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By Tony Holtz
Design is a matter of balance: weight vs. strength, cost vs. quality, speed vs. accuracy. As development progresses, prototyping is an essential part of the balancing process, and prototyping itself presents the designer with choices, all of which should be part of a designer’s “toolkit.”
In plastic part design, advancements in manufacturing technologies have created numerous quick-turn, 3D CAD-based prototyping tools from which to choose including 3D printing, CNC machining, and injection molding.
Industrial-grade 3D printing (3DP), or additive manufacturing, continues to stretch its utility as a complementary product development component to traditional options like machining and molding. Commonly used processes are stereolithography (SLA), selective laser sintering (SLS), Multi Jet Fusion, PolyJet, fused deposition modeling (FDM), and more. Each of these builds parts by joining—fusing, curing, extruding—layers of material to create durable prototypes.
CNC machining is also regularly used in the prototyping realm for form, fit, and functional testing of plastic parts. Many aerospace and medical industries leverage the production-grade material selection that machining brings to the table to accelerate to, and through, gate approvals and development phases.
Another prototyping option is rapid injection molding, which uses CNC machining to mill aluminum molds, rather than steel mold, to gain efficiencies in production time and cost, without sacrificing much in part quality.
Finally, there is traditional injection molding, which is used primarily for production, but could conceivably be used to create prototypes, granted at a significant upfront, and potentially risky, tooling investment.
Each method has strengths and weaknesses:
- 3D printing is fast and can reproduce complex and organic shapes impossible via traditional means. With no upfront tooling costs, it can be inexpensive as long as only a few parts are needed. However, there are limited economies of scale as costs can rise with quantity. Also, a limited (but growing) range of materials are available.
- Rapid injection molding uses metal molds to produce truly functional parts. It has a wide material selection of resins and color options and multiple surface finishes, from textured to high gloss, are attainable. It is similar to traditional injection molding, though far faster and brings with it a much more affordable point of entry—think $10,000 (aluminum) vs. $100,000 (steel) for tooling, for example.
- Traditional injection molding wins 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.
Considerations during prototyping include quality, cost, and speed. The required “quality” of a prototype can vary greatly. In early design stages, the resemblance to production parts can be approximate, but as development moves closer to launch, prototypes must more closely match finished parts.
There are a few areas from which prototypes can be evaluated. The first is form and fit—resemblance in shape, size, finish, and possibly even color to production parts. The other is function—resemblance in strength, durability, chemical resistance, heat tolerance, and other required material properties. Generally stated, the characteristics of the various prototyping methods are as follows:
|Injection Molding (Rapid)||Injection Molding (Traditional)|
|Form and fit||Good||Good||Good to excellent||Excellent|
|Recurring costs||$10s||$10s||$0.10s to $1s||$0.01 to $10s|
|Lead times||1-7 days||1-3 days||1-15 days||Weeks to months|
|Other considerations||Good for complex shapes; limited but growing material choice||Commercial-grade material selection||Can be used for bridge tooling or low-volume production||Most cost effective in long production runs|
A designer’s tools
As mentioned, many designers develop a “toolkit” of 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 to potentially speed up operations in another. For example, members of a design team may produce one, several, or even dozens of iterations of a part on paper or in their CAD program before creating a prototype. They may then create a series of physical prototypes using 3D printing or machining. These quick and relatively inexpensive prototypes can be used to adjust the look and feel of the piece.
Once that has been determined, the designers can move to functional testing, using rapid injection molding to produce a few hundred molded pieces. Because these parts can be produced quickly and from more than a hundred different thermoplastic resins, they are ideal for testing the strength, durability, chemical resistance, or heat tolerance of a part in real or simulated use. If a similar resin is required, the same rapid injection molds can be reused to produce the parts in a different material. Or, if flaws are discovered in the design itself, new molds can be quickly and inexpensively produced, or the same mold can often be altered if the change is minor and metal-safe. In some cases, rapid injection molds can even be used to produce longer runs for market testing.
A production option for a prototype mold?
If parts are ready to move into production, steel molds are considered the traditional route. However, in some cases, the original aluminum molds created for injection molding may serve as a production option—aluminum molds become production molds. This is especially the case if:
- The final production run ranges between 10,000 and 100,000 parts
- Bridge tooling is needed for preliminary production while steel molds are being produced
- Time to market is critical
This low-volume, on-demand production option can help companies manage demand volatility, reduce overall cost of ownership, and ultimately optimize their supply chain.
Strategic integration of different prototyping tools
As the aforementioned table illustrates, when choosing prototyping methods, you’ll need to consider a number of elements: form and fit, function, costs, volume, and more. Having a range of prototyping options in your toolkit can help streamline the design process. The right method at the right phase of development saves time and money, allowing for a more effective and cost-efficient iterative development process. Dollars saved can be reallocated, time saved brings products to market faster, and better prototypes mean a better end product. In today’s competitive markets, speed, quality, and affordability are a hard combination of benefits to ignore.
About the author:
Tony Holtz is an applications engineer at Protolabs. Holtz has been with Protolabs for more than 15 years with roles ranging from CNC mill operator to mold designer to customer service engineer. While his formal education is in industrial machinery operations, he has extensive knowledge and experience in both traditional and advanced manufacturing processes and materials. Throughout his tenure at Protolabs, Holtz has worked with countless designers, engineers, and product developers to improve the manufacturability of their parts.
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