Polishing 3D Printing (Additive Manufacturing) Parts
Okay, now that you’ve printed your Additive Manufactured (AM) parts, what’s next?
(For powder-bed fusion AM) Powder Removal: Check ✓
(For non-liquid based AM) Support Removal: Check ✓
Surface Polishing: Grit Blasting… or something more?
Grit blasting is a typical process that AM practitioners utilise to smoothen or texture AM surfaces. As you (should) know*, AM surfaces has distinct layer-by-layer surfaces due to the nature of AM (layer-by-layer printing)*.
*Check out my previous post for my introduction to surface finishing for AM: https://jeremyho93.medium.com/rough-realities-of-3d-printing-additive-manufacturing-9339f728e8ed
But other than aesthetics, why would we need surface finishing?
Easily said — Industries adopting AM for their critical, complex or high valued parts such as aerospace or medical parts would require very shiny surfaces. E.g. Turbine blades, Blisks or Vanes for good airflow in an aerospace industry or knee implants in the medical industry.
Imagine the implant rubbing against the skin INSIDE your knees — Ouch!
Before I get into the surface finishing processes, I would need to acknowledge that sometime rough, as-printed surfaces are a good thing for some parts. An example would be the other end of a knee implant. A rough surface is required for bio-adhesion to your muscles, thus making AM a likely choice.
But wait! Rough on the inside while smooth on the outside? Seems like selecting an appropriate surface finishing process is getting more complicated.. Let’s use our knee implant as an example to compare these surface finishing processes shall we?
Generally, the question one need to ask:
1. What’s the roughness value I desire?
2. Which are the areas I need to finish? Note that I don’t use the term ‘polish’ as it’s likely we require different textures… such as the knee implant.
3. What is my part material?
There are way more in-depth technicalities to consider in the process selection such as possible debris impingements, risk of Intergranular Oxidation (IGO) / Intergranular Attack (IGA) or Intergranular Corrosion (IGC). I won’t go into such details as it’s usually for very critical components like aerospace parts exposed to under extreme conditions… and this article probably isn’t really the best place to come to seek for advise for critical products.
1. What’s the roughness value I desire?
The figure below roughly explains processes that could achieve your desired surface finishing. One thing to note for new practitioners, and technologist alike — Do consider processes holistically such as cost, manufacturing process chain, supply chain, production line planning. Also, it is imperative to understand that surface finishing processing for conventionally-made parts might not be similar for AM-built parts. For example, a knee implant made by machining might only need basic buffing (front surface) and grit blasting (back surface) but an AM-build part needs a ‘stronger’ or more aggressive surface finishing process to remove the layered as-printed surface.
2. Which are the areas I need to finish?
For situations such as the knee implant, since there are mainly two areas — very polished and very rough. There are 2 choices to make:
A. Choose a process that is able to target specific areas on a part.
B. Mask the areas that we do not want polishing and choose a process polishes the whole external surface.
Processes that fall under Choice A that might suit knee implants could be milling, lapping, grinding, or rotary (mass) finishing.
Choice B looks at mass finishing (abrasive) such as vibratory or barrel finishing, electro-polishing, chemical polishing, or even a combination of both: Electric + Vibratory = Metal Dry Lyte ; Chemical + Vibratory = ‘Isotropic Superfinishing’
Also, for the rougher side of the knee implant, if AM as-printed surface has not met the roughness requirement or profile, one could look at grit blasting but using a higher grit size and harder material (Silicon Carbide maybe) with masking on the other side.
I should mention that since AM is useful for complex parts, it’s likely people might explore parts with internal features. Some of the processes mentioned such as electro-polishing could work, but inaccessibility makes it a challenge. One conventional process that seems to be popular is the Abrasive Flow Machining (AFM), which utilises a slurry to carry abrasive particles across the internal channel for polishing.
3. What kind of material is my part?
Plastic is not metal. Obvious right? Then it should also be obvious if I say Aluminum is not Titanium. Why did I say that? A common misconception is that surface finishing processes are a means to an end.
Obviously it’s not that simple, and if it was, there wouldn’t be any R&D for it. (And in my opinion, it’s still lacking especially for the AM industry)
As mentioned earlier, there are many potential issues a part might face after polishing, such as making the surface more prone to rust (IGO), corrosion (IGC/IGA) or even material from the grit (in grit blasting) or media (in mass finishing). One key thing to take note is the influences for material-to-material interactions.
Is the material plastic or metal? Is it ferrous? Is it chemically inert or will it corrode from the chemicals used in the surface finishing process?
Conclusion
Surface finishing process selection is a vigorous and critical process to enable the desired outcome of your part or product. It is quite ironic that the perception (and I do get this feedback quite a lot in the industry) of surface finishing is that it’s just a means to an end.
Think about it — If you had already spent so much effort on procuring raw material, sending it to manufacturing, carefully preparing the design and machine, preparing the material such as pouring powder (if it’s PB-F and hopefully the powder doesn’t explode), and finally spending hours, or days even, printing it, removing support and powder… and doing basic grit blasting or throwing the whole lot into a mass finishing machine (and hoping the parts don’t accidentally hit against each other to cause holes in them and making them unusable). Whew.
Luckily, as more industries are adopting AM, there seems to be more traction in the understanding of how critical surface finishing is. It is imperative that more research and development is needed to improve existing capabilities to suit AM surface finishing, along with more understanding that surface finishing for AM isn’t just a ‘means to an end’, and that it really isn’t that simple.
