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3D printing and other rapid prototyping technologies can lead designers astray because they don’t have the same constraints that injection molding, machining, and other production technologies are subject to. In some cases, a 3D printed prototype may satisfy a project’s functional requirements but may have overhangs or other features that make it difficult to cast or injection mold. In other cases, some a rapid prototyping system may not be able to achieve the accuracy or resolution needed that a production process can deliver.

We asked several experts in prototyping for tips on avoiding capability mismatches, and strategies for choosing the right prototyping process for a particular application and published an abbreviated version of their answers in the October 2017 print edition of PD&D. Thanks to the magic of the web, we’re able to post the full text of their responses here.

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Naftali Eder: - Stratasys Ltd: First, consider the part’s physical characteristics produced by the 3D printing technology used, which can vary greatly in terms of strength, detail and finish. PolyJet inkjet-based 3D printing, for example, uses materials with high detail surface finish that accurately represent all of the very fine details of your CNC or injection molded parts, whereas FDM technology based upon the fused layers of real plastic filament would better represent the functionality of the end-use part. When designing parts requiring high tolerance mating surfaces or aligning parts – PolyJet would be ideal. When designing products that require load bearing or high–temperature capability, then FDM is a stronger fit. Holes should typically be designed undersized, while mating surfaces should be designed oversized and then subsequently machined.

For elements featuring thin walls, consider that the z-axis strength of 3D printed materials may not stand up to the functional requirements of the end product. For such cases, use buttresses or ribs at the base of walls, or bulk the walls up, just for the prototyping stages. Consider a ‘temporary’ clamp in place of screw threads that you would add to the prototype design just for the sake of keeping the two parts together in testing, which would then be removed in the final design. The correct selection of composite materials such as Stratasys Nylon 12 Carbon Fiber may also help here.

Just because you can 3D print it, doesn’t mean you can manufacture it using traditional processes. 3D printing vendors often make the mistake of showing off certain capabilities of the printer that don’t translate into the real world of production. Try to avoid geometries that are challenging for subtractive manufacturing, such as blind holes, internal voids and undercuts. Even if you can prototype as a single integrated part, for the sake of fit and functional testing, break the part along the same seam-lines as the final product would be. And finally, use 3D printing for more than just the product you’re designing. Use it to build the manufacturing aids, the jigs, tools and positioning equipment that can help within the subsequent production line itself.  

Naftali Eder is an application engineer with the Rapid Prototyping Solutions Business, Stratasys Ltd.

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Colin Blain - 3D Systems: If a product is to be ultimately manufactured by anything other than Additive Manufacturing (AM), there is a distinct possibility that the design freedom afforded by AM may inadvertently produce a design that may be difficult and costly to manufacture.

One way to try and avoid this is for the designer to engage with the manufacturing engineers at the earliest opportunity - especially at the prototype stage where mistakes are less costly to address.

In the first instance, and in order for the most appropriate technology/material to be selected, it’s essential for the designer to understand and communicate the requirements (of the end product) to the engineers responsible for the prototype. Specifically, the final material and manufacturing process, and any specific accuracy and surface finish requirements. Armed with those facts, the prototyping engineer should be able make informed recommendations.

Where function of the prototype is to be assessed (as well as form), invalid results may be obtained, and unnecessary design changes may subsequently be made, if the wrong material and/or technology are used. This even applies to the orientation of the component, which can affect performance, both from a structural standpoint as well as surface finish – a key fact that is often overlooked, or possibly not even understood, by the designer.

In addition, if specific features are more crucial than others, it’s imperative this information is communicated at the prototyping stage. Part orientation (in AM) can often be a juggling act, and it’s also not uncommon for recommendations to be made (by the prototyping engineer) regarding adjustments to the geometry to make printing easier – these potential changes must, in turn, be discussed with the manufacturing engineer to ensure additional cost and unnecessary complexity isn’t being designed into the part.

Colin Blain works in advanced application development at 3D Systems. He  has been involved in both new and existing applications development in many sectors including medical, consumer goods, aerospace, automotive and motorsport. He was instrumental in Formula 1's early adoption of 3D printing - particularly Stereolithography - and was part of the team that developed 3D Systems' first composite SLA material, specifically developed for wind tunnel applications.

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Eric Utley - Proto Labs: This topic cuts to the core of effective design, because the functionality of prototypes are pivotal to the eventual creation of accurate parts that go into well-designed, final products for the marketplace.

Moving between 3D printing—or additive manufacturing—and injection molding, machining, and other production methods can be tricky business at times.

To ensure a smooth transition from additive to molding and machining, designers should consider machinability and moldability fundamentals early in the development process. This would include looking at items such as draft, radii, uniform wall thickness, and other elements. A first stop should be using design software or online tools to help analyze digital part designs for manufacturability before printing that first copy.

Material considerations are also important because 3D printing now offers several distinct processes to choose from. For instance, technologies such as selective laser sintering (SLS) and Multi Jet Fusion (MJF) use commercial-grade nylons, direct metal laser sintering (DMLS) produces parts in fully dense metal (that is, if you’re considering metal rather than plastic), and PolyJet can be used for early stage prototyping of flexible elastomeric part designs.

Although it may seem that the emergence of a variety of 3D printing technologies could cause confusion, the expansion of platforms actually helps solve the challenge of rapid prototyping with 3D printing technology, which can achieve an accuracy or resolution that is comparable to a production process. The best recommendation here is that designers should ask themselves what elements are the critical criteria of the part, such as tight tolerances or material properties, and then use the 3D printing technology that addresses those criteria the best.

In fact, having several 3D printing processes available, helps designers and developers choose the technology that’s best suited for their needs, and provides more options than ever to help those prototype designs move seamlessly from additive manufacturing to molding, machining or other methods.

Eric Utley is a 3D printing applications engineer at Proto Labs.

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Dávid Lakatos - Formlabs: The biggest issue in any product design or engineering company is getting a project delivered on time and on budget. At its inception, 3D printing was focused on helping engineering and designers achieve just that by making prototyping in-house easier than ever.

As with all manufacturing tools, when 3D printing, an engineer or designer needs to be aware of the limitations and features of the tool. While 3D printing is quickly gaining traction in manufacturing, the vast majority of “things” produced today are still made using non-3D printing technologies. Even during the prototyping phase, engineers and designers should be asking themselves: what tool will I use to manufacture this part? Then, they should design the part with that tool’s constraints in mind. Iterating between a 3D printed prototype and a prototype developed with a different process (usually closer to the final manufacturing method) can help decrease the gap between prototype and final part.

Today, 3D printing is a great choice for concept exploration, testing design iterations, developing proofs of concept, and designing for manufacturability. In some cases, 3D printing may even be the best option for production -- for instance, in cases of low volume production of individually customized parts like hearing aids, dental aligners, or manufacturing jigs. In other cases, maintaining design-for-manufacturability in whatever methods are ultimately going to be used in the product (investment casting, injection molding, sheet metal, etc.) in printed parts can seem counter-intuitive, but will keep your process honest to what those end capabilities can produce.

Dávid Lakatos is chief product officer at Formlabs. He is responsible for delivering desktop 3D printing experiences for engineers, designers, and professionals.

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John Sidorowicz - Xcentric Mold & Engineering: Each manufacturing process has advantages and limitations, especially when you factor in speed, accuracy, materials, and cost. Defining what is most critical to your project may help as you begin the design process, and can certainly prepare you in choosing the best method of manufacturing. Designing for each process often has a different set of rules and making design considerations at the start of your project will impact the costs associated with prototyping and production.

Firstly, when designing a part to be prototyped using additive manufacturing (or 3D printing), it is important to understand up front that some design features may need to be altered for injection molding, or production. Building these design considerations into design versions may help you reduce the needs for re-design at each stage of the project. A few items for consideration:

  1. Large parts outside the space limitations of your chosen process can be broken up into smaller pieces and assembled later.
  2. Avoiding sharp edges in your design can improve accuracy.
  3. Geometric angles larger than 45 degrees will require support in 3D printing, which could limit the complexity in your prototype version.
  4. Thick walls and hollowed interiors can reduce print time; however, they are features that will need to be re-designed for a process like injection molding.

Secondly, it’s important to remember each manufacturing process has its own selection of materials that can be used. Even similar materials may behave differently from process to process. For instance, a part made from ABS plastic using an FDM method will not be nearly as strong as one that was injection molded due to layering. It's important to understand the availability of these materials by process, especially if accurate material representation is critical to your part or assembly.

Finally, don’t forget to ask an expert! Lean on the technical support of your manufacturing partner, rapid prototyping to injection molding, we can help answer your technical questions or make sure to put you in contact with one of our materials partners to help with any compatibility needs. 

John Sidorowicz is VP of Sales and Customer Service at Xcentric Mold & Engineering.

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Greg Paulsen - Xometry: I want to make sure we un-blur the edges between rapid prototyping and direct digital manufacturing using additive technologies (3D printing). Additive manufacturing is another set of tools in the shop to build parts, and just like injection molding has wall thickness considerations, or CNC machining dislikes sharp internal corners, there are ways to design for additive that can make the process a solid step in production. Designing for an additive process and using it for end-results is direct digital manufacturing and can give significant design flexibility versus subtractive technologies. Unfortunately - the knowledge for most users is not as solid and there is a lot of trial and error!

With 3D printing it is the printer, and not the print, that dictates how your part will turn out. Tolerances are global and the machine is reading off the 3D CAD you have provided. As a general rule, I usually expect any powder bed fusion processes like SLS or DMLS to run large by a few thousandths of an inch. So, if your design has a critical edge that should be +0.000, -0.010”, I would recommend the CAD be configured to the minimum end, allowing for up to 0.010” growth versus designing to the nominal (or heaven forbid, the maximum extent). I know that creating a special CAD configuration just for 3D printing on a rapid prototype sounds like unnecessary labor, but it’s often the difference between your prototypes mating or needing to manually rework.

Additive can be a serious contender in the manufacturing marketplace – especially as “mass configuration” becomes an expectation, and technology updates happen more frequently. It pairs perfectly with other traditionally manufactured components. For example, I see 3D printing’s strength in designs that may require frequent rev changes like IOT devices with PCB brackets. Custom tools you use in assemblies and end-effectors in robotics. With metal printing, we are seeing a rise in parts that used to be cast or metal injection molded. You can take advantage of more relaxed design considerations inherent in the technology. If you’re apprehensive, talk to application experts and read design guides for best practices!

Greg Paulsen leads Xometry’s applications engineering team which consists of additive (3D printing) and traditional manufacturing experts. Xometry.com is an online manufacturing platform that serves as a one-stop-shop for custom parts.

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