More than Scratching the Surface
23 August 2011
Nigel Willcock. Abrasive Systems Division, 3M United Kingdom plc explains the role of abrasives in metal surface cleaning and finishing.
Removing contamination from metal surfaces is integral to the quality of subsequent processes, such as phosphating, painting, powder coating or welding, and so ultimately to the appearance of the final product.Oil, grease, rust, sealants, adhesives, paints, coatings and dust are some of the most common forms of contamination which can be present, all of which can create their own problems if not removed.
If left unchecked, contamination can also build up within the surface layer, due to the migration of various compounds and elements to the surface during manufacture. This of course can lead to corrosion and other problems, with the potential to adversely affect service life.
Even microscopic burrs, if not removed, can allow the build-up of dirt and moisture, negating the possibility for a high-quality finish.
The problems of not using the right products
Making the wrong choice of cleaning and finishing product can result in inconsistent finishes, excessive stock removal and serious surface scratching. As a result the final finishing stage may end up being more concerned with rectification – reversing the damage caused by over-aggressive abrasives and human error at earlier stages. Similarly, using products whose finish is inconsistent, such as polishing mops and low-quality belts, can deliver a poor quality finish which is unreceptive to further treatments.
Meanwhile, coated abrasives, wire brushes, wheels and some other finishing tools can, if used incorrectly, introduce harmful tensile stresses into the metal surface – which may lead to cracking during subsequent forming operations or fatigue. However, the scratch patterns introduced by modern abrasives – in particular non-woven products – can induce beneficial competitive stresses which reverse the effects of tensile stress.
The right type of abrasive cleaning can enhance the properties of the surface in other ways too, for example by increasing the true surface area of the metal, which in turn improves the adhesion of subsequently applied coatings.
Meanwhile, in the case of aluminium alloys, abrasives have been shown in testing to deliver a thinner oxide layer than is the case with chemical etching -resulting in reduced electrical resistance, giving improved weldability. Abrasive treatment has also been shown to be more effective than chemical etching as a pre-treatment to abrasive bonding, with greater bonding strength achievable.
Developments in abrasive technology
The conformability of modern abrasives for surface treatment and refinement means they offer a controlled cutting edge, allowing them to more closely follow the contours of the metal surface, thereby removing only contaminants on the surface with a negligible amount of substrate material.
The selection of abrasive product will ultimately depend on which solution can best remove any contaminants without damage to the underlying surface.
For example, non-woven abrasives are particularly well adapted to removing the surface carbon layer from steel – a layer which, if left in place, can inhibit reactivity to chemical anti-corrosion treatments and other processes. With steel, these abrasives are also able to produce surfaces which are uniform and similar in their chemical composition. Uniform cleanliness of the abraded surface will also enhance the reactivity of the steel to phosphate treatments applied to improve corrosion resistance.
Where aggressive cleaning is required, a fibre disc angle grinder fitted with a range of discs and a suitable back-up pad to support the disc offers a portable and versatile solution. Fibre discs offer the best below-surface stock removal, and non-woven surface conditioning discs are best for light duty cleaning applications, where a better finish is required. These products offer a reduced risk of gouging or undercutting, meaning less re-working is needed.
Where greater conformability is needed, a modern alternative to wire brushes which can be fitted to an angle grinder is a one-piece moulded disc, impregnated with mineral, which gives a more consistent cleaning action than wire brushes throughout the product’s life. This product’s construction also eliminates the risk of flying wires associated with wire brushes, making it far safer to use, while minimal pressure is required.
For larger workpieces where the tool is brought to the job, a straight grinder system can also be used, with discs selected depending on whether the need is for surface or sub-surface cleaning.
For detailed work, mini grinder systems offer a versatile solution, allowing greater access to the workpiece compared with conventional disc attachments. A comprehensive variety of mini discs is available, from specialist coating removal discs to unitised discs designed for tasks such as blue weld removal, where they can replace hazardous acid-based systems. Unitised discs can also deliver a bright satin finish.
An alternative for processing small parts, particularly where volumes are higher, is a backstand or linisher. As the part is taken to the belt, these systems offer greater control, a particular benefit when complex parts are involved. Once again, the choice of consumable will depend on the type of treatment needed; cloth belts will remove heavy scale and pitting, while non-woven surface conditioning belts offer a soft edge cleaning action without altering the dimension of the base metal. For hard to reach areas, there are several options, the main ones being a file belt tool with a range of contact arms and a portable flexible shaft machine. The latter can be fitted with a range of discs, brushes and wheels – heavy duty discs for rapid cleaning; radial brushes, which are ideal for weld blueing removal due to their conformability; and unitised wheels for a bright satin finish.
Finally, for cleaning flat accessible areas, specialist systems are available, which can be fitted with either brushes or belts to provide a complete solution for cleaning, graining and finishing.
From these examples, it can be clearly seen that the sheer range and versatility of modern abrasive tools and consumables have the potential to consign more cumbersome, inaccurate and potentially dangerous processes to the past. Given the productivity and quality advantages achievable with modern abrasives, cleaning and finishing has moved a long way from its previous role in rectification to become a key value-adding process stage.
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Products manufactured in the UK include coated abrasives, occupational health and environmental safety equipment, adhesive tapes, industrial microbiology products, drug delivery systems, high-performance coatings, secure documents and passport scanners. .For more information visit www.3M.co.uk/abrasives