Design is a matter of balance: weight vs. strength, cost vs. quality, speed vs. accuracy, and so on.  As development progresses, prototyping is an essential part of the balancing process, and prototyping itself presents the designer with choices.

The Options
In plastic part design, technology has given us a variety of prototyping options.  Rapid prototyping (RP) includes stereolithography, selective laser sintering, fused deposition modeling, laminated object manufacturing, and three dimensional printing.  Each of these builds parts, one-by-one, from 3D-CAD models, joining layers of material to create the finished prototype.  Rapid tooling (RT) uses rapid prototyping to create an initial part and then creates, from that part, a mold in which additional parts can be made.  Mold materials can range from silicone rubber to composites.  A third prototyping option is rapid injection molding (RIM), which works directly from a 3D-CAD model, using CNC machining to mill aluminum molds in which true injection-molded parts can be made.  Finally, there is traditional injection molding, which is used primarily for production, but could conceivably be used to create prototypes. 

Each method has strengths and weaknesses.

• Rapid prototyping is the quickest, and can reproduce very complex shapes.  With no up-front tooling costs, it can be inexpensive as long as only a few parts are needed.  However, because each part is made from scratch, RP offers no economies of scale and its costs rise rapidly with quantity.  Parts can only be made from a limited range of materials and are typically left with a coarse finish. 
• Rapid tooling can sometimes produce better quality parts than rapid prototyping, though materials choice is still somewhat limited.  It is also slower and more costly due to the extra step required to create a tool from the original prototype.  The need to create molds also increases up-front cost and can limit the complexity of shapes that can be effectively duplicated.
• Rapid injection molding uses metal molds to produce truly functional parts with good finish and in a wide variety of resins.  It is similar to traditional injection molding (though far faster and much less costly). It is competitive with rapid tooling for speed and offers better economies of scale than rapid prototyping or rapid tooling. 
• Traditional injection molding can produce the ultimate in part complexity and finish, but is generally considered too slow and expensive for prototyping, though it may be used when there is a high likelihood that the molds will go directly into large-scale production. 

Key Characteristics
Characteristics of a prototype include quality, cost, and speed.  The required “quality” of a prototype can vary greatly.  In early design stages, the resemblance to a production part can be approximate, but as the process moves toward completion, the prototype must more closely match the finished part.  There are two measures of quality.  The first is form and fit – resemblance in shape, size, finish, and possibly even color to a production part.  The other is function – resemblance in strength, durability, chemical resistance, heat tolerance, and the like.  Generally stated, the characteristics of the various prototyping methods are as follows:

	Rapid prototyping (RP)	Rapid tooling (RT)	Rapid injection molding (RIM)	Traditional injection molding
Form and fit	Good	Good to excellent	Good to excellent	Excellent
Function	Poor to good	Fair to good	Excellent	Excellent
Non-recurring costs	$0	$100-1000s	$1000s	$10,000s
Recurring costs	$10s	$1s to $10s	$0.10s to $1s	$0.01 to $.10s
Ideal quantities	1-10	10-100	25-5,000	10,000+
Lead time	1-5 days	5-10 days	3-15 days	Weeks to months
Other considerations	Good for complex shapes; limited material choice	Limited material choice	Can be used for bridge tooling or production	Most cost effective in long production runs

Obviously, there is no single best choice for all needs.  Rapid prototyping, for example, can be a good choice for quickly determining form and fit, but would generally produce poor parts for functional testing.  Rapid injection molding, on the other hand, is somewhat more expensive, but produces ideal prototypes for functional testing.

A Designer’s Tools
Many designers are developing a “tool kit” of plastic prototyping methods, choosing a specific technology to fit the needs of a project or a particular project phase.  This lets them allocate resources, using money saved in one phase speed up operations in another.  For example, a team may produce one, several, or even dozens of iterations of a part on paper (or on-screen) before creating a prototype.  They may then create a series of physical prototypes using a rapid prototyping or rapid tooling method.  These quick and relatively inexpensive prototypes can be used to adjust the look and feel of the piece. 

Once approximate look and feel have been determined, the designers can move on to functional testing, using rapid injection molding to produce a few hundred injection molded pieces.  Because these parts can be produced quickly and in any of hundreds of resins, they are ideal for testing the strength, durability, chemical resistance, or heat tolerance of a part in real or simulated use.  If a particular resin is wanted, the same rapid injection molds can be reused to produce the part in a different material.  Or, if flaws are discovered in the design itself, new molds can be quickly produced.  In some cases, rapid injection molds can even be used to produce longer runs for market testing.

If the part passes testing, traditional steel molds may be ordered for the final production run.  In some cases, the original aluminum molds created for rapid injection molding may be pressed into service as bridge tooling for preliminary production while steel molds are being produced.  Or, if the final production run is not too large, or if time to market is critical, the aluminum molds may actually become the production molds. 

Making Choices
In choosing plastic prototyping methods, you need to define both technical requirements and business constraints. 

• If form and fit are your primary issues, some rapid prototyping or rapid tooling options will work, as will rapid injection molding.
• If you need parts for functional testing, the materials limitations of rapid prototyping and rapid tooling may be a problem.  Injection molding, rapid or otherwise, is more likely to support the resins that will be used in production.
• If you need fewer than 10 parts, rapid prototyping will be more economical than rapid tooling or rapid injection molding.  For 10 to 100 parts, either rapid tooling or rapid injection molding may be a better choice.  For more than 100 parts, rapid injection molding is typically the best option.
• For parts in less than three days, rapid prototyping may be your only choice.  If you can wait three days to two weeks, either rapid tooling or rapid injection molding can meet your need.
• If you need more than 10,000 parts – a rare requirement in prototyping – traditional injection molding is probably your best bet. 

Having a range of prototyping options in your tool kit can help streamline the design process.  The right method at the right phase of development saves time and money, allowing more (or more effective) iterations.  Dollars saved can be reallocated, time saved brings products faster to market, and better prototypes mean better end product.  In today’s competitive markets, faster, better, and less expensive is a hard combination to beat.

References
For further information, see the following:

American Precision Prototyping Design Prototyping Technologies The Protomold Company

Xpress 3D, Inc. Wohlers Associates Castle Island RP

American Precision Prototyping Design Prototyping Technologies The Protomold Company

Xpress 3D, Inc. Wohlers Associates Castle Island RP

1)  American Precision Prototyping     2) Xpress 3D,Inc

3) Design prototyping technologies    4) Wohlers associats

5) The Protomold company                  6) Castle Island RP

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