Process and device for applying a plastic cover layer onto the inner weld seam of a preserve can (tin) body

摘要

The invention relates to a process for applying a cover layer of thermoplastic onto the inner weld seam of a preserve can body or the like. The plastic powder is electrostatically charged and applied as a powder trail onto the weld seam, to be covered, of the preserve can body which is connected (earthed) as a counter-electrode. The powder trail is then fused to form the cover layer. Continuous monitoring of the cover layer from the point of view of corrosion protection is necessary. For this purpose an infrared beam is directed at the powder trail by means of optical waveguides and infrared rays reflected out at the powder trail are measured. They are evaluated as a measure of the thickness of the plastic powder in the powder trail. A threshold value can be specified which ensures that the plastic cover layer fused from the powder trail satisfies all requirements from the point of view of corrosion. The invention also includes a device for carrying out the measures described, which device operates with a conventional powder application gun. An optical waveguide of an infrared radiation source with transmitter and receiver is installed in said powder application gun, the beam aperture of which is arranged downstream, in the transport direction of the can bodies, of the powder discharge slit of the powder application gun. <IMAGE>