Apparatus for depositing a coating on the internal surface of a container

摘要

In apparatus for coating a thermoplastic container interior, using low pressure plasma created in the container by excitation of precursor gas with ultra-high frequency (UHF) microwaves, the gas injection tube (13) penetrates into the container by a length of 25-50% of the overall length of the container between the inlet (18) and the base (17), forming an aerial for receiving UHF waves. A UHF short circuit (19) defines a zero amplitude point. Apparatus for coating the interior of a thermoplastic container, using low pressure plasma created within the container by excitation of a precursor gas with ultra-high frequency (UHF) microwaves, comprises: a cavity (2) of conductive material in which the container is placed and microwaves are introduced; a generator (3) for UHF electromagnetic waves; a waveguide (4) connecting the generator to a window (5) in the side-wall of the cavity, allowing generation of central field(s) in the cavity; a precursor gas injector, comprising an injection tube (13) extending partially into the container; an evacuation pump for the cavity and the container interior; and an inner casing (6), coaxial with the cavity and of microwave-permeable material (e.g. quartz), defining a chamber (7) in which the container is placed during excitation of the gas. The novel feature is that the injection tube penetrates into the container by a length of 25-50% of the overall length of the container between the open end (18) and the base (17), this length of tube forming an aerial for taking up the generated UHF waves. A UHF short circuit (19), in the form of a plate (21), on the injection tube is such that the hollow side face of the plate defines a zero amplitude point for the waves propagated along the tube. The length between the short circuit and the free end (13a) of the tube corresponds to an odd number of quarters of wavelengths, so that a maximum amplitude (loop) is obtained at the tube free end.