Hot stuff

Manufacturers are increasingly turning to laser welding in certain applications for the advantages it provides over other thermoplastic joining methods. The push, says Mike Johnston, Dukane Corporation’s national sales & marketing manager, is being provided by hosts of sophisticated and complex applications, primarily in the automotive and medical fields, which require precise, clean and cosmetically appealing welds.

“One of the main advantages to laser welding is that the weld joints have no flash or particulates outside the weld,” Johnston says. “Particulates can cause problems in applications such as an automotive transmission filter housing or a medical fill tube.”

Because the part is not subjected to mechanical forces, laser welding can produce visually appealing joints on parts with complex geometries.

Dukane introduced a fully integrated laser welding workstation at the last NPE show. The PolyScan system is based on a patented, diode pumped solid-state laser. A beam-steering and focusing head provides the capability for both fixed and dynamic beam welding.

“The system allows an operator to just raise or lower the beam head depending on the part,” Johnston says. “This permits a very quick changeover of tooling and a corresponding cost reduction.”

A laser CAD software is integrated into the graphical user interface that runs on Windows NT. All menus and commands can be selected in one of five different languages. The feature, Johnston says, makes it possible to very quickly draw a beam path for a given part. The system is ideal for shorter production runs and other specialized applications, such as sensitive electronic components, medical instruments and portable telecommunication devices. Typical workpiece size is one to 20 cm, but larger custom machines can be made to order. Dukane will provide customers with an application feasibility evaluation.

HIGHER VOLUME SYSTEMS

Laser welding is more expensive than vibrational, ultra-sonic and other types of welding, so there has to be market drivers compelling greater use of the technology. According to Paul Rooney, product and marketing manager, Branson Ultrasonics Corp., these drivers include materials that are difficult to join (e.g. LDPE), desire to eliminate particulates and part geometry.

Branson’s Laser IRAM Assembly System uses multiple lasers to illuminate the entire welding surface simultaneously. The lasers are mounted remotely and the energy is conveyed with fibre optics through a tool that matches part geometry.

“The system gives part designers greater flexibility, as it is not limited to line-of-sight scanning of the weld joint,” Rooney says. “It can weld on two opposite sides of a box.”

The IRAM system can weld pre-assembled parts, and the method allows for parts to be placed into the machine in the same position and orientation as the final, assembled position. A typical weld cycle time ranges between one and five seconds.

Rooney says most of the current applications for laser welding are in electronics and automotive.

In Europe, the method is used extensively in the automotive industry to manufacture sensors. Rooney says one of the largest potential markets for laser welding is medical, where there are stringent cleanliness requirements. Laser welding requires both a transmitting and absorbing material. One technical challenge to furthering laser welding applications in the medical market is that many approved materials are only clear or transmitting. This can be overcome by the use of a laser absorbing ink, such as Clearweld, an additive produced by Gentex Corporation. Clearweld employs specialty coatings and resins to focus laser energy and produce heat. While the need for FDA or other regulatory approval may still apply, the additive overcomes the technical barrier of laser welding certain non-absorbing materials.

IT’S ALL IN THE DELIVERY –AND THE LASER

For laser welding to work, materials must transmit or absorb light only at the specific wavelength of the laser source used. The wavelength can be selected for a specific application. Parts that appear to be black can be either transmissive or absorptive, depending on the formulation of the pigment. In general, laser welding is more forgiving of resin chemistry or melt temperature differences than most other plastic welding processes.

There are three basic types of lasers used in thermoplastic laser welding systems. Diode lasers are essentially high-powered LEDs that operate at a specific wavelength. They are relatively compact, light and cost-effective. Diode lasers are typically available in a wide range of power levels, usually at shorter wavelengths, ranging from 800 to 950 nanometers (nm).

Diode-pumped Nd:YAG lasers use a solid rod of rare-earth material as the laser-inducing medium. The longer wavelength (over 1000 nm) light produced is useful in more applications than basic diode lasers, and lasers of this type are generally available with a higher range of power levels.

CO2 lasers are essentially only effective for welding thermoplastic films, but are available in very high power levels to accommodate fast weldline speeds.

Beam delivery is another important difference in the choice of a laser welding system. Direct beam delivery uses optics mounted directly on the laser housing and fixed in focal length and beam position relative to the housing. They are used primarily in systems using the moveable part concept or with a diode laser mounted on robot arms.

Fibre delivery is used to get the beam into a tight spot where the laser housing may not be able to reach. Galvo heads are used to steer the laser beams at high speed to follow complex contours or provide for quasi-simultaneous operations.

Beam splitting is a technique in which a single higher power laser beam is split into two beams each at half the power of the original beam. The beam can be divided repeatedly as needed. The method allows for multiple-part production from a single motion source. This process is also useful for welding larger parts.

Courtesy of Dukane Corp

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Leister’s Globo Welding uses a laser beam focused at a point on the joining plane via an air bearing, frictionless, freely rotating glass sphere lens. The glass sphere focuses the beam and also serves as a mechanical clamping tool. While the sphere rolls on the component, it applies continuous pressure at a point on the joining plane. This ensures that the laser beam is only incident at the point where contact pressure is applied. The air bearing glass sphere replaces the mechanical clamping device and expands the scope of laser welding for both continuous and for three-dimensional applications. It is suitable for complex components used in automotive and other applications.