Precise Absorbent ink Deposition and Optical Inspection System

Abstract

Foils from the ethylene-tetrafluoroethylene (ETFE) copolymer are used as transparent, humidity resistant and UV-stabile facade and roof coverings, e.g. for stadia, indoor swimming pools or greenhouses [1]. With pneumatically supported cushions, large translucent structures can be realized. Until now, they are assembled through a thermal welding process [2]. The development of welding techniques using laser irradiation is under way.

Laser welding of transparent polymer foils requires an optical absorber placed in the interface between the two welding pairs. Usually, dye molecules with absorption properties adapted to the laser wavelength are used as absorbers. At a well-defined temperature, the dye molecules will be chemically modified, and transparent laser welding seams can be achieved. To get reproducible laser welding results, a homogenous layer of absorbent molecules or materials at the welding interface have to be realized, which is often very hard to achieve by wet deposition of dye molecules dispersed in a solution.

In our contribution, we report on an inkjet printing system that can be mounted on a R2R manufacturing setup. The main challenge to this approach is to find the right ink that is compatible with this highly hydrophobic ETFE foil. Therefore, both the pretreatment of the substrate as well as the utilization of different inkjet technologies are dealt with in this contribution. It is demonstrated that the inkjet printing of a laser absorbent ink in a defined way onto the substrate is possible.

For quality assurance, an optical inspection system has been developed to ensure a proper deposition of material. This ensures the quality control for the inkjet printing process of the special functional material. In that part the results of the comparison of the use of a dedicated 14-bit grey level CCD line camera is compared to a high quality webcam.

Laser irradiation of the foil with printed laser absorbent lines together with the untreated joining partner was performed by a continuous wave diode laser at a wavelength of 808 nm using a defocused laser spot. A nearly transparent welding seam was achieved. Mechanical tensile tests of the laser welding seams have demonstrated that their tensile strength is comparable to conventional thermal welding seams.