Additive Manufacturing: Building parts one layer at a time
It’s easy to see from the many types of available systems and the innovative technologies being developed that additive manufacturing (AM) is an extremely active area. Although operating details vary widely, all of the approaches build an object in layers under computer control.
One of these methods, stereolithography (SLA), was invented by Charles Hull in 1983. SLA cures photopolymer resins layer by layer using UV lasers and is one of the technologies used in machines made by 3D Systems, a company Hull co-founded.
In May 2016, the American Society of Mechanical Engineers designated SLA-1, the first commercial rapid prototyping machine introduced by 3D Systems, as an Historic Engineering Landmark.1 In comments related to this recognition, Hull said, “Although I expected 3D printing to be embraced by manufacturers, I never could have anticipated how widespread 3D printing is today or the types of things that people are doing with it.” Hull’s words succinctly describe the current state of AM—a term now used almost interchangeably with
Rapid prototyping remains a major market for AM tools and has attracted attention through applications such as making parts for Jay Leno’s classic cars. In a 2009 article,2 Leno said, “Let’s say you have an older Cadillac or a Packard, and you can’t get one of those beautifully ornate door handles. You could go to the big swap meet in Hershey, PA, every day for the rest of your life and never find it. Or, you could take the one on the left side of your car, copy it, use the computer to reverse it, [3D print it,] and put that new part on the other side.”
For example, the feedwater heater on Leno’s 1907 White Steamer needed to be replaced, but the badly corroded part was an aluminum casting. It took 33 hours to 3D print a plastic replica of the original, which previously had been scanned and the mechanical imperfections removed in software. After checking the fit of the plastic replica, Leno sent it to a foundry that made a mold and cast the required replacement part.
In another example, 3D Systems provided the printer used to make new air-conditioning vents for the custom Ecojet car.3 Leno concluded, “It’s an amazingly versatile technology…. We used plastic parts we designed, right out of the 3D copier. We didn’t have to make these scoops out of aluminum—plastic is what they use on a real car. And, the finished ones look like factory production pieces.”
Some things you can try at home …
In contrast to many high-tech tools, AM has become so popular that machines are appearing with prices low enough to appeal to the hobby market. For example, Amazon currently is selling a FlashForge Finder model 3D printer for just under $500. This type of printer uses the fused deposition modeling (FDM) technique in which, according to a white paper4 from Stratasys, a professional-level 3D printer manufacturer, “Thermoplastic filament feeds through a heated head and exits, under high pressure, as a fine thread of semi-molten plastic. In a heated chamber, this extrusion process lays down a continuous bead of plastic to form a layer.”
A recent PC Magazine blog5 compared the FlashForge Finder with other low-cost printers from XYZPrinting and LulzBot. The Finder has one extrusion nozzle so it can only produce a part in one color. The higher price FlashForge Dreamer model has two extrusion nozzles and can print in two colors.
LulzBot uses an all-metal “hot end” and a wide range of 3-mm diameter filament materials. The FlashForge and XYZPrinting machines work with 1.75-mm diameter filaments made from either acrylonitrile butadiene styrene (ABS) or polylactic acid; XYZPrinting machines also can extrude a flexible filament material. Layer thickness is adjustable from 100 µm (about 0.004 inch) to 500 µm (about 0.020 inch) on the FlashForge and XYZPrinting machines and from 0.002 inch to 0.020 inch on the Lulzbot printer.
… and others you can’t
FDM machines necessarily produce a less well-controlled surface than SLA because the layer thickness is greater. However, there are lots of parameters to consider when deciding on an AM technology. A Stratasys white paper4 compared and contrasted the properties of FDM and the company’s trademarked PolyJet SLA-type printers.
The SLA printers deposit layers of photopolymers that solidify when exposed to UV light. As the white paper states, “Polyjet gives you a near-paint-ready surface right out of the 3D printer…. That’s not true for FDM. The extrusion process can produce visible layer lines on side walls and tool paths on top and bottom surfaces.”
The Stratasys J750 3D printer simultaneously works with up to six materials, mixing them as required at the print head and forming layers as thin as 0.00055 inch. This is a high-end machine weighing 335 lb and requiring about 1.4 kW of power. Objects as large as 19.3 x 15.35 x 7.9 inches can be printed.
New materials are developed continuously, so machine capabilities frequently change. Nevertheless, as the company’s white paper explained, FDM machines can work with a wide range of real plastics from low-cost ABS for basic models to nylon for engineering parts and even Ultem for high-temperature/high-performance components. SLA printers are more limited in the applications they address because of the lower-performance materials that are available.
Figure 1 shows an SLA-printed structural model of an exhaust manifold made from 3D Systems’ Visijet M2 RWT (rigid white) material. As described on the company’s website, “This material yields plastic parts that look and feel like white injection-molded plastic and allows for rigorous testing and use. It is durable and stable under varied conditions, making it ideal for functional testing and rapid tooling applications.”
SLA upside-down—inverse SLA
Another approach to AM uses a liquid photopolymer resin in a vat at the bottom of the machine. Rather than selectively dispensing the build material only where it will form part of the completed object, as is done in both FDM and conventional SLA printers, inverse SLA systems cover the entire build area with a layer of resin. The material is polymerized by a light beam selectively deflected to the required locations. A similar method is used in high-tech electron-beam (e-beam) and laser-sintering AM processes in which layers of plastic or metal powder are laid down at the top of the build and selectively melted.
Because AM is such an active business area, many technology variations exist. As an example, the Carbon3D M1 printer uses a relatively deep resin pool with a special oxygen-permeable glass bottom. The resin is formulated so that polymerization occurs only when the material is exposed to both light and oxygen. So, when an LED light source scans the current 2D slice, only the thin layer of material in contact with the glass plate is polymerized. The build platform at the top of the object in the M1 printer moves upward in synchronism with the succession of layers being scanned. As the printed object emerges from the resin pool, the effect is reminiscent of the Terminator rising up out of molten metal in the movie Terminator 2—an inspiration for the product according to Carbon3D co-founder Joseph DeSimone.6
Formlabs makes the lower-cost Form 1 and Form 2 printers that also use the inverse-SLA technique. These machines include an automated squeegee that traverses the clear window beneath the resin pool before each layer is printed. A video on the Formlabs website shows a part being raised, the squeegee traversing, the part being lowered, and then another layer polymerized. Finished parts must be cleaned in alcohol to remove the coating of uncured resin.
Sintering and melting
By definition, thermoplastics can be melted, and several 3D printer manufacturers have developed laser sintering machines that selectively fuse portions of successive thin layers of thermoplastic powder. The basic approach also has been applied to 3D powdered metal sintering.
As a 2012 article7 reports, GE Aviation purchased a small company called Morris Technologies because Morris had invested in laser sintering technology. The article states, “… [This] involves spreading a thin layer of metallic powder onto a build platform and then [selectively] fusing the material with a laser beam. The process is repeated until an object emerges. Laser sintering is capable of producing all kinds of metal parts, including components made from aerospace-grade titanium.”
In 2013, GE acquired the Italian aerospace company Avio,8 which had developed a type of e-beam melting AM process. Compared with sintering, e-beam melting produces a completely solid part—sintered parts only approach a solid part’s density, although they can come close.
At about the same time, GE was perfecting a similar direct metal laser melting process capable of forming 100% solid parts. For a new jet engine fuel nozzle, shown in
Figure 2, the company used a cobalt-chromium alloy fused in 20-µm layers. According to a GE report,9 “The process can take as long as 120 hours, and the workers use big data analytics to monitor everything from the size of the weld pool [to] temperature and the stability of the laser.”
Todd Rockstroh, a laser processing expert at GE Aviation, commented, “When we designed the nozzle, we wanted to make it from an alloy that was mature, well-known, and thoroughly tested—nothing exotic.” Cobalt-chromium materials have a long history of successful use in replacement knee and hip joints as well as dental implants. Rockstroh continued, “Because of their medical applications, there has been a tremendous amount of research done on these alloys. They are also pretty common because they serve such a large market, which makes them cheaper.”
Fast-forward a few years, and in 2016 the CFM LEAP jet engine, jointly produced by partners GE and Snecma of France, is being tested in new airplanes. A GE report10 states that the 19 3D-printed fuel nozzles per engine will take the place of previous nozzles, each assembled from 20 separate parts. In addition, the new nozzles will be 25% lighter and 5x more durable. The article also notes that GE Aviation will make 100,000 3D-printed parts by 2020.
Rather than interpreting 3D printing as the formation of a 3D object, you could understand the term to mean more conventional 2D printing on a 3D substrate—a surface that isn’t flat. Optomec has developed the Aerosol Jet technology, which deposits a focused beam of atomized ink from a print nozzle. Relative motion between the print head and the substrate allows nonplanar applications to be addressed—such as printing the interconnections for a smart card before it is conformally coated.
Commercially available metal-loaded inks from a number of sources either cure at low temperature or, for substrates that can tolerate higher temperatures, are sintered. Sintering in an oven produces conductivity approaching that of the bulk metal. Sintering with a built-in laser avoids heating the entire part but results in resistance 2x to 3x higher.
An Optomec white paper11 referred to multilayer Aerosol Jet applications such as creating a capacitor from a dielectric layer sandwiched between two conducting layers, microsensors for avionics, and high-density interconnect backplanes for flat-panel displays. This technology excels at depositing a few layers of material to form a circuit or a 5-GHz carbon nanotube transistor as the University of Massachusetts and Brewer Science have demonstrated.
As a separate initiative that is described on the company’s website, Optomec also has developed a 3D metal sintering machine, which creates parts layer by layer from powdered metal. The interesting aspect of this printer is the use of technology similar to the Aerosol Jet technique. Metal powder simultaneously is deposited by a spray nozzle and sintered by a high-power laser. This method would appear to require significantly less powdered metal than is used by machines that lay down complete rather than selective layers. The approach also can be used to repair parts.
AM in action
Hull’s expectation that 3D printing would revolutionize prototype production has become reality. Nevertheless, AM involves equipment, processes, and materials that are distinct from conventional machining. Training and experience are necessary before you can get the best results. One way that companies can take advantage of 3D printing without much risk is to submit their digital design files to a printing bureau.
For example, 3D Systems provides a range of capabilities via the company’s on-demand parts-manufacturing Quickparts 3D design-to-manufacturing service—“the world’s leading provider of unique low-volume and high-volume custom-designed parts,” according to the 3D Systems website. If you do not have AM experience, partnering with a company that uses a number of technologies can be an advantage.
On the production side, clusters of 3D printers are being used—in the case of LulzBot, 140 3D printers run for at least 100 hours per week to manufacture parts for more LulzBot printers. According to comments made by Stan Middlekauff, cluster production supervisor, in an article on the LulzBot website, “The alternative to having [a] cluster producing those parts would be injection molding everything, but the injection molding cost for a die could be thousands of dollars and upward,” Middlekauff said. “With the cluster now in place, the up-front cost has been paid for us to make any type of part. Plus, if at any moment in time we need to change a part, we can change it quickly.”
AM has revolutionized product development time scales and economics for a wide range of manufacturers. And, it’s being used in production to make parts that can’t be conventionally manufactured as well as provide immediate inventory control.
- Millsaps, B. B., “Celebrating Chuck Hull & the SLA-1 Original 3D Printer—Now a Historic Mechanical Engineering Landmark,” 3D Print.com, May 19, 2016.
- Leno, J., “Jay Leno’s Printer Replaces Rusty Old Parts,” Popular Mechanics, June 7, 2009.
- Newman, J., “3D Printing in Leno’s Garage,” Design Engineering Rapid Ready, Dec. 11, 2015.
- “FDM and PolyJet 3D Printing: Determining which technology is right for your application,” Stratasys, Dec. 18, 2015.
- Hoffman, T., “FlashForge Finder 3D Printer,” PC Magazine, April 11, 2016.
- Tilley, A., “How Carbon3D Plans To Transform The Way We Make Stuff,” Forbes, Nov. 23, 2013.
- “GE Is So Stoked About 3D Printing, They’re Using It To Make Parts For Jet Engines,” The Economist, Nov. 24, 2012.
- Milkert, H., “GE Uses Breakthrough New Electron Gun For 3D Printing—10X’s More Powerful Than Laser Sintering,” edprint.com, Aug. 18, 2014.
- Kellner, T., “Joined At The Hip: Where The 3-D Printed Jet Engine Meets The Human Body,” GE Reports, July 1, 2013.
- Gilman, C., “A pragmatic view of additive manufacturing,” GE Global Research, February 2014.
- “Aerosol jet printed electronics overview,” Optomec, May 2009.