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Medical device manufacturers are constantly seeking new and better methods with which to design and manufacture medical devices. Recently, they are looking to a traditional prototype process that is enabling the fabrication of commercially ready components. This article looks at low-volume layered manufacturing.

 

COS_Figure1
Figure 1: Rapid prototyping machines now use materials strong enough for use as production parts.

Low-volume layered manufacturing (LVLM) is a design-through-manufacturing method already known by several different names. Whether called “rapid manufacturing,” or the term coined by The Society of Manufacturing Engineers, “direct digital manufacturing,” LVLM has the potential to redefine the way machines and products are designed.

This technique provides a method to improve quality, and decrease the costs and lead times of products and machines. It reduces cost by eliminating tooling and assembly part count through part consolidation. It increases the efficiency of the development process because new, slightly different parts can be manufactured in just a few days. Use of LVLM methods enable faster and better machine designs and quicker machine deployment.

A Closer Look
LVLM is the method of using Rapid Prototyping (RP) equipment to manufacture end-use parts. RP machines make parts through additive fabrication. Parts are made from the bottom up by adding material to the build space. This layer-by-layer process nearly eliminates all part design constraints or rules that exist with traditional manufacturing processes, like CNC machining and injection molding.

Currently there are three RP technologies (Figure 1) that can manufacture parts suitable for use as end-use parts: FDM (fused deposition modeling), SLS (selective laser sintering), and SLA (stereolithography).

Each RP technology has its strengths and weaknesses. To be a viable replacement for traditionally manufactured parts, layered manufacturing parts must meet application needs for strength, function, accuracy, and appeal. All three current technologies–FDM, SLS, and SLA–meet those needs. Selecting the best one for an application depends entirely on the specific needs of an application. If strength is a part need, FDM and SLS systems may 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 will 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 must also be free of undercut or die locked features. 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 layered manufacturing techniques is insufficient expertise in how to design parts and assemblies that take advantage of the design freedoms it provides.

...To Expertise in Flexibility
Layered 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 not a problem. Similarly, reverse draft can be included in the part, or internal, hidden channels can be handled.

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. Layered manufacturing, however, does not involve a process that requires expensive, long lead time tooling. Thus, it encourages active redesign as engineers 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 layered manufacturing have no tooling commitment, changes can be made on the fly based on customer or performance feedback. Such proactive evolution helps engineers stay focused on customer needs.

In addition, LVLM 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 layered 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. LVLM, on the other hand, enables part consolidation, combining several parts in an assembly into a single part.

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Figure 2: The original robotic wrist is “consolidated” into a single part, manufactured using the SLA process in high-impact ABS-like material. The original design called for three plates, three standoff posts, and two adapters, for a total of eight parts, not including the screws. In addition to reducing part complexity, layered manufacturing also eliminated tooling for those eight parts.

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 layered manufacturing, that assembly is combined into a single part; a part that would be impossible to make with CNC or molding methods. Layered manufacturing eliminated tooling for those eight parts, and the bill of materials is reduced by seven parts.

Made to Fit
Layered manufacturing excels when parts are designed to be made together. This is a new way to think about design-for-assembly. Look at the hand of the robotic arm in Figures 3A and 3B. Its original design requires separate parts for each finger, palm pads, joint pins, and washers. The 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, reducing inventory. The design also eliminates unique tooling for each of the parts, reducing cost and lead time. Changing the hand “on the fly” to suit customer needs, such as shrinking or expanding its size, is simple.

 

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Figure 3A (Top): With LVLM, 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. This assembly is manufactured with the SLS process in glass-filled nylon.
Figure 3B (Bottom): LVLM enables the engineer 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.

In most cases of layer-based manufacturing, if the part can be designed in 3D CAD software, then the part can be manufactured in an RP machine.

Appropriate Choices
All manufacturing processes have limitations, even layer-based manufacturing. The most notable limitations involve the capabilities of the materials used to make parts.

Rapid prototyping machines have been making parts for more than 15 years, but only recently have the materials been strong enough for end-use commercial applications. Medical and food grade ABS, polycarbonate, Nylon, and epoxy offer mechanical properties on par with production injection molded plastics.

Surface finish can be a limitation too. LVLM 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 as good as CNC or molded parts.

To successfully use layered manufacturing, it’s important to clear the mind of previously learned constraints. Instead, imagine parts with obscure organic shapes, or with internal volumes. 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 subassembly. Apply the layered manufacturing principles to create a design free from constraints.

Brian Ford is the director of strategic marketing at Quickparts. He is responsible for leading the marketing efforts at the company while developing each of the product lines the company offers. Ford can be reached at 770-901-3216 or bford@quickparts.com.

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