GB1013895A - Externally coating elongate articles with particulate materials - Google Patents

Abstract

Elongate articles such as pipes and rods are coated by immersion in a bed of dry particulate material which is maintained in a state of aeration between that of a fluidized bed and of a loose settled unaerated bed, the articles being supported and rotated while immersed in the bed, the adhering particles being subsequently converted into a coherent coating layer. The articles are preferably preheated to cause adherence of the particles but in the case of non-thermoplastic materials the articles may first be coated with an adhesive and while still tacky immersed in the bed of particles. Further adhesive material may then be sprayed over this coating layer. As a further alternative electrostatic coating methods may be employed to cause adherence. Pipes or rods may be rolled through an aerating vessel containing the bed of particles while supported on narrow-edged or toothed rails which minimize the damage to the coating (Figs. 1-3, not shown). Any such damage may be repaired by subsequent heating and compacting operations. Alternatively the ends of the pipes may be supported in and rotated by suitably shaped cup or cone members which may be suspended from overhead rails or supported on lateral supporting rails, Figs. 9-12 (not shown). Immersion in the bed may be achieved by raising the bed or by lowering the articles into the bed and is preferably to a depth of at least two inches but to less than two-thirds of the pipe or rod diameter. The bed of particulate material may be stirred at its base, Figs. 13-15 (not shown), to ensure uniform conditions throughout. The articles may be heated further after immersion to fuse the coating particles into a coherent layer. In a preferred method the material adhering to the surface of the article is compacted before or during final fusion of the coating by contact with a smooth surface, e.g. that of a presser roller, Figs. 18-21 (not shown), or of a ramp on which the article may roll from the coating vessel, Figs. 1 and 16 (not shown). After fusion of the coating is complete the article is preferably cooled by immersion in water while supported on a resilient material, such as foam rubber, Figs. 1, 15 and 16 (not shown). Air cooling may also be employed. Coating materials referred to include thermoplastic powders such as polyethylene, polyvinylchloride, chlorinated polyethers, fluorocarbon resins, nylons, heat-curable resin powders such as partly cured epoxy resins, wax powders, and powders of bituminous substances such as coal tar pitch and petroleum bitumen. Unfusible materials such as glass beads, sand, abrasive minerals and the like are referred to. Two aerating vessels may be arranged in succession to produce multi-layer coatings and a bath of primer substance may precede the aerating vessel.