Metal polishing & matting: Metal surface treatment

1. What is meant by the surface treatment of metal?

When a workpiece made of stainless steel or another metal is cast, forged or moulded, it is still in its raw state. Further processing is necessary so that it can fulfil its final purpose and, for example, glide seamlessly as a ball bearing, drive an engine as a piston or shine as a door handle on a cabinet. There are very different types of surface treatment. It usually involves the removal of larger or smaller layers. Sanding can give a workpiece the right surface finish: It loses burrs, unevenness and impurities such as rust or scale. It is then prepared for use: either by polishing or matting. In most cases, blasting technology also has an abrasive effect. A distinction is made between compressed air blasting plants, which are usually operated manually, and automatic wheel blasting plants. The abrasive, consisting mainly of many millimetre-sized metal balls, is accelerated and propelled against the surface to be treated, creating a uniform surface.

2. What are the most important reasons for polishing or matting metal?

From electric ovens to implants: Metal workpieces are used in numerous applications. Their surface must be treated differently depending on the intended use and the requirements of the final application. The workpiece is often given a paint finish, for which an optimally prepared surface is the basis. The metal must first be cleaned or its surface homogenised. In many cases, this is done using blasting technology. In other cases, such as decorative parts or jewellery, workpieces are intended to have an effect through their design or to support a design. Vibratory finishing, polishing and matting set the appropriate accents.

3. What are the benefits of polishing and matting metal?

The aim of polishing metal is to create the lowest possible surface roughness with the least possible material removal. This is because smooth surfaces lead to less friction when mechanical components engage with each other, for example, and therefore to less wear. Polishing also makes surfaces particularly even so that subsequent electroplating or painting adheres optimally. In both cases, the aim is to ensure a long service life and perfect functioning of the components installed in the end product. This is not only the basis for the required performance of a component or connecting part, but often also for safety. For example, vehicle components made of aluminium must be protected against corrosion. An optimised surface is the basis for effective corrosion protection. As the demands on the performance of components are constantly increasing, the requirements for surface treatment are also becoming more complex. Even the smallest irregularities can have a negative impact. High-gloss polishing leaves a surface that fulfils these requirements and creates the basis for further machining processes. However, just like matting, metal polishing can also have a purely decorative function. For example, in the design of handles, trims or decorative elements that are intended to emphasise the character of a piece of furniture or vehicle. Last but not least, the jewellery industry also traditionally uses the various surface finishing options to make gold and other precious metals shine. In mass production – as in component manufacturing – this is primarily done using appropriate machines and plants.

4. What methods are used for polishing matting?

Small metal parts can usually be polished by hand, for example with special attachments for hand drills. However, if you want to polish or matt components on a larger scale, small and medium-sized companies use special machines. On an industrial scale, the polishing and smoothing of large quantities is sometimes also carried out using blasting technology on larger plants. During mechanical polishing, a loose abrasive is pulled over the surface with pressure, thus removing any unevenness. The polishing paste bonds with the workpiece surface due to the high energy application, making it even smoother. In this way, the roughness that is specified for the respective application is achieved. Individual pieces or smaller workpiece surfaces are polished by hand. Stationary CNC polishing machines, contact polishing machines or high-gloss polishing machines are used for the surface treatment of metal in larger quantities. A line finish is when stationary, fine-grained grinding belts always move in the same direction and thus roughen the metal from one side. The line finish leaves fine, evenly aligned lines on the top of the component. This type of surface treatment is particularly popular for metal sheets that have a decorative function, for example as panelling. If, on the other hand, the component is to appear as shiny as possible without any alignment, a non-directional finish is chosen. This can be achieved by eccentric or orbital grinding. Satin finishing (also known as brushing) is used when metal surfaces should appear matt rather than shiny. The post-treatment is carried out using special rotary brushes that produce a matt to matt-gloss finish. The grit of the abrasive also determines how fine this sanding pattern is. Brushed surfaces generally also have a decorative function. In industry, workpieces are usually manufactured in large quantities or as mass-produced parts. In order to achieve the desired quality for further processing, industrial companies therefore use finish blast machines. Various blast machines are available for different purposes: screws and rivets are processed in drum shot blast plants or rubber belt tumble shot blast plants and large parts such as welded constructions or profiles in hanger type shot blast plants or roller conveyor shot blast plants. The surface finish must be as economical and favourable as possible, but must still deliver a qualitatively precise result. Here, it is the composition (round, angular, granular) and type of abrasive that determine the degree of surface roughness. For example, abrasives in the fine range 0.4 mm are ideal for polishing blasting. Which finish blasting is used depends on the respective workpiece shape and size as well as its material.

Conclusion

Components always fulfil a specific purpose, which is often linked to safety requirements. Whether blasting, grinding or polishing metal: The surface finish is an essential step on the way to the best possible quality and service life. But the perfect surface treatment of the metal is also important when it comes to visual effects for a successful design. The choice of the right tool or system depends on the respective requirements and the workpiece itself. Finish blasting with suitable blasting plants leads to optimum results when large quantities are to be produced in high quality.