Future success depending on innovation
29 May 2007
Dr Hywel, Senior Researcher and Consultant at the Materials and Engineering Research Institute (MERI) at Sheffield Hallam University, looks at new Roxton Coated Nozzles from Metro Technologies
As SMT nozzles become smaller and the market becomes ever more competitive the challenges faced by manufacturers are ever growing. To compete on costs alone is not a viable option for most western European manufacturers. For them success in the future lies in quality and innovation. One such company that has been at the forefront of both is Metro Technologies Ltd. By working with the Materials & Engineering Research Institute (MERI) at Sheffield Hallam University they have pushed the frontiers of nozzle materials and design in order to compete and grow.
One of the developments that shows how Metro are taking SMT design forward is Roxton, a new and unique development for SMT nozzles. Roxton has been the fruit of a research project which began when Metro approached MERI for help and guidance on how to develop better nozzles. Together MERI and Metro explored the issues surrounding SMT nozzles and their manufacture. Key issues that were identified were; machining challenges for smaller and smaller nozzles, costs and properties of raw materials, quality of end product, colour and maintenance of colour, nozzle lifetime and blockages. MERI helped Metro identify and explore technologies that existed in other applications areas such as anti-wear coatings for racing car engines, and carried out feasibility studies to determine whether such technologies could also be used on SMT nozzles.
Many different approaches have been taken by those attempting to produce a better nozzle. This has primarily focused on making the nozzle from a harder material in an attempt to increase wear resistance. Nozzles made from hard tool steel, nozzles with PVD coatings, nozzles made from bulk ceramics such as zirconia and nozzles with ¡§diamond¡¨ tips are all available in various designs. All have increased hardness but all also have draw backs. Diamond tipped nozzles are not actually diamonds but a complex composite of diamond and metal and can be prohibitively expensive and prone to cracking or fracture. Zirconia nozzles are also expensive and are less tough than metallic materials and can be prone to fracture and chipping if abused. PVD coated parts can be exposed to significant heat during the coating process and require a rigorous cleaning procedure. It is also a line of site process that cannot coat down holes, especially the small holes needed in nozzles. Tool steel is hard to machine and relatively expensive.
In order to create a better nozzle but also avoid the pit-falls of the material systems mentioned above Metro Technologies have developed Roxton. Roxton is a black, hard, wear resistant surface treatment that can be applied to aluminium substrates. This allows aluminium, a material which would otherwise be too soft and have too low a wear resistance, to be used to manufacturer quality nozzles. The use of aluminium can reduce both raw material and machining costs compared to hard tool steels or bulk ceramic nozzles. It can achieve significantly longer lifetimes and also reduces overall weight putting less strain on machine heads.
Roxton is formed by a proprietary Electrophoretic Ceramic Forming (ECF) Technology. The Electrophoretic Forming utilizes the motion of electrically charged ceramic oxide powder particles in an electric field to produce highly dense layers. Due to close-packed structures of the layer this method enabled to produce low defect ceramic forming on complex surfaces. ECF of oxide powder is performed in highly loaded suspensions. The stability of the suspension is of critical importance for the resultant coating. Optimal suspension properties are determined and achieved via electrokinetic and electroacoustic measurements supplemented by phealogical investigations. ECF enables the deposit of uniform thickness layers of 10s of microns in several minutes.
The thickness of the surface layer is easily controlled by careful control of the processing parameters and is highly reproducible and as such tolerances can be easily maintained.
In essence the surface becomes alumina (Al2O3), which is well known for its hardness and wear resistant properties. However, unlike bulk ceramics, Roxton is not brittle as it is supported by the tough aluminium substrate. Alumina is also most commonly an off-white colour but Roxton has been developed to have a matt black appearance, a necessary requirement for many machines with automated vision system. Roxton has excellent adhesion strengths (50MPa) and it can easily reproduce the shape of the substrate, even at sharp corners and down holes where PVD or anodising can be problematic.
Scanning electron microscopy (SEM) was used as a key tool in this and other development projects. SEM provides a powerful and unique way of examining nozzles and its analysis capabilities aids the diagnosis of problems such as coating spalling, coating wear, nozzle blockage or nozzle fracture. It allows the examination of nozzles in unprecedented detail and can enhance quality control and provides stunning images especially of the detail of the smaller nozzles. It has been especially useful while developing Roxton to assess the quality and properties of the surface layer. Roxton has been examined using SEM to check its integrity (no pores or cracks), its thickness and consistency and its composition.
The hardness of Roxton has been measured at MERI using a micro-hardness technique with a load of 50g (HV0.05). Micro-hardness is necessary because Roxton is a thin coating (100ƒÝm) and thus we need to avoid the influence of the (soft) substrate. For comparison the aluminium substrate was also measured. The result shows that Roxton is over 10 times harder than the aluminium substrate, with Roxton HV0.05 = 1570 compared with the aluminium substrate hardness of HV0.05 = 145. Tool steels nominally have hardness value of HV30=900 and in fact Roxton is more akin to ceramics such as silicon nitride (Si3N4) in hardness.
Extended testing of Roxton nozzles has taken place with up to 3 million simulated touch downs being carried out with no appreciable wear or damage being observed to the nozzle as observed using SEM. Roxton nozzles have also been trialed in the field with over 18 months of operation resulting in a medium volume production environment with no failures and excellent reported performance.
Roxton is one of the ways in which Metro is working with MERI to push the envelope of nozzle materials and design. The company is also engaged in several research and development programs with the university exploring everything from the way air flow can be effected by nozzle design to innovative new heat treatment processes to improve nozzle properties.
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