STEEL SHEET COMES COLOURED
Reproduced from Engineering (London). Vol. 200. No. 5196 (19th Nov. 1965) with kind permission of the publishers
By continuous paint and plastic coating, the steel Industryoffers another useful material intended for building, appliances, shelving, road signs, and the like. It also provides another outlet for sheet.
The building industry can expect a major sales campaign to break upon them shortly from the direction of South Wales. Richard Thomas and Baldwins Limited, the state-owned steel company with one of the largest wide strip mills in the world-Spencer works-have laid down a ￡600000 plant which coats their wide strip or, to be more correct, sheet, with paint or plastic. The material is to be known as Colour Coated Steel.
The new production line is claimed by RTB to be the most versatile in the world; it is also said to be the widest colour coated steel strip line in Europe. The paint is situated at Gorseinon, near Swansea. It is now producing coated coldly reduced and galvanized steel sheet up to 52 in wide and in the gauge range from 16 to 32 g. It is capable of applying coatings at a maximum speed of 150 ft per min.
There are other production lines for covering steel with coloured protective coating finishes in the UK, these being based on plastic finishes exclusively. The RTB line is regarded by the firm as the first to make available in the UK wide steel strip finish-coated in paint or plastic.
Markets
The chief market at present visualized by RTB is the building industry which they expect will probably take about 70 per cent of output. Plant output is expected to rise to 300 tons per week by January 1966, on a one-shift basis. The ultimate is a three-shift system which would produce some 1 000 tons per week and which it is hoped will be operating by 1967.
There are, initially, seven coating systems on offer comprising: alkyds, acrylics, epoxy phenolics, solution vinyl’s, organ sols, plastisols and PVC film laminates. So far as colour is concerned, the whole of the BS range is available on the product intended for interior use; for exterior use, the line can produce 16 colours at present. More coating systems are expected to follow from 1966 onwards.
The coated steel industry appears to take its commercial lead from the USA, where the production of coated materials reached a figure of 700000 tons last year, representing a 50 per cent increase over the figures for 1962. The RTB line embodies 11 the most advanced techniques known to the international colour coating industry, together with several new features evolved from RB's own research work, which has been going on for some five years.
The line was designed in consultation with Pre-Finish Metals Incorporated, of Illinois, USA, and was engineered by B & K Machinery Limited, of Bilston, Staffordshire, together with several specialist sub-contractors.
Some special features which distinguish this coating line, according to RTB, include the facility to apply three coatings in a single pass; precision levelling, at the entry end of the operation; a Scotchbrite cleaning unit for surface preparation; the capacity to apply pre-treatment primer as an alternative to spray phosphating (this is RTB's own development, called etch priming) high-efficiency ovens which incorporate 9f of air flotation (not used continuously, but to support the catenary of sheet under certain condition); and the ability to apply printed and embossed surfaces and also plastic laminates.
The RTB process starts with one of two substrates, either cold-reduced or hot-dipped galvanized wide steel strip, received in coils from Ebbw Vale and Spencer works. These coils may be between 24 and 52 in wide and have a maximum weight of 10 tons.
The full sequence of operations should be evident from the diagram and needs only selective elaboration. Two alternative systems of spray phosphating may be utilized: iron phosphate is applied to cold-reduced steel and zinc phosphate to the hot-dipped galvanized material. In the third alternative, etch priming, the pre-treatment is made in the form of a film.
Both priming and finish material application are performed by reverse, or forward, roller coating machines developed from the pilot-line machine at RTB's central research laboratories at Aylesbury.
The priming coater is equipped with two-roll coating heads. It coats first the top and then the bottom of the sheet. Each head has a steel roll which picks up paint from a trough and a synthetic rubber roll which applies the coating. Film thickness is controlled by roll speeds and by the gap setting between the rolls.
The finish coater has three-roll heads, a pick-up roll, a metering roll and an applicator. For low film weights, the head is used as a two-roll coater. There is a duplicate top coating head which may be used alternatively to minimize paint change down-time.
The continuously moving sheet is centralized on its way
through the plant by steering rolls which are air-jet controlled off the sheet edge and which tilt appropriately in response to variations in sheet position. The final coiler at the end of the line moves bodily from side to side along its rotational axis to make the trimmest possible coil.
The total power needed for the whole plant is about 1 200 hp, of which the line itself at the bridles takes about 300 hp. This is used to keep the sheet tight and to hold up the catenaries.
Corrosion-resistant
Since RTB expect the bulk of their market to lie in industrialized building, they are preparing for the demand for exterior grade PVC plastisol. This is some 0.012 in thick on the galvanized substrate. All doubts about rapid corrosion have so far been refuted by RTB, who has already had four years' experience with samples, and who believe from accelerated weathering tests (on the basis of DEF 1053, No. 26 modified to new draft BS Specification) that the material will be able to withstand at the very least, 15 years' exterior life without a noticeable change in properties.