2 January 2009

Laser Welding Acrylic (PMMA) Discs

Polymethyl methacrylate (PMMA), commonly called acrylic, is an exceptionally transparent thermoplastic sold under many trademarks including Plexiglas® and Lucite®. Bonding or welding PMMA can be accomplished using chemicals (liquids such as methylene chloride or cyanoacrylate), ultrasonic welders, or by using CO2 lasers. Of the three, lasers provide the greatest flexibility for controlling weld width, weld depth, and weld path.

To accomplish the welding process, a rotating fixture was set up that clamps two 1.14 mm (0.045”) thick PMMA discs in position underneath the beam path. Because traditional laser welding does not introduce additional material to fill gaps between pieces, proper part fit up is crucial to achieving a strong mechanical bond.

The Synrad laser beam delivery consisted of a 63.5 mm (2.5”) focusing lens fixed in place over a rotating stage operating at 100 revolutions per minute (RPM). The Synrad positive meniscus lens provides a 100-micron (0.004”) focused spot with a 1.8 mm (0.07”) depth of focus; however, in this instance, the Z-axis was adjusted and defocused the beam to create the customer-specified 0.51 mm (0.02”) diameter weld bead. Clean, dry air was chosen at 0.7 bar (10 PSI), as the weld assist gas.

The rotational speed and diameter of the acrylic discs translates to a weld velocity of 37.7 meters per minute (1484 inches/minute). To achieve the desired weld penetration using 60 watts of power, the discs rotate through five complete revolutions during the weld process. Overall weld cycle time for each set of discs is 3.0 seconds.

This photograph shows two 0.045” thick PMMA discs that are welded together circumferentially to form a single disc using 60 watts of power at a weld speed of 1484 inches per minute.

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