An article of cookware and method of forming the same

0001The present invention relates to an article of cookware having a hard-anodized surface, and to a method of forming the same.

0002Traditional non-stick cookware in particular is often made of aluminium alloy which is soft, porous and stains easily if not covered with the combination of a non-stick coating on the cooking surface and a porcelain enamel or silicon-polyester paint on the exterior. Besides providing a decorative appearance and non-stick convenience, these coverings prevent the food from interacting with the aluminium surface, which can change the taste and colour of food. Since the non-stick interior is soft and will wear out, the pan has a limited life.

0003In the case of the aluminium cookware, since aluminium is such a nnft material it is known to subject the surface of such an article to an anodizing or hard-anodizing process. In such a process, the aluminium surface functions as an anode in an electrolytic bath so that the surface becomes exposed to and reacts with the very active oxygen which is released at the anode. Hard-anodizing requires the bath temperature to be lowered. Such an anodizing or hard-anodizing process results in a much harder surface which is less susceptible to scratching or to other damage. The applicant's EP 0424072A describes a technique for hard-anodizing, and the contents thereof are incorporated herein by reference.

0004Although the hard-anodizing technique greatly hardens the surface such that it becomes almost twice as hard as stainless steel, and this prevents interaction of the aluminium with foods, a problem is that the hard-anodized surface is very coarse and hard, such that it can easily abrade other surfaces such as worktops, stovetops, shelves, kitchen utensils eLc which it comes into contact with and the surface quickly appears marked or marred. In addition, foods can penetrate into the hard porous surface easily staining it.

0005The present invention seeks to overcome these drawbacks.

0006According to a first aspect of the present invention there is provided a method of surface treatment of an article of cookware having an aluminium surface comprising subjecting the aluminium surface to both a polishing technique and to hard-anodizing.

0007Preferably, the polishing is performed both prior to and subsequent to hard-anodizing.

0008The polishing prior to hard-anodizing may require at least first and second polishing steps using progressively finer polishing cloths.

0009In one embodiment, the polishing prior to hard-anodizing requires at the exterior side wall of the article an initial polishing using an abrasive cloth of grit about 280-320. The article is subsequently polished with a finer sisal cotton buff, followed by a polish with a cotton buff.

0010The polishing subsequent to hard-anodizing is preferably performed in a single polishing step employing a fine cotton buff.

0011The process results in an article of cookware which has a very hard surface, and yet one in which the surface is very dense and smooth and does not suffer from the problems of associated with conventional hard-anodized cookware.

0012In a further aspect the invention resides in an article of cookware having a surface treated according to the aforementioned process.

0013According to a further aspect, the invention resides in an article of cookware having at least an outer surface comprising aluminium which has been subjected to both polishing and hard-anodizing.

0014Embodiments of the present invention are now described, by way of example only, with reference to the following detailed description.

0015An article of aluminium cookware is generally formed from a flat blank of metal which is stamped into the desired shape in a screw press or similar in which the side wall of the article is drawn through a die upwardly relative to the base. As well as cookware of aluminium (which is taken to include aluminium alloys) the invention is also applicable to steel clad aluminium where the steel forms the interior, and the aluminium is the exterior.

0016In a conventional hard-anodizing process the article would subsequently be subjected to a surface roughening by some grit-blasting procedure and/or etching process by chemical or electrochemical means. This removes any surface imperfections. According to the invention however, instead of surface roughening, the surface is first subjected to a polishing technique.

0017In particular, in the case of an exterior surface this is subjected to a series of polishing steps employing highspeed polishing wheels employing progressively finer cloths. A polishing agent is also employed. It is important that an agent is utilised which is not too caustic such that it eats into the hard-anodized layer, and is not too mild otherwise insufficient polishing effect is obLained. One suitable material is sold by Robitor &amp; Philadelphia srl under Number 3519 type AP/31C. The polishing sequence is as follows: <u>Side wall</u><ul id="ul0001" list-style="none" compact="compact"><li>1. An abrasive cloth grit 280-320, 20-25 seconds.</li><li>2. Sisal+ buff, 50-60 seconds.</li><li>3. Cotton buff, 25-30 seconds.</li></ul><u>Bottom</u><ul id="ul0002" list-style="none" compact="compact"><li>1. Sisal cotton buff 25-30 seconds.</li><li>2. Cotton buff, 25-30 seconds.</li></ul>

0018In the case of the side wall, the exterior surface is often of relatively poor surface quality since this is the region which has undergone the greatest deformation. This therefore benefits from use of an initial more abrasive cloth. The polishing is able to remove surface imperfect.ions prior to hard-anodizing.

0019In the case of the pan interior the sequence is: <u>Side wall</u><ul id="ul0003" list-style="none" compact="compact"><li>1. Sisal+ cotton buff, for about 100 seconds.</li><li>2. Cotton buff, for 25-30 seconds.</li></ul><u>Bottom</u><ul id="ul0004" list-style="none" compact="compact"><li>1. Sisal+ cotton buff, for 50-60 seconds.</li><li>2. Cotton bath, for 25-30 seconds.</li></ul>

0020The polishing times indicated are to be considered generally as minimum times to give an acceptable finish. The resultant surface is of considerably greater density and smoothness as compared to the roughened surface of the conventional technique.

0021The pan is subsequently subjected to the hard-anodizing process, typically requiring 20-200 minutes in an acid bath at below about 5°C. This has the effect that the aluminium reacts with oxygen creating a hard surface layer, which may be up to 30-40 microns deep.

0022Subsequent to hard-anodizing, the pan is then subjected to a further polishing step using a cotton buff wheel for at least about 25-30 seconds. A polishing agent such as Number 3533 type LUX ROSA of Robitor &amp; Philadelphia srl is used. This polishing step is applied to at least both the exterior side wall and bottom surface, although in the case of an article having a steel interior the further polishing can be omitted from the steel interior. Although the polishing results in some reduction in thickness of the hard-anodized layer this reduction is small, generally below 5%, This final step produces a final surface brightening.

0023The final resultant surface has greatly increased density as compared to that resulting from a conventional hard-anodized process having a much less porous and more closed surface structure. This gives the product an elegant marble-like finish. Typically a surface roughness of the order of 0.1 to 0.2 microns (roughness arithmetic Ra) is achieved. The smooth surface means that the surface cannot easily abrade other surfaces, whilst food cannot easily stick thereto, greatly reducing staining.