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Parts without limits – additive manufacturing

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Article citation: , (2011) "Parts without limits – additive manufacturing", Assembly Automation, Vol. 31 Iss: 3, pp. -

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Article Type: Mini features From: Assembly Automation, Volume 31, Issue 3

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.

A closer look

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^®).

Figure 1 RP machines now use materials strong enough for use as production parts

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.

From expertise in constraints […]

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.

[…] to expertise in flexibility

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.

Thought shift

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.

Figure 2 The original robotic wrist is “consolidated” into a single part, manufactured using the FDM^® process in the ABS material

Made to fit

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.

Figure 3 With additive manufacturing, the robotic hand was designed to be manufactured in the RP machine as a single assembly, with the movable parts designed with clearance and “grown” together

Figure 4 Additive manufacturing enables you to go from design concept to 3D CAD to production parts, without design constraint or tooling investment, but with the freedom to consolidate parts and design for one-build assemblies

Appropriate choices

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.

Top seven common mistakes in designing parts for additive manufacturing

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.

1. The part design has thin features or walls that are less than 0.030″ for standard resolution or 0.015-0.020″ for high-resolution machines – due to the “layer-by-layer” approach of the additive manufacturing process, anything smaller or thinner than this will often times not build and will not be present in the final model. Pay very close attention to raised or recessed logos and areas of small text, “knife edge” features which taper down to zero thickness, and curvy sections of any design where thickness can fluctuate.
2. The native CAD model is converted to.STL format with a very low resolution, resulting in heavy faceting in the model – if the resolution of the.STL file is too low, the model will be faceted instead of having smooth surfaces and curves. This can be quite common and produces unattractive parts. Typically, to achieve a smooth finish on a model there should be an edge-to-edge distance of less than 0.020″ between facets on the.STL file. Check the parameters on the native CAD program being used to determine the best method of exporting acceptable.STL files.
3. The original CAD data has numerous unstitched surfaces (rather than solids), resulting in errors when converting to.STL format – make sure that the surfaces in the original CAD model are “water tight”, in that only solids are modeled. The.STL file can also be inspected to ensure that all dimensions, part volume, and surface area all appear to be correct.
4. The part design has an enclosed hollow space from which support and build materials cannot be removed – any enclosed hollow void in the design will contain support materials which cannot be removed through the finishing process. This area may also be filled with unused resin or powder depending on the selected prototyping process. Consider filling in voids to be solid, building the design in halves to allow access to the enclosed space, or adding a hole of some kind in the model to allow for the removal of the support materials.
5. Assemblies, threads, and mating features are designed with improper clearance – the standard tolerances for most additive manufacturing processes start at ±0.005″ and compound from there as the design increases in size. It is not uncommon for first time customers to receive parts that, while within the published tolerances of the manufacturing process, do not “fit together” or mate up as intended. Typically, there should be a 0.015-0.020″ clearance between mating parts, which is different from what is required for traditional injection molding. This is an important point to remember when the success of the project depends on how well different designs mate up or assemble with one another.
6. The design includes a living hinge which needs to function – living hinge designs on most parts produced via additive manufacturing do not typically function as intended. The build material involved is often too rigid, especially in such a thin section, and will break. While there have been a few materials developed that look to address this need (the Duraform EX material using the SLS process can often work well), expect limited usage from a living hinge design produced via additive methods.
7. The units of measurement for the.STL file differ from what was intended – double check the.STL file properties to ensure that the correct unit of measurement is selected. This is especially true when there is more than one design with varying units of measurement being built together. Some CAD packages also have default settings where.STL files may be exported in a different unit of measurement from what was used during the design process. When there is a tight time line and the project is on the line, it can be difficult to see the comedy in dramatically oversized or undersized parts as they come out of the box.

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.

Conclusion

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