Lifting the LID on AM naming conventions

[TECH]

Though viewed negatively in its beginnings as a threat, Additive Manufacturing (AM) has become a true alternative to conventional production processes throughout many industry sectors. In addition, over the last decade additive manufacturing has become an area of considerable focus throughout the academic world, creating cross-functional research groups and extending the possibilities of materials development.  So whilst it is still viewed as a disruptive process, it has now become a revolutionary force that is galvanising modern manufacturing, and allowing more objects to be manufactured than has ever been previously possible. Perhaps changing the way of the manufacturing world for ever more.

But does everyone really understand what is meant by the term Additive Manufacturing, and is it the same as 3D Printing, or Materials Deposition, or Free Form Fabrication, or any one of the name that has been used?

The use of AM as an umbrella term to describe several different techniques is in relative terms still a recent thing. However, the adoption of this single term still hasn’t prevented the many misconceptions and misunderstandings about the true capabilities of each individual AM technology. One of the very peculiar aspects of the change in perception of AM over time is that is has been both aided and hindered by the very parties seeking to convince the world of its benefits. From the first emergence of objects from a bath of liquid polymer, thus capturing the world’s imagination, to the various technologies that have been developed in the decades since, it is all the names that came with them that created murkiness in each individuals understanding of AM. Furthermore, the lack of clarity on the capabilities of the separate techniques also fed the misconceptions about the effectiveness of AM as a production tool. People still believe it’s possible to press [Print] and have a fully formed, fully finished part at the end of the run. Therefore, irrespective of the technique being considered, the cause of any confusion today is most likely rooted in the lack of consistent naming conventions when that technology was first introduced. It’s true that some of the names that have been devised to launch a type of AM technology have quite frankly been baffling.

In an effort to bring harmony to the fast-growing sector of additive manufacturing, or 3D Printing, the ASTM F42 Committee made an attempt to curb the multiplication of naming conventions with its draft document, F2792 -12a, Standard Terminology for Additive Manufacturing Technologies. This document listed seven process categories, under which it should have been possible to identify any given machine type or technique. These were: binder jetting, directed energy deposition, material extrusion, material jetting, powder bed fusion, sheet lamination, and vat photo-polymerisation. Today these terms do cover most available technologies, and have helped a great deal to standardise the language used within AM, but there has been further developments, and even cross-breeding for want of a better term. For instance, there could now be systems that rely on a form of vat photo-polymerisation at the start of their process, but then go on to carry out a sintering or fusion step, which in itself may not be the final part of the process to achieve a fully dense part.

Now as we start to look at AM from the perspective of a production process for real parts, whilst for each of the seven technologies a new layer of material is generated onto the previous layer, and this continues for each layer in the same manner until the 3D object is complete, the application of the technologies have not always been thought of as manufacturing. In the beginning we have seen the methods being referred to collectively as Rapid Prototyping, and then re-invented in the following years as Additive Layer Manufacturing (ALM), Free Form Fabrication, 3D Printing, Rapid Manufacturing, Near Net Shaping, and so many more.  In fact, a lot of today’s industry still only consider the technology being used for rapid prototyping (RP). Hence, the hindrance to fast adoption of AM technologies came from the first commercially available systems for metal powder bed being launched for producing rapid prototypes. In fact, nearly all systems in the late 1990’s and early 2000s were generally referred to as “rapid prototyping equipment”. A further fall-out from this terminology is that sub-contract manufacturing services that do exist are still derogatively referred to as bureaus, when in fact many are fully equipped manufacturing companies. Thankfully, nowadays there is more understanding about the diversity of the techniques, and the full capabilities of the materials, and the stigma associated with it only being for rapid prototyping is beginning to disappear. What now needs to be avoided, as each new vendor brings their equipment to market, is the temptation to create a new term along with it. Especially, if the only aim is to try and describe their process, and make it sound different to any others. One can’t imagine a new car manufacturer telling the world that instead of a motor they have a Linear Inertial Drive (LID) under the bonnet.

The confusion that was caused by the various ways to refer to AM has meant that it has taken more than a decade, and probably two, for all of these terms to be collectively referred to as just Additive Manufacturing. Though 3D Printing is still ever present and used somewhat interchangeably, it would be more correct to recognise this as a subset of AM. During all that time, the major problem faced by the sector was in gaining acceptance that AM processes could actually be classified as realistic alternatives for production. By 2015 it was already becoming apparent that these techniques were not going to live up the hype of being able to produce fully finished parts, and we look back now in astonishment that people had truly believed they could 3D print something and use it right away. The hype was inevitably followed by disillusionment for some causing them to turn their backs on AM, or to just take a big step backwards to wait in the wings.  Nowadays, it has become clear that there is a very significant difference in those technologies that have the potential for industrial use and mainstream production, and those that are suited to model making and, perhaps only rapid prototyping.

However, how should production be defined?

Many only ever see AM as being worthwhile, if it’s possible to achieve high volume production numbers. That includes those that have decided to adopt the technology early on, and those that are currently only considering it for future use. However, no one would refer to 5-axis machining as rapid prototyping simply because the part being produced was a one-off. Take the recent spacecraft sent to Mars, for example, virtually everything was a one-off, but no one is calling the rover a prototype. Hence, if the part has a real world (or off-world) application then how it is made can be called production. If AM is the most suitable, or even only way to produce that part then that is classed as production work. What everyone in the sector should realise is that the experienced gained from each and every type of AM build, whether that is a one-off or part of a series production run, is crucial to furthering the success of AM globally, and they shouldn’t be shy about telling the world.

Con-Fusion

To finish, let’s concentrate on just one of the main AM techniques. When a layer of powder that has been uniformly deposited, or spread, over a certain area, that is then fused by an intense heat source following a 2D pattern (or slice), and repeated umpteen times to create a 3D part, this would fall under the category of powder bed fusion (PBF). Fusion occurs where two bodies are joined together becoming one without any form of separation, as opposed to simply bonding that can be achieved with an additional material, such as an adhesive, and where the original surface interfaces remain intact. Where the contention arises with PBF techniques, is the need to distinguish between sintering, where fusion only occurs at contact points between powder particles, and melting, where the resultant material is nominally fully dense after complete solidification. At the time of the ASTM draft standard both techniques were being marketed, but the dominant term being used across the sector was sintering. This was reflected in the standard itself, which made little or no attempt to draw a distinction between the two processes, and in the case of laser based systems melting was hardly referred to at all.

Historically, the first of the PBF systems used lasers and were developed to sinter plastic powders, in a process that became known as selective laser sintering (SLS®). Systems using a fibre optic laser are now the most commonly available, though the early polymer systems used low power CO₂ lasers. Similarly, when metal powder was first introduced the systems were also developed to simply sinter the metal powders. However, it was not very long before the first laser melting systems were introduced to the market, and this gave rise to two more significant terms being used in the rapid prototyping sector, Direct Metal Laser Sintering (DMLS®), and LaserCUSING®. The major problem faced by the ASTM by the time they had decided to try and introduce some standardisation, was that the first of these terms had already become engrained in common parlance. DMLS was for a long time thought of as the industry standard terminology. The ASTM then chose to include this trademark term in the list of definition of terms, whilst seemingly ignoring that of the second vendor. However, shortly after these first two vendors had launched their first systems, a group of other laser system vendors starting using another similar term, later trademarked by one of them, Selective Laser Melting (SLM®). Lastly, there was yet another system using an electron beam to heat and melt metal powders, EBM®. It is, therefore, possible that the reason why the first draft standard had to be abandoned, was because reaching agreement amongst a wide group of stakeholders, with different interests, was simply too difficult. Even if there was some recognition that each of the trademark terms was being used to describe essentially the same processes within the PBF category of AM.

Putting a lid on it

So as things stand today, there is no doubt that additive manufacturing continues to be a disruptive force in the world of modern manufacturing, but it is no longer viewed in only a negative way. Recent years have seen many companies adapting their capabilities to include one or more AM techniques as part of their business. Specific forms of AM, like powder bed fusion, are now recognised as enabling technologies, that increase the ability of design engineers to deliver more efficient, and yet more complex designs. Laser Powder Bed Fusion (LPBF), and E-beam Powder Bed Fusion (EPBF) are no longer presented as a threat to conventional manufacturing techniques. This has come about by the gradual acceptance that AM can be used for much more than just rapid prototyping. AM is seen as a way to release businesses from the constraints of designing for traditional production processes, and after proving that desired materials properties can be achieved, the manufacturing sector now also accepts these new technologies as being complimentary processes. There are even companies that understand how AM is essential to their future growth and success.

Under a single name and common understanding, industry and academia are now working closer than ever before to fully understand the interaction between processes and materials, and to further develop Additive Manufacturing technology. Technology that has developed so quickly over the past two decades that now there is a very large choice of equipment, from a greater number of suppliers, with a much wider choice of materials, and for an ever growing number of applications.

 

EBM® is a Registered trademark of Arcam AB. SLS® is a Registered trademark of 3D Systems Inc. DMLS® is a Registered trademark of EOS GmbH. SLM® is a Registered trademark of SLM Solutions GmbH. LaserCUSING® is a Registered trademark of Concept Laser GmbH