Rough Realities of 3D Printing (Additive Manufacturing)
By: Jeremy Ho
Tags: Industry, Manufacturing, Surface Finish, 3D Printing, Additive Manufacturing

Now, for most parts, AM surfaces are not ideal, especially parts requiring very shiny surfaces such as turbine blades or vanes in an aerospace industry or even knee implants in the medical industry. Image the implant rubbing against the insides of your knees — Ouch!

Before we proceed to the solution to solve this rough (pun-intended) situation, I would like to highlight that AM surface, as shown in the pictures earlier, is not technically ‘rough’. A surface is classified into 3 categories — waviness, form and roughness.

Decomposition of the measured profile in roughness, waviness and form error (Source: DOI:10.14393/19834071.2012.13669)

Therefore, the layer-by-layer AM surface is not actually ‘rough’, but technically ‘wavy’- Imagine waves at the beach. Although in some powder metal printing processes, improper sintering/melting of powder metal causes unexpected pimples to form on the surfaces, especially underside of a surface, which could then technically be called a ‘rough’ surface if looked at it as a whole.

Rough surface at the down-skin of a powder metal printed part due to improper powder sintering (Source: http://additivemet.com/wp-content/uploads/2016/07/design.pdf)

Generally, there are two ways to solve this issue — Additive or Subtractive methods.

An additive method is basically coating additional material onto the surface. This coating is usually something that augments the functionality of the part. An example would be a thermal coating for aerospace engine part that could be subjected to high heats during operations. One thing to note is that coatings are usually bonded to the AM surface, and mechanically, requires as much surface area as possible for a better bonding. This means that a rough AM surface is actually a good feature for such a coating process, albeit this is truly an over-simplification of the coating process for AM.

Subtractive method is basically to remove enough material from the surface until the desired surface roughness outcome. To put it in a simpler, layman understanding — Imaging a closed-up image of an AM surface is a series of mountains. By removing the tips of the mountains, you are effectively flattening the mountains into a flat and smooth surface.

Cutting the tips of the mountains would get you flat and smooth surfaces

There are multitude of additive and subtractive methods available in the market. The key things to note before selecting the right method, if you are already an AM practitioner or an aspiring one, are:

What is your part’s use case? Is your part production high? Does it have a highly complex surface? Any internal surfaces? (this will reduce the number of methods available tremendously)

At the end of the day, inexperience practitioners don’t consider the surfaces generated by AM until it is actually printed. Considerations such as the selection of a surface finishing solution would enable your part design to more comprehensive. This means that before even considering how to print, one should already consider what to do after printing — Finishing.