Laser Welding Adds New Dimension to Plastics Processing



Laser Welding Adds New Dimension to Plastics Processing

(Dec 28, 2005)

|[pic] |

|[pic]Gordon Graff |

|Introduction |

|Advantages |

|Processes |

|Equipment |

|Material |

|Types of welding |

|Applications |

|Recent advances |

|Conclusion |

|Introduction |

|Laser welding is a technique for joining thermoplastic sheets, films or molded parts using the heating power of laser beams|

|to melt plastic at the interfaces. First demonstrated in the 1970s, laser welding was for many years too expensive to |

|compete with older plastics joining techniques such as vibration and hot-plate welding. Since the mid-1990s, however, laser|

|welding has grown in popularity as the costs of the required equipment have declined. |

|Laser welding systems are most useful when the parts being joined are delicate (electronic components), or require sterile |

|conditions (medical devices and food packaging). The relatively high speed of laser welding makes it valuable on assembly |

|lines for plastic automotive parts. Laser welding can also join parts with complex geometries that would be hard to link |

|with other welding methods. |

| |

|Advantages |

|Laser processes have many benefits. Among them: |

|Welding equipment does not make contact with parts being joined. |

|High speed |

|Equipment can easily be manipulated robotically for complex parts. |

|No flash is produced |

|Welds are strong. |

|High-precision joints can be produced. |

|Technique is vibration-free. |

|Gas-tight or hermetic seals are possible. |

|Thermal damage and distortion are minimal. |

|Resins of different compositions and colors can be joined |

| |

|Processes |

|The most common form of laser welding is called transmission laser welding. In this process, two plastic parts are clamped |

|together and a laser beam, in the short-wavelength infrared (IR) region, is directed at the section to be joined. The beam |

|passes through the top layer, which is transparent; it is absorbed by the bottom layer, which is laser absorbing. |

|Absorption of the laser energy causes the bottom layer to heat up, melting both upper and lower layers of plastic and |

|causing them to fuse. The upper layer can be clear or colored, but must be sufficiently light-transmitting to allow the |

|laser beam to pass through it. |

|[pic] |

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|Figure 1: How transmission laser welding works. |

|(Source: DuPont) |

| |

|In the past, it was not possible to transmission-weld two transparent plastic layers, because neither absorbed enough laser|

|energy. Nor was it possible to transmission-weld two black layers, because the light beam could not penetrate far enough |

|through the upper layer to heat up the interface. Recent technological advances, however, make both types of welding |

|possible (see 'Recent Advances'). |

| |

|Equipment |

|Transmission laser welding uses two main types of lasers: neodymium:yttrium aluminum garnet synthetic crystal (Nd:YAG); and|

|diodes, which are made of semiconductors. The Yd:YAG lasers have a wavelength of 1,064 nanometers (nm), which is easily |

|absorbed by plastics with special fillers or pigments. The light can be transmitted by optical fibers, making it easy to |

|reach tight corners, especially if a robot is used in the welding. |

|Diode lasers, which possess wavelengths between 800-1,000 nm, are the most energy-efficient lasers used in welding. They |

|are highly compact, so they are easy to mount on a robot. Diode absorption characteristics are similar to those of Nd:YAG. |

|Carbon dioxide (CO2) lasers are also used in plastic welding. They emit light at a wavelength of 10,600 nm, which is more |

|easily absorbed by plastics than emissions from Nd:YAG and diode devices. However, light from CO2 lasers is not as |

|penetrating as light from the other two lasers, so CO2 units are typically used in film applications. |

|Laser Type |

|CO2 |

|Nd:YAG |

|Diode |

| |

|Wavelength (?) |

|10.6 |

|1.06 |

|0.8-1.0 |

| |

|Max. power (W) |

|60,000 |

|6,000 |

|6,000 |

| |

|Efficiency |

|10% |

|3% |

|30% |

| |

|Beam Transmission |

|Reflection off mirrors |

|Fibre optic and mirrors |

|Fibre optic and mirrors |

| |

|Minimum spot size * (mm) |

|0.2-0.7 diam. |

|0.1-0.5 diam. |

|0.5x0.5 |

| |

| |

|Table 1: Comparison of commercially available laser sources for plastics processing |

|(Source: TWI, Ltd.) |

|[*Approximate figures for general case.] |

| |

|Transmission welding with Nd:YAG or diode lasers can join plastics of more than 1 mm thickness at linear speeds exceeding |

|20 m/min. CO2 welding of films can be done even faster - at rates of up to 750 m/min. |

| |

|Materials |

|Nearly all thermoplastics and thermoplastic elastomers can be welded with lasers. Common materials often joined with the |

|technique include polypropylene, polystyrene polycarbonate, ABS, polyamides, polymethyl methacrylate, acetals, PET, and |

|polybutylene terephthalate. Certain engineering plastics, such as polyphenylene sulfide and liquid crystal polymers, are |

|not well suited for laser welding because of their low transmission of laser light. Carbon black is often added to the |

|lower plastic layer to make it absorptive enough for transmission laser welding. |

|[pic] |

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|Figure 2: Laser-weldable polymers. |

|(Source: BASF) |

| |

|Both unfilled and glass-reinforced polymers can be laser welded. But increased concentrations of glass fillers scatter the |

|IR radiation of lasers, reducing the overall light transmission through polymers. Colored plastics can be laser welded, but|

|penetration of laser beams through plastics declines as pigment or dye concentrations increase. |

| |

|Types of welding |

|Laser welding of plastics is done in several different ways. |

|In contour welding, the layers are gradually joined by a laser beam that moves and melts the plastic along the contours of |

|the seam; alternatively, the clamped layers move past a fixed laser beam. |

|In simultaneous welding, laser beams from multiple diodes are directed in a line along the contours of the seam to be |

|welded, causing the entire profile to be melted and welded simultaneously. |

|Quasi-simultaneous welding combines contour and simultaneous welding. Mirrors guide a laser beam at great speeds (at least |

|10 m/sec) along the section to be joined. The entire section gradually heats up and melts. |

|[pic] |

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|Figure 3: Contour, simultaneous and quasi-simultaneous welding (left to right). |

|(Source: BASF.) |

| |

|Another approach is mask welding, in which a laser beam is directed through a patterned mask, which exposes only small, |

|detailed sections of the underlying plastic layers that are to be melted and joined. High resolution welds, down to 10 |

|micrometers, are possible with this technique. |

| |

|Applications |

|In the automotive industry, laser welding of plastics has been used in the assembly of fuel injectors, gearshift housings, |

|engine compartment sensors, cockpit housings, hydraulic oil tanks, filter housings, headlights and taillights. Other auto |

|applications include production of air intake manifolds, and of auxiliary water pumps. |

|[pic] |

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|Figure 4: Auto headlight assembly is laser welded using a glass sphere that focuses the laser beam and acts as a clamping |

|tool. |

|{Source: Leister Technologies) |

| |

|In the medical area, laser welding is useful in the assembly of fluid reservoirs and filters, tube-to-tube connectors, |

|ostomy bags, hearing aids, implants, and microfluidic devices used in analyses. |

|[pic] |

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|Figure 5: Microfluidic device made with laser welding exploits the great precision of the technique. |

|(Source: LaserQuipment.) |

| |

|Because laser welding is a vibration-free technology, it is particularly valuable for assembling delicate electronic |

|components. Devices fabricated by laser techniques include keyboards, mobile phones and connectors. Automotive electronic |

|components made with laser welding include automatic door locks, keyless entry devices and sensors. |

|Lasers can also weld thin plastic films together at their edges to form packaging enclosures. The operation can be done |

|extremely rapidly. According to one source (TWI Ltd.), a 100W CO2 laser can weld 100 micrometer polyethylene films at 100 |

|m/min. |

|[pic] |

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|Figure 6: Micrograph of two polyethylene films welded with a laser. |

|(Source: TWI, Ltd.) |

| |

| |

|Recent advances |

|In the early days of laser welding, it was not possible to join two transparent plastics with the technique. Now this type |

|of welding is accomplished with the aid of infrared-absorbing coatings at the interfaces of two plastic layers. The |

|coatings heat up when exposed to laser light and melt the adjoining layers. |

|[pic] |

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|Figure 7: Laser absorbing coating at interface of plastic surfaces heats up and allows joining of normally non-weldable |

|transparent layers. |

|(Source TWI, Ltd.) |

| |

|Welding two black-colored plastic layers with lasers also used to be impossible because the laser energy was absorbed |

|before it could reach the joint interface. But it is now possible to add special pigments to the upper plastic layer that |

|appear black to the human eye, but which are relatively transparent to laser light. The pigments allow sufficient laser |

|energy to penetrate the upper layer so that melting can occur where the two layers meet. |

| |

|Conclusion |

|Laser welding of plastics is a specialized joining technique that can be used to best advantage when high-speed is desired |

|and the parts being joined are delicate or require sterile conditions. The technique was once cost-prohibitive, but |

|declining prices of the equipment have made laser welding competitive with ultrasonic and hot-plate welding for many |

|applications. Industries that employ laser welding include medical, automotive, electronics and packaging. Wider uses of |

|laser welding are likely due to advances in additives and coatings that have overcome some of the early limitations of the |

|technology. |

|References: |

|Website: |

|BASF, Bielomatik, Branson Ultrasonics, DSM, Dukane Corp., DuPont, Gentex, Herfurth Laser Technologies, Jenoptik, |

|LaserQuipment, Leister Technologies, Rofin, Ticona, TWI, Ltd. |

|Publication: |

|Jan H. Schut, "Plastics Welding: Laser and Infrared Systems Expand Capabilities," Plastics Technology, Oct., 2004 |

|Peter Mapleston, "New Laser Welding Systems Debut," Modern Plastics, Nov., 2003. |

|Robert Leaversuch, "Laser Welding Comes of Age," Plastics Technology, Feb., 2002. |

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