Web Exclusive Feature--The Basics of Rapid Prototyping
The pressures on companies to differentiate themselves in the market by designing and developing sure-fire products-and to do it faster, better and cheaper-has never been greater. The number of product variations continues to escalate, as does the complexity of the product. At the same time, products have a shorter shelf-life, giving companies less time to recapture their investment. The product development pressures fall squarely on the engineering community. The stress to get it right the first time, to compress the time from concept to market quickly and to take costs out of the process, have never been greater.
Recent figures show that a simple six-month extension in development time can cut into profits by an astounding 30%. Or increasing development costs by 50% can cut the bottom line by 5%. Neither option is a viable alternative in today's high-stakes market.
One of the technologies that is allowing engineers to meet these demands is rapid prototyping. It is a tool that is revolutionizing the way companies- particularly in the automotive, aerospace, medical and consumer products industries-design and build products.
What Is It?Rapid prototyping refers to a class of technologies that emerged in the manufacturing community in 1987. Taking images directly from three-dimensional computer-aided designs (CAD), rapid prototyping transforms the data into a physical model. Think of it as a "three dimensional printer."
As opposed to traditional prototyping methodology, in which craftsmen begin with a hunk of material and remove material from it, rapid prototyping is an "additive" process, combining layers of paper, wax or plastic to create a solid object.
"The challenge for design engineers is that all design work must be done as three-dimensional solid modeling," says John Choren, director of the Rapid Prototyping Consortium at the Milwaukee School of Engineering. "That is quite a change for somebody used to working with orthogonal views-that is, the view from the front, side or top of the object."
Rapid prototyping techniques employ the same basic five-step process. First, the product is designed using a three-dimensional CAD software package. The designer can use a pre-existing CAD file or create one expressly for prototyping purposes.
The second step converts the CAD file into .stl format. Indeed, most conventional CAD programs will output in .stl file format without difficulty. This .stl has been adopted as the industry standard interface between three-dimensional CAD solid models and the rapid prototyping system. "It converts the solid object on the screen into a mesh of thousands or millions of triangles that, when put together, gives surface representation of a smooth curve that the computer can read," says Choren.
Next, a pre-processing program that is proprietary to each rapid prototyping technology prepares the .stl file for construction. Several programs are available, and most allow the user to adjust the size, location and orientation of the model. This is important because the properties of rapid prototypes vary from one coordinate direction to another. There is an optimum orientation based on the shape of the object. For example, circles may not be smooth or remain circular if built in a particular plane. Additionally, build time is strongly dependent upon height in the z-axis. Knowing this in advance helps engineers reduce the amount of time it takes to build the prototype by placing the shortest dimension in the z direction.
Next, comes the actual construction of the part, one layer at a time, from polymers, paper or powdered metal. Most machines are fairly autonomous, needing little human intervention.
The final step is the finishing. This involves removing the prototype from the machine and sanding, sealing and/or painting the model to improve its appearance and durability.
The Benefits"The five-step process allows engineers to quickly create tangible prototypes of their designs, rather than just two-dimensional pictures that traditionally have gone the more time-consuming route of hand-crafting and modeling," says Greg Kruger, vice president of Kelly Engineering Resources, a business unit of Kelly Services. "They make excellent visual aids for communicating ideas with co-workers or customers."
Rapid prototyping allows for the creation of objects with sophisticated internal features that cannot be manufactured by other means. Prototypes dramatically improve communication because most people, including engineers, find three-dimensional objects easier to understand than two-dimensional drawings.
This physical model conveys more complete information about the product earlier in the development cycle. The turnaround time for a typical rapid prototype part can take a few days. Conventional prototyping may take weeks or even months, depending on the method used. Rapid prototyping can be a quicker, more cost-effective means of building prototypes as opposed to conventional methods.
Beyond appearance, these prototypes can be used for design testing. For example, an automotive engineer might mount a model in a wind tunnel to measure aerodynamic forces, or a medical engineer might be able to take a prototype valve and measure flow.
Rapid prototyping can lead to more effective strategic planning and concurrent engineering. By exchanging prototypes early in the design stage, manufacturing can start tooling up for production while the sales or marketing team starts planning the packaging, all before the design is finalized.
"It's action-oriented and results-driven," Kruger notes. "Engineers have always used prototypes, but rapid prototyping expands their capabilities. It is now easy to perform iterative testing: build a prototype, test it, redesign, build and test, and so on. Such an approach would be far too time-consuming and costly using traditional prototyping techniques, but it's easy using rapid prototyping."
ChallengesDespite its inherent advantages, there are still numerous kinks that must be worked out of the system. Depending on the machine, parts are usually limited in size to less than 24 inches on a side. Metal prototypes remain difficult to make, though this should change in the near future.
In addition, the accuracy of some prototypes lacks precision because the .stl files use planar slices and are unable to represent curved surfaces with precision. Increasing the number of slices improves the approximation, but at the cost of bigger file size. Large, complicated files require more time to pre-process and build.
Rapid prototyping systems depend on good CAD data to produce the parts. In other words, garbage-in--garbage-out. Care and precision must be exercised by the technician creating the necessary files for rapid prototyping.
The computer support-both hardware and software-do not come cheaply. As a rough rule of thumb, Choren suggests that companies need to be spending $50,000 to $100,000 annually on prototypes before bringing the process in-house. Although rapid prototyping technologies are highly customized and therefore difficult to predict, most companies will spend a minimum of$150,000 a year on acquisition and maintenance costs for typical rapid prototyping equipment.
Perhaps even more daunting than the technological shortcomings are the challenges of implementing the technology into the workforce, according and to Kruger. He predicts fewer, but more costly, professionals.
"We are seeing a reduced need for artistry and model builders, while at the same time, more demand for traditional information technology and CAD skills on both the design and modeling platforms," says Kruger. "Concurrently, we are seeing more activity from customers for increased training to help the workforce become fluent in the new design process."
Rapid prototyping also knocks down the walls between disciplines and forces a more collaborative atmosphere internally. "You can't have a tool that is fast and saves time, without people that share that same urgency," says Kruger. "The stronger and more effective the communication, the more effective the process. Different functions within a company-from design to engineering, from marketing to production-must cooperate more closely towards a common goal.
"The physical prototype is a perfect communication tool," says Kruger. "If a picture is worth a thousand words, then a physical model is worth a thousand pictures."
Future DevelopmentsOn the horizon, though, are several developments that will help to revolutionize and enhance the value of rapid prototyping as we know it.
Although most of the machines are approaching the upper limits of speed, due to mechanical or physical limitations, Choren says further gains in accuracy and surface finish can be accomplished. Today's commercially available machines are accurate to -0.08 millimeters in the x-y plane at best, but less in the z direction. Improvements in laser optics and motor control should increase accuracy in all three directions. In addition, rapid prototyping companies are developing new polymers that will be less prone to curing and temperature-induced warpage.
The introduction of non-polymeric materials, including metals, ceramics composites, represents another much anticipated development. These materials would allow users to produce functional parts. Today's plastic prototypes work well for visualization and fit tests, but they are too weak for functional testing. More rugged materials would yield prototypes that could be subjected to actual service conditions. In addition, metal and composite materials will greatly expand the range of products that can be made by rapid manufacturing.
Another important development is increased size capacity. Currently, several "large prototype" techniques are being developed.
Further Down the Road"As these advances indicate, the term 'rapid prototyping' is rapidly becoming somewhat of a misnomer," according to Kruger. As the precision of such systems continues to improve and the choice of materials grows, the "prototypes" are increasingly being used for functional testing or to derive tools for pre-production testing.
This new development-rapid tooling-automatically fabricates production quality machine tools. This is one of the slowest and most expensive steps in the manufacturing process because of the high degree of precision required. Some estimates say tooling costs and development times can be reduced by 75% or more by using rapid tooling technologies.
Closely related to rapid tooling is the next generation-rapid manufacturing-which refers to the automated production of salable products directly from CAD data. Currently only a few final products are produced in this way, but as the number of materials becomes more widely available, that number will no doubt increase.
"Rapid manufacturing will never completely replace other manufacturing techniques," says Kruger. "In large production runs, mass production remains more economical. For short production runs, however, the process is much cheaper since it does not require tooling. Rapid manufacturing is also ideal for producing custom parts tailored to the user's exact specifications."
Regardless of whether a company is thinking about rapid prototyping or has advanced down the path to rapid manufacturing, there can be no escaping the pressure to get products off the design table and to market faster and faster. While rapid prototyping provides the tools to help the engineering community accomplish the objective of reduced time to market, the technologies themselves can accomplish nothing without the input of a highly skilled and trained workforce that understands and can manage the process.
This article was reprinted from Kelly Engineering Resources' newsletter, "Issues & Trends," copyright 2002 Kelly Services. Inc. Kelly Engineering Resources provides all types and levels of engineers, technicians, designers, drafters and manufacturing specialists, logistics, project controls and other engineering support personnel. For more information, visit Kelly Engineering Resources' website.