Nadeem Rizvi, managing director, Laser Micromachining
Modern-day designers of micro-products are able to choose from a vast array of manufacturing technologies to bring their designs to life. Methods include mechanical machining, chemical etching, diamond turning, wire erosion, stamping, forming, moulding and laser machining. 3D printing or additive manufacturing (AM) can now be added to this extensive palette as the latest and fastest-growing technology to revolutionise many industries.
The term additive manufacturing can be used to cover a wide range of concepts but all essentially use some energy input (electrical, light, thermal) to convert, fuse, melt or chemically change a material so that the final desired shape is built up layer by layer. Due to this bottom-up (additive) approach— starting with no material and adding thin layers—AM can produce complete three-dimensional parts with complex inner geometries such as voids or chambers, something that no other top-down machining technique can match. It is this powerful ability to produce any 3D design that is driving the explosive adoption of AM in most industries.
The most commonly-used industrial materials—namely polymers and metals—can be used in AM techniques, and research into extending AM to accommodate other materials is being actively pursued. AM with ceramics and glass has recently been demonstrated and it cannot be long before any design will be able to be produced in almost any material or combinations of materials. Alongside material coverage, great advances are also being made into increasing the quality, reproducibility and feature resolution of AM-produced parts and these developments are set to be key enablers for using AM even more widely in many micro-manufacturing applications.
Polymers are the ubiquitous material for automotive, biotech, medical, optics and other micro-products, with thousands of types of plastic micro-components being produced in their millions each year. This is done in a highly cost-effective fashion, most commonly using a mass-replication method such as injection moulding or hot embossing. Although micron-level precision is possible with such high-volume replicated parts, the path to arrive at the mass manufacturing phase can often be a long and expensive one. The arrival of polymer AM has opened up interesting options for micro-technology applications, which might make the product development route less complex and burdensome.
Injection moulding and similar plastic replication methods require some form of mould master from which the replica parts are produced. Once such a mould tool exists, it can be used to produce millions of copies of the part quickly (often thousands of parts a minute) at a very low unit cost, so injection moulding is a very cost-effective method for mass micro-manufacturing. Since there is a lot of value in this mould master, it is always an expensive item and can cost many (tens of) thousands of dollars or euros each. Hence it is only worth investing in a mould master once the final design has been fixed, i.e. at the end of the developmental cycle. It does not make economic sense to waste money in producing mould masters while different designs are being investigated during the product development phase.
Take a typical example of a micro-fluidic device; it is made of a polymer, is the size of a large postage stamp and it contains some micro-features such as channels, mixers, junctions and feedthrough holes. Eventually the device will be injection moulded but during the development phase the layout of the device's features have to be investigated; for example, different sizes and shapes of the channels might need to be compared. It is often important at this stage of testing that as few variables as possible are introduced so that only the single design change being tested affects the results. This might mean, for instance, that the channels’ width and shape are maintained with micron precision while different channel depths are tested (also to micron-level tolerances).
Prior to AM, the test devices would usually have been mechanically machined each time to form the overall ‘macro’ part, with the ‘micro’ features (channels, junctions, etc.) being added using laser machining. Not only is the mechanical machining step a laborious one and hence relatively time-consuming and costly, but the ultimate repeatability and precision may not always be at the required micron level. This lack of consistency in the macro part production is a potential source of variability, but the mechanical step also prolongs the development cycle—if a change in the macro part is made, another set of parts has to be produced using the same laborious process.
Now AM can be used to produce macro parts quickly, reliably and simply, the critical micro features can be added using laser machining (Figure 1) and then the real R&D effort can be concentrated on testing the final (highly exact) parts.
Figure 1: Additively manufactured polymer fluidic device bases (left) and laser-machined channels (right).
Starting with identical AM macro parts means the micron-level changes made using laser machining can be tested with a high degree of confidence that only the effects of the single change (channel depth in this example) is being considered. Such testing is also crucial in defining the final manufacturing tolerances—if channel depth variations of 10 percent, say, do not produce any performance difference then that gives an idea of how tightly the depths of the final parts need to be controlled. Defining production and measurement tolerances are important factors in the high-volume manufacturing phase and affect the production yields and costs significantly.
At present, polymer AM parts cannot match the quality, control and repeatability that polymer laser-machined parts can deliver, at least where micron-tolerance features are concerned (Figure 2). There is no doubt that AM will become comparable in these regards soon enough, but for now a definite distinction exists. Using combination processing can, however, bring together the best of both; the simplicity and speed of AM, the precision and repeatability of laser machining.
Figure 2: An additively manufactured polymer part, with the central laser-machined region of visibly higher quality.
This hybrid manufacturing is shortening product development cycles and also making them more cost-effective. In an increasingly complex and competitive manufacturing landscape where functionality, materials and features all need to be optimised, any leverage offered by novel methods is a big advantage and using more than one technology can bring about real benefits.
Nadeem Rizvi
Laser Micromachining
Biography
Nadeem Rizvi is the managing director of Laser Micromachining and has been active in the field of laser micro-manufacturing for 25 years. He holds a PhD in laser physics from Imperial College London and has previously worked at the Rutherford Appleton Laboratory, the Institute of Optics in Madrid, the ORC Southampton University and Exitech.