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Understanding Surface Finish in Metal 3D Printing of Valves

While metal AM provides a host of positive advantages, it’s important to understand the realities of as-printed surface roughness. This understanding will help valve design and engineering teams understand the costs associated with bringing 3D printed parts to a suitable finish.

Among the benefits of metal 3D printing are product-development flexibility and the ability to produce parts so complex they’d be impossible using conventional methods. While these positives are significant, there’s one area in which metal AM has historically struggled: surface finish.

Why Surface Finish Matters

How much a disadvantage metal 3D printing has as far as surface finish varies significantly by application. Sometimes, roughness may not matter at all, and it can even be a good thing in certain situations. However, most components, especially valves, will eventually end up as part of an integrated system that requires a level of internal smoothness to allow precise control over flow.

Metal 3D printing, regardless of material, has struggled to produce those smooth finishes.

Reducing friction on the internal channels where fluids and gases travel is a priority for any metal additive manufactured valve. Therefore, the surface finish on internal channels of additively manufactured valves in the “as-printed” state will be a challenge for the foreseeable future.

Printed Metal Parts vs. Conventional Manufacturing

It’s important to quantify the trade-offs associated with 3D printing a component as opposed to using CNC machining, casting or metal injection molding. While many variables influence the final roughness average (Ra) of a part, metal 3D printing produces parts with greater surface roughness than do the other methods. Metal injection molding typically delivers the smoothest finishes (with a typical Ra of 30−50), while casting is relatively rough (ranging between a 100 and 500-micrometer Ra).

surface finishAn example of the rough finish of AM-printed titanium alloy brackets. Note the roughness of the surface before machining. (Image courtesy of NASA.)

Metal AM, on the other hand, has a very rough surface finish—typically in the 250−400+ Ra. Part of the challenge is that there are many types of metal AM processes, and each has its own roughness. The specific type of printer used, part complexity and materials used can all play a major role in the final roughness of a component. The range of technologies has resulted in a range of surface finishes between 200−1000 Ra, making it harder for manufacturers to pin down the exact smoothness they should expect.

Impact of Surface Finish on Part Costs

The bottom line for these issues is that the closer to the final required finish before post-processing, the lower the finishing operation will cost. If a part is 800 Ra and needs to be < 50 Ra, much work needs to be done to accomplish this.

Before settling on a production technique for a given component, manufacturers should factor surface finish requirements into the overall cost equation. No matter what production process is used, parts must meet design specifications that often include measures of surface finish. If the end-use application of a component calls for a roughness average of under 200 micrometers and the part exits the build chamber with a Ra of 500 micrometers, that’s a problem that will take time and money to rectify.

Nearly all 3D-printed components require some form of secondary finishing, which adds time and cost to the manufacturing process.

Common Surface Finish Post-Processing

A number of common post-processing techniques can bring a 3D printed part to the necessary smoothness. Disc finishing, high-energy centrifuge and stream finishing have all been applied with success in reducing the roughness associated with metal AM. These processes aren’t always controlled in-house, however. Anytime a manufacturer has to outsource post-processing, frictional costs are incurred: shipping and administrative expenses come into play on top of the actual costs of having the finishing done. There’s also an increased risk of quality issues creeping in when multiple parties are involved. At the extreme, post-processing costs can erode the advantages 3D printing brings in the first place by adding steps (and time) to the manufacturing process.

Using Metal AM to Manufacture Valves

The design limitations of traditional manufacturing, especially machining, are clear any time a component needs to twist and turn internally to help direct the flow of liquids or gases. If the machining tool cannot reach an internal feature, the part must be created as two halves and then be brazed together. Additively manufactured valves can be made in one print instead, regardless of internal geometric complexity. As more engineers start to embrace additive manufacturing in the design phase, they will begin to utilize the true potential of the technology and create valves that are impossible to produce any other way.

To continue to widen the addressable range of manufacturing needs metal AM can meet, it’s imperative that surface finish in metal AM improve dramatically. There is now technology that can create a level of finish on par with the smoothest casted surfaces. By moving beyond design for manufacturing (DFM) and towards design for additive manufacturing (DFAM), valve producers can take full advantage and take a major step forward in making valves through this technology.


Matt Sand is president of 3DEO.

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