Copper Polishing Quality – Austral Wright Metals
Application Data Sheet – Copper Alloys
Copper Polishing Quality
Copper and copper alloys, such as brass and bronze, are often used for decorative items with an excellent finish. The shiny finish may be produced by polishing, or by plating with gold, silver, nickel or chromium for protection or aesthetic reasons, all of which require an excellent polished surface. However consideration must be made on how to keep the surface tarnish free.
The high gloss surfaces achievable on copper may highlight features in the microstructure, even on what appears to be a clean copper surface, which attracts the attention of the eye and may be seen as defects on the surface of the polished copper item.
Suitable Quality to Polish Copper
Reliable production of a high quality polished or plated finish requires the use of material that has been specially produced for polished items, copper that has been manufactured for the purpose of polishing. Copper used for polishing is usually known as ‘polishing quality’ copper. It can usually only be produced by the mill in limited quantities, with extra processing steps in the manufacturing process, and consequently is significantly more expensive. Most material supplied by Austral Wright Metals is not of polishing quality, as very few users need or are willing to pay for the extra quality. Austral Wright Metals may be able to supply special material, at a price premium, usually at longer lead times as special manufacture will be required.
Austral Wright Metals copper sheet
Copper sheet supplied by Austral Wright Metals is clean copper finish, with paper interleaved to protect from scratches. The finish is uniform and moderately shiny. Whilst it is not mirror bright, it is possible to clearly see a reflection and is tarnish free.
Many users are able to achieve satisfactory results by careful attention to the polishing process they apply to the material. In particular, it is advantageous initially to cut the surface to a significant depth, to remove artefacts from the heavily worked layer of the copper. This requires the use of coarse abrasives, which in turn may require extra polishing steps to achieve the necessary smooth final copper finish.
Because the extra steps to polish copper metal not manufactured for polishing incur extra expense, some customers prefer to omit the initial deep cut, and are often able to achieve a satisfactory surface with only fine abrasives. However, not all batches of product supplied by Austral Wright Metals will allow this. Some batches of copper metal will show artefacts that mar the finish, which may be more or less easy to see depending on the gloss level of the polished material. The artefacts may be highlighted by streaks or ‘comet-tails’ which are left by the polishing process. Unfortunately, the presence of the artefacts may not become obvious until many items have been polished or even plated, and since the only remedy is to repeat the polishing after an initial deep cut, a significant expense may be incurred.
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It is therefore prudent to check the performance of each new batch of material by processing and polishing a few copper pieces to completion before doing any processing on the bulk of the batch. It is also essential to review the performance of the polishing process carefully before a batch of material is condemned – numerous instances have been encountered of material being blamed for problems that were resolved by reviewing the polishing process.
Customers are responsible for choosing to use mill finish quality for high surface finish applications. For these high surface finish applications, polishing quality can be supplied, for sufficient quantity to allow a manufacturing batch, and if sufficient notice is given. Typical minimum quantities could be as much as 2 metric tonnes or greater. Lead time is also lengthy and can take up to 6 months to supply.
Austral Wright Metals stock a range of sizes in copper sheet , however this is all in mill finish. Should you require a customer size it is also subjected to minimum mill runs as the surface finish. So when planning your project it is recommended to use thickness of 0.55, 0.7, 0.9, 1.2, 1.6, 2.0 and 3.0 mm as these are commonly available. The sizes in brass sheet is more limited.
Copper and its alloys are extremely resistant to corrosion, but a superficial discolouring tarnish eventually forms with exposure to the atmosphere or handling. The appearance of these metals can be preserved by applying thin clear protective coatings to their surfaces. These layer coatings are organic chemicals that harden at room temperature or with baking and are usually applied in a solvent.
There are hundreds of useful clear copper coating products on the market, formulated from numerous polymers, solvents, and additives. The user selects from these a coating based upon economics, intended life, desired transparency, and the expected service conditions. The effectiveness of a finish also depends on the procedures used when coating.
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