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2 almost arbitrary geometries, which unleashes a new level of design freedom, such that product designers can focus on the functionality of the product. This design freedom also extends to materials. In traditional manufacturing, usually only one ma- terial is used per component. In 3D printing, parts can be made with multiple materials or even functionally graded materials (for example, the hardness of a part may change gradually from one location to another). This freedom can enable simplifica- tion of system design with fewer components, better function- ality, and lower cost. The other major way 3D printing brings changes to manu- facturing capabilities is digitization of manufacturing, which fundamentally changes where, when, and how items are pro- duced. From a business perspective, digitization brings many benefits, such as reduced inventory, ease of distribution, fast turnaround, reduced waste, and ease of customization. As the Industrial Revolution brought manufacturing technologies for mass production, the digital revolution enables customization. Digitization of manufacturing is moving toward an age of mass customization, in which everyone can get customized products at the same or lower cost as mass-produced products. State of the art 3D printing has found applications in multiple industries. Some of the most exciting progress in recent decades is listed below. • The first 3D-printed car, Strati, was printed successfully in 40 hours by Local Motors in collaboration with Oak Ridge National Laboratory. • Made In Space, Inc., working with NASA, 3D-printed the first part ever made off the Earth. • 3D-printed houses, pioneered by contour crafting invented at the University of Southern California, have been at- tempted by various start-up companies, and results dem- onstrate the promise to reduce building time from months or years to days. • General Electric, enabled by the design freedom of 3D print- ing, redesigned and 3D-printed its LEAP jet-engine nozzle as one single unit, replacing the original 20 components. The redesign, which has been approved by the U.S. Federal Aviation Administration (FAA), improved fuel efficiency by 20% and achieved 10% more power, saving millions of dol- lars of fuel cost every day for airline companies. • Align Technology, one of the first successful examples of mass customization, streamlined the process of making customized clear aligners used in orthodontics with 3D printing. • Adidas®, working with Carbon (a 3D-printing start-up company in the San Francisco Bay area), makes customized 3D-printed sneakers. 3D printing has been widely adopted in many industries. The U.S. movie industry uses 3D printing to make scaled models. Doctors use 3D printers to make customized implants, hear- ing aids, and prosthetics. Artists use 3D printing to replicate artwork and to realize unconventional fashion and jewelry designs. Engineers use 3D printing for rapid prototyping and tooling. Chefs use 3D printing to make unique foods. Students use 3D printers for various projects in their education. Bioprint- ing has been used to print animal or human organs for research and drug testing. It is expected that 3D printing will continue to make inroads into almost every major industry and have a profound impact on our lifestyle in the coming decades. Additive manufacturing The term "additive manufacturing" stems from the manufac- turing philosophy of joining materials together, in contrast to subtractive manufacturing (that is, machining), which removes materials. The core idea of joining materials to make objects is ancient. Some examples include various adhesives, Lego® blocks, and welding. 3D printing uses computers to control the joining of materials based on a digital model. The first 3D printing technology, called stereolithography, was invented in 1984 by Charles Hull, the co-founder of 3D Systems, Inc. Stereolithography used a computer to control the scanning of a laser to selectively solidify a liquid photopolymerizable resin one layer at a time to form a 3D object. Since then, more than a dozen 3D printing technologies have been invented. ASTM International Committee F42 on Additive Manufacturing Tech- nologies divided the technologies into seven major categories based on the mechanisms of joining materials as listed in the table. In 3D printing, the materials are joined together to form differ- ent patterns on each layer. There are three different ways to cre- ate the patterns: (1) patterning materials directly, (2) patterning energy to join materials, and (3) patterning both materials and energy. The patterning can be done by point scanning, line scanning, or area filling. Point scanning moves the printhead point by point to fill a layer and is considered a 1D patterning technique. Line scanning and area filling can pattern an area by moving the printhead in a single pass or not moving the print- head at all. These are faster than point scanning and are consid- ered to be 2D patterning techniques. The table summarizes the main differences among the major 3D printing processes. 3D Printing (continued) + ward ' s science