Investigation of interaction mechanisms in laser drilling of thermoplastic and thermoset polymers using different wavelengths

In this paper a comparison of the percussion drilled holes in fiber reinforced polymer and non-reinforced thermoplastic sheets using three wavelengths, 1064, 532 and 266 nm is presented. At near infrared and visible wavelengths, 1064 and 532 nm, most of the bulk thermoplastic and thermoset polymers are partially transparent. The degree of transparency is dependent upon the degree of crystallinity. The most amorphous thermoplastics, such as PMMA have the highest transparency. In comparison, the fibre reinforced thermosets – graphite fibre reinforced epoxy – have low transparency. If the IR or visible laser beam is strongly focused, it can cause thermally induced degradation and sometimes burning and other damages (cracking, boiling, etc.) around the focused laser spot and the interaction zone. By using the fourth harmonics ultraviolet light (266 nm) of a diode-pumped and AOQ-switched Nd:YAG-laser with pulse duration of about 100 ns, the hole drilling process was significantly improved and the risk for thermal damage has been significantly reduced for both materials. Around the irradiated spot or drilled holes the material is influenced. This was investigated by both scanning electron microscopy and optical microscopy. There was clear evidence of a change in the interaction mechanism if UV light with high enough photon energy was used. By using UV light, bond breaking rather than thermal material removal occurred, i.e., a change from thermal to photo-chemical dissociation or photo-ablation becomes the dominant interaction mechanism.