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April 22, 1999

7 Min Read
Lasers take aim at welding plastics

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Laser technology is seemingly everywhere, from the microsurgery suite to your portable CD player, and plastics processing is no exception. We see lasers protecting molds, measuring parts for quality control, microwelding damaged molds, building prototypes from polymer, making mold inserts from steel, marking plastic parts, and more. Now lasers are entering into plastics welding.

Dr. George Pinho of Cosma-Powerlasers Ltd. (Kitchener, ON) told us his company is in the development stages of several automotive applications. An application developed by the Fraunhofer Institute in Germany has already been commercialized there. Laser welding was specified for the housing of a keyless automobile entry system because the system's electronic circuitry was endangered by other welding techniques. Although applied laser welding of plastics may be like a wave just breaking on the beach, the technology for this specific application has already arrived.

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Figure 1. Welding one transparent piece and one absorbing piece. The parts must overlap and no special joint design is required.

How Lasers Weld Plastic Parts
Laser welding consists of focusing a laser beam on a plastic part that absorbs it (Figure 1). The resulting heat generates melt that permits bonding to an adjoining part. The fact that most un-colored plastics are transparent to diode or Nd:YAG laser radiation is essential to CD technology, and it is the same for laser welding plastics. The wavelength of the longer-established CO2 lasers, by contrast, will not pass through an exterior plastic part to reach an interior surface, so they can't weld plastics. The beam generated by a diode or Nd:YAG laser, by contrast, passes through polymer with no effect, unless the plastic part contains an absorbing material, usually a color pigment. The basic premise is that a laser can weld two plastic pieces in an overlap configuration if the top/exterior piece is transparent to the beam and the bottom/interior piece is pigmented to absorb the radiation.

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Figure 2. Welding two colored parts. The top part contains a radiation transparent pigment, and the bottom part contains a radiation absorbing pigment.

A laser beam can be controlled very precisely as to depth and surface area. Among other things, this means the top piece can be virtually any thickness and can even be colored as long as the pigment is not of the absorbing type (Figure 2). If both layers are transparent to the laser beam (Figure 3), a bead of pigment can be laid on the weld site of the inner layer to absorb the beam. The amount of pigment can be so small the laser vaporizes it, usually leaving no trace of color.

Why Weld with Lasers?
Because a laser beam can be focused very precisely, it can be made wide or very, very narrow. Because this is a noncontact process, part thickness is not a factor, and it has no impact or effect on delicate mechanisms near the weld. The shape and size of the parts are also not factors so long as the laser can be aimed through the outer piece and onto the weld point of the absorbent inner piece. The parts must overlap. The outer part must be transparent to the beam, and the inner one must be absorbent or have absorbent material applied to its surface.

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Figure 3. Welding transparent parts containing a layer or bead of absorbing pigment.

Pinho says almost any thermoplastic material can be welded as long as fiber content is 40 percent or less. The effects of additives such as flame retardants and impact modifiers are minimal. Dissimilar polymers can be welded as long as they have similar chemical groups and melting points. Among materials welded at Powerlasers are nylon 6 and 6/6 up to 40 percent glass filled, transparent nylon, PP, PE, PC, acrylic, TPO, and TPE. The process also works with PET, PBT, ABS, TPU, and PC blends. The company is protecting its expertise regarding pigments that absorb or transmit laser radiation, but says both types exist in virtually every color.

Welded parts can literally be any size or shape. Very thick parts can be welded unlike with ultrasonic welding where the effect is diminished by increasing part thickness. Heat generation is localized, so parts are not prone to deform. Thin parts can be welded without surface effects. There is no vibration or motion, so sensitive assemblies such as electronics modules or circuitry can be welded without disturbing the contents. There is no need for any flash traps, flanges, energy directors, or ultrasonic welding horns, nor for any angle considerations along the joint. Designers need not provide space for tooling that other processes require.

Pinho says laser welds are as strong or stronger than the parent material, and the seal will be hermetic. Besides being able to vary the width of the weld by varying the width of the laser beam, the laser can produce a very clean, aesthetically pleasing weld line. Because it is a noncontact process, there is no tool wear, nor any potential damage from contact with the welding device.

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The complete robotic welding system includes a 50 to 100W laser with an 800 to 1000 nm range and a beam manipulator.

Precise, Flexible Systems
A laser welding system requires two pieces of equipment: the first is a 50 to 100W laser with an 800 to 1000 nm range such as diode and Nd:YAG lasers. The second part is a beam manipulator. At present, says Pinho, the best physical platform is the six-axis industrial robot. Integrating the laser and the robot creates a flexible, automated welding tool that can be programmed to easily move around even large, complex assemblies to complete the required welding. The robot's programming flexibility means a variety of products can be welded in succession simply by switching programs.

Beam splitters and fiber optics can further extend the flexibility of a laser welding system. A single laser's beam can be split and carried by fiber optics to various points for simultaneous multi-point welding. Though that divides the wattage of the laser, Pinho says the range of today's lasers includes higher powered units that can handle such situations without changing the welding system's configuration. Welding speeds can be as fast as 10 m/minute, depending on the power of the laser and dimensions of the weld.

Powerlasers Ltd. was formed in 1976 and later became part of Triam Automotive. Last year Triam was acquired by Magna International, which also owns Decoma. Magna, with more than 40,000 employees and 146 manufacturing plants, is the largest global automotive supplier, and Powerlasers is now part of its Cosma group. Powerlasers developed the world's first fuzzy logic controller for laser welding. Its proprietary laser welding techniques produced the first aluminum tailored blank, which is when two or more flat sheets having varying thicknesses, coatings, and/or alloys are welded into a single unit. The blanks are then formed into a part whose specific properties are positioned exactly where needed. The company has also designed a variety of laser welding systems now used by car makers.

Will laser welding displace other welding techniques? In some cases it will, but a more likely scenario is that the technology's design possibilities will open new applications in markets like computers, telephones, entertainment devices, and medical equipment. It can support the creation of more performance-functional and/or radically designed physical packages that can be welded with no effect on the delicate equipment inside. The investment in a laser welding system has already been justified for some applications. Flexibility will allow those systems to handle other applications, thus amortizing the investment faster. The driving force is that laser welding will permit production of many previously unweldable assemblies. As Pinho notes, "You can do things with the laser that you just cannot do with other technologies."

Contact Information
Cosma-Powerlasers Ltd.
Kitchener, ON
Dr. George Pinho
Phone: (519) 578-0517
Fax: (519) 578-0137
E-mail: [email protected]

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