An MD&DI September 1997 Column
Designers often need to develop parts early to determine which materials will maintain necessary tolerances in manufacturing complex devices.
Designing complex disposable products can be a formidable challenge. For some devices, such as an automatic corneal shaper, designers face the task of finding a rapid tooling solution that can produce the plastic injection mold to deliver surgical precision. Because the reusable medical device must be disassembled and sterilized for each patient, the risk of assembly error is increased. The cutting blade of the shaper needs to be manufactured with a fixed slice thickness, eliminating the possibility of miscalculation.
Stereolithography and 3D Keltool provide an integrated approach to rapid tooling, such as in the production of sintered metal mold core and cavity inserts.
A device such as the corneal shaper, which consists of 18 unique and intricate components, requires a method to mold parts in the early phases of the development cycle for testing. A rapid tooling process such as 3D Keltool (3D Systems, Valencia, CA) creates plastic injection molds from master patterns in as few as eight days.
Developing a complex product adds steps to the product development cycle. The shaper, for example, needed to be tested in advance in an end-use material that resembled its final physical and cosmetic characteristics. The material had to be optically clear and easily sterilized. And, most important, it needed to hold the final dimensions and tolerances between the cutting edge and the cutting surfaces during assembly. Since the part contained numerous compound curves, the designers used the rapid tooling process, which held tolerances of ±0.005 in.
Rapid tooling offered the designers a hedge against program slips in the product development process. While the engineering team was finalizing the design, the tooling created with 3D Keltool made it possible to injection mold parts and continue functional testing and real-world troubleshooting with production quality parts. As a result, design delays were minimized because any design flaws could be detected visually and corrected less expensively at an early stage in the product cycle and prior to final production.
Another challenge the project faced was demonstrating the feasibility of making the device out of plastic. Since the original design tolerances were not possible using conventional tooling, the design team needed a way to assemble and test various project concepts. Using the mold created with the rapid tooling process, the team was able to injection mold various plastic materials for part evaluation in an exact representation of the intended design. The parts were subjected to radiation sterilization at various doses and then evaluated for color and clarity.
After the mold inserts were designed, mold insert masters were produced using stereolithography, a rapid prototyping process that produces plastic parts. These were handfinished and used to make the core and cavity steel inserts (positive and negative of design) to create the tool for the plastic injection mold. In three weeks, the team was able to mold the parts and begin critical subassembly of the instrument: the blade alignment.
Joe Frantz is president of Frantz Industries, Inc. (Mentor, OH).