The current digital revolution is affecting almost every aspect of our lives. Many of the revolutionary changes, such as our latest mobile phones and the emergence of driverless cars, are obvious. Other changes, while much less visible, are no less important. The impact on product development has been especially profound. In particular, the emergence and popularity of digital, three-dimensional (3-D) technologies, such as computer-generated solid models and additive manufacturing (3-D printing) techniques, have greatly expedited the preparation of prototypes. Likewise, subtractive manufacturing techniques, such as computer numerical control (CNC) milling and laser cutting, have made prototype generation faster and less expensive, although these techniques have not received as much recent attention as additive manufacturing. But in either case, all of these production improvements are enabling multiple iterations of cost-effective prototypes to be made, which in turn has facilitated the development of exquisitely optimized manufacturing designs. And the solid model that results from these designs is the starting point of the “digital thread” that provides the digital data link through manufacturing and sustainment in model-based engineering (MBE) [1]. MBE is a process that is increasingly being adopted by many DoD and related organizations (including the National Defense Industrial Association [NDIA]) to manage the manufacture of complex products.

Rapid prototyping is defined as a group of digital techniques that facilitate the quick fabrication of a prototype physical part or assembly from a 3-D computer file. The first rapid prototyping technology was developed more than 30 years ago, when the first 3-D printer was invented [2]. Since then, many other 3-D printing techniques have been developed. These 3-D printing technologies, where a part is built up layer by layer, are collectively known as additive manufacturing. Other 3-D prototyping techniques, such as CNC milling and computer-controlled laser or water-jet cutting, are collectively referred to as subtractive manufacturing. With subtractive manufacturing, material is removed from a block or sheet until only the part remains. It is important to note that, in practice, it is often necessary to supplement digital prototyping techniques with manual methods, especially in the early stages of prototype development [3].

MANUFACTURING AND THE 3-D MODEL

MBE is a concept being adopted by many organizations to manage the manufacture of complex products. A 3-D computer-aided (CAD) model is the official data record and forms the starting point of the previously mentioned digital thread. It is produced during the concept and design phases of manufacture and will progress through the manufacturing process to the product’s end of life (see Figure 1). Currently, two-dimensional (2-D) blueprints are often used to officially track changes while “unofficial” 3-D models are created from these as needed, with all the inefficiencies that this approach entails. Therefore, 3-D model development, aided by the creation of iterations of prototypes during the concept and design phases of an MBE manufacturing process, is increasingly important.

PROTOTYPE COMPUTER MODEL DEVELOPMENT

A 3-D CAD model is required before either additive manufacturing or subtractive manufacturing techniques can be used to create a physical prototype that is suitable for evaluation. High-quality solid modeling CAD programs, such as SOLIDWORKS, PTC Creo Elements/Pro, and Autodesk Inventor, have greatly decreased the time needed to create 3-D models. A solid 3-D model is assumed to have volume, and all the faces of the model are connected to other faces. Solid-modeling software usually has the ability to test structural integrity of the model using finite element analysis (FEA) and to perform other analyses [4]. Because the models are feature-based, their geometry can easily be changed to accommodate design enhancements.

A 3-D CAD model may be developed from “scratch” from design requirements, or it may be reconstructed from existing blueprints. Often, these blueprints do not represent the part exactly as the part has been adjusted or adapted during previous production runs. In some cases, only a physical part is available, which must be captured and adapted to meet current requirements (see Figure 2). To create a 3-D CAD model from an existing part of any complexity requires the use of either a contact or noncontact coordinate measurement machine (CMM).

A contact CMM could be a fixed-bridge CMM, an articulated measurement arm, or a laser tracker. All of these CMMs touch the part to make the measurement and collect individual points or streams of points. Particularly useful for model creation from these types of point measurements is feature-based reverse engineering (RE) software, such as DezignWorks or Point2CAD, which facilitates creation of CAD models from parts using measurement arms.

A smaller part is generally scanned with a noncontact CMM, such as a laser scanner or a structured light scanner. A laser scanner, which is often attached to a measurements arm, projects a laser line onto the part, which is captured by a sensor offset from the projector. The projected line is distorted by the part’s surface topography, thus providing information about its curvature. The line scans are referenced into the global coordinate system by the encoder readings on the arm. In the case of a structured light scanner, a 2-D pattern is projected onto the part surface, and the pattern’s distortion is captured by an offset image sensor. A surface point cloud is then computed from this distortion.

Point cloud data are generally collected and integrated in specialized RE software, such as PolyWorks or Geomagic Verify. The points are processed to reduce noise and remove duplicate and erroneous data and converted into a more compact polygonal model. It is usually necessary to extract the geometric primitives from the polygonal models and to replace freeform surfaces with CAD-friendly lofted or Non-Uniform Rational Basis Spline (NURBS) surfaces. The model can then be exported to solid-modeling CAD software for adjustment and numerical simulations.

CREATING THE PROTOTYPE

Once a digital model has been generated, the first prototype can be produced using either additive or subtractive methods. As mentioned previously, additive manufacturing builds up the part layer by layer whereas subtractive manufacturing creates the part by cutting away unneeded material.

ADDITIVE MANUFACTURING (3-D PRINTING)

The basic process of creating a prototype using additive manufacturing is straightforward. The CAD file must be converted into an appropiate file format, usually stereolithography (STL). In the process of generating an STL file, the 3-D model’s geometry is converted into a series of planar triangles, which are closer together in areas of curvature and further apart in flatter areas. The STL file is then imported into 3-D printing software, where it is sliced into 2-D sections so that it can be printed layer by layer. The resulting file contains the low-level program (G-code) that provides instructions to the printer. The exact form of the file depends on the particular technology of the printer and may include printing support material to prevent overhanging sections warping while the printing material cures.

Additive manufacturing has some important advantages for prototype creation. In general, parts can be quickly and (for small parts) inexpensively made, which encourages the creation of multiple iterations so that the optimum design requirements can be achieved. Additionally, complex parts are as easily produced as simple ones because they are all produced in the same manner, layer by layer. And printed parts can easily be fitted with electronic or mechanical components.

However, additive manufacturing does have some restrictions. The range of materials from which parts can be printed is limited, as is the size of parts that can be produced by typical commercial printers. And the cost of a 3-D printed part increases rapidly with size.

The following sections detail some common technologies that are categorized as additive manufacturing.

Stereolithography (SLA)

SLA, the pioneering 3-D printing technology, works by focusing an ultraviolet (UV) laser onto a bead of UV-curable resin to create a 2-D cross-section on the build platform. The platform is then lowered so the next layer can be cured. SLA printers produce extremely smooth parts with almost no stepping between the layers because of the high precision of the focused UV layer. Parts produced by SLA printers are particularly useful when the detail of the prototype is required to closely resemble the final, mass-produced part. The voids in the center of a part are typically filled with additional resin, and parts may require additional finishing/curing after the print has been completed.

  Figure 2: Prototype Development From Scratch, Blueprint, or Existing Part.

Fused Deposition Modeling (FDM)

The predominant solid-based 3-D printing technology is FDM, which works by extruding material layer by layer. The print head outlines the base layer of the object and then fills in the contour. After the first footprint layer is printed, the print head is raised slightly and deposits the second layer. The printer continues depositing one cross-sectional layer of the object after another (as illustrated in Figure 3). High-end FDM printers can produce large parts for prototyping. Parts can be made from plastics, generally acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA). Overhangs are supported by layers of temporary material, which is easily removed once the print is complete. To achieve smoothness, parts must be post-processed by hand.

Powered Bed Printing

Powder bed printers work by spreading powder on a bed and then solidifying the layer with a liquid binder. There are many variations of this technology, but the most common one is the inkjet printer, which prints (colored) binder into the powdered material instead of ink. By using multiple print heads, it is possible to print in multiple colors. Overhangs, which may be a problem in other types of printing, are supported by the underlying powder. However, the prints tend to be brittle and require considerable cleanup to get rid of excess powder. In a similar manner to SLA prints, voids in the print are filled with powder.

Selective Laser Sintering (SLS)

SLS—and, similarly, direct metal sintering (DMLS) and selective laser melting (SLM)—use high-powered lasers to melt powdered particles. As illustrated in Figure 4, the powdered material is laid down and melted by the laser, and then the next layer is added. No binding material is required, and parts can be created out of numerous types of materials, such as metals, ceramics, and plastics. Further, the resulting parts can be as strong as conventionally manufactured parts. However, DLS, DMLS, and SLM printers are expensive and use potentially dangerous materials that require careful handling.

Polyjet Printing

Polyjet printers deposit a layer of UV-curable resin onto the print bed, which is instantly cured with a UV light, located on the print head. Overhangs are supported with gel, which is easily washed away after the printing is complete. The parts produced are strong and can include different colors and materials when produced with the appropriate resin. Many types of resin are available, and the print can resemble any material, from hard plastic to rubber. For example, a hard plastic inner shell, covered by a soft grip or protective layer of a rubber-like material, can be printed.

Figure 3 (left): Fused Deposition Modeling. Figure 4 (right): Selective Laser Sintering.

Digitization has also profoundly enhanced milling and cutting capabilities, although this method has not captured the popular imagination to the same extent as 3-D printing or additive manufacturing. High-quality 3-D or sheet metal prototypes can be created directly from a processed computer model.

CNC Milling

Milling machines in their simplest form consist of a motor called a spindle that spins an end mill, which cuts away material from a block until the desired part remains. This block of the material is attached to a platform that can move in three or more directions with respect to the end mill. The operator removes material in pockets, revealing one feature at a time. Digitization has resulted in the replacement of many of the tasks of an operator with a computer and the creation of CNC mills.

The CNC milling operation is controlled by computer-aided manufacturing (CAM) software. Increasingly, CAM programs are sold as a module that works within solid modeling programs, such as Creo Elements/Pro or SOLIDWORKS. This software takes the 3-D model and processes it to create a series of instructions for the mill. However, CNC mill operation requires a considerable amount of skill and experience, with much operator interaction being required, depending on the material and the shape of the part being made.

When equipped with a suitable end mill, CNC mills can create parts from many materials, including steel, aluminum, titanium, wood, and plastic, making these mills highly versatile.

Computer-Controlled Laser Cutting

Laser cutting has emerged as an exciting technology that is useful both for prototyping and manufacturing. High-powered lasers are able to quickly cut complex metal profiles. Smaller CO2 lasers are sufficiently powerful to cut plastic and wood. Laser-cut parts are particularly suitable for sheet metal prototypes or may be part of the assembly used to create the prototype, depending on the application [2].

COMPARING ADDITIVE AND SUBTRACTIVE TECHNIQUES

A CNC mill, equipped with the appropriate end mill and cooling system, is capable of making parts in many more materials than 3-D printers. It is also able to achieve extremely high dimensional accuracy. However, 3-D printers can create parts with a great deal of complexity. Depending on the 3-D printing technology adopted, internal geometry can be just as complex as external geometry. Furthermore, it is possible to print preassembled assemblies, such as the assembled chain shown in Figure 5.

Additionally, CNC mills are capable of producing high-precision parts (less than 0.001 inch in size). On the other hand, the accuracy of 3-D printing relies heavily on the particular technology and is generally unable to meet the accuracy achievable by CNC mills. The general prototype iterative approach can be used to improve the accuracy of the part created by additive manufacturing. As shown in Figure 6, the part is printed and measured. The geometry of the solid model is then adjusted to compensate for those areas that are out of tolerance. The model is reprinted and selectively scaled until it provides a print of the required accuracy.

Machines that combine 3-D printing and CNC capabilities are becoming available. A part can be printed, and then select features can be machined for the highest accuracies.

A CNC mill, in the hands of an experienced operator, can produce smooth surfaces. Most 3-D printing techniques can as well; however, some 3-D printer techniques require post-processing to achieve a smooth finish.

Figure 5: Assembled Chain Printed With Polyjet Technology.

Figure 6: Iterating to Improve Printed Part Accuracy.