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Article citation: , (2011) "Parts without limits – additive manufacturing", Assembly Automation, Vol. 31 Iss: 3, pp. -
Additive manufacturing is a design-through-manufacturing method that is becoming increasingly popular amongst design engineers, and has generated a significant amount of dialogue lately within the design and manufacturing community. By providing a means of producing low quantities of various designs and eliminating the need to leverage economies of scale and produce higher and higher quantities of a fixed design, additive manufacturing has demonstrated that it has the potential to redefine the way machines and products are designed and manufactured.
This technique provides a method to improve quality, while decreasing the costs and lead times of products and machines. It reduces cost by eliminating expensive tooling and assembly part count through part consolidation. It increases the efficiency of your development process because you can now manufacture new, slightly different parts in just a few days. Use of additive manufacturing methods drives innovation, enables faster and better machine designs, and provides quicker machine deployment.
Additive manufacturing is the method of using rapid prototyping (RP) equipment to manufacture end-use parts. RP machines make parts through a variety of additive fabrication processes. Parts are made from the bottom-up by adding the appropriate material to the build space. This layer-by-layer process nearly eliminates all part design constraints or rules that currently exist with traditional manufacturing processes like CNC machining and injection molding.
Currently, there are three primary RP technologies (Figure 1) that can manufacture parts suitable for use as end-use parts: fused deposition modeling (FDM®), selective laser sintering (SLS®), and Stereolithography (SLA®).
Each RP technology has its strengths and weaknesses that must be considered. To be a viable replacement for traditionally manufactured parts, additive manufacturing parts must meet all of the typical application needs for strength, function, accuracy, and appeal. All three of the primary current technologies, FDM®, SLS®, and SLA® meet those needs. Selecting the best one for an application depends entirely on the specific needs of the project. If your top priority is the overall strength of the parts, you may find that FDM and SLS systems have a slight edge over SLA systems. All three systems make parts that meet tolerance and accuracy requirements. SLA systems, however, offer the best surface smoothness and manufacture parts the fastest. SLS systems resist heat more than FDM and SLA systems.
Traditional design has always required a good understanding of the constraints the manufacturing process imposes on parts. Training courses in design-for-manufacturing (DFM) and design-for-assembly (DFA) have helped provide the needed knowledge of these constraints.
For example, parts that are designed to be made by CNC machines must not have narrow, deep pockets because the rotating cutter of the machine cannot cut such features. Parts designed for injection molding need drafted walls in the direction of the tool movement to release the part from the tool after molding. Injection-molded parts are often designed specifically to be free of undercut or die-locked features in order to avoid more expensive tooling and per part charges. The DFM and DFA rules exist to enforce the constraints of the part’s manufacturing process.
One reason for the delay in the broad adoption of additive manufacturing techniques is insufficient expertise in how to design parts and assemblies that take advantage of the design freedoms it provides. However, this new school of thought is gaining ground rapidly.
Additive manufacturing enables a part to be made from the bottom up, layer by layer, significantly reducing design constraints. Those narrow deep pockets, for example, are no longer a problem. Similarly, you can include reverse draft in your part, or handle internal, hidden channels.
Part design is no longer compromised by machine tooling. Often, parts requiring an investment in tooling become locked in an unchangeable design to avoid the cost of reworking the tooling or making new tooling. Additive manufacturing, however, does not involve a process that requires expensive, long lead time tooling, and does not require high quantity economies of scale in order to be cost-effective. Thus, it encourages active redesign as you iteratively learn what works and what does not.
This capability not only handles intricate product designs, it promotes product flexibility, allowing customers to change features or continuously improve products without penalty. Since parts made with additive manufacturing have no tooling commitment, changes can be made “on the fly” based on customer or performance feedback. Such proactive evolution drives innovation and improvement, and allows engineers to remain focused on the needs of the customer.
In addition, additive manufacturing enables a design to be manufactured within a few days of creation. Thus, companies no longer need to face a warehouse full of obsolete products, and instead, can reap the benefits of tighter inventories.
To take advantage of additive manufacturing, engineers need to shift their design process. For example, many manufacturing processes force the use of multiple parts because they cannot accommodate certain types of complexity. Additive manufacturing, on the other hand, lets you consolidate parts considerably, combining several parts in an assembly into a single part.
For example, consider the robotic arm (Figure 2). The original design for the wrist consists of three plates, three standoff posts, and two adapters, for a total of eight parts, not including the screws. With additive manufacturing, that assembly is combined into a single part; a part that would be impossible to make with traditional CNC or molding methods. Additive manufacturing eliminated tooling for those eight parts, and the bill of materials is reduced by seven parts.
Additive manufacturing excels when parts are designed to be made together. This is a new way to think about DFA. Look at the hand of the robotic arm of Figures 3 and 4. Its original design requires separate parts for each finger, palm pads, joint pins, and washers. The layer-by-layer-based manufacturing version, however, provides a complete single hand part that still meets the product requirements for function, accuracy, and strength.
In this example, 15 separate parts are reduced to one, which reduces inventory. The design also eliminates unique tooling for each of the parts, which reduces cost and lead time. Changing the hand “on the fly” to suit customer needs, such as shrinking or expanding its size, is simple.
In most cases of additive manufacturing, if you can design the part in 3D CAD software, then you can manufacture the part in an RP machine.
All manufacturing processes indeed have limitations, even layer-based additive manufacturing. The most notable limitations involve the capabilities of the materials used to make parts.
RP machines have been making parts for more than 20 years, but only recently have the materials been strong enough for end-use commercial applications. Medical and food grade ABS, polycarbonate, Nylon, and epoxy, can all offer mechanical properties on par with production injection-molded plastics.
Surface finish can be a limitation too. Additive manufacturing parts cannot produce a smooth surface finish comparable to CNC machined or molded parts. Tolerances in layer-based manufacturing are good and well established based on part size, however, they are not quite yet at the level of CNC machined or injection-molded parts.
Design for manufacturability (DFM) is the general art of creating new designs in such a way that they are easy and inexpensive to manufacture. Anyone who has ever designed a product to be injection molded likely learned along the way that small changes to the design could significantly impact the cost, time frame, and overall success of the manufacturing project.
This is true for any additive manufacturing project as well. Being aware of a few common mistakes made throughout the design process can help minimize costs and delays, and help prevent the creation and delivery of unsatisfactory parts that require further changes and rebuilds in order to meet the needs of the customer.
Pay close attention to not only the native CAD design of what is to be produced via additive manufacturing, but also the converted.STL version which is often required. The.STL file format is the standard data interface between CAD software and most additive manufacturing machines. A.STL file approximates the shape of a part or assembly using triangular facets.
“Even well-conceived designs with the best of intentions can present a potential problem when coverted to.STL format and submitted for additive manufacturing”, says Patrick Hunter, VP of Sales and Marketing for industry leader Quickparts. “This is why we make a point to review the files our customers submit to us, and address any issues we find before parts are built, rather than after they are delivered”.
Before submitting a design for any additive manufacturing project, keep an eye out for these seven common mistakes concerning part design and file conversion.
Keep these seven common mistakes in mind when considering any additive manufacturing project. Be careful to confirm the integrity of the original CAD data, and be mindful of living hinge designs, enclosed or trapped hollow spaces, clearance between mating features, and any features or walls that are smaller or thinner than 0.030″. After exporting the.STL file from the native CAD file, take time to confirm that the overall resolution of the file is sufficient and that the selected units of measurement are correct.
To successfully use additive manufacturing, it is important to clear your mind of previously learned constraints. Instead, imagine parts with obscure organic shapes, or with internal volumes. Think about multiple iterations of an evolving design available in a short time frame, rather than months in advance for high quantities of a fixed design. Consider past approaches and the constraints imposed by other manufacturing processes and ask what parts can be consolidated into one.
Then, identify a candidate project, such as a current sub-assembly. Apply the additive manufacturing principles to create a design free from constraints.
For more information, please visit the web site: www.Quickparts.com