Shadowgraphy of pulsed CO2 laser ablation of polymers

Heavy plume expansions in air, helium and argon produced by pulsed CO2 laser ablations of poly(methyl methacrylate) (PMMA) and polyimide (PI) were imaged by shadowgraphy. The PMMA was melted at 1-2 mm depth beneath the surface followed by an outward expulsion of heavy plumes. A shock front was formed which detached from the first plume after several microseconds. A second, mushroom-shaped plume and a narrow ejection were also observed. In contrast, the mushroom-shaped plume structure was absent in PI ablation, but it was in the form of a single blob, which collapsed back to the target.