It's a Lot More Than RP Now
Art to part with no machining
By Robert B. Aronson
"Rapid prototyping" is no longer a suitable catch-all term that defines an entire industry. It is just one of many applications for a field more properly called additive-fabrication (AF) technology. (Machining operations are subtractive in that a product is created by cutting away material. With AF, products are made by adding material.) In addition to prototypes, this technology's applications include short-run production, replacement parts, tooling, and casting patterns. Other terms used for this type of work are rapid manufacturing (RM) and additive manufacturing (AM).
Whatever the title, this industry is no longer dominated by service bureaus. Lower costs and greater ease of use has made much AF equipment both more attractive and affordable. Printers that make products in 3-D are now available for $15,000, and there are hints that $10,000 units are in the works.
Another major trend is that usable parts, either for prototypes or production, are now practical. This is due chiefly to the ability of AF units to make parts from high-strength polymers and metals.
In units that can make metal parts, a laser or electron beam focused on a steel preform creates a molten-metal pool. Then a stream of powdered metal or polymer is injected into the melted metal. The stream of powdered material, under computer control, traces out the desired shape and solidifies into a thin layer. The layers build up to form a part.
Currently metal parts are made using a process offered by EOS, a German company with offices in Novi, MI. According to company Vice President Jim Fendrick, "EOS systems work with two stainless steels, cobalt-chromium, maraging steel, and titanium, and efforts are under way to include the Inconel family of metals as well as aluminum. AF technology has advanced to the point where it is a viable option to more traditional manufacturing processes."
The unit now available from EOS has a work envelope of 10 x 10 x 12" (254 x 254 x 305 mm) with an accuracy of ±0.001" (0.025 mm).
The EOS direct-metal process produces parts with properties said to be superior to those made by conventional casting, chiefly because of better grain structure.
A major market for EOS equipment is dentistry. Dental repair items, such as bridges, are individualized, highly complex shapes, and such products are what AF excels in making.
Electron-beam melting (EBM) is the process used by the metal-part making units from Arcam (Gothenburg, Sweden). This process is similar to that employed by units based on a laser-heating system, but instead Arcam units use an electron beam. The Arcam A2, the larger of the two units the company builds, uses a 4000-W electron beam to heat metal powder to 1100°C. The molten stream is used to form the part under computer control.
The Arcam A2 offers two build volumes. One measures 200 x 200 x 350 mm, the other is cylindrical with a 300-mm diam and a 200–mm height. The other Arcam unit, model S 400, has a work volume measuring 200 x 200 x 180 mm.
Because of the high heat produced by the electron beam, the process is said to be up to seven times faster than any laser-based system. It can be used with any conductive metal, including cobalt, chrome, and several titanium alloys. One of the benefits of this process is that the resulting parts need not be stress relieved, as is required in some other processes.
Research is now underway to find ways to control the hardness and modulus of the parts produced by Arcam units. Initial results are said to be promising.
AF has also provided a major improvement to mold makers. Conformal cooling is often used when it is critical to keep close control over where and how accurately a molded part is cooled. This is` achieved, to a degree, by drilling water channels within the mold. But in some cases this approach did not produce the optimal cooling results. With molds made by an AF process, cooling channels are positioned as needed for better part cooling. The result can be a 30–50% reduction in cycle time, and a corresponding improvement in part quality.
A major advance for AF processes is the ability to form parts using the organic polymer polyetheretherketone, or PEEK. It's good for demanding engineering applications, such as bearings, pumps, valves, and medical implants. PEEK's key specifications: tensile strength, 92 MPa; compressive strength, 140 MPa; melting point, 343°C; and operating temperature 249°C.
The Selective Laser Melting (SLM) process used by MTT Technologies Group (Staffordshire, UK) produces fully dense metal parts direct from a 3-D CAD program using a high-powered laser. Parts are built layer-by-layer in thicknesses ranging from 30 to 100 µm. The process uses a range of atomized powder metals including stainless steel 316L and 17-4PH, H13 tool steel, aluminum alloys such as Al-Si-12Mg and Al-Si-10Mg, titanium alloys including Ti-6Al-4V and Ti-6Al-7Nb, commercially pure titanium, and cobalt.chrome (ASTM75).
"The latest SLM machines from MTT, the SLM125 and SLM250/300, have fully seal- welded vacuum chambers," explains Group Manager Robin Weston. "That enables lowpressure purging and operation at minimum oxygen concentrations. This feature contributes to material integrity and mechanical performance.
"The largest machine currently offered, the SLM 250/300, has a build volume of 250 x 250 x 300 mm," says Weston. The machine is available with a choice of fiber lasers, with output power ranging from 100 to 400 W.
The SLM 125 machine is offered with a choice of a 100 or 200-W laser, and a focused spot size of 30µm in diam. Build volume is 125 x 125 x 125 mm, which is said to be well-suited for the manufacture of smaller, detailed components. "This machine builds lattice geometries down to 200 µm, for example, and makes holes as small as 80 µm in diam," Weston explains.
"Build speed and surface finish are both material dependent. Tool steels take longest to make a part and titanium and aluminum parts are made faster. For fully dense parts, build speed reportedly ranges from 5 to 20 cm3/hr. Surface finish is in the range of 15–30µm for horizontal surfaces and 13–25µm for vertical sides. Laser scanning strategies can control surface finish," Weston concludes.
One of the first successful AF processes was stereolithography. It uses a UV-curable photopolymer and a UV laser to build parts. To make a layer, the laser traces a part pattern on the liquid resin. The UV laser light cures the pattern. Subsequent layers form the part.
Stratasys (Eden Prarie, MN), with its FDM (fused-deposition-modeling) technology, is one of the more experienced companies working in this area. Its Model 900mc, the company's latest machine in its high-end Fortus line, makes parts from productiongrade thermoplastics. Build envelope is 3 x 2 x 3' (1 x 0.6 x 1 m) and is said to be the largest work area currently available. The unit has made parts as large as an SUV door, but is more often used to make batches of parts.
"The Fortus 900 mc is specifically designed for direct digital manufacturing," says the company's Joe Hiemenz. "It is designed for applications that meet one or more of these criteria: production volume is relatively low, parts are highly complex, or there is a high probability there will be near-term design changes."
The unit currently uses ABS plastic and polycarbonate as building materials, and one big advance is the introduction of Ultem, a polyetherimide amorphous polymer with characteristics similar to PEET. Because of its strength and solvent resistance, Ultem is a preferred material for medical applications. Specifications include a continuous-use temperature of 170°C, and compressive and tensile strengths of 22,000 psi (154 MPa). The Ultem 9085 blend has been certified for use on commercial aircrafts.
Stratasys also has a line of 3-D printers and recently offered a unit from its Dimension division called the uPrint for $15,000.
The category of AF machines called 3-D printers, or just printers, evolved from the long-established inkjet printing process. But instead of making 2-D copies, the machines "print" 3-D parts. It is chiefly these low-cost, clean, user-friendly printers that have been responsible for the much wider use of AF equipment.
Z-Corp (Burlington, MA) is the largest of the US 3-D printer manufacturers. "Our design goals have always stressed simplicity," explains Director of Product Management Joe Titlow. "We want our units to be easy to use and accepted in the office environment just like a conventional copier.
"There are many RP applications where our products offer a solution. For example, our standard parts are great communication tools. We can make parts with fine details, and in any color or with a full spectrum of colors. So, explaining a new product to a single client or a team of engineers is greatly simplified with the use of concept models."
In the production process it's possible to add color pixel by pixel. The control system gives each pixel, called a "voxel" (for 'volumetric pixel' in the 3-D realm), its own individual color so with 600 dpi (236 pixels/cm) resolution. "Color helps communication. With color, we can highlight certain areas of a part to help in the design evaluation, identify different parts of a complex assembly, or make the part look lifelike," Titlow concludes.
Objet Geometries (Billerica, MA) 3-D printers use the FullCure Materials family of proprietary acrylic-based photopolymers to create models and parts. For example, FullCure720, a translucent acrylic-based photopolymer material, is used where visibility of liquid flow or internal details are needed. For opaque models, or those requiring enhanced mechanical properties, the Vero material family is used.
Models are used for both dynamic testing and limited-volume production. When a model has to be conductive, a metallic coating is applied over the printed models.
Service bureaus continue to be a large part of the AF industry. One example is RedEye (Eden Prairie, MN), a business unit of Stratasys. It functions as a service bureau specializing in producing production-grade plastic parts, especially for low-volume manufacturers that do not want to absorb the costs of buying and operating their own AF systems.
The company produces parts that have a near injection-molded appearance in a process called RedEye on Demand. An optional Overnight Build service promises part delivery the next business day.
C.ideas Inc. (Cary, IL), uses the FDM, Polyjet, and Urethane Casting processes for most of their work, according to Mike Littrell, company president. "About 30% of our assignments are for clients utilizing Direct Digital Manufacturing. We match the correct process with materials, to complement the product's function."
Some of these parts can be nearly as accurate and detailed as actual production methods. They are not only useful for design development, testing, and presentations, but can help shorten lead time. One company used C.ideas to more quickly get products from outside suppliers. "With the model we supplied, the customer was able to tell vendors where to put mounting holes, wiring harnesses, and mounting hardware," Littrell concludes.
This article was first published in the April 2009 edition of Manufacturing Engineering magazine.