Using only the following information, answer the question: "Explain the 8 main steps of the Additive Manufacture process in detail."

 3.2 The Eight Steps in Additive Manufacture
The above-mentioned sequence of steps is generally appropriate to all AM
technologies. There will be some variations dependent on which technology is
being used and also on the design of the particular part. Some steps can be quite
involved for some machines but may be trivial for others.
3.2.1 Step 1: Conceptualization and CAD
The first step in any product development process is to come up with an idea for
how the product will look and function. Conceptualization can take many forms,
from textual and narrative descriptions to sketches and representative models. If
AM is to be used, the product description must be in a digital form that allows a
physical model to be made. It may be that AM technology will be used to prototype
and not build the final product, but in either case, there are many stages in a product
development process where digital models are required.
AM technology would not exist if it were not for 3D CAD. Only after we gained
the ability to represent solid objects in computers were we able to develop technology to physically reproduce such objects. Initially, this was the principle
surrounding CNC machining technology in general. AM can thus be described as
a direct or streamlined Computer Aided Design to Computer Aided Manufacturing
(CAD/CAM) process. Unlike most other CAD/CAM technologies, there is little or
no intervention between the design and manufacturing stages for AM.
The generic AM process must therefore start with 3D CAD information, as
shown in Fig. 3.1. There may be a variety of ways for how the 3D source data can be
created. This model description could be generated by a design expert via a userinterface, by software as part of an automated optimization algorithm, by 3D
scanning of an existing physical part, or some combination of all of these. Most
3D CAD systems are solid modeling systems with surface modeling components;
solid models are often constructed by combining surfaces together or by adding
thickness to a surface. In the past, 3D CAD modeling software had difficulty
44 3 Generalized Additive Manufacturing Process Chain
creating fully enclosed solid models, and often models would appear to the casual
observer to be enclosed but in fact were not mathematically closed. Such models
could result in unpredictable output from AM machines, with different AM
technologies treating gaps in different ways.
Most modern solid modeling CAD tools can now create files without gaps (e.g.,
“water tight”), resulting in geometrically unambiguous representations of a part.
Most CAD packages treat surfaces as construction tools that are used to act on solid
models, and this has the effect of maintaining the integrity of the solid data.
Provided it can fit inside the machine, typically any CAD model can be made
using AM technology without too many difficulties. However, there still remain
some older or poorly developed 3D CAD software that may result in solids that are
not fully enclosed and produce unreliable AM output. Problems of this manner are
normally detected once the CAD model has been converted into the STL format for
building using AM technology.
3.2.2 Step 2: Conversion to STL/AMF
Nearly every AM technology uses the STL file format. The term STL was derived
from STereoLithograhy, which was the first commercial AM technology from 3D
Fig. 3.1 The eight stages of the AM process
3.2 The Eight Steps in Additive Manufacture 45
Systems in the 1990s. Considered a de facto standard, STL is a simple way of
describing a CAD model in terms of its geometry alone. It works by removing any
construction data, modeling history, etc., and approximating the surfaces of the
model with a series of triangular facets. The minimum size of these triangles can be
set within most CAD software and the objective is to ensure the models created do
not show any obvious triangles on the surface. The triangle size is in fact calculated
in terms of the minimum distance between the plane represented by the triangle and
the surface it is supposed to represent. In other words, a basic rule of thumb is to
ensure that the minimum triangle offset is smaller than the resolution of the AM
machine. The process of converting to STL is automatic within most CAD systems,
but there is a possibility of errors occurring during this phase. There have therefore
been a number of software tools developed to detect such errors and to rectify them
if possible.
STL files are an unordered collection of triangle vertices and surface normal
vectors. As such, an STL file has no units, color, material, or other feature
information. These limitations of an STL file have led to the recent adoption of a
new “AMF” file format. This format is now an international ASTM/ISO standard
format which extends the STL format to include dimensions, color, material, and
many other useful features. As of the writing of this book, several major CAD
companies and AM hardware vendors had publically announced that they will be
supporting AMF in their next generation software. Thus, although the term STL is
used throughout the remainder of this textbook, the AMF file could be simply
substituted wherever STL appears, as the AMF format has all of the benefits of the
STL file format with many fewer limitations.
STL file repair software, like the MAGICS software from the Belgian company
Materialise [1], is used when there are problems with the STL file that may prevent
the part from being built correctly. With complex geometries, it may be difficult for
a human to detect such problems when inspecting the CAD or the subsequently
generated STL data. If the errors are small then they may even go unnoticed until
after the part has been built. Such software may therefore be applied as a checking
stage to ensure that there are no problems with the STL file data before the build is
performed.
Since STL is essentially a surface description, the corresponding triangles in the
files must be pointing in the correct direction; in other words, the surface normal
vector associated with the triangle must indicate which side of the triangle is outside
vs. inside the part. The cross-section that corresponds to the part layers of a region
near an inverted normal vector may therefore be the inversion of what is desired.
Additionally, complex and highly discontinuous geometry may result in triangle
vertices that do not align correctly. This may result in gaps in the surface. Various
AM technologies may react to these problems in different ways. Some machines
may process the STL data in such a way that the gaps are bridged. This bridge may
not represent the desired surface, however, and it may be possible that additional,
unwanted material may be included in the part.
While most errors can be detected and rectified automatically, there may also be
a requirement for manual intervention. Software should therefore highlight the
46 3 Generalized Additive Manufacturing Process Chain
problem, indicating what is thought to be inverted triangles for instance. Since
geometries can become very complex, it may be difficult for the software to
establish whether the result is in fact an error or something that was part of the
original design intent.
3.2.3 Step 3: Transfer to AM Machine and STL File Manipulation
Once the STL file has been created and repaired, it can be sent directly to the target
AM machine. Ideally, it should be possible to press a “print” button and the
machine should build the part straight away. This is not usually the case however
and there may be a number of actions required prior to building the part.
The first task would be to verify that the part is correct. AM system software
normally has a visualization tool that allows the user to view and manipulate the
part. The user may wish to reposition the part or even change the orientation to
allow it to be built at a specific location within the machine. It is quite common to
build more than one part in an AM machine at a time. This may be multiples of the
same part (thus requiring a copy function) or completely different STL files. STL
files can be linearly scaled quite easily. Some applications may require the AM part
to be slightly larger or slightly smaller than the original to account for process
shrinkage or coatings; and so scaling may be required prior to building.
Applications may also require that the part be identified in some way and some
software tools have been developed to add text and simple features to STL
formatted data for this purpose. This would be done in the form of adding 3D
embossed characters. More unusual cases may even require segmentation of STL
files (e.g., for parts that may be too large) or even merging of multiple STL files. It
should be noted that not all AM machines will have all the functions mentioned
here, but numerous STL file manipulation software tools are available for purchase
or, in some cases, for free download to perform these functions prior to sending the
file to a machine.
3.2.4 Step 4: Machine Setup
All AM machines will have at least some setup parameters that are specific to that
machine or process. Some machines are only designed to run a few specific
materials and give the user few options to vary layer thickness or other build
parameters. These types of machines will have very few setup changes to make
from build to build. Other machines are designed to run with a variety of materials
and may also have some parameters that require optimization to suit the type of part
that is to be built, or permit parts to be built quicker but with poorer resolution. Such
machines can have numerous setup options available. It is common in the more
complex cases to have default settings or save files from previously defined setups
to help speed up the machine setup process and to prevent mistakes being made.
3.2 The Eight Steps in Additive Manufacture 47
Normally, an incorrect setup procedure will still result in a part being built. The
final quality of that part may, however, be unacceptable.
In addition to setting up machine software parameters, most machines must be
physically prepared for a build. The operator must check to make sure sufficient
build material is loaded into the machine to complete the build. For machines which
use powder, the powder is often sifted and subsequently loaded and leveled in the
machine as part of the setup operation. For processes which utilize a build plate, the
plate must be inserted and leveled with respect to the machine axes. Some of these
machine setup operations are automated as part of the start-up of a build, but for
most machines these operations are done manually by a trained operator.
3.2.5 Step 5: Build
Although benefitting from the assistance of computers, the first few stages of the
AM process are semiautomated tasks that may require considerable manual control,
interaction, and decision making. Once these steps are completed, the process
switches to the computer-controlled building phase. This is where the previously
mentioned layer-based manufacturing takes place. All AM machines will have a
similar sequence of layering, including a height adjustable platform or deposition
head, material deposition/spreading mechanisms, and layer cross-section formation. Some machines will combine the material deposition and layer formation
simultaneously while others will separate them. As long as no errors are detected
during the build, AM machines will repeat the layering process until the build is
complete.
3.2.6 Step 6: Removal and Cleanup
Ideally, the output from the AM machine should be ready for use with minimal
manual intervention. While sometimes this may be the case, more often than not,
parts will require a significant amount of post-processing before they are ready for
use. In all cases, the part must be either separated from a build platform on which
the part was produced or removed from excess build material surrounding the part.
Some AM processes use additional material other than that used to make the part
itself (secondary support materials). Later chapters describe how various AM
processes need these support structures to help keep the part from collapsing or
warping during the build process. At this stage, it is not necessary to understand
exactly how support structures work, but it is necessary to know that they need to be
dealt with. While some processes have been developed to produce easy-to-remove
supports, there is often a significant amount of manual work required at this stage.
For metal supports, a wire EDM machine, bandsaw, and/or milling equipment may
be required to remove the part from the baseplate and the supports from the part.
There is a degree of operator skill required in part removal, since mishandling of
parts and poor technique can result in damage to the part. Different AM parts have
48 3 Generalized Additive Manufacturing Process Chain
different cleanup requirements, but suffice it to say that all processes have some
requirement at this stage. The cleanup stage may also be considered as the initial
part of the post-processing stage.
3.2.7 Step 7: Post-Processing
Post-processing refers to the (usually manual) stages of finishing the parts for
application purposes. This may involve abrasive finishing, like polishing and
sandpapering, or application of coatings. This stage in the process is very application specific. Some applications may only require a minimum of post-processing.
Other applications may require very careful handling of the parts to maintain good
precision and finish. Some post-processing may involve chemical or thermal
treatment of the part to achieve final part properties. Different AM processes
have different results in terms of accuracy, and thus machining to final dimensions
may be required. Some processes produce relatively fragile components that may
require the use of infiltration and/or surface coatings to strengthen the final part. As
already stated, this is often a manually intensive task due to the complexity of most
AM parts. However, some of the tasks can benefit from the use of power tools, CNC
milling, and additional equipment, like polishing tubs or drying and baking ovens.
3.2.8 Step 8: Application
Following post-processing, parts are ready for use. It should be noted that, although
parts may be made from similar materials to those available from other
manufacturing processes (like molding and casting), parts may not behave
according to standard material specifications. Some AM processes inherently create
parts with small voids trapped inside them, which could be the source for part
failure under mechanical stress. In addition, some processes may cause the material
to degrade during build or for materials not to bond, link, or crystallize in an
optimum way. In almost every case, the properties are anisotropic (different
properties in different direction). For most metal AM processes, rapid cooling
results in different microstructures than those from conventional manufacturing.
As a result, AM produced parts behave differently than parts made using a more
conventional manufacturing approach. This behavior may be better or worse for a
particular application, and thus a designer should be aware of these differences and
take them into account during the design stage. AM materials and processes are
improving rapidly, and thus designers must be aware of recent advancements in
materials and processes to best determine how to use AM for their needs.