12.01.08
Striving for Perfection
Prototypes are now expected to be as close as possible to the finished product.
Michael Barbella
Perfection takes practice. Lots of practice. No industry is more indicative of this simple but palpable assertion than the medical device sector, which has been held for years to near-seamless standards created by the FDA. Whether these standards beget “perfect” devices is up for debate, but the process by which these gadgets are manufactured is perhaps best described as an ardent pursuit of perfection.
Prototyping and production providers in the medical device industry—the orthopedic sector, in
particular—are at the center of this perpetual pursuit. These providers have a tall order to fill: They
must reduce lead times on projects while meeting customers’ growing demands to create a device or component that is as close as possible to the finished product.
“Depending on the industry, prototypes have to have the same fit quality as the final product,” said Jim
Fendrick, vice president of EOS of North America Inc., a Novi, MI-based manufacturer of laser-sintering systems. “In many areas of medical product development, the properties of the prototype and finished product must be identical.”
Evolution of Prototyping
Prototypes have existed in the medical device industry for more than four decades. Early prototypes were often crafted by hand and took several weeks or months to create. Though the process was painstakingly slow, industry leaders recognized the potential of creating a sample device or component to determine its potential value to customers.
Laser-sintered prototypes of open hooks that attach to vertebral bodies/ribs. These hooks are produced from stainless steel material by direct metal laser sintering. Photo courtesy of DePuy Spine. |
In the 1970s, Voelcker developed the mathematical tools that eventually led to the algorithmic theories
that form the basis of modern computer programs used to design nearly all mechanical devices. While his theories changed designing methods in the 1970s, they did not change the well-established, drawnout practices for building these devices or components.
In the mid-1980s, however, a researcher from the University of Texas pioneered the idea of layer-based manufacturing, in which a prototype is built in layers. The researcher printed three-dimensional models by using laser light for fusing metal powder in solid prototypes, one layer at a time. This process eventually came to be known as laser sintering.
3D Systems, a Rock Hill, SC-based provider of solid imaging solutions, developed the first rapid prototyping device, the SLA-1. Today, the company has a complete line of SLA (stereolithography) products that use three-dimensional computer-aided design (CAD) data to convert liquid plastic materials and composites into solid cross-sections, layer by layer. The process creates a highly accurate three-dimensional part. 3D Systems currently has four different types of SLA centers; the
latest incantation—the iPro 8000 MP SLA Precision Center—was introduced earlier this fall. That system can process hundreds of files at once, making it ideal for the mass customization and production of medical parts, including orthodontics, implant drill guides, dental restorations, hearing aid shells, and ear molds.
More importantly though, 3D Systems designed the iPro 8000 to quickly build large and small parts
with smooth surfaces, edge definition, feature resolution and tolerances that rival the accuracy of CNC-machined parts. Possessing the capability to produce such clean, accurate prototypes is becoming increasingly necessary in the orthopedic device sector as customers demand archetypes
that can be run through various testing methods.
“If you are just going to hold a prototype in your hand, then you really don’t need all the refining and the advanced properties,” noted Nic Seymour, technical director at Sandvik, a Swedish engineering
group. “But customers want prototypes that are nearly implantable-ready so they can run it through various machines and perform fatigue and wear testing. To do that, you can’t have any rough surfaces.”
Rapid Prototyping: A Smooth Operator
Rough surfaces on medical device prototypes have become a rarity in recent years, as companies turned to a process called rapid prototyping to reduce costs and expedite the speed-to-market process. Defined as the automatic construction of objects using solid freeform fabrication, rapid prototyping was first used in the late 1980s to produce models and sample parts. Over the last 20 years, however, the practice has grown to encompass a wide range of applications, and now often is used to manufacture
small amounts of high-quality parts.
The rapid prototyping process takes virtual designs and transforms them into a full-fledged physical
model on an incremental basis. The process creates horizontal cross-sections of the product or component to be prototyped and then reproduces that cross-section in physical space. Material is usually added to each layer of the prototype as it is being made, and the layers are either joined together or fused automatically to create a final shape.
Though the term “rapid prototyping” implies a relatively quick process, this method can take anywhere
from several hours to several days to complete, depending on the complexity of the device or component and its material characteristics. With customers demanding shorter lead times, however, companies such as Sandvik have had to find ways to develop a prototyping process that is quick and produces a product that is virtually impeccable.
Rapid tooling dramatically can reduce lead times and enable companies to produce larger volumes of prototypes. Photo courtesy of Sandvik. |
To produce the wax prototype, Sandvik uses a wax thermojet, a machine that is similar to an inkjet
printer. Instead of ink though, the machine uses drops of wax to build a three-dimensional model from a virtual design. The process, while simple, usually produces a prototype with a rough surface, a major disadvantage for customers that want near-perfect models. Other disadvantages to using the wax thermojet include the unmanageable nature of the product’s dimensional stability, the amount of time it takes to manufacture the prototype (it is typically a slow process), and the enormous amount of work it
takes to repair a sample and prepare it for processing. Seymour said this process is most effective for customers that don’t need many samples and are not concerned about the product’s surface quality.
Despite its ability to reduce lead times, Sandvik uses the wax thermojet strictly for initial samples or to produce only a few prototypes for customers. Though the machines and materials used for the wax prototype process have evolved and the surface quality of the archetypes have improved, Seymour
said his company has developed other methods to provide customers with high-quality samples.
One such method is rapid tooling, which uses a resin-based, rapid prototyping machine to manufacture production quality machine tools. Due to its high quality requirements, tooling is typically one of the slowest and most expensive steps in the manufacturing process. Most tools have complex
geometries, yet must be dimensionally accurate to within a hundredth of a millimeter. Tools also must
be hard, resistant to wear, and have smooth surfaces. To meet such demands, molds and dies traditionally have been made by CNC-machining, electro-discharge machining, or by hand. These methods, however, are expensive and time-consuming, so manufacturing firms have incorporated
rapid prototyping techniques into their production systems to try and accelerate the manufacturing process.
Rapid tooling can reduce tooling lead times from two to three weeks to two to three days. This method enables Sandvik to produce larger volumes of prototypes with a smoother surface finish and improved dimensional accuracy. The process also permits the manufacture of complex tooling to the same design standards as hard tooling in a fraction of the time. In addition, resin-based prototyping machines can make multi-piece tooling where undercuts, negative tapers, moving slides and loose
pieces need to be incorporated into the design, according to information on Sandvik’s Web site. Redesigns to existing tooling also are possible because it is easy to revert to the original design while revamped samples are tested.
“We believe that the resin-based [rapid prototyping] system gives us better results,” Seymour said. “We’ve been using the wax [thermojet] machine far less. Twelve months ago, we would have used the wax prototype machine for a lot of our prototyping projects. But we are confident that the resin-based system is a better route for us right now.”
Selective Laser Sintering: Bottoms Up
A better route for many companies that offer prototyping services is selective laser sintering (SLS), a technique that uses a computer-guided high power laser to fuse or sinter small particles of plastic, metal or ceramic powders into a three-dimensional prototype.
Compared to other manufacturing techniques, SLS can produce parts from a wide range of commercially available powder materials including polymers (such as nylon or polystyrene), metals (such as steel, titanium and alloy mixtures), and green sand. During production, a three-dimensional
CAD model is sliced into layers and built using laser-sintering technology. The energy of the laser
solidifies the powder-based materials, thus producing the prototype.
Direct metal laser sintering (DMLS) is a technology that is increasingly being used by companies to dramatically reduce their time to market for rapid tooling and direct metal applications. The technology
works by sintering extremely fine layers of metal powders layer by layer from the bottom of a prototype up until the model is complete. This method churns out medium to small parts and inserts within hours or days rather than days or weeks.
“It is very expensive to make traditional tooling to create one-off products and prototypes,” EOS’s Fendrick said. “Production tooling often requires an investment of between a quarter of a million and a million dollars. Eliminating that cost and cutting down on the amount of wait time is an attractive benefit for any customer. There’s a huge niche out there for production-grade prototypes that we are just starting to discover. Many large companies do not necessarily want to spend money for hard tooling,
especially for limited first production. In the automotive industry, where you make a million parts a year, it’s cheaper to make a tool to produce that part. But that’s not the case in the medical device industry.”
The equipment developed by EOS GmbH Electro Optical Systems in Krailling, Germany (the parent company of Fendrick’s firm) uses plastics and metals in the rapid prototyping process. The metals
include stainless steel, titanium Ti64 and cobalt chrome alloys, which has helped the company enter the fast-growing orthopedic device market.
DePuy Spine Inc. has used the DMLS technology to dramatically reduce its lead time and produce prototypes for evaluation purposes. The Raynham, MA-based developer of spinal pathology products has produced more than 1,200 parts using DMLS technology and has reduced lead times in some projects by up to 50%.
With the orthopedic device market expected to grow at an annual rate of 10% over the next five years,
EOS is facing a daunting challenge: keeping up with capacity. Fendrick said he has noticed an increased interest in the company’s sintering machines and technology from medical design engineers.
The introduction of non-polymeric materials, including metals, ceramics and composites presents a
different kind of challenge to companies. These materials, particularly ceramics, enable rapid prototype users to produce parts that are both functional and durable. C5 Medical Werks, a medical-grade components manufacturer based in Grand Junction, CO, recently expanded its
capabilities to include ceramic injection molded components. Andrew Nield, sales and marketing director for the company, said the added capability enables C5 Medical Werks to meet customers’ demands for high volume, tight tolerance and intricately-shaped components.
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Companies that provide prototyping services are faced with the formidable task of producing a near-perfect model in a short amount of time. While there is no universal formula for accomplishing
this task, prototyping techniques such as rapid tooling and selective laser sintering can help companies quickly produce models of medical devices or components that are as close to perfect as possible. But not too perfect.