April 14, 2015

3D printing: a new dimension for components

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3D printing is revolutionising manufacturing processes in industries ranging from construction to medical and Formula 1 motor racing. Rail is also benefiting from this Technology. However, as Kevin Smith discovers, the complex process of approving new materials for use in rolling stock components is preventing widespread adoption.

IN a dedicated facility at Parker Hannifin's Fluidic Manufacturing division in Milton Keynes, Britain, away from conventional machining and manufacturing activities, is a sector dedicated to the company's Additive Manufacturing (AM), or 3D printing, processes.

Here four machines are busy utilising three different AM techniques to produce components and products while supplementing four other machines that produce Parker's patented module moulding process.

Parker3D printing as a process has been around since the 1980s. However, it is only in recent years with home applications that it is has reached the mainstream consciousness; the 2012 news story from the United States that it was possible to print a working gun at home sent shockwaves around the world at what this technology could potentially offer.

Large multinational corporations such as Ford, Mercedes and Jaguar, as well as some universities were the first to tap into the technology's commercial potential in the mid-1990s, mainly as a means of producing prototypes and experimenting with its capabilities. Parker, was also one of the early pioneers, with its efforts led by Mr Paul Gray, Parker's manager for AM Technologies.

Gray, who previously worked as a design consultant and with universities pioneering the development of AM, joined the company in 1998 and convinced its managing director and chairman to invest in the technology.

"It appeared to me that the technique could be used for production and for manufacturing. I fell in love with it," Gray says. "At that time there was a movement to look at plastic pneumatic modules instead of metal as a way of reducing the weight of components and there were various ways of doing it. 3D printing combined with vacuum casting was initially a prototype technology but from this we developed a production process, which is a patented technology still used by Parker today."

Gray says the benefits of 3D printing include freedom of design which can be as "complex as you like" because there is not the restraint of traditional manufacturing techniques. Indeed as an additive rather than subtractive manufacturing process, there are often reduced cost implications for the manufacturer because rather than starting from a block of metal or plastic and taking material away through drilling, milling or turning to produce the final product, 3D printing only uses the material that will form the final product. "You are producing what you want, and not lots of waste material." Gray says.

He adds that while the obvious benefit of the process is to deliver a lightweight component, this is not necessarily at the expense of the final product's strength; instead it is dependent entirely on the process control and material used in manufacturing. Nylons, some metals and even carbon fibre are all suitable for AM and results have shown their performance is comparable or even better than some conventional materials. Many also meet the strict fire safety standards, such as UL94VO, and do not emit any harmful gases during combustion.

AM processes

Parker currently utilises three AM processes in Milton Keynes all of which compile layers of material to produce the final product.

Gray says these AM processes are best suited to producing small to medium batches of items because, depending on complexity, the cost of large volume production is often prohibitive. However, as the technology develops, mass production is conceivable; Shanghai-based WinSun Decoration Design recently unveiled a five-storey apartment building in Suzhou, China, that was made entirely using a 150m-long 3D printer. 3D printing has also been used to produce products ranging from shoes to medicine and even food like chocolate and crackers.

Parker's own activities actually began in the rail sector in an unsuccessful project for a Korean train in the early 2000s. Gray says that the project utilised the conventional twin plate method but the adhesive used between two separate parts was not up to task in operating conditions. However, he used this setback to springboard the new AM technologies into the company and turned a failure into a success. Since then the technology and in-house knowledge has advanced significantly, with Parker producing products and selling its AM process as a service for clients in the aviation, medical, and transport industries as well as for Formula 1 motor racing teams, where there is a strict emphasis on using lightweight components.

In rail, the company has produced thousands of components ranging from pantographs to door control systems, with some using conventional and others using AM derivative technologies. Increasingly its emphasis on AM in the rail sector is on reverse engineering for components used on aging rolling stock for example, grab rails and cab interiors for which the production tools to are no longer in use or the drawings and manuals are out of print.

Parker2However, adoption of plastic and 3D printed components beyond obsolescence applications to front line production of components for new rolling stock, or as spare or replacement parts for existing units, remains a long way off.

Gray says innovative processes like 3D printing face the "grandfather rights" challenge, whereby there is a prevailing attitude in the industry that things have always been done a certain way, they have a safety precedence and they work, so there is a reluctance to innovate.

He reflects on experiences with the aviation industry and a visit to a factory during the 1990s where this was apparent. "It was like going back to my apprenticeship," Gray says. "They were making things exactly as they have done for the past 50 years. They know it works, and it is understandable, because when you are putting a 200-tonne tube into the air carrying 200 passengers, you want to be able to have a good night's sleep when you go home."

"While health and safety has dramatically enhanced the regard for safety in the workplace, it has also developed a culture where it can be daunting for people developing systems that are safety-critical because of the hurdles they have to jump through to get somewhere. It is often so much easier just to stick to what you know."

Gray says that aviation has moved on since then and in some areas, particularly where there are substantial cost implications, the industry is adopting AM, albeit through a strenuous approvals process. He notes that many of the components on the new Boeing Dreamliner use AM technologies while Airbus is using Sintered metal parts on its new A350-XWB passenger jets.

In contrast rail is currently where the aviation industry was 15 years ago and at this stage he says no one appears ready or willing to take the risk to innovate.

"There is a slow pick-up on this because often it relates to a single component or spare part," he says. "This costs maybe £20,000 and because they know it's a proven technology, and that they might only want one and may only use it once over the next 20 years, the case for developing something new that uses different technologies or materials is very weak. However, when thinking about the long-term benefits, someone has to be bold and bite the bullet and go for it. Accountants will never see any sense in this but the longer we wait, the more detrimental it is to progress. The rail industry needs to get its head around this challenge."

Adopting a long-term outlook may be beyond the current generation of engineers and decision makers in the rail industry because of the perceived risks of deserting known practices. However, the arrival of a new generation, who would have been exposed to these techniques during their university training, offers the possibility of bridging this gap.

In the meantime, Gray says awareness of what is achievable must be increased. The current emphasis of rolling stock manufacturers to take weight out of their vehicles is a challenge made for AM. The hope for a company like Parker, which is leading this process in rail, is that one manufacturer makes the leap and others follow suit. This is certainly the preferred scenario. However, for Gray, past experience has shown that it sometimes takes an unforeseen event to change traditional thinking and deliver the paradigm shift required.

"In the Falklands War there was a requirement for a new refuelling probe," he says. "Under normal circumstances this would have taken the aviation industry months, even years to approve because of the requirement to go through the various regulations and obtain the right paperwork. In 1982 they did it in less than three weeks."

Parker's 3D printing processes

  •  Stereolithography (SLA): SLA employs a vat of liquid ultraviolet curable photopolymer resin and an ultraviolet laser to build a component one layer at a time. For each layer, the laser beam traces a cross section of the part pattern on the surface of the liquid resin with exposure to the ultraviolet laser curing and solidifying the pattern traced on the resin and joining it with the layer below. Following the completion of this process, the SLA's elevator platform descends by a distance equal to a single layer typically 0.05mm to 0.15mm and a resin-filled blade sweeps across the component's cross-section recoating it with fresh material ready for the process to be repeated until the complete 3D part is created. After this, the component is immersed in a chemical bath where excess resin is cleaned and is subsequently cured in an ultraviolet oven. The process utilises supporting structures identified during CAD which must be removed manually.
  •  Fused deposition modelling (FDM): FDM begins by processing an STL and mathematically slices and orientates the model for the build process. Again the machine builds the component in layers but this time extrudes small beads of thermoplastic. To do this a plastic filament or metal wire unwinds from a coil and supplies material to an extrusion nozzle which is heated to melt the material and can also switch the flow on or off. The nozzle is mobile and follows a tool-path controlled by a CAM software package to build the component bottom up, one layer at a time. Parker utilises an X-Y-Z rectilinear design for its FDM production processes.
  • Selective laser sintering (SLS): SLS uses a high-power laser to fuse small particles of plastic, metal, ceramic, or glass powders into a three-dimensional mass. The laser selectively fuses the powder by scanning cross sections generated from a 3D digital description of the component. Like SLA, after each cross section is scanned the powder bed is lowered by one layer and a new layer of material is applied on top, which is repeated until the part is complete. SLS machines use a pulsed laser due to laser power determining the finished part's density. The machine also preheats the bulk powder in the powder bed just below its melting point to ease the process for the laser. Unlike SLA and FDM, SLS does not require support structures because the part is surrounded by un-sintered powder at all times, which enables construction of complex geometrics.

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