The world of additive manufacturing, or 3D printing, is constantly evolving. The technology was invented less than 35 years ago yet has come a long way. What began as a unique, though limited, way to develop low-end prototypes, has exploded into a critical component of the product development and manufacturing process with the ability to produce end-use parts for critical applications in markets such as industrial and aerospace and defense.
To help our customers and the larger technology community stay abreast of the changing world of additive manufacturing, we launched a glossary of the most important terms in the industry that you can bookmark here for easy access. To make it easier to digest, we’re also starting a blog series outlining ten terms to know in different sub-categories.
For our first post in the series, here are the top ten terms
for Additive Manufacturing Processes that our experts think everyone
Any additive manufacturing process that uses a binder to
chemically bond powder where the binder is placed on the top layer of powder
through small jets, usually using inkjet technology. One of the seven standard
categories defined by ASTM International (www.ASTM.org) for additive
A type of vat photopolymerization additive
manufacturing process where a projector under a transparent build
plate shines ultraviolet light onto the build layer, which
is against the transparent build plate. The part is then pulled
upward so that a new layer of liquid fills between the build
plate and the part, and the process is repeated. Digital light
synthesis is a continuous build process that does not create distinct layers.
A type of powder bed fusion additive manufacturing
process where a laser beam is used to melt powder material. The
beam is directed across the top layer of powder. The liquid material
solidifies to create the desired part. A new layer of powder is
placed on top, and the process is repeated. Also called laser powder bed
fusion, metal powder bed fusion, or direct metal laser sintering.
An additive manufacturing process where metal
powder is jetted, or wire is extruded from a CNC controlled three or
five-axis nozzle. The solid material is then melted by an energy source,
usually a laser or electron beam, such that the liquid metal
deposits onto the previous layers (or build plate) and then cools to a
solid. One of the ASTM defined standard categories for additive
A type of material extrusion additive manufacturing process
where a continuous filament of thermoplastic material is fed into a heated
extruder and deposited on the current build layer. It is the trademarked name
used for systems manufactured by the process inventor, Stratasys. Fused
filament fabrication is the generic term.
A type of powder bed fusion additive
manufacturing process where a laser is used to melt material on
the top layer of a powder bed. Also called metal powder bed
fusion or direct laser melting. Most often used to melt metal powder
but is used with plastics as with selective laser sintering.
A type of direct energy deposition additive
manufacturing process where a powder is directed into a
high-energy laser beam and melted before it is deposited on
the build layer. Also called laser powder forming.
Any additive manufacturing process where build
or support material is jetted through multiple small nozzles whose
position is computer controlled to lay down material to create a layer.
One of the ASTM defined standard categories for additive
A type of vat photopolymerization additive
manufacturing where a laser is used to draw a path on the
current layer, converting the liquid polymer into a solid. Stereolithography
was the first commercially available additive manufacturing process.
A class of additive manufacturing processes that utilizes
the hardening of a photopolymer with ultraviolet light. A vat of liquid is
filled with liquid photopolymer resin, and ultraviolet light is either traced
on the build surface or projected on it. Stereolithography is the most common
form of vat photopolymerization. The build layer can be on the top of the vat
of liquid or the bottom. One of the ASTM defined standard categories for
additive manufacturing processes.
We hope this new blog series will help to firm up your
knowledge of the ever-evolving world of additive manufacturing. For a list of
all of the key terms and definitions in the additive manufacturing world,
please visit our new glossary page at https://www.3dprinting-glossary.com/.
The glossary allows you to search by terms or download a PDF of the glossary in
its entirety to use as a reference guide.
Subscribe to the
PADT blog or check back soon for the next installment in our series of “Top Ten
Terms to Know in Additive Manufacturing.” We also welcome your feedback or
questions. Just drop us a line at here.
The most important discussions around installing and operating a metal 3D printer involve safety. The requirements can be difficult to pin down since they depend on several things: whether you are using reactive or non-reactive alloys (read a previous post on this subject here), the risk perception of your local building safety director and fire marshal, local regulations (and exceptions) and the volume of material you are dealing with. As with all things safety and more so because of how nascent metal 3D printing technology is, I list a few disclaimers at the bottom of this page.
There is so much to say about safety in this process that half-way through writing this post, it became clear it would not fit in one post. Therefore, I have split the content into two: in this post (3A) I talk about the risks: where they come from and why they matter. In the next post (3B), I will discuss how these risks can be mitigated.
1. Sources of Risk
Broadly speaking, I like to think of two sources of risk in this process since as an operator of these machines you have to think differently about how you interact with these sources.
1.1 Metal Powder
Metal 3D printing involves fusing together powder in a bed. Typical metal powders used for laser based 3D printing are spherical in shape and range from 10-70 microns in diameter, as shown in Figure 1. At this size, a metal can be prone to fire and explosion (under the right circumstances) and there is also the physiological concern of long-term inhalation of, and contact with, these powders. The powder also has a long life cycle and requires human interaction at many steps – from arriving in a container (as shown in Figure 2), through multiple recycling steps through final disposal. These risks come into play just when handling the powder (independent of its use in the process) – an additional risk comes from the melting process itself.
1.2 The Laser Fusion Process
The powder in the bed described before is fused together into a solid using a laser that locally melts the powder one layer at a time. This is conducted in an inert atmosphere (Argon or Nitrogen) and is the second source of risk since these gases can displace Oxygen from a closed environment. Additionally, the process of laser melting of metals creates vaporized soot (see video below), some of which deposits on the process chamber and in the extraction module and filter. The smoke particles can be even finer than the powder itself, and need to be cleaned out with care on a regular basis.
There are 4 main risks arising from the laser powder bed fusion process: fire and explosion, powder inhalation and contact, inert gas asphyxiation and the environmental impact of the wastes generated.
2.1 Fire and Explosion
In May 2014, OSHA cited a 3D printing company for 10 violations deriving from the workplace safety standards surrounding the operation of a metal 3D printer (including not having the proper Class D fire extinguisher). The disregard of multiple safety measures during a routine build setup procedure resulted in a fire which caused life-threatening burns to the operator of the printer. While this incident was the result of gross negligence, it is nonetheless a cautionary tale that should drive us to understand the fundamental reasons why a metal 3D printer can cause fires and to appreciate the underlying reasons for why suppliers recommend the safety measures they do.
Fire and explosion require a combination of conditions as shown in the commonly cited image below used by OSHA and other agencies to communicate risks of powder handling.
As shown in Fig. 3, when handling powder in ambient atmospheres (with oxygen), all that is needed is a suitable ignition source to initiate a fire. Further, if this occurs in the presence of a dust cloud with many particles dispersed in a contained area (such as a small room or an air duct), this could lead to a more damaging explosion.
Fire: When handling metal powder, the user needs to be aware that she/he already has 2 of the 3 requirements of a fire met and the main aim must be to ensure protection against any ignition source. There are several sources that could cause an ignition, the most likely one for a user of a metal 3D printer is static electricity. Additionally, it is possible that a fire can be initiated by hot surfaces, flames, hot gases and particles, mechanically generated sparks and strayelectrical currents.
Explosion: With regard to explosions, in addition to the 3 requirements above, dust clouds in contained areas can exacerbate any ignition to a much larger impact within milliseconds. Therefore, the prevention of the formation of metal dust clouds (as unlikely as that may seem), is of paramount importance.
In addition to the requirements above, there are levels associated with each requirement that need to be met together for an actual fire or explosion to occur. The risk of ignition increases with reducing particle diameter and given a certain particle size, the most significant factor governing risk is the nature of the metal. Reactive metals (Aluminum alloys, Titanium and its alloys, as well as smoke particles from both reactives and non-reactives) pose much higher risk than non-reactive metals (steels, Inconels, bronze, Cobalt Chrome alloys) – this is a subject I wrote about in more detail in a previous post.
2.2 Powder Inhalation & Contact
As discussed before, most metal 3D powder particles range in size from 10-70um. This is at the very edge of what is considered respirable and damaging to our lungs. While contact physically is to be avoided since it may initiate irritation and potential dermatitis, there is greater concern about the long-term inhalation exposure risks of these powders. Particles of the size range in this process can get deposited in the tracheo-bronchial region per Jenson  and Goldich . Ultimately, these particles are discharged from the body or swallowed, but effects of long term exposure for the wide range of metals and alloys is not fully studied – which is why suppliers insist on respirators (more on that in the next post). It is worth pointing out though, from the work published by Jenson and Goldich, that it appears that while metal 3D printing powders are small enough to travel past the nasal cavity if inhaled, their sizes are large enough that respiratory damage in the lungs is highly unlikely – only particles under 2 microns are at risk of making it all the way to the alveoli and causing lung disease .
2.3 Inert Gas Asphyxiation
Inert gases are used in laser metal 3D printers to reduce the reactivity of the metal for processing purposes. Most metal 3D printers either use Nitrogen or Argon. Inert gas asphyxiation is the main risk due to oxygen being displaced by either of these gases that have leaked for some reason. Since both gases are not detectable by humans, victims do not realize that they are inhaling air depleted of oxygen and as a result this can have a serious impact. The human body is used to atmospheric percentages of oxygen (21%) and values below 19.5% can be harmful and are defined as oxygen-deficient per OSHA . Thus, any user of nitrogen or argon gas (and this applies not just to any process using inert gases), especially in small spaces such as a closed room, needs to be aware of this risk and protect against it.
2.4 Environmental Impact
A key challenge with powder based processes lies in collecting and disposing the stray or “fugitive” powder from different locations such as the tool, PPE, containers and vacuum systems into temporary storage, during which the above risks of fire/explosion and inhalation remain. Additionally, the storage typically results in loose powder and solid waste as well as water with powder particles, both of which need to be disposed into the outside world and could pose an environmental hazard. I will discuss this further in a future post, when I attempt to look at some of the environmental aspects around this technology.
This is intended to supplement the supplier training you must receive before using the equipment and not meant to replace it – in case of conflicting information, your supplier’s training and equipment requirements override any discussion here. PADT assumes no legal responsibilities for any decisions or actions taken by the readers of this document.
My personal experience derives specifically from the use of Laser-based metal 3D printing tools, specifically Concept Laser’s MLab Cusing R equipment. I expect majority of this information to be of use to users of other laser based powder bed fusion metal systems and to a lesser extent to Electron Beam systems, but have no personal experience to vouch for this.
Local, state and federal regulations vary, and are important – partner with your local fire marshal (or equivalent authority) as a starting point and take them along with you every step of the way. If in the US, familiarize yourself in particular with OSHA’s guidance on dust explosions  and NFPA 484 , the National Fire Protection Association’s standard for combustible metals (links below).
Continue to Part 3B here, where I address mitigation strategies to address the risks described in this post. In the meantime, please read my prior posts below if you haven’t already, or send your inputs to me via message on LinkedIn. Thank you!
One of the first concepts you come across in metal 3D printing is the notion of reactivity of the powder metal alloys – in this post, I investigate why some of these powder alloys are classified as reactive and others as non-reactive, and briefly touch upon the implications of this to the user of metal 3D printing tools, scoping the discussion to laser-based powder bed fusion. Ultimately, this boils down to a safety issue and I believe it is important that we, the users of these technologies, truly understand the fundamentals behind the measures we are trained to follow.
Figure 1 below is indicative of the range of materials available currently for the laser-based powder bed fusion process (this selection is from Concept Laser). I have separated these into non-reactive and reactive metal alloys. The former includes steels, Inconels, bronze and CoCrW alloys. The reactive metal alloys on the other hand are Aluminum or Titanium based. The question is: what classifies them as such in the context of this process?
Reactivity in this process really pertains to the likelihood of the alloy in question serving as a fuel for a fire and/or an explosion, which are two related but distinct phenomena. To truly understand the risk associated with powder metals, we must first understand a few basic concepts.
1. Fire and Explosion Criteria
Figure 2 is a commonly used representation of the criteria that need to be met to initiate a fire (fuel, oxygen and an ignition source) and an explosion (the same three criteria for a fire, plus a dust cloud and confined space). When handling reactive metal alloy powders, it is important to remember that two of the three requirements for a fire are almost always met and the key lies in avoiding the other criterion. When not processing the powder in the machine, it is often subject to ambient oxygen content and thus all precautions are taken to prevent an ignition source (an ESD spark, for example). When the metal is being processed with a high power laser, it is done in an inert atmosphere at very low Oxygen levels. This thought process of appreciating you are one criterion away from a fire is useful, if sobering, to bear in mind when working with these powders. Well in case of fire explosion which can harm that should be resist or plan to resist.
2. Terms Used to Describe Fire and Explosion Risk
There are several terms used to describe fire and explosion risk. I have picked 5 here that tie into the overall “index” I will discuss in the following section. All these parameters are in turn functions of the material in question, both with regard to its composition and its size distribution and are co-dependent. These definitions are adapted from Benson (2012) and Prodan et al. (2012).
Fire Related:These two terms describe the sensitivity of a metal dust cloud to ignition.
Ignition Temperature: This is the lowest surface temperature capable of igniting a powder or dust dispersed in the form of a dust cloud
Minimum Ignition Energy: This measures the ease of ignition of a dust cloud by electrical and electrostatic discharges.
Explosion Related: These terms describe the severity of an explosion arising from a fire once ignited.
Minimum Explosion Concentration (MEC): This is the smallest amount of dust which when suspended in air, under a set of test conditions, will initiate an explosion and propagate even after the action of the ignition source has ceased.
Maximum Explosion Pressure: This is a measure of the highest pressure that occurs during of an explosion of a flammable mixture in a closed vessel.
Maximum Rate of Pressure Rise: This is the maximum slope of the pressure/time curve during a flammable mixture explosion in a closed vessel.
3. Index of Explosibility
Having defined these terms, the question is how they can be tied together to give some sense of the hazard associated with each metal powder. I came across a 1964 US Bureau of Mines study that defined an Index of Explosibility as a measure of the hazard risk posed by powder metal alloys. The index represents both the sensitivity of the powder to ignition, and once ignited, the severity of the resulting explosion. Since this is a subjective metric, it is normalized by comparison against a “standard”, which was selected as Pittsburgh coal dust in the 1964 study. Importantly though, this normalization enables us to do qualitative comparisons between metal powders and have some sense of the hazard risk posed by them. Figure 3 is the equation reproduced from the original 1964 report and shows how this term is estimated.
The study also showed how the index was a direct function of particle size. Most powders for 3D metal printing are in the 20-100um range, and as shown in Fig. 4 for atomized Aluminum, the risk of an explosion increases with reducing particle diameter.
The authors tested a range of metals and computed the different variables, which I have compiled anew in the table in Figure 5 for the ones we are interested in for metal 3D printing. The particle sizes in the 1964 study were ones that made it through a No. 200 sieve (less than 75 microns), but did not include sub-micron particles – this makes it an appropriate comparison for metal 3D printing. It is clear from the Index of Explosibility values, as well as the Cloud Ignition Temperatures in the table below why Aluminum and Titanium are classified as reactive metals requiring special attention and care.
4. Implications for Metal 3D Printing
So what does this mean for metal 3D printing? There are three things to be aware of that are influenced by whether you are working with non-reactive or reactive alloys – I only provide a general discussion here, specific instructions will be provided to you in supplier training and manuals and must be followed.
Personal Protective Equipment (PPE): There are typically two levels of PPE: standard and extended. The standard PPE can be used for non-reactive alloy handling, but the reactive alloys require the more stringent, extended PPE. The main difference is that the extended PPE requires the use of a full bunny suit, ESD grounding straps and thermal gloves.
Need for Inert Gas Handling: Many tasks on a metal 3D printer require handling of powder (pouring the powder into the chamber, excavating a part, cleaning the chamber of powder etc.). Most of these tasks can be performed in the ambient for non-reactive metal alloys with standard PPE, but for reactive alloys these tasks must be performed in an inert atmosphere.
Local authority approvals: It is important that your local authorities including the fire marshall, are aware of the materials you are processing and review and authorize their use in your facility before you turn on the machine. Local regulations may require special procedures be implemented for preparing the room for use of reactive metal alloys, that do not apply to non-reactive metals. It is vital that the authorities are brought into the discussion early on and necessary certifications obtained, keeping in mind that reactive metal alloy use may drive additional investment in safety measures.
Safe operation of metal 3D printers requires installation of all the necessary safety equipment, extensive hands-on training and the use of checklists as memory aides. In addition to that, it helps to connect these to the fundamental reasons why these steps are important so as to gain a clearer appreciation of the source of the hazard and the nature of the risk it poses. In this article I have tried to demonstrate why reactivity in metal 3D printing matters and what the basis is for the classification of these metal alloys into reactive and non-reactive by leveraging an old 1964 study. I wish to close with a reminder that this information is meant to supplement formal training from your equipment supplier – if there is any conflict in the information presented here, please revert to your supplier’s recommendations.