3D Printing Materials

A: “Additive manufacturing” and “3D printing” have become a phenomenon in the engineering and manufacturing communities. The terminology may refer to one of a vast number of different manufacturing methods and machines, however. This can lead to confusion because some people picture small plastic models while others imagine metal lattice structures and still others visualize complex jet engine parts in exotic materials.

In general, additive manufacturing refers to the concept of building a part—be it a valve component, jet engine nozzle or desk toy—in small increments until a near-net shape component is produced. It would take too much space to cover all additive manufacturing processes or go into much detail on any one in this column, but the table below summarizes the most common processes, the types of materials they typically use, what those processes are currently being used to produce, and how they might be used in the future—all within the context of the valve industry.

The most well-known advantage of additive manufacturing is the ability to produce complex geometries that would be difficult or impossible to produce with traditional manufacturing techniques. Machining a tortuous path into a severe-service cage limits the possible geometries; using small cores in a casting process is time-consuming, expensive and may lead to quality issues. Welding, brazing or assembling multiple components to make a single cage may be cost-prohibitive or unreliable.

When discussing metal parts, laser-powder bed fusion (L-PBF) is the process that stands out. Because parts start as powdered metal and are then fused by a laser, leaving a path for process fluid to flow through a cage requires that the laser be programmed to skip over the areas that won’t be solid metal in the finished part. When the machine finishes a build, the loose powder can be removed, and no machining is necessary to create a complex internal flow passage.

A major limitation of L-PBF is the equipment required. Current machines have fully sealed chambers that are purged with inert gas to prevent the powder from reacting with air as it is printed. The size of the chamber limits the size of the part that can be produced, and the technology to use larger chamber sizes effectively is under development. Directed energy deposition (DED) machines can bypass this limitation. While some do have sealed chambers, others blow inert gas only on the area at which the energy source is directed. Essentially, this is the same as using shielding gas in a welding operation. Because only a small area is shielded at a time, the limit on part size is the range of motion of the arm holding the material feed and energy source. Complex geometries can be produced this way, but the resolution of DED machines is much lower than L-PBF.

Binder jet printing is a process similar to L-PBF except that in place of a laser-melting metal, a nozzle sprays an adhesive binder to fuse powder particles together. When printing is completed, the loose powder is removed, leaving a 3D object. Currently, the most common application of this technology is in the production of sand molds for metal casting. Traditional sand molds are made by forming sand around a pattern, which means a pattern must be produced before a part can be cast.

Binder jet printing bypasses the need for a pattern, reducing the time required to produce a casting. This is especially useful for parts with low production volumes such as specially engineered valves or replacement parts for legacy products. In addition, since castings poured into printed sand molds are tested and certified the same way as those made with traditional molds, there are fewer barriers to the adoption of this technology than other additive manufacturing processes. Sand is not the only material that can be printed with binder jet technology. Ceramics and metals that cannot be welded can be formed into complex geometries as well. After printing, the parts are sintered and the binder is removed to produce a solid object. Typically, parts produced this way contain porosity, unlike with L-PBF and DED, which can produce parts with very low porosity. However, binder jet printing allows materials to be produced in shapes not possible with traditional manufacturing techniques.

Additive manufacturing of polymers has become commonplace with a variety of printers manufactured and sold for anything from home to industrial use. For valve manufacturers, the two primary technologies, fused deposition molding (FDM) and stereolithography apparatus (SLA), have not made their way to producing finished parts, but are used extensively to rapidly create prototypes and fixtures, jigs or other manufacturing aids. One of the largest hurdles for these processes to clear before they can be used in finished parts is the limited selection of materials. When materials suitable for components such as gaskets, diaphragms or other seals can be printed, innovative designs for sealing components made possible by additive manufacturing will emerge.

3D printing has already begun to transform the valve manufacturing industry, but standardization of these new manufacturing processes is lagging in implementation. Currently, it remains largely up to individual valve manufacturers and end users to ensure the quality and suitability of additively manufactured parts. However, some ASTM standards have been written that address additively manufactured 316L stainless steel and titanium: ASTM F3184 and ASTM F3302, respectively. Within ASME, there is a Board on Pressure Technology Codes Standards Special Technical Committee on the use of additive manufacturing for pressure retaining equipment.

BRETT HOFMAN is a materials engineers, Flow ­Control, for Emerson Automation Solutions (www.emerson.com). Reach him at brett.hofman@emerson.com.