Edited by Leslie Langnau, Managing Editor
Your fellow engineer has a design problem similar to yours. He solved it by using a material in an innovative way. Sometimes, all that’s needed is a fresh perspective to find the solution to your design issue. Following are several stories about using unusual materials to solve a design need.
Laser sintering plastic into rugged airplane parts
Jerry Clark, manager of the Air Vehicle Configuration Design, Integration and Rapid Development Department of The Boeing Company in Mesa, needed to reduce cycle times, build tools more quickly, and in some cases, eliminate the need for tooling and some post-processing steps altogether. His solution was to purchase an SLS system from 3D Systems. “We saw areas where the SLS system could help us with research,” said Clark. “We saw it as an opportunity to significantly reduce cycle time.”
Clark liked the SLS system’s flexibility and the variety of materials it uses, including its increasing potential with metals.
In the span of five months, Clark’s department built more than 400 parts on the SLS system. Most were made with DuraForm Polyamide (PA) material. DuraForm is filling many of the direct manufacturing as well as initial prototyping needs Boeing has in Mesa, as the company works to design, test and produce a variety of products for air vehicles including the AH-64D Apache Longbow helicopter.
Many of the DuraForm parts go directly on prototype aircraft, vehicles and mock-ups. Clark noted that, “DuraForm works well in areas where we have to make functional parts for the secondary structure, such as ducts, fairings and other non-load carrying components and parts.” Boeing also is looking into using DuraForm to create load-carrying components and parts.
These parts usually undergo a variety of tests, including those measuring tensile strength, heat resistance, fatigue, material consistency and resistance to moisture and various fungi. The combination of tests performed depends on how and where the parts are used, what conditions and hazards they may be subjected to, whether humans will be in contact with them, and other factors.
“If I were to add up all the tooling costs we’ve eliminated, all the secondary rework we didn’t have to perform, all the post-processing steps we’ve eliminated, all the parts we’ve made, and all the man hours we’ve saved by using our SLS system to create prototypes and parts, I could easily say we’ve saved enough to pay for the system—and potentially even a second machine,” said Clark.
“With our SLS system, we often go directly from computer data to prototype part to installation on the aircraft. It’s another type of paperless system. If a picture is worth a thousand words, then a physical 3-D mockup is worth 10,000 words,” added Clark.
“Plus, we have the ability to customize a baseline quickly and rapidly and produce parts per customer needs without adding in a lot of tooling time or costs to the process.”
Clark noted that Boeing has used DuraForm parts and prototypes for a long and growing list of applications, including creating visualization samples for suppliers; providing quotes; producing samples of existing parts that could not be produced any other way except through dissection of a complex assembly; conducting internal design review and technical review sessions with upper management; performance reviews (with customers’ sample parts provided); and producing scale models for testing.
“Probably one of our greatest challenges here is getting our people to understand that our SLS system is not just a machine for making pretty models,” Clark said. “Sure you can do that, but right now we are using it for actual parts, functional parts, rapid prototyping, rapid tool development and rapid manufacturing.”
Extending part life with additive manufacturing
Ulterra Drilling Technologies designs and manufactures polycrystalline diamond compact drill bits used in the construction of oil-and-gas wells. When the design engineers sought to extend the life of rotors and stators for down-hole applications, they looked into metal 3D printing/additive manufacturing.
The part the engineers were working on was originally made from 4145 Steel with conventional subtractive machining at a cost of $400 to $500 each. The engineers chose ExOne’s 3D metal printing technology to print the components in an S4 stainless/bronze matrix. This material has a higher abrasion resistance than conventional 4145 Steel. The 3 to 5-in. part dropped in cost to $75 to $150 each, which was fine for a short production run.
Components produced by metal additive manufacturing can reduce weight, integrate multi-piece assemblies, enhance product function and reduce lead times for prototype and short-run production.
Custom 3D printed polymeric cranial implants speed the healing of head injuries
Say you’re in a car accident and you hit the windshield, injuring your skull. Once you reach the hospital, they clean up that area. After the swelling goes down, surgeons discuss various ways to restore your skull to normalcy. If you did not have access to 3D printing technology, the chances of long, costly medical processes are high.
Recently, though, a 3D printing material specific to skull injuries received the first Food and Drug Administration (FDA) 510(k) clearance. Oxford Performance Materials (OPM), South Windsor, Conn., received clearance for its polymer laser-sintered OsteoFab Patient-Specific Cranial Device (OPSCD). The customizable implant restores voids in the skull caused by trauma or disease.
Data from a CT or MRI scan are turned into what is essentially a slice file that is not too dissimilar to the data used by a laser sintering system to build parts. Upon review by a physician, that slice file is sent to OPM. Using 3D design software, a team of design engineers creates an implant based on the file to precisely fit the patient’s anatomy. “Once you have that, you get approval of the implant design from the surgeon, and then print or ‘grow’ the part,” said DeFelice.
Once the implant is removed from the leftover powder, it’s ready for quality inspection. “In addition to mechanical and analytical testing, we use a structured light scanner to do 100% line-of-sight metrology inspection to certify the dimensional accuracy of the final product,” DeFelice said, “The total process from receiving the data to shipping the implant took less than two weeks.”
Built in hours with Additive Manufacturing (AM) technology from EOS, the implant was successfully used just a few days later, when the device was implanted in a patient missing a significant portion of cranial bone. “It was very large, measuring nearly six inches across,” said OPM President and CEO Scott DeFelice. “The fit was perfect.”
Quicker recovery, lower costs
The right implant means less time on the operating table, quicker recovery, and reduced possibility of infection. Hospitals benefit as well—typical operating room rates run upwards of $60 per minute, so pressure is high to manage the costs of patient care.
The OsteoFab technology consists of additively manufactured medical and implant parts produced from PEKK material. PEKK (Poly-Ether-Ketone-Ketone) is DeFelice’s favorite molecule, and is a high-performance thermoplastic with many exceptional properties, including a number of mechanical and thermal qualities that make it suitable for cranial reconstruction. It has a density and stiffness similar to bone, is lighter than traditional implant materials such as titanium and stainless steel, is chemically inert and is radiolucent so as not to interfere with diagnostic imaging equipment. An interesting note—bone has an affinity for this material.
Noted DeFelice, “If polymers were ice cream, PEEK would come in vanilla, strawberry, and maybe chocolate. That’s it. PEKK, on the other hand, is more like an entire ice-cream shop, with dozens of flavors.”
The difference is structural: PEEK is a homopolymer, made up of identical monomer units. By contrast, PEKK is polymorphous. That means PEKK has lots of molecular “knobs” that the team at OPM has learned how to turn. By adjusting the polymer’s manufacturing process, or incorporating different additives, they can move melt points, crystallization levels, and mechanical properties. The result is a rich toolbox capable of supporting different applications and a variety of custom materials from the same base molecule.
“PEKK as a molecule is unique,” said DeFelice. “Based on research studies, it is osteoconductive, meaning bone cells will grow onto it, unlike some other materials.”
DeFelice added, “Because of these osteoconductive properties, long-term implant stability may be easier to achieve than with other materials. And given the correct implant design, results are better. You can get a multiplying effect by increasing surface area, and achieving intimate contact between the implant and native tissues. That’s a portion of what led us to laser sintering, and ultimately, to the OsteoFab process itself.”
Why laser sintering?
Because OPM needed to produce low-volume parts with complex shapes, AM was a logical choice. Laser sintering lifts manufacturability restrictions that traditional processes impose—for instance, draft angles in molding and corner design for CNC tooling. It also doesn’t require the upfront costs of tooling and molding, so it is suitable for creating one-off, patient-specific parts.
PEKK (like its cousin PEEK) has a high melting point relative to other polymers, necessitating a high-temperature laser sintering system. OPM went with the EOS EOSINT P 800 system.
The path to approval
It wasn’t just a matter of buying a system one day and making a product the next. The path to commercialization of patient-specific implants was arduous. Aside from the not-so-trivial prerequisite of the right molecule and the right process, DeFelice said climbing the mountain of regulatory requirements was a daunting task. “For starters, you need an ISO 13485 compliant facility that has design controls and an appropriate clean manufacturing environment. That’s a pretty big step. You need to be compliant with CFR 21 cGMP (current Good Manufacturing Practices). Add to that a completely validated process and ISO 10993 biocompatibility data on your finished parts.”
Using soluble cores to build hollow parts
Hollow composite parts that trap a pattern can present a manufacturing challenge. Traditional composite part manufacture involves winding, wrapping, molding and laying up various combinations of materials and resin systems on molds, bucks, patterns, cores and mandrels. To obtain a part within a hollow composite, the usual technique has been to use clamshell tooling to lay up two halves (and bond) or lay up a single piece by working from the inside of the tool’s cavity.
An alternative is to produce these patterns using Fused Deposition Modeling (FDM) in a novel way. This new approach replaces the typical mold process with an FDM soluble core. Soluble cores reduce delivery leadtime and labor expenses by eliminating the need for making a mold. This technique also reduces the time required for laying up the part. Instead of laying up the two mold halves, then laying up the part in each half, and then bonding the two halves together, the composite cloth can be wrapped around the 3D printed soluble core “form.” After the part has cured, the core is simply dissolved. This process is especially suitable for low–volume production.
The nature of additive manufacturing makes it possible for FDM to produce much more complex geometries than are possible with other cores. FDM soluble cores are strong enough to withstand the loads of composite manufacturing processes. And there is no risk of damaging the part during core extraction because the core simply melts away as it soaks in a liquid bath.
Manufacturing FDM soluble cores requires two changes to the standard FDM process. First, the core is designed to make its internal structure mostly hollow. Second, the strong thermoplastic of a standard FDM part is replaced with a soluble material that is normally used for the construction of a part’s support structures. The core can be designed in two ways. One way is to create a solid 3D model and use the sparse fill option. The Insight program—the Fortus build preparation software—will automatically create an internal structure that minimizes the internal volume of the core. The second approach is to create (in CAD) an internal structure that keeps the core stable under the temperatures and pressures of composite molding while promoting flow of the solution to accelerate core removal. Both of these approaches minimize material consumption, build time, and washout time.
It’s relatively simple to integrate FDM soluble cores into the manufacturing process. No modifications to the process are needed prior to composite curing and core removal. The cure cycle is also unchanged, but temperatures must be limited to avoid distortion. In general, composite parts with FDM cores must be cured at temperatures below 250 °F (121 °C) and at pressures less than 50 psi (345 kPa). The only process change is that, after the composite part has cured, the core is removed by dissolving it in a solution bath, such as the Stratasys WaterWorks soluble support system.
This technique helps keep racing cars on the NASCAR circuit. After each race, Joe Gibbs Racing (JGR) engineers have just three days to diagnose a problem, find a solution, and implement it before the car ships to the next race. JGR’s ability to go from concept to production part has helped the team win three championships.
One Sunday, a tire blew out in a JGR car and engineers identified a problem with the duct outlet supplying air to cool the tire. In the past it would have taken more than a week to develop a new duct outlet concept design, build and evaluate an FDM prototype, build a mold using an FDM pattern and lay up a composite part. This process could not be completed in time to produce a new part before the next race.
JGR used an FDM soluble core to substantially reduce the time required to redesign the duct and build a production part. On Monday, a JGR engineer designed a new duct outlet to deliver air over the tire bead exactly as needed to keep it cool. Then the engineer built a concept model in four hours using the Stratasys Fortus system. After completing several iterations on the concept and confirming its fit on the car, the engineer produced an FDM soluble core. The final carbon fiber part was laid up on the composite core. After the part had cured, the soluble core was dissolved away. The new part was ready on Wednesday, in time to be bolted onto the car before it was shipped to the next race.
Filed Under: Design World articles, Materials • advanced