Try to laser weld two pieces of clear plastic together and they may not remain clear for long. The carbon black and other opaque additives that make laser welding of clear materials possible in the first place—by absorbing light energy—also mark the joint with a visible line. Gentex engineers have now found a way around this welding paradox with a new transmission laser welding process that creates joints without the use of opaque additives.
The Clearweld process instead relies on a proprietary material that's both optically clear after welding and capable of absorbing near-infrared laser light, according to L.P. Frieder, manager of the Gentex's light management products. Applied in a thin layer at the joint interface, this solvent-based welding aid acts as a focal point for laser light, allowing its energy to melt the plastic surrounding the joint. The resulting joints match or exceed the strength of joints produced by vibration welding and solvent bonding, Frieder says.
Gentex has conducted strength tests of overlap joints in various sheet stocks and films. Polycarbonate samples, to take one example, held up to 7,705 psi. In PMMA, the Clearweld joint reached 2,119 psi. And in polyethylene film, the joint held out to 1,929 psi. "In all cases, the samples failed in the parent material rather than the weld," says Gentex materials engineer Nicole Woosman.
Gentex has tried the new process on a wide variety of plastics, including polycarbonate, acrylic, and polypropylene. "We haven't found any material limitations yet," Frieder says. And though Clearweld was initially conceived as a way to join clear plastics, the process also targets colored thermoplastics like glass-filled nylon. In these cases, Clearweld helps offset many plastics' poor IR transmission and absorption properties by enhancing the amount of energy soaked up at the joint interface, Frieder explains. "The majority of our work may ultimately be in colored plastics," he adds.
Regardless of the plastic used, the laser welding process does offer some design flexibility that can otherwise be tough to achieve. Frieder points out that it can produce contoured joint lines in three dimensions. Or it can be used to create stacked structures in a single step. Finally, Frieder says the process has also demonstrated a knack for joining some dissimilar plastics. "We're still trying to understand the science behind the dissimilar substrates, but this capability looks very promising."
Formulated to work with infrared light in the 940- to 1,000-nm wavelength range, Clearweld is compatible with commonly available laser equipment. The only capital-equipment barrier to the process can be found in the precision dispensing equipment needed to lay down a microns-thick coating of the ink-like welding material. "You have to apply the ink just where you want to create a weld—and nowhere else," he says. To come up with precise dispensing systems, Gentex has teamed up with makers of industrial ink-jet and needle dispensing equipment. The company will ultimately offer Clearweld as a turnkey process. "We want to keep the engineering as transparent to the users as possible," Frieder says.
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