Coating line

De-coil strip. To maintain continuous operation successive coils are stitch welded; strip is held in a catenary form to give maximum area for vaporization of the solvents to occur Time to allow this operation to be completed is provided by means of an accumulator system at the feed ends of the line.

Pretreatment

Remove oil, grease and surface residues by degreasing and zinc passivation with intervening water wash and scrub.

Precoating

A solvent-based primer is applied to both sides of the strip by means of a reverse roll coater

First oven

The sheet is now suspended (under tension) in a large oven (temperature 260°C), in which it moves at a maximum speed of 30 m/min. Electronic signalling ensures a nearly constant catenary for the strip so that the vaporization of solvents from the coatings can take place.These solvents are incinerated to prevent atmospheric pollution. The heat generated from the combustion of these solvents is returned via a heat exchanger to preheat the fresh air circulating in the ovens.

Base coat or top coat application

The principal coating is now applied using rubber rollers if both sides of the sheet are to be painted. A second oven then cures this coat. Electronic signalling of the approach of the stitched joint enables the operative to raise the paint applicator rolls to avoid damage to the roller surface by the joint.

The speed of the sheet through the second oven controls the overall speed of the line. It is dictated by

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5.17 Typical steel rolling and coating plant.

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5.17 Typical steel rolling and coating plant.

tensioning rollers, which control the catenary in the second oven to ensure that the sheet is suspended correctly and is evenly heated on both sides of the sheet.

An accumulator allows a 1.5 min delay at the end of the line, and a steering mechanism is provided to ensure that the sheet is straight in the line. A backtension device allows re-coiling of the sheet, and at this stage prepainted sheets can be fed into the line for embossing if required. Sheets are re-coiled prior to being fed into the rolling mill.

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