Thermal Processes for Additive Manufacturing
Thermal processing is destined to play a significant role as the market for 3D printing expands.
According to Process Heating:
“For many years, 3D printing technology — or additive manufacturing, as it is often called — has been gaining ground against long-established manufacturing processes. In the early days, additive manufacturing was relegated to the realm of prototyping. More recently, myriad uses have evolved, including tool-and-die manufacturing, custom implants, pilot-line manufacturing and even mass production. One manufacturer asserts that their process is cost competitive with established alternative manufacturing techniques for lot sizes of up to 50,000 parts.
“Additive manufacturing encompasses several distinct manufacturing processes. What is more, each process is supported by multiple equipment manufacturers that use their own trade names and terms. It is a rapidly changing technological landscape.
“This article is not intended to be a buyer’s guide for 3D printers, or a comprehensive comparison of manufacturing techniques. Rather, it will discuss the various thermal processes involved in each manufacturing technique.
“By its nature, additive manufacturing is a thermal process. At some point in the process, material is heated to — or near — its melting point and deposited in a prescribed geometry to form a part. How the material is delivered and how the heat is applied is what differentiates one additive manufacturing process from another. Some of the processes require thermal post-processing as well. That topic is the primary focus of this article.
“Generally speaking, additive manufacturing technologies can be divided into two groups: direct-deposit processes and bulk-deposit processes.
Direct-Deposit Processes for Additive Manufacturing
“With direct-deposit processes for additive manufacturing, material is laid only where intended.
“Material Jetting. Material jetting is comparable to common inkjet printing. Droplets of photopolymer material are jetted in progressive layers onto a build platform and cured using heat or ultraviolet (UV) light. This process has the capability of generating very fine detail and provides the added benefit of being able to deposit multiple materials in the same part. Thus, a part could be made with both a rigid plastic and a flexible plastic in a single manufacturing step. Material jetting is limited to plastics.
“Material Extrusion. Material extrusion uses rolls of material filament that is drawn through a nozzle, where it is heated, and deposited to build layers that form the product. The nozzle moves back and forth and deposits material where needed. The platform lowers to allow new layers to be added.
A batch oven is used to perform thermal post-processing on 3D-printed metal parts.
FIGURE 1. Multiple parts can be manufactured inside a single build box of the binder-jetting process, making it suitable for mass production.
“Material extrusion is a precise, automated process akin to building up a shape with a hot glue gun. Many entry-level additive manufacturing systems available for home and educational use employ this technology, but it is widely used for higher-end manufacturing as well. Some systems are even capable of extruding a metal matrix for a metallic end product.
“Directed-Energy Deposition. Directed-energy deposition (DED) is a process in which material is added by melting or sintering it with direct heat from a laser, electron beam or plasma arc. It primarily is used to repair or add material to existing metal and alloy parts.
“Directed-energy deposition is the opposite of machining in that it adds material rather than removes it. It sometimes is combined with CNC machining systems. Five-axis DED systems are available with the capability of processing parts as long as 16’. Imagine a complex turbine weldment requiring repair. DED can be used to build up material where needed followed by traditional machining to provide the surface finish.
“Using a range of materials, parts are printed from 3D CAD drawings to make prototypes as well as mass-produced components.
Bulk-Deposit Processes for Additive Manufacturing
“With bulk-deposit processes, material is spread in a bed throughout the workspace, or in a liquid form in a vat, and fused or bound together only where the solid part will be.
“Vat Polymerization. Vat polymerization uses UV light to cure layers of photopolymer resin within a liquid vat to form the product. This method is limited to plastic materials and is a top-down layering process. As the material is cross linked within the pool of monomer, the part in-process is slowly lifted out of the vat, looking much like the Terminator rising from a puddle of liquid metal.
“Powder-Bed Fusion (PBF). Powder-bed fusion builds products by fusing layers of powder material using a laser or electron beam. This process can use either plastic or metal powder; a given system is dedicated to one material or the other.
“Binder Jetting. The binder-jetting process uses two materials: a build material and a binder. The build material is powder based, and the binder is usually in liquid form. Alternating layers of powder material and liquid binder are deposited to form the product (figure 1).
“Binder jetting offers several advantages to the user:
- “It is faster than other 3D printing technologies.
- “It can print plastic parts in multiple colors using multiple print heads.
- “It is becoming a leading technology for manufacturing production-grade metal alloy parts.
“For many applications, binder-jetted parts have an acceptable finish without requiring additional surface treatment. It can use widely available metal injection molding (MIM) powder effectively, minimizing material costs. It can print with a variety of media, allowing printing of sand castings and even edible sugar creations.
“Improved binder formulations and new production-grade binder-jetting systems have been developed that will continue to spur this technology’s use. Advancements in materials also have allowed for an increasing number of alloys to be printed.
“Metal powder is added to a 3D metal printer in some additive manufacturing applications.
Thermal Steps in Additive Manufacturing
“Two of the processes — material jetting and material extrusion — require no further thermal processing after printing. Material jetting creates a finished part that needs only minimal cleaning to remove support material. Similarly, material extrusion requires mechanical removal of support material as well as some chemical cleaning and additional mechanical polishing, but no additional thermal processes are needed.
“For other additive manufacturing methods, depending on the material, thermal post-processing may be needed. One example is powder-bed fusion. The PBF process often is referred to as laser sintering, but that is something of a misnomer. Rather than sintering, the laser used in the process melts the material. Thermal post-processing is not needed for PBF-produced plastic parts. When using this technology for metal parts, however, thermal post processing such as age hardening is required. In fact, for a given alloy, the same age-hardening processes required with an equivalent cast or machined part are required after PBF manufacturing to obtain the desired Rockwell hardness.
“Parts created with directed-energy deposition also require the same heat treatment as machined parts to harden the material and relieve stresses.
“Different manufacturers take divergent approaches to the vat polymerization method. One manufacturer includes a UV post-processing cure. Low temperature bake steps at 120 to 140°F (50 to 60°C) are used to melt residual wax from the parts after mechanically removing most of the wax support material. A final chemical cleaning step is used in all vat polymerization processes, and cleaning also can be done at temperatures up to 140°F (60°C).
“Of all the additive manufacturing processes discussed, binder jetting is unique in that it does not rely on applying heat to the materials during the printing step. Some heat is applied to evaporate solvents in the binder during the print process, however. Parts come out of the printing step buried in a bed of powder inside of a build box, and the first thermal process is to cure the binder. This gives the green parts the strength to stand up to the de-powdering process. The green products then must be sintered in a high temperature furnace or autoclave. The sintering step in the binder-jetting process is similar to the sintering process in the metal injection molding process. The amount of material and its composition determine the sintering process and conditions required.
“Sintering is done in a protective atmosphere or in a vacuum at temperatures up to 2,650°F (1,450°C) but below the melting point of the metal. Depending on the alloy to be sintered, the furnace may need to be equipped with hydrogen, nitrogen, argon or a mixture of those gases along with piping and safety systems. Residual binder is captured using a cold trap.
“For many years, 3D printing technology has been gaining ground against traditional manufacturing processes.
“Up to 99 percent of the loose powder recovered after the de-powdering step can be reused after a thermal de-binding. The de-binding process bakes out any residual binder before the powder is loaded back into the printer.
“In conclusion, by offering new alternatives for manufacturers, 3D printing is changing the landscape of manufacturing. Although we are still a few steps away from Star Trek replicators, it is likely that machine shops everywhere and manufacturers large and small will soon have additive manufacturing capabilities of one type or another. No longer relegated to small prototyping applications, 3D printing is being used for mass production of parts made from a variety of metals and composites. The ability to make forms in ways that were never before possible has opened up opportunities for innovation in many industries. Thermal processing is destined to play a significant role as the market for 3D printing expands.”