Introduction

With increased competition from the global economy, manufacturers face the challenge of delivering new customized products more quickly than before to meet customer demands. A delayed development or delivery can mean business failure. Several technologies collectively known as Rapid Manufacturing (RM) have been developed to shorten the design and production cycle, and promise to revolutionize many traditional manufacturing procedures.

Before production of a product begins, a sample or prototype is often required as part of the design cycle, to allow demonstration, evaluation, or testing of the proposed product. The fast creation of a prototype is known as Rapid Prototyping (RP) [1], and is generally carried out before specialized molds, tools, or jigs are designed. Prototyping traditionally required considerable skilled hand labor, time, and expense, typically applied to cutting, bending, shaping, and assembling a part from standard stock material. The procedure was often iterative, with a series of prototypes being built to test various options. For many applications, this process has been revolutionized by a relatively recent technology known as layer manufacturing or Solid Freeform Fabrication (SFF) [2], in which a part of an arbitrary shape can be produced in a single process by adding successive layers of material.

RM also includes the fast fabrication of the tools required for mass production, such as specially-shaped molds, dies, and jigs. Many different layer manufacturing processes have now been developed, using an increasing range of materials. The parts produced have been of steadily increasing size and durability, and as the quality has improved layer manufacturing is being used more and more frequently to fabricate the parts both for production tools and functional prototypes. The application of layer manufacturing to make the components used in production is termed Rapid Tooling (RT) [1]. It has been applied to injection molding, investment casting, and mold casting processes.

Variety of parts producible by Rapid Manufacturing Source: Phoenix Analysis and Design Technologies http://www.padtinc.com

For some products, it can be economical to use layer manufacturing to produce the final products themselves, sometimes in a matter of days instead of weeks or months. Although the layer fabrication process itself is typically not as fast as traditional mass production techniques, it eliminates tooling, setup, and assembly processes, can produce parts of superior quality and complexity, and can be ideal for making custom parts based on a customer's special requirements. More manufacturers are taking advantages of these techniques.

Layer manufacturing allows parts of completely arbitrary 3-dimensional (3D) geometry to be fabricated, offering designers a new freedom to shape parts optimally without the constraints imposed by forming, machining, or joining. Another important advantage is that the process utilizes the computer description of the part shape directly, and allows integration of the Computer Aided Design (CAD) with the Computer Aided Manufacture (CAM) of the part. It therefore allows a manufacturing cycle with a seamless transition through the computer design, simulation, modeling, and fabrication procedures. In addition, the profiles used by the fabrication process are straightforward for the designers and customers to understand, thus facilitating technical communications.

How is Rapid Manufacturing Performed?

The first commercial layer manufacturing system was presented at the AUTOFACT show in Detroit, MI in November 1987 by the 3D Systems company [3], and intended primarily for rapid prototyping application. Several other processes were subsequently developed through the 1980s and 1990s. The technologies now available include a variety of different processes, such as Stereolithography, Selective Laser Sintering, Shape Deposition Manufacturing, and Laminated Object Manufacturing.

A Rapid Manufacturing machine Source: Phoenix Analysis and Design Technologies http://www.padtinc.com

Basically, all layer manufacturing systems consist of a combination of a computer CAD system with an operation machine to perform the fabrication of a layer under computer control. First, a 3D CAD representation of the part is created by a computer software package such as ProEngineer, SolidWorks, or Autocad. The computer representation of the part is then sliced into layers of a certain thickness, typically 0.1 to 0.25 mm, and their two-dimensional (2D) profiles stored in a triangulated (tessellated) format as a .STL file. Second, the software converts the .STL data to machine data, which are sent to the operation machine to generate each layer of the part by the specific fabrication process. The process is repeated many times, building the part layer by layer. The final step is finishing, removing the part from the machine, detaching support materials, and performing any necessary cleaning or surface finishing. Polishing, sealing, or painting the parts can improve their appearance.

In some respects, layer manufacturing is related to Computer Numerical Control (CNC) manufacture, in which a cutting machine such as a lathe or milling machine is controlled by computer to cut a specified shape, often with many different steps and cutting tool changes. However, in layer manufacturing the fabrication process builds the part systematically by adding material instead of cutting it away, and a much wider range of shapes can be achieved, including cavities or intricate geometries that would be difficult or impossible to machine. Also, an experienced machinist may be required to design or assist the CNC sequence, whereas layer manufacturing can be more highly automated.

Stereolithography (SL) was the first commercialized fabrication process, producing parts from photo-sensitive polymer resin [4]. It operates by scanning the liquid surface of a bath of the resin with an ultraviolet (UV) laser beam that causes the resin to cure in the shape of a layer of the part. The lowest layer is carried on an elevator platform that is lowered by the slice thickness after each new layer is formed at the surface. The layers combine to form the desired 3D shape of the part. The SL process can fabricate plastic molds for pattern making or blocks for metal sheet forming, as well as produce a wide range of polymer prototypes.

Selective Laser Sintering (SLS) is another process, with a wider range of material than SL [4], [5]. SLS can produce highly complex parts from materials such as metal, plastic, ceramic, and sand. The material in powdered form is deposited on a platform, and a carbon dioxide (CO₂) laser is used to selectively melt or sinter powder into the desired shape for each layer. The layers are lowered on a platform, with loose powder around the growing structure acting as a support for the top powder layer. The strength and porosity of the material can be controlled by adjusting various process parameters, such as laser scanning speed and power. Products have ranged from turbine rotors to medical inserts.

Shape Deposition Manufacturing (SDM) is another layer manufacturing process that combines the techniques of deposition and CNC machining [6]. Each layer is machined after it is deposited, and support material is added and machined to receive subsequent layers. The incremental machining allows a smooth surface to be achieved, even with thick layers, and the use of support material allows layers with overhanging, undercut, and separated features to be supported during the fabrication. The support material is removed at the end by melting or dissolving, and final machining is not usually required. SDM is a good choice for custom tooling, precision assemblies, structural ceramics, and wax molds for casting. It allows a high quality surface finish, intricate undercut features, and multi-material structures with inserts.

Turbine rotor produced by SDM Source: Carnegie Mellon's The Robotics Institute http://www-2.cs.cmu.edu/~sdm/

The Laminated Object Manufacturing (LOM) process was developed by Helisys of Torrance, CA [2]. It produces parts from a sheet material bonded together in layers to form a laminated structure. The original material used for the layers was paper, but several other sheet materials are now available, including plastic, water-repellent paper, and ceramic and metal powder tapes. The process has been used to make casting dies for automotive parts.

The 3D Printing process is based on ink-jet printing technology [2], [4]. A group of print heads moves across a powdered material in a scanning pattern, distributing a liquid binder to bond the material in the shape of each layer. The part is lowered, additional powder is added, and the process is repeated. At the end, the part is removed from the powder bed and cleaned. The field of potential application ranges from functional metal parts to small-series parts and mold inserts. Such mold inserts are suitable for plastic injection, metal die casting, extrusion tooling, etc [7].

Applications

The cost saving potential of RM techniques may be illustrated by a research program carried out at Northrop Grumman Corp, Pico Rivera, CA in 1997. The program studied the application of several layer manufacturing technologies -- SL, Fused Deposition Modeling (FDM), and LOM -- to the production of tools for sheet metal forming. Sheet forming involves plastic deformation of sheet metal blanks by one or more operations into required shapes, usually by pressing the metal against a mold or die by fluid or elastic pressure. The tooling required is relatively expensive to produce by traditional machining, but layer manufacturing offered great savings. For the most complex dies, used in hydraulic forming and rubber pad forming, fabrication by hand machining required 96 hours whereas SL was able to produce the same shapes in 2 hours, a time saving of 98%. The dies were ready to shape metal as soon as they cured [8].

The applications of RM are numerous and varied. Researchers at the University of Texas at Austin have reproduced human bone shapes using SLS to form titanium castings during the late 1980s [9]. Casting dies for automobile deck parts were fabricated by LOM [10]. IBM used SL to produce operating display units of its ThinkPad tablet computer for the annual COMDEX show. Coca-Cola used a RP process to design contemporary 20-ounce plastic Coke bottles [11]. A turbine part with a complex shape was built by 3D Printing using a ProMetal RTS-300 machine. A research project at the University of Delaware used a 3D model of a person's head to construct a custom-fitted helmet. NASA is testing RM processes to produce spacesuit gloves fitted to each astronaut's hands [2].

Conclusion

Numerous commercial RM systems for various materials and sizes are now available on the market around the world. RM technologies have seen rapid development and improvement in capability, and have been in widespread use for well over ten years. They have gained tremendous success by practical verification, and will no doubt see further development and application in the future.

1. http://www.me.psu.edu/lamancusa/rapidpro
   (Bill Palm and John S. Lamancusa, Penn State Learning Factory Rapid Prototyping website, last revised July 30, 2002)
2. http://www.ims.uconn.edu/~hmarcus/info.htm#what
   (University of Connecticut, SFF program)
3. http://www.3dsystems.com
   (3D Systems company)
4. http://www.techok.com
   (Tech, Inc. company)
5. Juha Kotila, Olli Hyrhila, Tatu Syvanen, Jan-Erik Lind, "PM [Powder Metallurgy] Prototype Component Manufacturing Using Direct Metal Laser Sintering," Rapid Tooling, 2001, pp 303-305.
   back to article
6. http://www-2.cs.cmu.edu/~sdm/
   (Carnegie Mellon's The Robotics Institute, Shape Deposition Manufacturing)
7. Haiko Pohl, Frank Petzoldt, Peter Gosger, "New features in three dimensional printing of metal powders," Powder Metallurgy, 2001, pp 309-312.
8. Boris Fritz, Rafiq Noorani, "Form sheet metal with RP tooling," Advanced Materials & Processes, ASM International, Volume 155, Number 4, April 1999, pp 37-39.
9. "Rapid Prototyping Advances Medical Bone Implant Technology," Foundry Management & Technology, 2002, p 96.
10. Yuhua Song, Yongnian Yan, Renji Zhang, Da Xu, Feng Wang , "Manufacture of the die of an automobile deck part based on rapid prototyping and rapid tooling technology," Journal of Materials Processing Technology, Number 120, 2002, pp 237-238.
    back to article
11. http://www.foundryonline.com/
    (FoundryOnline.com