Improved process for applying a protective coating to metal containers
Publication Date: 2005-Nov-30
The IP.com Prior Art Database
According to the present invention there is provided a process to reduce stress fracture in polymer coated metal containers subjected to at least one heating step during manufacture by any press operation for container formation wherein during at least one heating step the metal container temperature is raised to a first temperature between the glass transition temperature (Tg) and the melting point of the polymer coating, then immediately quenched by rapid cooling to a second temperature less than the Tg of the polymer coating, thereby minimizing crystal formation and improving the flexibility of the polymer coating. Quenching and heating can also take place in multiple steps.
IMPROVED PROCESS FOR APPLYING PROTECTIVE COATING TO METAL CONTAINERS
FIELD OF THE INVENTION
This invention relates to processes for the coating of metal containers. Particularly it relates to processes which improve the integrity of the internal coating of metal containers.
The present invention is to be understood in light of what has previously been done in the field. However, the following discussion is not an acknowledgement or admission that any of the material referred to was published, used or part of the common general knowledge in
Australiaas at the priority date of the application.
The metal beverage can is currently manufactured, on a global basis, using essentially the same process and in virtually the same manner. The most common manufacturing methodology is referred to as a ‘draw and wall-iron’ (DWI) process used for longer beverage cans whereby the metal — usually aluminum or steel, uncoated — is fed from a coil into a body-maker that firstly draws a shallow cup which is then subjected to a sequential process of stretching and wall-ironing. The process, being basically a press operation, involves the use of lubricants, washing agents, chemical preparation of the metal surface and subsequent application of both internal and external coatings. The application of these coatings usually requires the use of ovens for heat curing of the organic materials used. Heat is also required for drying subsequent to washing. One desirable side-effect of heating, in the case of aluminum, is to cause annealing and strengthening of the dome at the base of the can.
manufacturing processes used for different can products include the 30
“draw-redraw” (DRD) process used for shorter cans having greater wall strength
and “draw-stretch redraw” (DSFID). Whilst both steel and aluminum may be used as the base material in the DWI process, aluminum is subject to large price variation and most supply contracts are linked to movements on the London Metal Exchange (LME). Steel, whilst a lower cost material, requires additional outlay for the manufacturing process capital equipment and is technically more demanding. The DWI process, regardless of metal used, requires a large ‘footprint’ i.e. floorspace, and requires
significant quantities of energy and materials such as water, alkali (or acid), lubricants, coatings, inks, varnish. Since the process requires the use of lubricants and coolant hence a washing and drying process is necessary to remove the residual material and prepare the metal surface for later coating.
This manufacturing process results in an open-ended cylinder that must be 10 internally sprayed with a protective coating that requires heat-curing. The protective coating is present to protect against a chemical interaction between the metal of the can and the can contents. Any such interaction may result in spoilage of the can contents and/or corrosion of the can itself by the can contents lea...