Visibly transparent dyes for through-transmission laser welding

A method for transmitting a laser beam at a laser welding wavelength through an upper region of a first substrate to a joint region that upon assembly includes a first welding surface of said first substrate and a second welding surface of a second substrate and a laser absorbing section comprising the steps of:

• providing an upper region which is highly transmissive of the laser welding wavelength;
• selecting a dye with (i) an absorption band above about 350 nm that includes the laser welding wavelength, (ii) an absorption trough in the visible spectrum, (iii) strong transmission of light within the visible spectrum;
• dissolving the dye into the laser absorbing section, said dye being very soluble in the plastic so that it makes little contribution to the light scattering characteristics of the plastic and said dye efficiently absorbing the laser radiation and via vibronic relaxation transmitting heat to weld the plastics;
• and
• assembling all parts of the joint region together and transmitting the laser beam through said upper region to the laser absorbing section to heat and weld said welding surfaces together,
• wherein said selecting step comprises the step of quantitatively calculating an Absorbance Ratio of a candidate dye that yields optical density as a function of wavelength (wv) relative to a reference wavelength, the Absorbance Ratio is defined by the following equation A ⁢ ⁢ b ⁢ ⁢ s ⁢ ⁢ o ⁢ ⁢ r ⁢ ⁢ b ⁢ ⁢ a ⁢ ⁢ n ⁢ ⁢ c ⁢ ⁢ e ⁢ ⁢ Ratio ⁢ ⁢ ( OD ) = OD ⁡ ( Substrate + Dye @ wv ) - OD ⁡ ( Substrate @ wv ) OD ⁡ ( Substrate + Dye @ lww ) - OD ⁡ ( Substrate @ lww )
• where lww is the reference wavelength, and the optical density is one of {fraction (1/100)} or {fraction (1/1000)} less dense than a reference point.