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The Secrets to Designing for Metal 3D Printing Revealed

Posted on Oct 15, 2018 by Cullen Hikene

3Diligent Among Experts Featured in Design for Metal 3D Printing Article

Cullen Hilkene, CEO of 3Diligent, was interviewed by Ann Thryft of Design News about a topic seemingly on every designer’s mind these days: metal 3D printing.

In particular, the intention of the article was to speak with experts across the digital-manufacturing industry with expertise in metal additive about how designers need to think differently about the process. With a clearer understanding of design rules, designers stand not only to have more successful builds but also can take fuller advantage of the broader design possibilities of metal additive to deliver entirely new and better designs.

The article provides:

  1. General design guidelines
  2. Specific rules of thumb
  3. List of differences between metal and polymer additive-manufacturing design

3Diligent is featured alongside industry leaders such as GE Additive and 3D Systems in the article.

Click here to read the 22-page article.

Design for Metal 3D Printing Guidelines

Below, we’ve recreated the 10 Rules of Thumb, which drew significantly from 3Diligent’s design for additive-manufacturing experience:

Design Rules of Thumb for Metal AM Design

These top 10 design rules are aimed at powder bed fusion processes for AM—primarily direct metal laser sintering (DMLS) or selective laser sintering (SLS), which are the most commonly used metal AM technologies.

1. Avoid supports: post-processing hassles.

When considering laser powder bed 3D printing with metals, the most important thing design engineers need to be aware of is supports. The most dollars lost in 3D printing with metals occurs because of iterative design: too many iterations are needed just to get the supports to work or to get rid of them completely. In the laser powder bed, the part cools unevenly. So residual stresses are built into the metal as it hardens, which makes it ‘potato chip’ while it is being built and when you take the part out.

To cope with this, there are some things engineers need to do that drive design. The first is designing supports to hold the part down to the metal plate so it doesn’t potato chip. The other reason for supports is to conduct heat away from the top layers of the part.

But now, you have to cut out those supports with a tool when the part is done. This really becomes a problem when your part has internal passages; you have to get a tool in there to cut them out since it’s not like plastic, where they just break away or can be dissolved in water. And you need enough room to remove them. This is real design-for-manufacturing. You must either design the part to not need internal supports or design it so you can get a tool in there if needed.

—PADT’s Miller

2. Avoid supports: the 45-degree rule and overhangs.

Consider the process mentally as you’re designing. With powder bed, thermal stresses are created by the build, so you may need to use supports. As a general rule, you want to design to avoid supports. To do that, typically you want to orient the part in the build chamber to avoid overhangs of more than 45 degrees. Generally speaking, angles more vertical than 45 degrees are self-supporting, but angles under 45 degrees will need supports. Because these supports are made of the same metal material as the part, removing them can be time-consuming and costly.”

—3Diligent’s Hilkene

3. Avoid thin-walled features.

As a general rule, you don’t want walls less than 500 microns thick and we recommend a minimum of 1mm. That’s not the thinnest achievable with 3D printing—we’ve done 150 micron thick walls—but they’re not tall. A 40 to 1 ratio for walls is also a good rule: keep to a 40mm height for a wall thickness of 1mm. Operate at a ratio bigger than that and you’re putting that feature at risk.

— 3Diligent’s Hilkene

Design engineers need to know what wall thicknesses work with a manufacturing process. For example, with additive manufacturing, if it’s a big block of metal, you’ll get a lot of shrinkage. And the transition from thin to thick can cause shrinkage problems. There are certain minimums for different additive metals processes, for certain manufacturers of 3D printers for each of those processes, and even for different models within a manufacturer. For each manufacturer of laser powder bed 3D printers, the specific laser, and its settings often determine wall thickness. There are similar issues for other metal 3D printing technologies.”

—PADT’s Miller

4. Stair-stepping has post-processing consequences.

Engineers must be aware of the fact that because AM with metals is a layered manufacturing process, the main consequence is stair-stepping. How does this impact design? Engineers must be aware of the typically rough surface that results, and they must specify the desired surface finish. Can you and your customer live with that roughness? If not, you must specify the surface finish you can live with. That means knowing the machine’s parameters and how to tweak part parameters if you’re the operator. Design engineers aren’t taught in school about manufacturing issues, so they often don’t know this.

—PADT’s Miller

5. Beware additive’s orthotropic planes.

The other main consequence [of stair-stepping] is that the microstructure is unusual compared to other metal manufacturing processes. In the X-Y plane, the crystal may be quite large, but it’s also really thin. In the Z plane, of course, the metal crystallography is different and the material properties are different from those in the X-Y plane. In other metal manufacturing processes, the properties are isotropic in all planes. But with additive metal, they are orthotropic.” “Stair-stepping is the most visible aspect of the differences between different printer manufacturers’ product lines. It constrains some of the geometry since the minimum feature size in the Z direction is layer height. You can’t make features smaller than that height, and the feature has to be positioned at the beginning of that layer.

—PADT’s Miller

6. Orientation matters.

Orientation matters: the design engineer should specify part orientation relative to build direction. Stair-stepping, supports, and not making features thinner than layer thickness—these constraints are all dependent on part orientation in the machine. For example, if you build a cylinder-shaped object standing upright in the powder bed, no supports are needed and you won’t get much stair-stepping. But if you build it on its side, you will need supports and stair-stepping will be more pronounced. When you design machined metal parts, thinking about setup steps is critical, since you want to minimize them, and it’s the same for additive.

—PADT’s Miller

Which direction the part is made in—oriented in either the X-Y plane or in the Z-axis—affects the area of the cross-section and therefore how much material shrink occurs in a given layer. Design engineers should try to have gradual changes in the cross-section to avoid drastic changes in material shrink.

—Protolabs’ Utley

7. Design at the system level, not the part level.

A trap we see lots of people fall into is thinking about part-for-part replacement of the current product: today I’m stamping a part and tomorrow I’m going to print it. But from a productivity perspective, it’s important to find the opportunity at the product level and not look for only a piece-part replacement. In the GE Catalyst engine, GE Aviation reduced 855 parts to 12 printed assemblies using 3D printing and the redesign is made possible. The innovation was not a piece-part replacement, but in thinking about this product different from the beginning—by looking at the system, at parts consolidation, at soft costs, at optimizing the business case.

—GE Additive’s Schuppe

8. Combine subassemblies.

You can combine subassemblies that don’t need to be separate into a single assembly. You can also make new features like captured features, where elements of a design are built inside or interlocked with other features. When doing this, you’ll just need to be mindful of how you’ll remove powder and whether it can be printed without supports.

—3Diligent’s Hilkene

9. Design for post-processing removal of excess material.

“Since 3D printing is good for small quantities, some customers want to give us a design they’re already machining and see if we can print it more affordably. But you only occasionally see a cost advantage in doing that, because the parts were designed with a different process in mind—especially if you’ve already invested in tooling to support that process. To unlock the potential of additive, you should really design with the process [of 3D printing] in mind. One thing it can do is remove excess material where it’s not contributing meaningfully to the part’s performance. This is commonly done in applications like aerospace and high-end cars. There is a class of software referred to as topological optimization or generative design that is focused on this area.

—3Diligent’s Hilkene

10. Design for post-processing removal of loose powder.

Loose powder in internal chambers will need to be removed, so you may want to design with drainage holes for it to escape. You also need to consider clearances between features for removing lightly sintered powder. For example, there may be lightly sintered powder near a wall feature that’s even harder to get out of crevices than loose powder.

—3Diligent’s Hilkene

 

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