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Machine Design

Advanced Manufacturing: 21 prototypes later, a nifty design aid for plastic resins

Balancing the conflicting demands of design — for example weight versus strength — can be difficult at best. So most companies build prototypes early in the development process. When Proto Labs in Maple Plain, Minn., recently faced a thorny design challenge, it built 21 prototypes for what would be a hinged, injection- molded “design aid.” The structure’s 30 individual pieces would go into a kit to be assembled by end users. The design aid would show users how six different resins would look with each of four different finishes.

The first challenge was designing the device to be fun and cool. As sometimes happens in design, the initial breakthrough came about almost as if by chance.

Proto Labs UK had recently run a student design contest for which Leonhard Klein and Florian Kössler of the HfG Offenbach University for Art and Design had created a rotatable 3D kaleidocycle. It was a complex, geometric construction that could be assembled from three identical molded plastic units. (See examples of kaleidocycles at

Proto Labs COO Don Krantz saw the piece and suggested that a different sort of kaleidocycle could be used to demonstrate different resins and finishes. As initial proofs-of-concept, he constructed several 3D prototypes out of cardboard and tape.

After building these, the next step was to create 3D CAD models of the structure’s parts and then build plastic prototypes to confirm fit and function of the assembled device.

Needless to say, a better connector than tape was needed for plastic prototypes. The connections forming each tetrahedron could be fixed and permanent, but connections between them would have to flex so the entire structure could showcase each material and finish.

A set of plastic prototypes was first machined out of polypropylene by Proto Labs’ Firstcut CNC machining service. Clips were added for snapping three sides of each tetrahedron together. Clips on the remaining flap of each unit allowed snapping the whole device together. The addition of hinges ensured the structure could flex sufficiently to showcase each resin. Because many of the design issues had been resolved using inexpensive machined prototypes, only two molded iterations of the framework were necessary.

Each resin sample was then molded in five-segment “stars.” Each star was made from a different resin and consisted of four black triangles (each with a different finish) and a “break test” tab. The break test was designed to show the relative performance of the resins in hinge applications. Some would flex easily, others would break immediately, and the rest would break only after repeated flexing.

Molding the resin samples presented a whole new set of challenges. The molded prototype for the samples addressed general design issues including: adjusting the clips on each triangular chip for ease of insertion into the structure; proper placement of cutouts on the chips so they could not be inserted incorrectly; and adding fillets so parts had no sharp edges.

In addition, although published shrinkage specs were used to machine the mold, fits varied from “suitable” to so loose that chips fell out of the frame. For chips needing modification for a better fit, careful thought allowed modifying the molds instead of remaking them from scratch.

All told, Proto Labs created 21 prototypes. This might seem like a lot, but every additional step produced a significant improvement in the final product. The Protomold Protogami is now in production, and kits are available at no charge here.

— Kevin Crystal

Kevin Crystal is a senior engineer at Proto Labs and lead engineer on Protogami

Edited by Leslie Gordon, [email protected] (She assembled the model shown.)

© 2011 Penton Media, Inc.

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