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The term perforated sheet is defined in the DIN 24041 standard. In practice, perforated sheets, coils and blanks are also used. This refers to a (usually metallic) plate with regularly arranged holes.
Perforated sheets are a proven construction and design element with a wide range of functions in various areas of application. They take on very different forms, have to be installed in complex environments and do their job there for many years. The unique feature of perforated sheets among thin sheets – the perforations or hole patterns – is also their corrosive point of attack for destruction by oxidation, which can be controlled with the correct post-treatment following the stamping process.
The perforated sheet itself is trivial. It is a metal sheet with punched-out openings. In detail, however, precise and customised solutions are required to bring the advantages of perforated sheets to bear in practice. Decisive factors in the durability of perforated sheets are the quality of the thin sheet used as the base material, the perforation process itself and, in particular, the post-treatment of the punched hole patterns.
A perforated sheet with the standard Rv5-8 round perforation (hole width 5 mm, hole spacing 8 mm) has well over 18,000 punched holes per square metre. These have a cumulative edge length of around 280 metres per square metre of perforated sheet. For large-area applications, for example in façade construction or road sound insulation in road construction, 1000 kilometres of punched edge lengths are quickly reached.
In the case of hexagonally perforated metal components, such as tractor radiator grilles, which are optimised for air flow but still need to have a high protective effect, there are as many as 200,000 holes per square metre. The hexagonal perforation of Hv 6.0-6.7 allows a free cross-section of over 80 per cent. The webbing between the perforations is then only 0.7 millimetres wide. A free cross-section of this size makes it clear that the surface conditions here are completely different from those of the unperforated base material.
A globally recognised test method for measuring corrosion protection is the salt spray test (NSS) in accordance with DIN EN ISO 9227-0 and ASTM D1654-2008 method A. The test specimens must withstand a defined salt spray for a specified period of time without corroding in order to meet the requirements of the standard. A high-quality primer and a final paint or powder coating of the steel sheets alone are not enough to achieve this quality feature. The production and type of metal also have an influence.
The base material can be obtained from EMW Steel Service Centre, for example, one of the largest plant-independent steel service centres in Europe. According to its own information, EMW has an average of over 300,000 tonnes of thin sheet (0.25 to 16 millimetres) readily available in almost all marketable grades. The perforated sheets business area belonging to the SCHÄFER Group also benefits from this.
In the perforation process for thin sheet metal, metal stamps are pressed into the smooth sheet surface by machine. This creates the specific hole patterns of perforated sheets. Process-related burrs occur on the punch exit sides on the underside of the sheet.
After perforating, it is advisable to break the edges, depending on the application. This procedure is a suitable preparation for the next finishing steps, as implemented by SCHÄFER in production. Following mechanical finishing, chemical or electrochemical processes, among others, can provide permanent oxidation protection for perforated sheets.
Cathodic dip coating (CDC) is ahigh-quality finishing option that is often used in the automotive and commercial vehicle sector. After this work step, the surface coating is not yet UV-resistant. In order to achieve permanent corrosion protection and resistance to ultraviolet radiation, powder coating is an option alongside a number of other processes, such as painting.
But even with the coating, the challenges are not obvious at first glance. The greater the number of holes per unit area, the higher the demands on the coating process are. In concrete terms, this means that measures must be taken to prevent the perforations from clogging during coating. Too little material application, especially at the edges of the holes, is just as problematic.
To ensure permanent corrosion protection of perforated components, sufficient edge coverage must be guaranteed without restricting the function of the perforation. The perforations must remain completely open so as not to distort the free cross-section.
Phosphate coating is a common post-treatment. This coating is mainly used for components made of steel and iron. Elements made of zinc or other non-ferrous metals can also be given a phosphate coating. This process is used for corrosion protection, wear reduction, adhesion promotion and electrical insulation. The substrate with a phosphate coating is ideal for the final surface finish.
Zinc-magnesium-coated flat steels are exceptional and available from EMW along with many other steel grades. Zinc-magnesium is a hot-dip coating made of zinc, aluminium and magnesium. The proportions of magnesium and aluminium together can be up to eight per cent.
At cut edges or scratches, the alloy reacts by forming a solid top layer, which delays the reaction of oxygen and iron. In this respect, the surface can almost "heal itself" in the event of injuries. With additional coatings, for example painted surfaces, the infiltration of scratches is significantly reduced.
Compared to conventional zinc coatings, the zinc-magnesium alloy is up to three times more resistant. This advantage is particularly important in highly corrosive environments, especially in salt-heavy applications, for example in the offshore sector.
Perforated sheets with a zinc-magnesium coating can be provided by SCHÄFER and finished in the standard RAL colours at the factory. The long-term corrosion resistance has been tested and certified by independent third parties using a salt spray test.
Even first-class perforated steel sheets in grades DC01 to DC06 can withstand the extreme conditions of the salt spray test for over a thousand hours after a high-quality cathodic dip coating and powder coating, as confirmed by the AkzoNobel test laboratory in Arnsberg, in the Sauerland region of Germany, and the paint factory Gross & Perthun GmbH & Co. KG in Mannheim, Germany.
Making oxidation and complexity manageable
Oxidation is an ongoing issue for anyone who works with steel or processes metal parts. Estimates put the damage caused by oxidation at EUR 100 billion per year in Germany alone. By complying with a series of standards, statements can be made about the corrosion behaviour of metallic components.
Manufacturers of perforated thin sheets are faced with very specific challenges. Many influencing factors – the base material, manufacturing process, surface process, paint or powder manufacturer, and test procedures for quality assurance, etc. – must be taken into account in order to find durable solutions that meet the requirements.
The product range of specialised manufacturers is not only extremely diverse. They have application expertise and offer a wide range of processing services, including laser cutting, edging, notching, punching, degreasing, powder coating, painting and anodising. This enables the delivery of ready-to-install components just-in-time.
Major manufacturers such as SCHÄFER Perforated Metal have over 400 hole patterns immediately available. In an in-house design department, application specialists are responsible for the development and optimisation of customer-specific sample solutions – which makes the complexity manageable.
Variants of post-treatment of perforated sheets.zip