In 1941, Arthur M Young demonstrated a model helicopter flying on a tether while working for Bell Aircraft Corporation and just five years later, Bell Helicopter (www.bellhelicopter.com) received the first-ever certification for a commercial helicopter. The Texas company has now made and sold more than 35,000 of the aircraft worldwide.
For some years, the company has used additive manufacturing (AM), otherwise known as 3D printing, to produce prototype components but wanted to use the technology to build functional parts. It turned to nearby AM bureau, Harvest Technologies (www.harvest-tech.com), which uses more than 40 AM machines, to provide the expertise.
Before production could begin, Bell Helicopter and Harvest needed to prove out the processing capabilities of the latter’s EOSINT P 730 plastic laser-sintering machine from EOS (www.eos.info) that would be used to make the helicopter parts and to certify the platform for use in the aerospace industry. Heat distribution, powder degradation, dimensional accuracy, repeatability, component quality and performance, and the economics of manufacture were all examined.
Elliott Schulte, Engineer III at Bell Helicopter said, “We characterised the mechanical properties of each additively manufactured build so that we could confirm that the EOS system met our specification requirements and produced the same quality product each time.
“The systematic testing was done with a number of different materials and across a series of individual builds to establish that EOS technology was robust and highly repeatable.”
Subsequently, Bell Helicopter and Harvest began the meticulous process of manufacturing aerospace hardware, taking advantage of the freedom of design that comes with applying AM.
Christopher Gravelle, head of Bell Helicopter’s rapid prototyping lab commented, “Material characterisation is a critical consideration for us during design. For instance, if we are creating bosses for attachment points in additively manufactured nylon rather than metal, it is a new material and process and you cannot just use the same configuration.”
After a final review of the first component design for producibility, Bell Helicopter sent a 3D CAD model to Harvest to develop a build strategy. Before each batch was produced, rigorous pre-production inspections were carried out by Harvest, including checking that nitrogen leak rate was low, which is important for reducing waste and ensuring part quality.
Caleb Ferrell, quality manager at Harvest added, “After every build, we test for tensile and flexural properties of the components. This is a requirement for process assurance that we continuously monitor.
“The parts that we get have very good feature definition and the mechanical properties have been good as well. We are especially happy with the larger platform size and the nestability we are achieving.”
Currently, the helicopter manufacturer 3D prints parts mainly for its environmental control system (ECS) using EOS technology, but AM production is expanding. Bell Helicopter is interested in employing 3D printed components throughout the aircraft systems of its commercial helicopters.
Schulte added, “The ECS engineers who have gained experience with the material and the process are now communicating with teams involved in other functions, and those teams are starting to incorporate additively manufactured hardware into their assemblies.
"The EOS technology produces a robust and highly repeatable process that complies with our specification. We have done a number of conversions of aircraft parts from previous processes to AM. With the EOSINT P 730, we often discover that the production cost per piece is substantially reduced compared to conventional manufacturing methods.”
Bell Helicopter will also be evaluating AM of high-temperature plastics intended for more demanding roles and environments.
Ferrell explained, “In addition to the design advantages, there are significant manufacturing benefits with EOS technology. Tool-less manufacturing means you do not face certain limitations or up-front costs.
“If you need to change something, you can build new revisions simply by changing the CAD file – no moulds, no new machining tool paths and very little wasted time and money.”
“Because of the large build platform in the EOSINT P 730, we can print bigger components in one piece rather than in sections, eliminating assembly costs.”
Another advantage of the EOS system is the clean surface it produces, according to director of business development at Harvest, Ron Clemons. He explained that the EOSINT P 730 incorporates a software fix that provides crisper detail and smoother surfaces. As a result, there is relatively little peripheral powder melting and adhesion, so the desired quality of finish is achieved. There is consequently a saving in post-processing cost compared with the bureau’s other AM systems and lead-times are shorter.
An important secondary benefit of EOS technology is increased recyclability of the plastic powder. Other AM processes used by Harvest leave behind a significant amount of partially melted and therefore unusable powder, whereas more of the EOSINT P 730’s leftover powder can be reused.
Harvest has since acquired a second EOSINT P 730 and an EOSINT P 760 and is currently working with Bell Helicopter to implement the manufacture of one-off or two-off orders for spares, nested within the build volumes of existing batch production.