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Design freedom at last?

Greg Blackman investigates the process used in laser sintering technology - and explains how it has advanced

Design freedom is a wonderful thing for parts manufacturers and end users alike. If you want to produce and test a highly complex, fullyformed, bespoke part – and if you have a 3D CAD model of it – then, within reason, it can be done. Laser sintering, an additive manufacturing technique that builds up a 3D structure by selectively fusing layers of powdered material with a laser, is paving the way for producing part geometries that, using traditional manufacturing methods – turning, drilling, milling, for example – would just not be possible. Virtually anything goes, with unique parts currently being developed within Formula One racing and aerospace industries to name just two.

The technology is being increasingly utilised to engineer medical implants and it is already widely used in the dental industry, with up to 120-150 customised dental caps being manufactured for each production run. Laser sintering lends itself particularly well to this application, as the constituent parts can be made-to-measure for individuals.

The process, in theory, is relatively simple. Take one 3D CAD model, that shows the details of the design of the part, and split this up into constituent layers, or ‘slices’. Next, deposit an initial layer of powder onto a Z axis movable table. The individual particles of powder are then selectively fused by a scanning laser according to the segmented CAD design, and the table is dropped down by a minute amount for deposition and fusion of the next slice. Upon completion, excess powder is then vacuumed away leaving only the fused material. In this way, an intricate 3D structure can be built up layer by layer.

Additive manufacturing or rapid prototyping, under which the laser sintering techniques fall, has been around since the late 1980s, when stereolithography was being developed. Here an object is built up layer by layer from a vat of liquid resin, using a laser to selectively solidify a pattern in the resin according to the design that has been input. Since then, the technology has come a long way and includes many variations on the additive manufacturing theme.

Laser sintering is a generalised term for a family of processes dealing with fusing powder-like materials with a laser to create a 3D object. The terminology used for classifying these processes depends on the materials used, the sintering method and the company producing the equipment.

Selective laser melting (SLM) and selective laser sintering (SLS) are two such processes, with subtle differences between them. ‘The fundamental difference is that SLM fully melts the substance, whereas SLS does not, in the majority of cases,’ explains Dr Chris Sutcliffe of the Manufacturing Science and Engineering Research Centre (MSERC) at the University of Liverpool. This full melting allows the production of metallic components from standard metals using SLM, while only polymers and some specially-developed metallic systems are possible using SLS, usually via a further process step.

There are also variations in the names given to the process depending on the company supplying the equipment: ES Technology has LaserCusing; EOS uses Direct Metal Laser Sintering (DMLS); and MCP/MTT Group uses SLM. Each of these processes though is essentially the same.

 

Fine aerospace parts, such as these produced by 3T RPD, are more accurately and easily processed via direct metal laser sintering.

Layer thickness varies depending on machine, but generally ranges from 20-50μm. SLM parts are accurate to approximately 0.1mm in 100mm and the smaller the build volume, the greater the accuracy. Fibre lasers allow for an accurate melt, producing a 99.7-99.8 per cent dense structure.

There are a wide variety of metals that can be processed in this way, including stainless steel, titanium, aluminium and inconel – a tough, nickel-based alloy that is commonly used for automotive parts. Cobalt chrome is also used, often in medical applications.

Graham Bennett, manager of CRDM’s Wycombe Sands rapid manufacturing production plant in Buckinghamshire, UK, which carries out DMLS, notes that recent developments in materials and laser technology have made it possible to manufacture parts in commonly recognisable metals for production applications. The increased use of fibre lasers, which, compared to CO2 lasers, have a smaller spot size (around 50μm) has allowed a fully melted part to be produced, as opposed to those manufactured using a CO2 laser that only partially melts the metallic powder. This, coupled with tighter atmospheric controls, allows metals such as aluminium, titanium, stainless steel, and cobalt chrome to be processed using DMLS.

The chemistry of the process, up until recently, was such that an amalgam of materials was necessary to overcome issues inherent in the sintering process, including stresses from heating and cooling, as well as from oxide brittlisation. Metal oxides, which form if oxygen is present during sintering, create high stress areas that weaken the component, and any alloys used would have to contain a suitable metal for mopping up excess oxygen in the system. Advances in atmospheric control and the development of high-powered fibre lasers that reduce stresses caused by heating and cooling, mean that amalgams of metals, which are costly and often unsuitable for manufacturing parts, can be avoided.

The production of sintered parts is still expensive, indicates Bennett. The process is costly, as are the materials, which leads to very specific applications for laser sintering. However, the range of geometries available using DMLS are practically unlimited. ‘You have entire design freedom on the machine,’ explains Bennett, going on to state that where very complex parts are machined out of multiple component parts, it can be less expensive to produce the complete part through DMLS.

Geometries that contain sharp internal corners benefit from production by DMLS. Traditionally, a block of metal would have to be drilled and hollowed out using computer numerically controlled (CNC) milling, which selectively removes material. Any sharp internal corners would have to be machined by spark erosion, a method of wearing away metals conductive to electricity. ‘This is a complex procedure which usually adds expense and significant amounts of time to the production of parts and tool dies,’ says Bennett. DMLS allows this type of geometry to be produced much more rapidly and with greater accuracy.

SPI Lasers, based in Southampton, UK, has been supplying fibre lasers to Sutcliffe’s team at the University of Liverpool for their work using SLM in conjunction with MCP/MTT Group. ‘SLM is one of the applications where fibre laser systems prove to be particularly advantageous,’ says Andy Appleyard, product line manager for cutting and welding systems at SPI Lasers.

‘The single-mode beam quality of fibre lasers ensures the beam can be focused down to a very small spot size,’ explains Appleyard, which is particularly suited to SLM as it allows for an accurate melt. This, coupled with the stability of fibre lasers, ensures a uniformly constructed part with high detail and resolution. Furthermore, fibre lasers can be directly modulated very precisely. This ability to be switched on and off with high stability and at high speed allows for a clean surface profile to be produced. Laser systems utilised for SLM operate at near infrared wavelengths, which are commonly used in metal processing due to the well-known absorption properties at these wavelengths.

Sutcliffe and his team are currently using SLM to produce unique porous materials made up of a fine, complex lattice that can be used in a number of applications, from increasing the surface area for a chemical reaction to providing a growth stimulus for bone regeneration. The University of Liverpool was the first in the UK to install an SLM system some five years ago and now runs three machines making MSERC the largest research centre of its kind in the UK.

 

The precision required in medical implants makes the process of selective laser melting ideal for this market. Image courtesy of SPI Lasers.

While there is a relatively healthy market for producing bespoke components using SLM, Sutcliffe is also looking to produce standard components that are used regularly within the medical industry. ‘There is a larger market for standard components,’ says Sutcliffe, suggesting that SLM could become increasingly used in the production of these parts.

The technique is also important for manufacturing tooling inserts containing conformal cooling channels. ES Technology, an Oxfordshire-based supplier of laser engraving, etching and marking systems, is the UK distributor for Concept Laser’s selective laser melting technique, dubbed LaserCusing. Concept Laser is a subsidiary company of the Hofmann Innovation Group, a manufacturer of tools for the plastics processing industry, and was created out of a necessity to service Hofmann’s in-house injection mould production unit.

‘LaserCusing was born out of a production need within its [Hofmann’s] own tool room,’ explains Colin Cater, product sales manager for ES Technology.

‘LaserCusing is a room-temperature process,’ says Cater, going on to explain that because the laser fusing occurs only locally, the object is cool enough to handle directly after processing.

‘It [LaserCusing] has really split in two directions – tools and parts,’ states Cater. The conformal cooling channels, which, traditionally, were drilled through as linear holes, can now be designed in any shape to draw heat away from the mould as effectively as possible. Rapid cooling of the cast assembly reduces the production time involved in the mass production of injection-moulded parts by 30-50 per cent, suggests Cater.

Whilst this technology has certainly carved out a niche for itself, Cater feels there is still a way to go in its development. Rigorous testing on additive manufacturing procedures is necessary to ensure the parts produced are manufactured to the same standard as those made through traditional machining methods. ‘It’s like bringing a new drug into the market – there are lots of stages and processes to go through before the product is released,’ Cater says.

The level of detail and timescale of tests carried out will depend on what the part is ultimately used for, with those utilised in high-risk situations, such as a critical piece of an F1 car or aircraft, or as a medical implant, being subjected to the most rigorous testing. Cater cites five to 10 years as a timeframe for testing new products – but in the case of medical implants, this could be up to 15 years.

Sutcliffe makes a similar point, although the timescales cited do differ. He feels that currently there is a lack of standardisation for producing components using these additive manufacturing methods. Regulations need to be put in place to verify the process and ensure that standards are adhered to. Sutcliffe has ‘no doubts that this will happen’, but believes that timescales for setting up these regulations will vary depending on the industry. ‘Dental caps are already being produced and put into the market place,’ he says, and predicts that implantable medical devices could come onto the market in the next 12 months.

Bennett views DMLS as a process that will remain in a relatively niche market for part production, at least in the near future. He feels that CNC milling is at an acceptable standard to satisfy a lot of part production. ‘The software and hardware required for CNC milling has improved greatly during the past few years, and is still more economical for many applications,’ says Bennett. ‘Sales [for DMLS] will continue to grow, but it will still be used in specialist applications until machine and material costs benefit from the economies of scale.’

Stuart Offer, sales manager at 3T RPD, which carries out both SLS and DMLS at its Newbury, UK site, shares similar views. His outlook is that components produced by more established manufacturing methods, such as CNC milling, will not be supplanted by DMLS as the two are different in their setup and operation. ‘DMLS is not looking to replace CNC milling,’ he says. ‘A lot of products are designed for CNC milling, which is a completely different process [to DMLS].’ DMLS will come to be regarded as a complementary process to CNC milling, he feels.

Demonstrating the benefits for the end user are important if the technology is to be more widely used, explains Bennett. Increases in building speed, as well as the range of metals that can be processed, will also drive the technology. In addition, customers will need process monitoring and quality control checks common to serial production systems, rather than those used for prototype applications.

One limitation of the process is the size of the SLM chamber. Currently, the largest SLM machine available has a chamber size of 0.25 x 0.25 x 0.25m, but the latest developments in the industry will see a 0.5 x 0.5 x 0.5m machine installed at the University of Liverpool soon.

Whilst almost any geometry can be produced within that space, the size of the chamber does limit the maximum size of component. Sutcliffe believes that increases in the physical dimensions of commercially available equipment, along with machine speeds, will allow further developments in the capability of this process.