Why does surface finishing matter in metal 3D printing?

3D printing is quick, incredibly accurate and low cost. But the trade-off is a slightly rougher surface. Emily Newton explains why surface finishing matters so much in metal 3D printing.
Why does surface finishing matter in metal 3D printing?
Surface finishing, which involves adjusting the roughness of metal finishing, is important for any type of manufacturing process, including traditional fabrication. Even with CNC machining, casting, and metal injection molding techniques, the surface finishing of produced components must be analyzed and optimized. But when you compare the other manufacturing techniques to additive manufacturing or metal 3D printing, the surface finishing is much less of a concern overall. Why?

It’s because of how additive manufacturing or metal 3D printing processes develop the items. 3D printing is quick, incredibly accurate, and comes with additional benefits — like cheaper production. But the trade-off is a slightly rougher surface.

Here are some reasons why that is, and why surface finishing matters so much in metal 3D printing:

Why are 3D prints rough?
Not all forms of manufacturing produce optimal results in terms of smooth surfaces and general roughness. Metal casting, for example, can create pretty rough surfaces at times — between 100 to 500 micrometers in Roughness Average (RA). That’s not always the case, however, and not all components need to have perfectly smooth surfaces or edges.

Metal 3D printing, however, will always produce harsher edges and surfaces, for several reasons.

First, the process is partly to blame, as most prints are done layer by layer and the system uses a filament or viscous material to create the component. Each layer of the material is applied and hardens, and the printer moves on to the rest of the component in the meantime. As the material settles and hardens, this creates an imprecise edge. Then the printer returns to the same surface area and prints another layer and the process is repeated until the item is finished.

It can create layering, or what’s known as the staircase effect. This is most evident in curved shapes, slanted pieces, fillets, holes, and other dimensional changes.

What’s more, the type of printer used, the materials applied, printing speeds, printing temperatures, and the complexity of the print can also play a role in the smoothness. Collectively, these elements lead to a more rough surface, which requires surface finishing before the part can be shipped or used. It’s so prominent that surface finishing costs can be up to 60% of the total to produce an additive manufacturing component.

Metal 3D printing or additive manufacturing will usually create parts with a surface finish of 250 to 400 RA, or more. It’s also true of all forms of metal printing, including powder bed fusion, metal binder jetting, and directed energy deposition.

Component application matters
If you’re putting two parts or components together, and you need them to have a strong bond to create a tight seal, then the surface finishing makes all the difference. Both pieces must have a smooth, even surface with no dents, defects, or openings when touching. That’s also true of mechanical parts that will be moving rapidly, as defects can increase friction and result in faster wear, potential damage to other components, or worse. Some components must meet surface finish standards — like with medical components or implants.

But not all builds need to be perfectly smooth. That’s why the component’s application is so crucial. How will the part be used? Sometimes, metal 3D printing won’t necessarily require extensive surface finishing or any kind of finishing for that matter. Specifications and the intended applications should always be shared with the development team or printing party. If the item is being printed in-house, all the better.

Most of the time the surface finish will be integral, while other times it will not. The point is that you need to know and understand how that piece is going to be used, to determine what standards the finish should be held to.

Determining the finishing type
Another element at play is the type of finish that must be used for the component or part. There are many finishing methods, for metal parts and other materials, and the preferred method may be determined by how a part was created. Some methods may be too harsh, increasing the risk of damage or failure and weakening the components in question.

The size of the component, its hardness and strength, the type of material, and even shading or color can all play a role in determining the appropriate finishing method. Machining, media blasting, shot peening, vibratory finishing, electrochemical polishing, and superfinishing are all commonly used with 3D printed metal parts. There are many alternative solutions too, like hot blackening, painting, and powder-coating.

Mitigating costs
Arguably one of the most important factors, depending on the component and the project, is the total cost to develop and finish. It’s even more important with components that are going to be mass-produced or created in bulk.

The costs to create a part, print it, and then apply surface finishing are often separate. But even so, they are still connected. The more work that needs to be done in one case, may also affect the costs in the other. For example, a 3D printed metal part that has an extremely high RA may increase the total production costs, if and when it needs to be extensively refinished to a better RA rating. It stands to reason that most development processes should put the completed parts as close to the final form as possible, including the inherent smoothness.

A balance must be struck between the work done to print or build a component, and the work done afterward during surface finishing processes. Even though the surface finish may be extremely important, it’s still crucial to find a low-cost or affordable process that doesn’t balloon the total production budget.

Surface finishing in metal 3D printing: A smooth conclusion
3D printing tends to create items that are rough or harsh, but the technology is improving. There may come a time when that is no longer as evident or even a problem. Right now, surface finishing is necessary for all types of 3D printed components and parts, metal creations included.

To determine the type of finish, and just how smooth the surface needs to be — in terms of RA rating — it’s important to consider the application of the item. What is it going to be used for? Is the finish key to its general operation, and will an unfinished surface expose the item to a higher risk of failure or damage?

The total development cost is also important, as production budgets can balloon when items are printed with high RA tolerances yet need extensive reworking to smooth out the edges or surfaces.

About the author:
Emily Newton is the Editor-in-Chief of Revolutionized, an online magazine exploring the latest industrial innovations.

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