Additive Manufacturing
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What is Additive Manufacturing?
As already mentioned, additive manufacturing is the key factor that will push people towards the fourth
industrial revolution. It represents actually the bridge between third and fourth industrial revolution.
It was known initially under the name of rapid prototyping, or 3D-printing: howevere, nowadays we
use these three terms to address different technologies.
The "Hype-Cycle"
Around 2013, additive manufacturing was one of the hottest topic discussed in scientific field.
It was discussed all over the world, and people started fantasizing about what could have been
possible to do. Hence, the Hype-Cycle began. It basically represents how something (in this
case additive manufacturing) is perceived by people. Such cycle is characterized by 5-phases:
1. Innovation trigger: All media starts talking about it increasing exponentially its popularity.
Basically the phase in which Additive Manufacturing started to become famous.
2. Peak of inflated expectations: It’s the phase in which people start thinking abount what
could be done whit this new technology.
3. Trough of disillusionment: All the fantasy and expectations of people can’t match the
level of development of the actual technology, leading a high delusion and decrease of
interest. It can also be seen as the phase in which the "slope" of expectation is much
higher than the slope of actual development, hence bringing people to reality and to what
is possible right now. We have to keep in mind that the hype cycle can be applied to
everything: this is alos considered the phase in which almost all patents fail.
4. Slope of enlightenment: It’s the phase in which people continue their path in the development of the technology, bringing some important results and actually showing that
the A.M. technology can possibly bring several advantages, and showing that it can be
applied for some industrial application. This phase is were the remaining survived patents
(from previous phase) moves ahead.
5. Plateau of productivity: It’s the phase in which the technology starts to get consolidated,
and used in many industrial’s reality. It’s also the phase in which the technological
development reaches a plateau, a phase in which technological development is stable.
Additive manufacturing has been developed independently in different company, even before the
term of A.M. was introduced. For this reasons, in the past there were various terms that were given by
company to address this kind of technology. Only after 2009 (date of the first committee about A.M.)
a rigorous literature was defined.
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Rigorous definition of AM
Missing
Let’s now provvide with an example of A.M. part, introducing also its benefits with respect
to conventional machining.
Figure 1: Left: A.M. Right: Conventional machining
Additive Manufacturing
• The whole of the piece was produced in a SINGLE operation, already fully assembled since it
was produced as a single part;
• Lower weight;
• Higher lifetime of the part;
• Lower lead time ;
• It was possible to design an internal Pilot gas-feed, automatically integrated in the structure.
Conventional machining
• 13 parts have been machined with different machining processes, and 18 welds were required
to joint all the parts together and obtain the final piece;
• Heavier;
• Way longer lead time
• Lower lifetime of the part - Because of the numerous welding that are required, which induced
thermal stresses and reduced the mechanical resistance;
• Only external pilto gas-feed. The internal one is really impossible to achieve with machining
operations.
Benefits of A.M.
Decreased number of required operations
Lower weight
Longer lifetime
Lower lead time
Achieving the impossible tasks of conventional operations
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Historical development
Additive manufacturing is basically fabbrication of structure done layer-by-layer, from 3D computer
models. It was born around 1980 as a Rapid Prototyping technique. It then evolved in Rapid Cast
and Rapid Tooling, as a technique used to quickly maunfacture mold casts and insert for tools. Only
after 2009, when the first committee was established, the concept of Additive Manufacturing was
born, and yet. Nowadays we use both A.M. and RapidP., but they are not the same thing!
• Rapid Prototyping is used for producing only 1 part! It represents a prototype of a concept,
physical translation of the idea.
• Additive Manufacturing is a technological process engaged in industrial production of a part
and supply chains, not a single piece.
It’s important to distinguish it from 3D printing, typically associated with homemade printing of
parts. The technique behind all three process is the same though (addition of layers).
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Classification of prototypes
Prototyping is the process of developing a piece/part behind the idea to have a rapid feedback from it.
There are many stages that goes from an idea to the final part, and this basically identifies different
phases of the prototyping. THere are 4 main steps:
1. Phase 0 - Not to be considered - When we turn on the "lampadina"
2. Conceptual prototype (Part 1): We think to the object, to its geometry and shape. We think to
the possibility of having different parts assemblied and what could be the technical issue of it.
We don’t focus at all about material and fabrication technique.
3. Conceptual prototype (Part 2): A first prototype is made, and a first evaluation of the performances is done. I have now to optimize the product to better perform its functions. Now, a
material class identification is required. We are restricting our area of interest.
4. Technical prototyping: The product is almost finished in its design, and real performance
analysis are done. A higher focus in fabrication technique and type of materia.
5. Pre-series prototype: Final evalutaion of the product, with a defined fabrication technique and
material.
Figure 2: Phases of prototyping
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It is important to notice that state also that A.M. technologies will never FULLY REPLACE
conventional machining processes, and the reason is simple: we adopt AM whenever it is convenient.
When parts that can be easily manufactured by conventional machining, going for AM will simply
be unproductive due to the very high costs of the process, if compared with the one of Conventional
machining.
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Digital Fabrication
For additive manufacturing, we have several step before we arrive to the part, and this has not to be
confused with the single-process of fabbrication. All this steps constitutes the steps of the Digital
Fabbrication (actually, a rigorous definition of digital fabbrication is missing, and you can interpret
it as simply Additive manufacturing). The steps are 4:
1. First step: we start from the 3D CAD model of the part we are interested of. It’s the beginning
step for everything else that comes after since it describes the geometry.
2. Second step: The CAD model represents a file wich describes the 3D solid model and surface
of the part, but in order to obtain the real part we need all the coordinates that costitutes the
surface, and this is obtain by sending the CAD file through a polygonization process, which
leads us to a "tasseleted" model, also called the STL file (which is different from the CAD). An
STL file uses triangle to describe a surface, where each triangle is defined by three points and
a normal vector facing the outside. These triangles, all summed up, define the surface. It is
important to check such files, sinche polygonization might bring some errors, like gaps, internal
walls, normal vectors facing the inside and intersection between triangles.
3. Once the polygonization is completed, the STL file is uploaded in the AM machine. This is not
the phase in which the process is started though: in fact, some manipulation is needed:
• Supports. Supports are needed to hold the part during the process, avoiding it to collapse.
Such structures are added by AM machine’s software and might be formed with same
material of the part, or different material (and are removed later). Actually, the structure
serves another important aspect, which is to dissipate heat, in order to avoid thermal
distortion. However, we can’t add supports as we want: they have to be minimized,
since the surface needs a finshing process. Hence, before the structure is fabricated, a
part orientation is required.
• The part orientation is done before everything else, becaues it has also to consider the
second manipulation: Slicing. The STL file si sliced by a sequence of parallel planes,
producing layers. These are the layers that will be deposed during fabrication. Slicing
could be difficult for some surface, like rounds surfaces: the stepping stair phenomena
could give us some problems.
That’s why part orientation is important: minimize surface contact and stair stepping. Everything
we said till now is all performed by software.
4. Once the file is fully completed, it is finally uploaded in the AM machine that begins the building
(hardware part). The building is a layer-by-layer fabbrication process: usually, powder is applied
to the platform and is melted and solified by means of a laser; next, platform is lowered and
process is repeated until the last layer has solified. Some constraints have to be specified, like
energy source, material constraints and timing. After building, a post processing phase begins
to clean, finish and remove thermal stresses.
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Figure 3: Polygonization
Figure 4: Layered Additive Manufacturing
Figure 5: Benefits
What we have described till now is the LAYERED ADDITIVE MANUFACTURING. It has some
drawbacks that needs to be pointed out:
• Very fine layers implies very low build-rate
• Non-productive time associated to limited speed of the machine (like powder)
• Machine size limits the product size (the chamber limits the potential size): However, this can’t
be considered that much of a problem since large parts are usually simple parts and we use
conventional machining for them
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• Limited material avaiable under powder form (a lot of polymers, but very limited of metal)
• Scarce superficial finish (post finishing is required, but not in all AM process);
• Powder production is higly expensive
• Metals that have limited weldability have no application for AM, since AM is based on welding
(cast iron can’t be printed by AM).
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Fused Filament Fabrication
It’s basically an Additive process (under the class of Power Feed processes), in which there is a
deposition based on the extrusion of a filament through a nozzle: by moving the nozzle, which
represents the deposition head, we can deposit wherever we want. It is important to clarify that
such method uses mainly polymeric material, and the two processes are extrusion and subsequent
deposition. This process was invented and patented by Scott Crump in 1989: however, after the
expiration of the patent, many FDM machines were developed and today ware called 3D machines,
but be careful that as we already said they are of a different bread! Here its is showed how FDM
Figure 6: FDM process
works:
• A filament of polymeric material (unspooled filament) is pulled and pushed by means of a
stepper motor to the print nozzle, or extrustion head, trhough the PTFE tube;
• Before the deposition happens (which by the way is still made in sequence to reproduce again the
layers obtained by the slicing process), an heating element melts the filament thanks to electrical
resistances. The heating element is part of the extrusion head, and it can be seen as a "chamber"
where the polymeric material is melted before the final act (the extrusion).
• The stepper motor transfers the power to a geared system, techically from a small gear (sharing
the same shaft of the motor) to a large gear, that is in contact with the filament.
• The polymeric material is so melted and extruded. Also, FDM machine have a standard cartesian
structure;
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• FDM machines might have multiple spools (praticamente le pulegge dove il materiale polimerico
è avvolto), and this allows to deposit different material so that we can deposit the holding
structure and then the part. Of course, to make this possible, multiple extruders are necessary
(like multiple nozzles).
• Different FDM machines are based on how the filament feeding mechanism is positioned (it
might be fixed with the machine or it might move with extrusion head).
The gap between the nozzle and the bed (the platform) represents the layer thickness. For this reason,
we have that:
• Nozzle height is important for surface finish (very far nozzle means very bad surface finish), as
well as nozzle diameter, since an higher diameter gives thicker layer, so less accurate surface;
• Bed livelling, to guarantee a good orientation of the part.
As we mentioned, FDM machines works like CNC machines!
FDM and CNC: a closer insight
FDM machines, like CNC machines, are governed by a main electronic board. While for CNC
the MCU used to control the three cartesian axes in speed and position, FDM machines follows
cartesian system as well and now the main parameters to be checked are:
1. Temperature of extrusion head (or deposition head, or nozzle)
2. Deposition rate, important for the surface finish
3. Height of the extrusion head
By having a feedback system for the temperature and speed, we can set up a closed-loop process
like the one we used for CNC.
We can also pass easily from a NC file to the one for FDM machines: given an NC file written
in ISO standards, we can derive all the coordianates and use G1 code to set up a point-to-point
motion for the extrusion head (straight lines) and M codes to control other parameters, as
filament feeding rate, temperature ecc.
It has to be specified that it’s not correct saying that higher feed rate bring higher surface quality:
actually having higher feed rate brings better surface finish, but its better to seek for the optimal speed!
Having low speed means instead bad surface finish. But.. how is the feed rate related to surface finish?
It’s because the feed rate is directly related to layer thickness: higher speed rate means we spend less
time over a certain area, depositing less material, and having so a thinner layer (which means higher
quality). One last thing: small parts have IT13, big parts have IT12.
Check slide 52-53
As we mentioned, material for FDM machines are mainly polymeric material. This FDM filaments
might costs even over 40 times the price of the raw material. Also, there are different type of materials,
as:
Soluble Rigid Static Bio compatible Rigid opaque Rubber like Rigid transparent
The
ST-130 ABS- ESD7
ABSi
ULTEM
TANGO
VERO CLEAR
ST-130, since is soluble, is used only for support structure (since it ca be easily removed). ULTEM
has higher performance, and a higher price of course. For engineers, there are avaiable tables where
is indicated the temperature at which the extrusion head should be so to have a good deposition,
for different material. Did you know that FDM found application for in-space manufacturing, since
controlling solid filament in space (where there is no gravity) is easier than controlling powder.
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Design for A.M
Figure 7: Comparison between price and complexity of a part: whether we should use AM or
conventional machining
Additive Manufacturing overcomes the problems connected to traditional manufacturing processes,
because it is possible to build a part without constraints pertaining to the geometry complexity. Topology optimization was developed as an advanced structural design methodology to generate innovative
lightweight and high-performance configurations that are difficult to obtain with conventional ideas.
Additive manufacturing is an advanced manufacturing technique building as-designed structures via
layer-by-layer joining material, providing an alternative pattern for complex components. The integration of topology optimization and additive manufacturing can make the most of their advantages
and potentials, and has wide application prospects in modern manufacturing. Such optimization in
the design of a part can result in:
• Minimization of part weight
• Maximization of static strength
• Optimal Dynamic and Thermal behaviour
5.1
What is Generative Design?
Generative Design is a design exploration process: given the functionality and the constraints (as
spatial requirements, material, manufacturing methods and cost constraints) we have during design
phase, softwares simulates different design solutions and explore all them, trying to find what could be
the best possible solution that is the most suitable for us. Such solutions are obtained after numerous
iterations, and the solutions are very complex shapes and solids. This lets us introduce AM again:
obtaining such shapes with conventional machining is almost impossible, that’s why AM represents a
key factor for achieving high performance components due to their particularly complex design.
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