Forget the details and unlearn the past. Direct Digital Manufacturing (DDM) presents a radical departure in design practices, techniques and methodologies.
Traditional manufacturing methods, like machining and injection molding, have many rules,
restrictions, and limitations. These rules don’t apply when using direct digital manufacturing. Engineers are free to concentrate on the best design and not concern themselves with manufacturability.
Direct digital manufacturing (DDM) is a process unlike any other. Through the use of additive fabrication technology to make products without tooling, molding and machining, DDM gives you a new set of capabilities to make what was once impossible or impractical a reality. These new capabilities eliminate constraints that have ruled the art of product design.
A fundamental advantage of DDM is often touted as the “freedom of design.” While in general terms, it is obvious what this advantage implies; the key concern is how far does this freedom reach? What do you now need to know?
Essentially, design for manufacturability (DFM) rules are discarded. Design is no longer
constrained by the limitations of conventional manufacturing processes. This frees you to design the part for the application without consideration of the manufacturing process. Here are a few
examples of typical injection molding rules that don’t apply to direct digital manufacturing.
• In injection molding, draft angles must be included in the tool or parts won’t eject properly.
• With DDM however, there is no need for a draft on the part. You can even have “negative (back) draft” on a DDM part.
• With injection molding, uniform wall thickness is required in order to minimize warpage and sink marks.
• Wall thickness can be varied throughout a DDM part and thick wall to thin-wall transitions cause no problems.
• Radiused (rounded) corners are required to reduce stress concentrations and improve plastic flow during injection molding. Radii are required on most inside and outside corners of a molded part.
• With DDM, you can have sharp corners wherever desirable.
In injection molding, each material has a specific shrink rate, so it may not be feasible to change materials once a tool has been made.
• With DDM you can change your material with each new build.
By consolidating components, Bell + Howell reduced the number of assembly parts from 26 to 13 and eliminated the need for screws. The photo on the left was the original design, the photo on the right is the redesign.
Although DDM can be implemented without change to existing design principles, a little education will go a long way in getting the maximum value from the process. To get started on the road to DDM success, consider these design tips.
Design Methodology
• Forget design for manufacturability
DDM rewrites product design doctrine, so the most critical step is to start by forgetting what you’ve learned about designing for traditional manufacturing processes. Complex can be fast, cheap and practical when manufactured with additive fabrication technologies.
It is essential to let your mind expand beyond what has been learned through years of education and practice.
• Focus on function
The design process begins with an intense focus on function. Ignore concerns for form, fit, and manufacturability and design parts to achieve the best performance possible. Make the parts as complex, intricate and detailed as needed. For industrial designers, the converse holds true — focus on form and let fit and function follow.
Because DDM uses an additive process to manufacture the parts, cost and time are no longer a function of complexity as they are with conventional manufacturing methods. For the same
reason, design features are rarely impossible to reproduce, so complexity and intricacy are no longer concerns. There are a few process constraints, which depend upon the particular brand of additive fabrication equipment used. They can be addressed at the production stage.
• Iterate
Increase the frequency of design iterations and plan to continue design refinement much later in the product development cycle. Continue to hone the design right up to the day that the product is launched.
Although all additive fabrication processes are suitable for rapid prototyping, not all are suitable for direct digital manufacturing. Some processes can make beautiful models, but not durable parts for end use. The Stratasys FDM® process, for example, creates durable parts from various formulations of engineering thermoplastics like ABS, polycarbonate, sulfone, and blends.
If you use a rapid prototyping process that is also suitable for direct digital manufacturing, then you may use the same equipment for both processes. If so, both the prototyping and the production process will be identical.
Each is completed with little effort, minimal cost, and no delay. The only difference between the final prototype and a production part is its intended use. This low-risk, rapid-turn cycle lets you be creative and push the envelope. There is no penalty for revisions late in the product development cycle.
• Refine the design
DDM can be performed with various additive technologies, so it is important to have a good understanding of the one you will use. Each technology has different specifications in areas such as minimum wall thickness, expected tolerance, producible surface finish, and deliverable material properties. Refine the product’s design to accommodate these characteristics. If you cannot produce the needed part qualities using the additive fabrication process at your disposal, you will have to either outsource production or purchase a system that can produce your product.
With no concern for manufacturability, BMW designed this automobile assembly tool for best function. It would be difficult and costly to machine such a tool.
• Question tradition
Do not let past practices, old traditions, or previous decisions dictate design options and process selections. Question everything. For example, a part previously made of sheet metal may be an ideal candidate for plastic because the rationale for the original decision may no longer hold true. With DDM, a sheet metal enclosure can be converted to a sophisticated, stylized plastic part since there is no tooling to amortize over a small production run.
Design Techniques
• Make it feature rich
With traditional manufacturing methods, each feature adds cost because it must be machined into the part, mold or die. This is not true with DDM. Consequently, never simplify a design for any reason other than product performance or aesthetic value.
• Rethink wall thickness
Many manufacturing methods have a narrow range of recommended wall thicknesses. For example, the sweet spot for injection molding is 0.40 to 0.80 in. When designing a part for DDM, the only consideration is to stay above the minimum wall thickness needed for the part to perform as specified. Also, there is no need to maintain a consistent wall thickness.
To maximize strength while minimizing weight, consider making the walls hollow. In the FDM process, this construction style is called sparse fill. A lattice structure is skinned with bounding surfaces to yield the mechanical strength needed for the application while decreasing material volume. Although the volume reduction can range greatly, a typical application might reduce volume by 60%. Hollowing out features also has the added benefit of reducing material cost and part construction time.
When the materials testing group at Stratasys needed a new test fixture, an engineer drew up this
design in CAD. She had the simple part machined and used DDM to produce the complex flanged sections. The part was in place in 24 hours. The flanged portions would have been difficult to machine.
• Consolidate or segment
Part consolidation is a big advantage of DDM that should be considered at all times. Rather than producing a multi-piece subassembly, the entire unit may be consolidated into a single component. Such consolidation can eliminate assembly and simplify inventory management, which results in lower manufacturing costs. Part consolidation may also overcome an overly tight tolerance specification. For example, a tight-tolerance interface can be avoided by simply consolidating mating parts.
The converse also holds true. A single piece can be segmented into several components without a significant increase in cost. With traditional processes, dissecting a component may not be justifiable because doing so may require more molds, which translates to higher expense. The ability to create a sub-assembly, rather than a single piece, can be an asset when addressing product design considerations like serviceability and replacement cost.
• Fill the envelope
Use every nook and cranny of your product’s available space. Twist, turn, and contort to maximize the use of space and minimize product size. Since machining and molding limitations are removed, think organically and let the design flow.
• Forget the details
When the design is complete, there is no need to invest time to adapt it to process-specific requirements like those common for machining or injection molding. For example, designers do not need to spend time defining parting lines, adding draft angles and determining how to incorporate them without changing form, fit and function. Also, there is no need to resolve undesirable sink marks, ejector marks or knit lines. These types of process constraints no longer exist.
Above all else, be creative. Stretch the skill set, push the design envelope, and challenge conventional wisdom. Never settle for a direct process substitution, because much of DDM’s value will be lost. Always allow the time necessary to design a part, sub-assembly or product to capitalize on the unique capabilities of DDM.
Finally, never stop redesigning. Equally powerful to the freedom of design is direct digital manufacturing’s freedom to redesign. There is no commitment to tooling and little investment of manpower, so a design is never frozen. It is perpetually fluid. Capitalize on this by continually refining designs to satisfy the customer, maximize manufacturing efficiencies and minimize production costs.
MPF
