Insight

Post Processing Metal 3D Printing

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Every Friday we’ll post a new video – each one giving you a deeper Insight into how to design better parts. We’ll cover specific topics such as choosing the right 3D printing material, optimising your design for CNC machining, surface finishes for moulded parts, and much more besides.

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Insight: Post Processing Metal 3D Printing

Transcript

Hi. If you watched our insight video last week last well, you will remember that we talked about 3D printing metal using direct metal laser sintering or DMLS.

One of the most fascinating aspects of additive manufacturing is how straightforward and clean it can seem. It can seem like all we need to do to get a great part is to send a CAD file to the machine, click the “go” button and pull out our phone to kill time until the process is done.

However, most of the time the work we put in doesn’t end the moment the DMLS - direct metal laser sintering – machine finishes up. Once a build is complete, there are still a huge number of post-processing options that can improve a part’s dimensional accuracy, surface finish quality, and mechanical properties.

This is becoming especially important as additive manufacturing begins to weave its way into production applications where this post-processing is needed to meet the more advanced kind of requirements that are out there. After all, there’s no room for error when your design is playing a vital role in a successful space launch or in some life-changing medical procedure.

Probably the simplest and most common of these is post-process CNC machining. This is usually needed because, like many 3D-printing processes, DMLS parts can come with support structures that help with the printing but aren’t wanted on the finished product. Typically, these parts are first removed from the build plate using wire EDM or a band saw, and then finally snipped off with hand tools.

This kind of process works well in many cases, but every now and then we’ll need some additional machining options when critical features require tighter tolerances or improved surface finishes.

In general, the DMLS process can produce parts with tolerances of plus or minus 0.1 to 0.2mm. But with post-process CNC machining, we can get these tolerances as tight as plus or minus 0.05mm.

This isn’t the kind of detail you need on every single part, but let’s say you’re designing a fuel delivery system that will be mounted to the body of a rocket, and that some combination of geometry, size and material means that you need to use mating features to make things work.

Now that is somewhere that precision really, really matters, and post-process machining provides a way to produce these incredibly precise mating features while still retaining the design freedom of 3D printing.

Just keep in mind that if your design does require post-process machining, it will need to be fixtured within the mill, so parts with curved or bevelled surfaces can be a little tricky. Your manufacturer’s applications engineering staff should be able to assess whether the part is suitable for post-process machining during the design review process, though, and in some cases, a sacrificial portion can be designed into the source file to aid in the machining process before being removed.

Some of the other big post-processing options out there are advanced heat treatments. These are used to deal with the internal stresses that can build up during the rapid heating and cooling involved in the DMLS process. And, while every metal part should undergo a standard stress relief treatment, additional heat treatments can help to improve a whole raft of mechanical properties like hardness, elongation and fatigue strength.

One of the most common of these comes in the form of Hot Isostatic Pressing, or HIP. This is a technique that involves applying high heat and uniform pressure to fully solidify the part and help to drive out any remaining internal microporosity.  It is another level of control to reduce failure.

Another well-known process is annealing, which I’m sure most of the engineers and material scientists are already rather familiar with. This works by heating the piece to a high temperature and then rapidly cooling it, which changes its internal structure and helps to improve ductility and is most often used for aluminium parts.

The final part of our common post-processing options isn’t the most exciting one out there, but sometimes it can be the most important – quality inspection.

Now, there are a variety of inspection methods and quality reports available to validate a part’s dimensional accuracy and mechanical properties. The simplest of these is first article inspection, where we essentially verify that final parts comply with the original drawing, purchase order, and other specifications, but we can also use dimensional inspection, which uses CMM equipment to ensure part dimensions are within tolerance and align with measurements provided in the original drawing.

However, because many DMLS parts take advantage of metal 3D printing for lightweighting or making complex internal structures, computed tomography scanning is becoming a go-to inspection method as it offers a non-destructive way to validate parts – even those with hollowed-out features or internal channels.

Right, I think I’m ready to call time on this video now, but we’ll be back with another one next week.

Thanks, and see you then.

 

 

With special thanks to Natalie Constable.


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