Importance

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Abstract

A flat strip-like metallic material having both surfaces coated with a first synthetic resin layer is passed between plural pairs of upper and lower rollers so that a U-shaped recess is formed at the central portion in the longitudinal direction of the strip material and ring-like projection at both edges of the strip material. Then, the strip material is bent into a spiral form. The spirally wound strip material is passed to a transferring means and first, second and third extruders to extrude molten resin and first, second and third pressing rollers operating in association with the repsective extruders, whereby a second synthetic resin layer is formed at the outer circumferential portion of the adjoining edges of the sprially wound strip material, a third synthetic resin layer is formed on the back side of the recess portion of the same, and a fourth synthetic resin layer is formed on the outer circumferential surface of the recess portion and the adjoining edges of the spirally wound strip material.

First Claim