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These micrographs show seams made with the mask welding procedure. The gray shaded area is welded. |
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A MEMS silicon mass flow sensor contains a highly sensitive thermal anemometer. The sensor membrane is made from silicon nitride, 100 to 300 nm thick. Deposited on the membrane are tiny heating elements and temperature sensors. The light-absorbing housing is molded from ABS and the optically transparent cover from PMMA. A 1-mm-wide weld line gives the gastight cover the required strength. The LTW process doesn't impart mechanical or thermal loads to the microstructure, a key requirement for this application. |
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Mask welding locates a reflective or absorbing mask between the laser source and workpiece. Areas not in the mask's shadow get welded. |
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The laser is scanned along the weld line. |
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Multiple lasers weld the entire part at once. |
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A self-priming micromembrane pump is built from two injection-molded polycarbonate halves. Microstructures form the inlet and outlet channels. The two halves are separated by a 2- m-thick polycarbonate membrane which serves as a valve. The upper part and the membrane are transparent — the lower part absorbs the laser radiation. Both contour and mask LTW techniques were evaluated for the assembly. The contour method produced a weld line less than 0.5 mm wide at speeds of about 5 mm/sec, resulting in a process time of roughly 3 min/pump. The narrow beam width was required to navigate around the microchannels. Areas with wider welds were scanned back and forth, which increased process time. In contrast, the mask technique welded the assembly in about 3 sec using 8 W/mm2 of laser power. The microchannels and chambers were protected during the procedure by the mask. |
Traditional methods to join plastics include adhesives, fasteners, and fusing by heat from ultrasonic vibration, hot gas, and laser beams. Although these techniques work well for larger components, microstructures or parts with tiny features can be problematic. Hot-gas welding, for example, tends to overheat the area surrounding the weld zone because the gas stream can't focus sufficiently. Laser beams, although more tightly focused, still can overheat temperature-sensitive parts. And, ultrasonic vibrations may induce stress cracks or otherwise damage fragile components. And, weld-line width is limited to about 0.5 mm. Moreover, this process leaves debris which is unacceptable for many applications, especially medical devices, that require a sterile package free of contaminants.
But there's another method better suited for joining smaller assemblies: laser transmission welding (LTW). Here, a laser beam transmits through a transparent part (at the laser wavelength) and is converted to heat in an adjoining, absorbing part. A precise amount of energy is delivered only to the weld zone so the area nearby stays cooler. Best of all, the process leaves no seams (no displacement or volume reduction) or debris. Joints appear to have been glued together, without the disadvantages of adhesives. Parts are clamped firmly together during the process to boost heat conduction and further encourage local melting at the joint. Welds can be shaped as a point, line, or an area. Part geometry and the clamping fixture limit the length of a weld line. Line width can be varied from several centimeters to less than 100 m, depending on the laser and optics. The use of absorbing or reflective masks — to fully or partially block the beam — gives further control over weld area. Masks allow extremely temperature-sensitive structures or materials to locate close to welds, without damage.
Weldable plastics
Most thermoplastic materials are compatible with LTW, including ABS, PC, PE, PMMA, PS, SAN, and thermoplastic elastomers. It is also possible to join different materials with similar melting temperatures. A typical combination is ABS-PC. Here, the mechanically tough ABS could serve as a housing and the optically transparent polycarbonate for a window.
It's also possible to weld materials with different melting viscosity, because LTW produces minimal material flow or spread. Melt spread is controllable to within 5 m, in most cases. Low flow also means the joining process is more precise, an important consideration for microstructures whose dimensions can be on the order of 100 m.
Essentially, plastics to be welded need only have different optical transmission at the laser wavelength. Wavelengths are typically in the near-infrared (NIR) between 800 and 1,100 nm. Because most plastics in natural form transmit in this range, carbon black is often added to the resin in low concentrations to produce the desired absorption.
But absorbing parts don't have to be black or transmitting parts, clear. Emerging NIR-absorbing transparent pigments allow the joining of two visibly transparent components. These pigments could make possible a wide range of new applications, especially medical devices. Conversely, NIR transparent and visible-light absorbing pigments allow two black parts to be welded. One company uses the technique to join halves of a remote access control for automobiles. An alternative to pigmented plastic is thin NIR-absorbing plastic foil made of the same material as the joined parts. These foils are placed between the two transparent parts at the weld joint in a separate step.
LTW process equipment
LTW process equipment consists of a part holder and a laser source with focusing optics. Some systems also monitor and optimize the welding process. For example, a noncontact temperature measurement taken at the weld zone is used in a laser power control loop to compensate for changes in welding speed and optical absorption. Clamping pressure is another accurate indicator of weld quality and is often monitored.
There are three basic types of LTW systems: contour, mask, and simultaneous. Depending on type and application, systems can use one of three different lasers: semiconductor diode, solid state, and diode array. High-power, semiconductor-diode lasers work well for contour welding. They emit NIR radiation at wavelengths between 808 and 980 nm and produce the required 10 to 100 W. With contour welding, the laser head moves along the weld line path in either two or three dimensions. A translating stage works for 2D jobs while a robot is best for 3D contours. Scan speeds range from 10 to 100 mm/sec, depending on the application.
Another factor is weld width. Microstructures typically require narrower weld lines — less than 100 m in some cases — so the laser beam must focus to a smaller spot. This requires a laser source with good beam quality. Because high-power diode lasers use several individual emitters, the resulting beam can't focus smaller than a few hundred microns. Those applications requiring a smaller spot size should instead use a high-quality, solid-state laser. Regardless of type, laser light can be delivered to the part via an optical fiber or directly from the laser head itself through optics.
Simultaneous welding systems use laser-diode arrays in combination with beam-shaping optics. The resulting "light curtain" illuminates and heats the whole weld line at once. Unlike contour welding, process times for the method are independent of weld length, and can be less than one second in some cases. However, with improved throughput also comes less flexibility because the light-curtain optics are typically custom made for a workpiece.
Mask welding, in contrast, locates a reflective or absorbing mask between the laser source and workpiece. A semiconductor diode laser, focused to a line, scans the mask, welding areas that aren't in the mask's shadow. A simple linear stage provides the required motion. Masks can be exchanged quickly making short production runs feasible. The arrangement isn't as sensitive to beam quality as the other methods which allows the use of lower-cost lasers. Moreover, sophisticated, beam-shaping optics aren't required. A downside to masks is they tend to defocus the beam and create a wider melt zone. Still, with careful control of laser power, clamping pressure, and scan rate, the melt zone can be held within 3 to 5 m wide.
Information for this article was contributed by Jei-Wei Chen and Thomas Hessler, Leister Process Technologies, Switzerland.