Good Vibrations

in Vol. 20 - November Issue - Year 2019
How To Overcome The Post Processing Challenges Of Additive Manufacturing?

Picture 1: Some components created by 3D printing

Picture 2: Metal parts with precise internal cooling channels

Picture 3: Cranial plate made from CAD model of CT scan

Picture 4: 3D printed part (blue) with support structure (grey)

Picture 5: Initial surface roughness of 3D printed metal part

Picture 6: 3D printed metal part finished by mass finishing

Picture 7: Value-added chain of additive manufacturing

No manufacturing technology is being talked about more than additive manufacturing: While some people expect AM, also known as 3D printing, to completely revolutionize today’s manufacturing environment, others are a bit more cautious and see AM as just another component-shaping technology like casting and forging. Without doubt, AM is having a great impact on how products are designed and produced. And the rapid technological advancements in this field are making AM more attractive and competitive. But like any other technology, AM must also overcome some downsides, which can be expressed in two simple words: Post Processing!

The AM manufacturing technology is coming of age

While until not too long ago, 3D printing was mainly utilized as a highly effective prototyping method for primarily plastic components (Picture 1), it is rapidly evolving as a technology for producing low to mid volumes of standard plastic but also all kinds of metal components.
3D printing offers significant advantages over conventional manufacturing methods:
- It allows the uploading of complex component designs in the form of CAD models to a dedicated production machine ("printer") that instantly produces the physical component without requiring any expensive molds or tooling. This helps reduce development lead times from months to just days.
- It allows creating highly complex components with a precision that is practically impossible with casting, forging or any other shaping method.
For example, it allows placing precisely located, intricate internal cooling channels in tooling components with diameters as small as 3 mm (Picture 2).
Or, medical implants can be individualized by creating a CAD model from a CT scan and sending this model to a 3D printer (Picture 3).
- 3D printing started out with the creation of plastic components. Today there are systems on the market that can create metal components from all kinds of alloys like stainless steel, titanium, cobalt chrome Inconel and tungsten carbide, to name just a few. Even zirconium, wax and silica sand can be used as raw materials; the latter, for example, for making sand casting molds.

Post Processing issues with additive manufacturing

But additive manufacturing presents a few technical challenges that must be overcome. Once a component has been printed, it must undergo quite a few post-processing steps before it is ready for its intended purpose:
Separation of the AM component from the build plate and removal of powder residues ("unpacking")
In case of metal components, the additive manufacturing process takes place on a so-called build plate. Once the additive cycle is complete, the component must be separated from the build plate. No such build plates are required for 3D printing of plastic parts.
AM uses loose powder that is either completely melted or sintered to create the desired plastic or metal component, built up layer upon layer. This process leaves residues, either in the form of loose and/or caked-on powder that must be completely removed.

Removal of support structures
A component is taking shape layer-by-layer. That is why, any sections extending horizontally pose the risk of collapsing during the build process. The solution is supporting these extensions with support structures, which become an integral part of the component (Picture 4). Upon completion of the build process, the support structures must be removed.

Smoothing of rough initial surface readings
Compared to casting and forging, AM creates much rougher initial surfaces with Ra readings as high as 40 μm or more (Picture 5). To cut down these surfaces to a smooth or polished condition with Ra values of sometimes 0.1 μm can be a daunting task.
Some approaches for resolving these issues are described below.

Solutions for the post processing challenges

Separating the build plate from the AM component
This can be done by saw or by wire EDM (Electrical Discharge Machining).
The part design must allow for additional stock of a few mm between the AM part and the build plate to allow separation without jeopardizing the dimensional integrity of the part.

Powder removal
Loose powder can be removed by simply tilting the build plate so that the powder can slide off.
Simple compressed air blowers or air suction systems can also be utilized. Since the powder generally has a very small grain size, it represents a health hazard and, if blown off by air, should be handled with caution!
Other possibilities are to completely rinse off the parts, remove the powder by vibration, or use brush systems.
Caked-on powder is a bit more difficult:
Shot blasting is one method. This applies to external surfaces as well as internal passages. These may, however, require very thin, flexible blast lances to reach the critical internal surface areas. As some metal powders like titanium and aluminum are highly reactive, the equipment must be protected against potential fires or even explosions.
Other systems are using electrochemical methods for removing the caked-on powder. Since such systems use aggressive acids, containment (the prevention of acid spillage) is of crucial importance.
Both shot blasting and electrochemical systems offer the advantage of achieving a certain surface-smoothing effect.

Support structure removal
Depending on the support structure thickness and material, different methods for their removal can be applied:
Mechanical methods:
Cutting the structure off with a saw, breaking it off with pliers, or shot blasting (if the structure is very thin).
Electrical / electrochemical methods:
Electrochemical removal with acids, EDM (Electrical Discharge Machining), TEM (Thermal Energy Machining).
Even ultrasound methods are sometimes used for support structure removal.
We can look forward to a lot more development work relating to this issue. And it is safe to assume that within the not-too-distant future, reliable, automated and cost-efficient support removal systems will become available.

Surface smoothing and polishing
Practically all known surface refinement methods like abrasive flow machining, electro-polishing, electrolytic deburring, laser polishing, etc. can be utilized.
However, it looks as if the dominant surface refinement methods for AM components will be shot blasting and mass finishing:
Both create homogeneous, all-around "isotropic" (multi-directional) finishes;
Both can handle all materials, from the toughest metals like titanium and cobalt chrome to all kinds of polymers and even ceramics;
Whatever the task, shot blasting and mass finishing produce consistent, absolutely repeatable finishing results at surprisingly low costs; and
Users can choose from a broad equipment spectra that allow treating components of practically any shape and size.
Occasionally, shot blasting and mass finishing are employed in combination, with shot blasting creating an initial surface cleaning and cut-down effect while mass finishing provides the final surface smoothing and polishing. This allows reduction of the initial surface roughness readings of AM components from Ra = 40 μm or more down to a final finish of as low as Ra = 0.1 μm.
Shot blasting and mass finishing are equally effective in cleaning the internal passages of 3D-printed parts.
No doubt, as with support structure removal, a lot of development work with regard to equipment adaptation and media development lies ahead. But shot blasting and mass finishing have already proven that they can handle practically any surface refinement task for AM components.

Post processing issues must already be addressed during the design phase

Whenever an AM product is designed from scratch, or an existing component is redesigned for 3D printing, post processing considerations must be an essential part of the design process. Here are some questions that must be addressed:
- What material allowances are required for build plate and support structure?
- What machining operations are needed? For example, AM is not very good in creating round holes; or critical mating areas must be machined.
- Is heat treatment required, for example, in removing residual stress in metal components?
- How will support structures be removed?
- Are extra holes required to drain loose powder from the component?
- How smooth does the component surface have to be? Does it have to be polished?
- Which surface finishing methods can be utilized?
- Must the components be dyed with a special color?
These questions can only be answered by experts. For this reason, it is advisable to cooperate with knowledgeable organizations studying the manufacturing process in its entirety -- from the initial design to the finished product (Picture 7); those having a clear understanding of the interdependence between the various AM production stages.
A leading manufacturer of surface treatment equipment has established a separate division doing exactly this, even offering 3D printing services for prototyping.

Good Vibrations
by Eugen Holzknecht , Contributing Editor MFN
andRösler Oberflächentechnik GmbH
E-mail: holzknecht.usa@gmail.com
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