Boston Device Blog

Prototyping Tips - Building Robust and Functional Prototypes

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Building functional prototypes is more than simply printing parts and gluing them together.  It requires a deep understanding of the prototype materials and fabrication methods that are used to construct parts.  In certain cases, part fabrication methods may result in insufficient resolution or materials that are too brittle to withstand certain types of usage.  The various prototyping methods are beyond the scope of this article, but following are some general prototyping tips for creating robust visual and functional models:

1. Use pins to align parts where fasteners are not possible.  In many production devices, parts are snapped or ultrasonically welded together in order to avoid using fasteners.  However, employing these methods in prototypes is typically not practical.  Reason being is that prototypes often need to be opened and tweaked multiple times in order to adjust fit and swap components.  So, the best method of holding parts together while allowing for repeat adjustment is the usage of fasteners.  However, fasteners don't always fit in tight places where a snap, weld joint or adhesive might be in a production design.  In these places we typically use press-fit alignment pins to keep the parts together.  Press-fit dowel pins typically come in increments of 1/32" and are available in hardware supply catalogues such as McMaster-Carr.
2. Build in reveals to disguise parting line mismatch.  When parts are fabricated, they slightly distort due to heating, external forces, painting and a variety of other factors.  If parts without reveals are fastened together, this distortion is quite evident.  Reveals disguise distortion that occurs along parting lines.  A person's eye is unable to detect mismatch between parting lines when a small gap exists between adjoining parts.  We typically design mating components with a lap joint that forces a bottom-out location and defines a reveal between the external parting lines. 
3. Use soft-touch paint to simulate rubberized texture areas.  Many production devices use TPE overmolding to achieve the rubberized texture in human touch points, such as handles, grips, button faces, etc.  This overmolding process can be replicated through urethane casting processes, but this often significantly increases the cost and time of prototype fabrication.  A quick way to simulate overmold areas is to simply finish the parts using a soft-touch paint.  This paint creates a matte finish with a rubber-like texture.  Soft-touch paint does not provide the compliance that an overmolded or low-durometer urethane cast part will, but it achieves the basic look and feel in a fraction of the time and cost. 
4. Build and finish separate parts instead of masking.  Very talented and careful modelmakers can mask parts to simulate color and texture breaks or parting lines.  However, we find that a much more reliable route to creating clean parting lines is to separate parts, paint / finish them separately and attach them together when complete.  Perfectly masking parts (especially those with compound surfaces) is extremely difficult, and overspray is often noticeable when finishing parts in this manner.  While it takes a bit more time on the engineering side to split out separate parts and provide sufficient clearnance, it results in a much higher quality finish.
5. Design parts with sufficient clearance.  Most additive rapid prototyping processes (i.e. SLA, SLS, FDM) run slightly on the large side.  So, when designing mating parts it is critical to integrate sufficient clearance.  You will be kicking yourself if you rely on sandpaper or files to help you out after the parts are fabricated.  For parts that would fit inside of an 8" cube, .010" of clearance is generally sufficient.  

In summary, good prototypes are a result of good engineering.  It requires a bit of additional time and effort to make sure that parts are designed for their respective prototyping methods and materials, but this time is typically much less than the time required to modify parts after they have been fabricated.