When it comes to rapid prototyping, product design presents you with a series of choices involving the function and aesthetic of the product, the methods of manufacture to be used to realize the product, and physical, financial, timeline, and other constraints on the choices. This month’s tip describes some of the choices facing one designer during a product’s development. We hope you will gain some insights you can use in your own development projects.

The product is Reptangles™, a building toy consisting of identical, faceted turtle-shaped blocks that can be connected in almost limitless ways to create complex three dimensional designs. The concept has been patented as “Multifaceted Nesting Modules.”

The project had three major design challenges: developing the shape of the plastic turtles; creating the connectors—56 per block—that allow the turtles to be connected in a variety of configurations; and designing the blocks so they could be produced in simple and inexpensive straight pull molds in order to keep the product cost low.

Figure 1: The Reptangles’ connectors snap together to resist separation, but are still separable.

The connectors had to be located in a variety of orientations on a faceted block (see Figure 1). This was part of the reason that a simple friction fit, like that of LEGO® blocks, was not workable. A friction fit requires extremely tight tolerances that can be lost with wear, and while LEGO blocks are typically stacked vertically so that gravity helps keep them connected, Reptangles can be connected in virtually any orientation, and thus connections can be pulled in almost any direction. Designer Jonathan Stapleton quickly realized that his connectors would have to snap together to resist separation, but still be separable. He also saw that, in making connections, the faces would not necessarily approach one another in a direction perpendicular to the faces, but could move obliquely toward one another as the faces met.

Moldability was quite a challenge. Because the turtles were to be three-dimensional and hollow, they would be molded in two parts—top and bottom, as in Figure 2—and then joined to form the finished block, but each half still had to be produced in a two-part mold. With faces positioned at 45°, 90°, or 135° to the direction of mold opening, connectors would have to be specifically designed to avoid undercuts.

Figure 2: Top and bottom pieces are joined to form a finished plastic turtle.

Through CAD modeling and prototyping, Stapleton found a solution based on triangular connectors. Right-angle arches on one face would fit into slots on the mating face, and catches in the walls of the slot would create an interference fit (Figure 3). The triangular shape of the male connector allows insertion in any direction within a 90-degree arc.

Figure 3: An arch on the face of the part fits into slots on the mating face.

This design requires a resin with enough flex to allow the catches in the slots to move slightly during attachment and separation of the mating parts but still hold tightly enough to prevent accidental disconnection.

Having solved the connector problem, Stapleton moved on to issues of molding. Dividing the finished block into top and bottom halves allowed connectors to be formed by the interaction of the A- and B-side mold halves. The underside of the male connector is formed by a protrusion through the part wall of the B-side mold half, which meets the A-side mold face in a sliding shutoff. As seen in Figure 4, this allows the molding of male connectors on faces both perpendicular and oblique to the direction of mold opening. Similarly, the female connectors are formed from the “inside” of the part by protrusions of the B-side shutting off against the surface of the A-side mold half.

Figure 4: The end walls of the slots follow the direction of the mold opening rather than the contour of the mating male connectors, allowing all the slots to be formed in a straight-pull mold.

The designer recognized that, if the female connectors were designed to follow the contours of the male connectors, those on the oblique faces would represent undercuts. He solved the problem by aligning the potentially problematic surfaces of those connectors with the mold opening direction (see Figure 4). Because the interference fit is accomplished by the catches of the female connector grabbing the loop of the male connector, this did not impair connector function.

Finalizing the part design was a multi-step process. After CAD modeling and prototyping in wood, Stapleton recognized that the only prototyping methods that would positively confirm moldability were injection molding. The first set of Protomold’s injection molded prototypes showed the need for minor modifications in the design. Fortunately, the needed changes involved increasing the size of some features (“adding plastic”), which could be achieved by modifying the first mold rather than making a new one. The final product was licensed to a toy company and is now on the market, and the designer has filed a patent application for the connectors themselves.

Proto Labs
www.protomold.com

MPF