasers that offer high reliability and compact packaging as compared with excimer, CO2, or solid-state lasers. These lasers also offer very low cost per watt, e.g., $80 per watt or lower for a fiber-coupled laser bar.

These diode lasers are semiconductor devices that directly convert electricity into laser light when driven by a high electrical current. High-power diode lasers are created by combining multiple diodes in monolithic, linear, and two-dimensional arrays to achieve output as high as kilowatts. Plastic welding typically requires power levels up to 100 W, which is easily obtained from a simple linear diode laser array. Such arrays, even when fully packaged with cooling and mounting, are only a few inches or less in size. For example, a vertical stack can be an inch or so long and less than ½ in. thick.

There are two basic ways in which the output of a diode laser array can be applied to plastic welding: direct writing or photomask and beam shaping (see Figure 5). In direct write, the beam leaves the fiber optic with a circular shape, which is then focused to a spot on the work surface using one or more lenses. For some welding applications, using a line-shaped spot rather than a circular spot may yield better results. This is achieved by using a cylindrical lens. The weld is then created by scanning the laser spot on the plastics or by moving the plastic parts relative to the laser spot. Direct writing takes advantage of the fact that diode laser output is readily coupled into fiber optics. In fact, most diode laser arrays are available preassembled with optional fiber-optic output.

There are several advantages to direct-write laser welding. First, it can be used on contoured parts. For example, a simple bottle seal can be created by rotating the bottle with a fixed beam. Or, more-complex weld shapes can be produced with robotic motion of the beam or part. Also, software control of the beam or part's relative motion means that a system can instantly switch between different products without the limitations created by application-specific tooling.

Laser plastic welding requires that the two parts to be joined are held in firm contact during weld creation. This is often accomplished using mechanical or vacuum clamping. Another advantage of the direct-write method is that the optomechanical arm that delivers the beam can be used in a contact mode. It can be used to physically press against the plastics, pinning the two parts together and thereby eliminating the need for an additional clamping mechanism.

The alternative to the direct-write method is to shape the beam and simultaneously weld an entire seam or pattern of seams. The simplest example of this is to shape the diode laser beam into a long straight line with the dimensions of the desired weld seam. More-complex weld patterns are created by using a photomask, which is a mask that blocks part of the beam. This creates a shadow pattern on the work surface that defines the area that will be laser welded. The main advantage of masking and shaping the beam is speed; a complex, elaborate weld pattern can be created in a single exposure. This approach is well suited to high-volume production of identical parts. In contrast, short production runs are problematic because each design change requires a new mask or a beam-shaping optic.

Conclusion

Laser welding of plastics offers significant advantages over other techniques. High-power diode lasers provide manufacturers with high reliability and compact packaging, and laser welding is versatile enough to be used for a variety of medical applications. As laser manufacturers improve the return on investment on diode lasers, this green sealing method will be available for an increasing number of applications.

Sri Venkat is director of marketing for Coherent Inc. (Santa Clara, CA). He can be reached at [email protected]. Andre McFayden is a product line manager at the company. McFayden can be reached at [email protected].

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