Rapid prototyping in the fast lane

For the purposes of engineering-based manufacture, rapid prototyping (RP) is essentially the automatic construction of physical objects using additive manufacturing technology. Put plain and simple, additive manufacturing is now viewed as the first major challenger to the three traditional ways of making things that have been used since ancient times: ‘subtractive’ (carving material away, as in flint tool making and modern machining); ‘moulding’ (pottery, casting and moulding); and ‘forming’ (bending, forging and stamping).

Today, the breadth and capability of RP techniques is little short of mind boggling: everything from laminated object manufacturing (LOM) and 3D printing (3DP), through to selective laser sintering (SLS), fused deposition modelling (FDM), stereolithography (SLA) and electron beam melting (EBM) are now widely accepted techniques. Each technology competes for market share depending on factors such as materials, strength requirements, quantities and of course, cost.

The main objective of automotive prototyping is to quickly learn how a new automotive product behaves in its natural working environment, before transferring the prototype to the production line. As a result, selecting the right RP process for the job is vital. And here, Tier one automotive supplier, Dana Corporation, has recently demonstrated best practice.

“By the time a conventional prototype can be made using machining or casting, it’s possible to invest large amounts of time and money in a design that doesn’t work,” says Bruce Vanisacker, designer for Dana’s rapid prototyping/CAE services. “It’s often difficult to determine, just by looking at one of our complex assemblies on a computer screen, whether or not it meets key form and fit requirements.” Seeking a better development method, Dana investigated several RP technology options for its products, which include under-the-hood filtration systems, cooling systems, differential cases, carriers and housings.

“We took a cross-section of parts from all of our divisions and sent them to five rapid prototyping system suppliers,” says Vanisacker. “We decided that the Fortus system from Stratasys produced the best parts because it could produce highly-accurate and much stronger parts in multiple colours. FDM is also very easy to use and economical, enabling us to produce accurate and specialist functional prototypes in a few days. A physical part gives everyone the opportunity to hold and touch and feel the part and determine exactly where we stand.”

Dana has found that its Fortus system excels at making accurate rapid prototypes of complicated assemblies like clutch assemblies. In fact, FDM prototypes are so strong that in some cases they even use the prototypes to help evaluate the performance of the part.

FDM works on an additive principle by laying down material in layers. A plastic filament or metal wire is unwound from a coil and supplies material to an extrusion nozzle which can turn the flow on and off. The nozzle is heated to melt the material and can be moved in both horizontal and vertical directions by CNC, controlled directly by CAM software. The model or part is produced by extruding small beads of thermoplastic material to form layers as the material hardens immediately after extrusion from the nozzle.

At the BMW plant in Regensburg, Germany, FDM is an important component in vehicle design prototyping, where the plant’s department of jigs and fixtures uses a Stratasys 3D production system to build hand tools for vehicle assembly and testing.

For hand-held devices used on the assembly line, engineers have discovered that there are great advantages that arise from the design freedom that FDM offers. In one example, BMW reduced the weight of a device by 72% with a sparse-fill build technique. Replacing the solid core with internal ribs cut 1.3kg from the device. “This may not seem like much, but when a worker uses the tool hundreds of times in a shift, it makes a big difference,” says engineer Günter Schmid.

Another advantage is improved functionality. Since the additive process can easily produce organic shapes that sweep and flow, the tool designers can maximise performance while improving handling characteristics. An example is a tool created for attaching bumper supports, which features a convoluted tube that bends around obstructions and places fixture magnets exactly where needed.

Pushing the envelope

Just how far can we go with RP techniques such as FDM – just for small parts, right? Not so. In a major advance, the Stratasys development partnership with Winnipeg engineering group, Kor Ecologic will see it create one of the world’s most fuel-efficient and environmentally-friendly vehicles. Code-named Urbee, it is the first car ever to have its entire body produced by additive manufacturing processes – the first full-scale prototype appeared at the SEMA exhibition in Las Vegas in November last year.

“Our goal in designing it was to be as ‘green’ as possible throughout the design and manufacturing processes.” says Jim Kor, President and Chief Technology Officer at Kor Ecologic. “FDM technology from Stratasys has been central to meeting that objective. It lets us eliminate tooling, machining and handwork, and it brings incredible efficiency when a design change is needed. If you can get to a pilot run without any tooling, you have advantages.”

Urbee is the first prototype car ever to have its entire body 3D-printed with an additive process. All exterior components – including the glass panel prototypes – were created using Dimension 3D printers and Fortus 3D production systems at Stratasys’ digital manufacturing service – RedEye on Demand.

Common FDM materials include ABS, polycarbonates, polycaprolactone, polyphenylsulfones and waxes, and this matches the trend of increasing numbers of car parts.

However, engine components are currently not common as they have to withstand high temperature and mechanical stress. However, this is changing and technology experts such as ProtoCAM have been using thermoset phenolic materials to make engine parts for a number of years as the material is strong and has excellent heat resistance.

ProtoCAM has particular expertise in SLA, an additive manufacturing process that uses a vat of liquid UV-curable photopolymer (resin) and a UV laser to build parts a layer at a time. On each layer, the laser beam traces a part crosssection pattern on the surface of the liquid resin. Exposure to the UV laser light cures or solidifies the pattern traced on the resin and it adheres to the layer below. The company says its automotive customers request parts that range from engine parts and body/trim components, through to instrument panels and lenses.

In a high-profile case study of SLA at work in the automotive sector, Porsche used a transparent SLA model of the 911 GTI transmission housing to visually study oil flow. Among the latest developments in this area, Rapid Product Development Group (RPDG) has announced the addition of an SLA resin from DSM Somos. ‘Somos NeXT’ is an advanced resin that is said to produce parts demonstrating a combination of stiffness and toughness that characterises thermoplastics, yet with all of the high feature detail, dimensional accuracy and aesthetics offered by SLA. It is already attracting the attention of the automotive sector.

“A major auto manufacturer accepted our offer to build them a free sample out of NeXT resin to test its heat deflection and impact strength,” says RPDG’s Business Development Manager, Steve Forney. “They chose to have us build a rear window wiper arm assembly.” The length of the main portion of the arm was 26”, which was longer than RPDG’s vat size so the part had to be fabricated/joined from two pieces.

“The part finished beautifully and was installed on an actual working model of the vehicle: the resin did not deflect after sitting in the summer sun and held up to the activation of the wiper without distorting. The client is now considering using NeXT resin to replace the traditional composite frames previously used in this kind of testing,” says Forney.

Material world

So what of SLS? Well, compared to other methods of additive manufacturing, this process can produce parts from a relatively wide range of commercially available powder materials. These include: polymers such as nylon, (neat, glass-filled or with other fillers) or polystyrene; metals including steel, titanium and alloy mixtures; composites; and green sand.

Case studies in the automotive sector are diverse and many have their roots in specialist projects. For example, a plastic laser-sintering machine from EOS has solved the problem of how to extract maximum thrust from batterypowered motors driving the front wheels of a Westfield hybrid sports car. The project is being funded jointly by Warwick Innovative Manufacturing Research Centre and Potenza Technology, a specialist in hybrid and electric vehicle systems.

To maximise both the power and the time that the electric drive is available to the front wheels, it is important to prevent excessive heating of the 396 lithium ion-phosphate cells that supply the axial-flux motors. This was a challenge, as the only space available for the batteries was in the enclosure under the driver’s seat.

At the outset, the cells were stored in rows, side by side, in two battery boxes machined from solid plastic, one for each motor. However, an alternative was suggested by engineers from EOS. Instead of open battery boxes, two arrays of 11 plastic modules were designed so that each cell could be retained in its own cylindrical cavity, 18 per module.

Rapid manufacturing in an EOSINT P-series lasersintering machine, driven by data derived from slices taken through the CAD model of the new battery box, was used to produce the modules from layers of fused EOS PA 2200 polyamide powder. The flexibility of design afforded by SLS allowed air channels to be created between each cell cylinder and between adjacent modules as they are bolted together. This would not have been feasible by any other production method. The result is significantly enhanced cooling of the 3.6V cells due to air flowing around them while current of up to 100 Amps is being drawn, boosting the performance of the batteries and hence of the electric motors.

From a standing start, the Westfield Hybrid in four-wheel drive mode achieves 60 mph in 3.5 seconds, compared with 5.5 seconds when only the rear wheels are driven. The bottom line is that an £8,000 adaptation of a £25,000 factory-built racing car delivers similar performance, albeit in short bursts, to that of top-end sports cars costing over £100,000, and with considerably reduced environmental impact.

“The project has led to the robust design and manufacture of reliable battery solutions which could potentially be used in many platforms,” says Paul Faithfull, Managing Director of Potenza Technology.

University of Warwick WMG principal fellow, Dr. Steve Maggs, adds: “Following the success of this prototype, there is potential for the system to be rolled out for production and sale by Westfield Sportscars. In fact, the system that has been developed is pioneering in that it can be integrated into any motor vehicle and therefore has numerous exciting potential applications in the wider automotive industry.”

From a metals perspective, new developments in this technology area include the recent introduction of MTT’s SLM125 and SLM250 machines. Both feature innovations such as inert powder handling, safe change filter system and vacuum assisted inert atmosphere, and both are approved for use with titanium, stainless steel and cobalt chrome.

Next up for MTT is the launch of the larger SLM500, which is currently in development having passed through the feasibility study stage. Having a half-metre square build platform would once have been the stuff of dreams — even today the technical challenges are great. Currently in testing using a single 400W laser system, the development team is eventually looking to ship the largest in the SLM range with a 1kW laser.

At the other end of the scale spectrum, Concept Laser’s soon-to-be-launched M1Lab machine is designed with smaller component sizes in mind. It will provide the capability to build fully dense metallic parts in stainless steel, cobalt chrome and precious metals, direct from CAD.

Past masters

Despite the dominance of additive manufacturing in RP, conventional production methods such as machining and moulding are fighting back, albeit not in their traditional forms. This is because systems such as 3D printing machines are generally unable to produce the component in the actual material designated for use in production and in certain instances, systems and technologies that can require secondary machining operations to achieve the required surface finish or remove supporting structures.

Able to testify to this fact is Norwich-based Scion-Sprays Ltd, which recently turned to the RP services of Proto Labs to help it produce fully functioning prototypes for an affordable, engine management system. With a 4 million order up for grabs, there was no margin for error.

“Our first commercial product is a fuel injection system specifically designed for a European manufacturer of light motorcycles and scooters,” says Richard Hoolahan, Manufacturing Manager. “This new system will cut emissions of HC+NOx (the cause of photochemical air pollution) by around 35%, and CO2 emissions by over 80% compared with a standard engine. It will also save 30% on fuel.”

Founder Jeff Allen came up with the idea of a small, low cost ‘constant volume displacement pump’ to control fuel flow rates. The pump proved to have greater market potential than the original research because of its efficient, clean-burn credentials and its simplicity. So with backing from an ‘angel’ investor, who saw both the business and the environmental benefits of the technology, the team set about developing Scion-Sprays’ engine management system called Pulse Count Injection (PCI), and with the help of RP services Firstcut and Protomold from Proto Labs, it is now moving out of development and into production.

“I used different RP services to produce less critical components,” says Hoolahan, “but chose Proto Labs for the throttle crank and the stepper motor arm because there could be no compromises in the mechanical qualities of those parts: and the production method and the materials would be very similar to what we would use in the final version.”

Hoolahan decided to use Proto Labs’ Protomold injection moulding service to produce the stepper motor arm from glass-reinforced nylon. However, he chose Firstcut’s machining service to make the throttle crank from the same material, partly because he wanted to compare the processes and partly because creating a mould for the throttle crank would not have been cost effective.

The machined parts were delivered within three days of Hoolahan accepting the quote. The moulded parts took slightly longer because of the tooling required. Nevertheless, they were delivered in a matter of days. As a result, the company secured its first commercial order from a European motorcycle maker worth €4 million over three years.

One new RP development in the technology area of machining is the Cybaman digital manufacturing system.

At the heart of this six-axis machine is a patented three-axis workpiece manipulation system invented by the company’s founder Joe McLean. This, combined with three Cartesian axes enables components to be manipulated in 3D around the process tool. Imagine machining a golf ball on a tee and it will provide some indication of capability levels.

The innovative reference datum within the machine, created at the intersection of its rotary axes, provides a real world 3D datum that emulates the datum point in the virtual CAD world. The machine’s software, which runs within a CAD package and forms the front end of the manufacturing system, is used to manipulate the workpiece in 3D simply by moving its virtual model in CAD in either a point to point mode or continuous 3D path presentation sequences.

And here is the twist – although the initial concept for the Cybaman digital manufacturing system centred on subtractive machining methods, more recent developments include the integration of laser technology and laser direct metal deposition.

This system, among others, looks set to take advantage of the growing acceptance of the latest RP methods within the automotive sector.