Method and apparatus for forming wall ironed articles

Metal deforming – Process – Deforming sheet metal

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428 344, 428698, 72347, 72349, 72467, B21C 2502, B21D 2220

Patent

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051687429

DESCRIPTION:

BRIEF SUMMARY
This invention relates to the manufacture of cans and like articles from laminates of polymeric film and metal sheet by blanking a disc from the laminate, drawing a cup from the disc, and mounting the cup on a punch which is passed through at least one ironing ring to thin and elongate the side wall of the cup; and more particularly but not exclusively to ironing rings suitable for ironing the side wall of cups made from laminates of polymeric film, such as polyesters, and sheet metal.
British Patent Application published No. 2003415A describes laminates of polymeric film adhered to a metal sheet and discusses characteristics necessary for the laminate to survive a drawing and wall ironing process. One of these characteristics is said to be that "it will be capable of reflow at temperatures from about 400.degree. F. to about 450.degree. F. (circa 204.degree. to 232.degree. C.) depending upon the particular resin and consistent with its degradation properties". Preferred laminates described include polyvinyl chloride or polypropylene film, adhered to aluminium or steel, by an adhesive such as maleic anhydride modified polypropylene. The film may be applied to both major surfaces of the metal: alternatively a laminate of film on one major surface and a partially cured epoxy-phenolic resin coating on the other major surface was drawing and ironed with the partially cured coating on the interior of the drawn and ironed workpiece. It is mentioned that "Reflow of the film laminate and coatings may occur during forming or subsequently during washing, decorating or interior coating may effectively heal and eliminate metal exposure" on both the inside and outside of the can. Such reflow indicates a severe reduction of sidewall thickness confirmed by example in which the cup sidewall thickness 0.26 mm was reduced to about 0.10 mm using a drawing and ironing assembly described in British Patent No. 1,517,732. Our work with laminates of polyester film and sheet metal, such as aluminium alloy, indicates that the heat necessary to cause reflow of a polyester film will cause undesirable change to the structure of the film.
British Patent Application published Nos. 2092931A and 2092932A describe press tools in which a precoated metal blank is drawn to a cup which is then concurrently drawn and ironed to make a container body having a side wall approximately 0.001" (0.25 mm) thinner than the blank. This corrective ironing is imposed by mounting the cup on a blank holder which surrounds a punch. The bottom of the cup is pressed against an end face of a combination die as entering of the punch into the die reduces the overall diameter of the cup against the die radius. Continued punch travel pushes the newly formed sidewall into a frusto-conical die portion which converges at an angle in a range of 1/2.degree. to 3.degree. to compress the sidewall before the sidewall enters a land of axial length between 0.25 and 2.25 mm. A relief portion supports the land and diverges from it at an angle. The work done in such combination dies is more than is done in separate drawing or ironing dies so that there is a risk of excessive heating of a laminate drawn and ironed in such dies. Furthermore, each press tool to achieve a reduction in overall diameter of a cup requires a dedicated punch, blank-holder and die. In contrast, a series of ironing rings or dies, each of smaller land diameter than the previous ring, can cooperate with a single punch and give opportunity for application coolant to the workpiece between the rings.
Using apparatus in which a single punch cooperates with such a progression of ironings, we have wall ironed cups formed from laminates of polyester and aluminium alloy. We have observed that it is necessary to prevent excessive heat at the ironing rings in order to avoid damage of the polyester film as the side wall of our cups was reduced in thickness by 10% or more. The degree of coating damage depends on the melting point of the polyester film; increasing as the melting point is reduced.
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