Rough Realities of 3D Printing (Additive Manufacturing)
3D Printing, or officially known as Additive Manufacturing (AM) by ISO and ASTM (ISO/ASTM 52900), seems like the end all to all manufacturing processes. The one true process that is able to replace every conventional manufacturing processes like CNC machining, forging, sheet forming, injection molding eg.
In reality, it is not.
AM has been around since the early 90s, previously coined as Rapid Prototyping, where manufacturers used this technology mainly for well, prototyping. It wasn’t until AM OEMs and powder metallurgical developments made sufficient progress that only major manufacturing industries started to look into AM adoption. Even as of writing, developments in AM is still a huge race worldwide.
One of the biggest challenges that AM is facing, be it plastics or metals, is the surface finishing of AM surfaces. The simplest explanation is that fundamentally, AM is a layer-by-layer process. This means that no matter how small your base material is (be it powder or liquid form), the surface of your AM part will always have a layering effect — Just image a beautifully made Mille Crepe Cake.
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.
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.
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.
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.
