Originally published July 1996
In January 1988, the first rapid prototyping system was installed at Baxter Healthcare for use in the development of medical devices. Now, eight years later, approximately 1500 systems have been installed worldwide, and an entire new industry has arisen to provide rapid prototyping services to industrial customers. Virtually every engineer involved in the development of mechanical components has a working knowledge of several systems, and many actively use rapid prototyping in the development of their products. This article, the first of two, will discuss the reasons for prototyping, present a brief overview of the available systems, and offer some considerations regarding the decision of whether to purchase a system or buy services from a service bureau. A subsequent article will cover the prototyping of injection-molded parts, the process of urethane casting, and the creation of prototype tooling.
THE COST OF DESIGN ERRORS
There are two primary reasons for prototyping a new product design. The first is to verify design adequacyto ensure that the design will meet the requirements of the product. The second reason, less well recognized, is to identify design errors and problems as early as possible in the design process. The later in the development process design errors are discovered, the greater the cost to correct them. The price tag associated with correcting design errors after work on the tool has begun can be staggering. Elements of that cost can include tool rework, shorter tool life, late delivery of product, reduced design flexibility, and loss of goodwill within the production team.
Tool Rework. The most obvious cost is rework of the tool in order to correct the design errors. Discussions with several tool builders indicate that the expense of rework averages 10% of the cost of the tool, although actual cost varies with the extent of the changes made. Furthermore, 75% of all tools built come back for rework. The implications of these numbers are significant: on average, for every single project, companies spend 7.5% of the cost of the tool on changes made after the tool is begun. Given the cost of tooling, this can amount to several thousand dollars per part.
Shorter Tool Life. Every time a tool is reworked, its life decreases. Welding, annealing, and repeat heat-treating all reduce the integrity of the tool and the number of shots available before it must be replaced. Considering that 75% of all tools must be reworked, an average tool-life reduction of only 3% adds 2.25% of the cost of the tool to every development project.
Late Delivery of Product. In most cases, the additional time required to redesign and rework a tool will result in a late product introduction. At a minimum, a three-month delay in introducing the product represents three months of lost sales. However, the true loss is usually higher: if a product is late to market, it will ultimately garner a lower market share than if it had been delivered on time. A leading management-consulting firm concluded in a study that if a medical product is six months late to market, the total profits generated by it will be reduced by one-third. Obviously, this can represent a potential loss of millions of dollars.
Reduced Design Flexibility. One consequence of late rework not often recognized is that if a company discovers a design error after work on a tool has begun, it typically will only consider design changes that allow it to salvage the tool; changes that would require scrapping it will not be considered. The result is that the final version is a compromise design, not the best designa liability affecting the quality of the part that goes to market.
Lost Goodwill. When a product must be reworked, goodwill is lost, regardless of whether the fault lies with the manufacturer, the tool builder, the molder, or some other processor. As a result, the chances of the same team of players working again on future projects is significantly diminished. A cohesive group working together on successive projects can learn how to be more efficient, reducing costs and lead times; a new team starts over again.
How can the tremendous costs accompanying late design changes be avoided? Arguably, the best way is to prototype the design before the tool is ordered. Critics maintain that prototyping only delays getting a product to market. However, the time and cost of correcting a faulty design far outweighs the time and cost of the prototyping process. In this sense, prototyping can be viewed as product development insurancea way to minimize the downside effects of design errors.
New rapid prototyping methods introduced in the last few years have provided a means of creating prototypes faster than was previously possible, to the point where prototyping can often proceed concurrently with tooling acquisition. The first few weeks of ordering a new tool typically involve specifying the steel and preliminary configuration before any machining of cavity or core surfaces takes place. Rapid prototyping methods can now be used to complete the prototyping process during this period, so that any design errors found can be corrected without rework costs and without delaying the development process.
RAPID PROTOTYPING METHODS
There are currently nine rapid prototyping methods commercially available in the United States. These technologies share several common characteristics:
- All build parts directly from a computer model, with no machining or tooling required.
- All use a file structure called an .STL file as a starting point. The .STL file is a faceted representation of the exterior surfaces of the part and can be created by most major CAD systems. It does require a complete three-dimensional representation of the design to start with; a two-dimensional CAD file is not adequate.
- All work by building the part in a succession of thin layers, each usually a few thousandths of an inch thick. Calculating these successive layers or cross sections is an integral aspect of the specific process and is controlled by software supplied by the manufacturer of that process.
Each of the methods will create reasonably accurate representations of a design directly from a CAD model. The systems do vary widely, however, in a number of areas, including the material used; the manner in which the layers are created; the size and accuracy of the part that can be built; the strength, surface finish, and functionality of the resulting model; and the ability of the model to be used as a pattern for a subsequent process such as urethane casting or investment casting. Which of the processes is best for a particular application will depend on the requirements of the prototype. Table I provides a short summary of each system.
THE MAKE-OR-BUY DECISION
As it does with many components or services, a company that needs rapid prototyping initially faces a "make-or-buy" decision. There are a wide variety of systems available, ranging in price from about $40,000 to well over $500,000. Whether it makes sense to purchase a system will depend on a number of factors:
- Obviously, the higher a company's volume of prototyping, the easier it is to justify the purchase of a system. The break-even point depends on the purchase price of the system. However, if requirements are for 10 or fewer parts a month, it will be difficult to justify the purchase even of a low-end system.
- Because each of the rapid prototyping systems works from a complete three-dimensional CAD model, a certain operator proficiency level is assumed. If a company does not routinely use solid modeling, it probably should not consider a system purchase.
- If the rationale behind the prototyping is to create simple model parts to be used to illustrate new designs and convey ideas, any of the systems can be used. Creating functional prototypes, however, requires one of the more expensive, higher-end systems as well as shop support for operations such as mold making and part finishing.
The alternative to purchasing a system is to contract out with a service bureaua company in the business of providing rapid prototyping services to industrial customers. Working with service bureaus has several advantages. By using different bureaus for different applications, a client can take advantage of the various rapid prototyping processes, choosing the best method for a particular job. In addition, most service bureaus have developed extensive experience in certain applications, and can greatly increase the chances of success in those types of projects. On the other hand, a manufacturer working with a service bureau cannot prioritize projects to the same extent as it can with its own equipment.
CHOOSING A SERVICE BUREAU
There are now more than 125 rapid prototyping service bureaus in the United States, and more than 230 worldwide.1 However, not all service bureaus may be capable of meeting a company's prototyping needs. Following are some of the questions a manufacturer might want to ask prospective service bureaus to help narrow the field:
- What type of rapid prototyping process do you use? Of the nine current technologies, some have significant size limitations, others may not be accurate enough to make models that can serve as patterns. The service bureau should be able to describe the type of equipment used and explain how applicable it will be for a particular project.
- Do you have the equipment yourself or are you subcontracting the work to another firm? A number of companies selling rapid prototyping services do not own equipment, but rather subcontract the work to other firms. Although they may be able to provide excellent parts, questions can arise about who is responsible should problems occur. In addition, it is generally more expensive to deal with a bureau that subcontracts compared with one that makes the prototypes itself.
- How long have you been providing services? Operating virtually any rapid prototyping system requires substantial experience before quality parts can be reliably and consistently produced. If a service bureau received its equipment two weeks ago, the chances are minimal that it can make parts of the same quality as can a company that has been using the equipment for several years.
- Can you handle our CAD data? Most service bureaus can handle a variety of CAD formats, but any incompatibility can add considerable time and expense to a project. If a customer can supply data in .STL format, there should be no problem; if it can't, the customer must make sure the service bureau can handle the data without having to remodel the geometry in its system.
- Can you provide customer references? When asked by a new client, any reputable service bureau will gladly provide the names of customers for whom it has completed similar projects. If it declines to provide the information, it is likely that the bureau has little experience with the type of job requested.
Few markets can match the pace of technological innovation now occurring in the medical device industry. The benefits offered by rapid prototyping respond directly to the importance the industry places on research and development and to its critical need to bring products to market quickly. The second article in this series will describe some of the specific ways in which prototyping is currently being used to improve the development of medical products.
1. Published sources for locating rapid prototyping service bureaus include the Rapid Prototyping Directory (CAD/CAM Publishing, Inc., San Diego); the Thomas Register (Thomas Publishing Co., New York City); and the MPMN Buyers Guide: The Medical Product Designer's Sourcebook (Canon Communications, Inc., Santa Monica, CA).
Tom Mueller is a founder and vice president of Prototype Express (Schaumburg, IL)a rapid prototyping service bureauwhere he is responsible primarily for sales and marketing. Prior to joining the company, he was manager of computer-aided engineering at Baxter Healthcare, and presided over the installation and operation of Baxter's stereolithography department.