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AM Notes

AM Notes
ASTM definition of AM : Per its ASTM definition, Additive Manufacturing (AM) refers to a process by which digital
3D design data is used to build up a component in layers by depositing material. It
describes a professional production technique which is clearly distinguished from
conventional methods of material removal. Examples are: Vat photopolymerization/SLA
(material is cured by light-activated polymerization), powder bed fusion/SLS/SLM
(energy, typically a laser or electron beam, is used to selectively fuse regions of a powder
bed), material extrusion/FDM (material is selectively dispensed through a nozzle and
solidifies), material jetting (droplets of build material are jetted to form an object). Other
methods are binder jetting, sheet lamination, and directed energy deposition.
Limit the adoption of AM
Additive manufacturing processes typically have a production rate of 0.01 to 1 kg/hr, while
injection molding and machining have production rates in the order of 10 kg/hr. The cost
is limited by the slow rate, and the high cost of input materials. Finally, the resolution of
3D printing processes is around 100 microns, while machining can produce features in
the order of 25 microns.
Advantages of additive manufacturing
Advantages of additive manufacturing are both the ability to create more versatile part
geometries than what many conventional processes are capable of, including undercuts
and sharp internal features, as well as the lack of molds or other specific tools, making it
both faster and cheaper for prototyping cycles than conventional processes. However,
certain Additive Manufacturing methods may result in anisotropic or less robust structural
properties than other manufacturing processes and many processes do not reach the rate
of certain mass production techniques, such as injection molding.
Extrusion process
Process Basics Nomenclature:
FFF - Fused Filament Fabrication
FDM - Fused Deposition Modeling
Support raft - an initial layer of material used to adhere the part to the build plate
and needed to assist in the separation of the part from the build layer.
Build plate/build platform- a flat sheet of material that the part is built on. In many
machines, the build platform is heated to give good adhesion between the part and the
flat support.
Image citations: [1] and [2]
In FFF/FDM printing, the nozzle dispenses molten thermal plastic onto a build platform
and the part is built up through the relative motion of the nozzle and the build
platform. There are typically three axes of motion, x y and z, all orthogonal. By the
coordination of motion between the nozzle and the build platform, FFF/FDM printing can
build curvilinear layers and complex geometries from the bottom up. The plastic is
typically fed into the machine as a spool of filament, about two millimetres in diameter,
and then melted as it is fed into the nozzle.
Image Citation: [3]
FFF/FDM parts show intrinsic features resulting from the tool path (the nozzle and build
platform motion). This is due to the discretization of the trajectory of extrusion and the
fact that the nozzle has a fixed size and has to deposit material pretty much wherever it
goes. If you cross-section any FDM feature that is fully solid, you can see it is made by
a finish pass around the outside as well as an infill pass which is crosshatched to
provide the requisite density of material. An example is shown below. Not only can you
see the boundaries between the finish pass and the infill in the example, you can also
see what happens when you make a feature smaller and smaller, but you can't change
the diameter of material that you extrude. The ever smaller gaps in the rectangular
features become wavy because of the intrinsic precision of material dispensing in the
FFF/FDM process.
Image Citation: [4]
There are also trade-offs between, for example, accuracy and strength, and tool pathing
and part properties. For the highest accuracy, you have to keep the tool paths just
barely touching each other, because you don't want the excess material to run into itself
and cause deformation. But if you want highest strength, you need to have the best
bonding between the consecutive depositions of material. Any parts made with
FFF/FDM printing need to be made with these trade-offs in mind.
FFF/FDM can use a variety of thermoplastic materials ranging from ABS and nylon up
to polycarbonate and ultem. Many materials companies have started developing special
plastics for 3D printing, and the range of materials continues to expand.
During printing, the material is heated similar to injection molding. The material starts at
room temperature, is heated past the glass transition temperature where the chains
loosen and can slip an align, and then we reach the melt temperature. The melt
temperature is typically the set point temperature for the extruder to create a
dispensable material without over heating the material. Unlike injection molding, the
material cools in open air rather than in a mold cavity.
Image Citations: [5], [6], [7]
Process Limitations
The build rate and quality are limited by the temperature of the material and the stepper
motors that feed the filament into the nozzle. As the temperature of the material lowers
and the viscosity increases, more force is required to force the filament through the
nozzle. As the feed rate, i.e. the velocity of the filament through the nozzle, increases,
more force is required. Eventually the force required exceeds the shear strength of the
material and the motors can no longer grip the filament.
In the image below, an experiment was run on a desktop FDM to measure the
relationship between force, feed rate, and temperature. As the feed rate increased, the
force required grew until the material shear strength was exceeded at 60 N and the
machine could no longer dispense material.
Image Citation: [9]
Another limitation on build rate comes from heat transfer to the filament. From the
diagram of the nozzle below, you can see that heating primarily occurs in the initial zone
(Zone I) of the nozzle. Then the diameter of the nozzle reduces until it matches the
width of the polymer extruded from the nozzle.
Image Citation: [10]
We ran finite element simulations of the nozzle geometry as shown below and
plotted the temperature along the centerline of the extruder at different feed rates. We
found that a feed rate of about 3 millimetres per second was the highest feed rate at
which the temperature at the bottom of the large diameter section was equal to the melt
temperature. And when you don't meet this heat transfer requirement, a much greater
force is required to extrude the polymer because it's too cold and too viscous when it
hits the nozzle, and therefore the print head fails.
Image Citation: [11]
From these experiments, you can see that the feed rate of a conventional desktop FDM
printer is limited by the melt temperature of the material, the heat transfer of the
material, and the shear strength of the material. These limitations on feed rate reduce
the amount of material the machine can dispense per hour, which in turn limits how
many parts per hour the machine can produce. However, many innovations are taking
place in the field of FDM printing to overcome these limitations and increase the
throughput of the process.
Design Considerations
To master FDM as a manufacturing process, there are several important
Understand the criteria for support structures. If the part has a certain angle that
becomes close to the horizontal, you're going to need to build secondary
structure to support that overhanging feature. And the criteria for support depend
on the geometry, the orientation of the part, the material, and even the machine.
Learn about the mechanical properties of parts that you build. One of the most
important things about FDM extruded parts is that they're anisotropic. That
means that the strength of a part will be different if you pull along the filament
direction versus perpendicular to the filament direction. Results are getting better,
processes are getting better, but typically parts are still a few tens of percent
weaker in terms of their strength perpendicular versus parallel.
Understand the tolerances produced by the machine and how tolerances are
impacted by printing orientation. From Manufacturing Engineering and
Technology 7th edition by Kalpakjian: "The layers in an FDM model are
determined by the extrusion-die diameter, which typically ranges from 0.05 to
0.12 mm (0.002-0.005 in.). This thickness represents the best achievable
tolerance in the vertical direction. In the x-y plane, dimensional accuracy can be
as fine as 0.025 mm (0.001 in.) as long as a filament can be extruded into the
features." [12]
Understand ways to improve surface finish on parts. There are established
methods using mechanical or chemical means such as solvent vapors to go from
the rough surface to the smooth surface, whether it's a small part or a large part.
1. 3D printed ship wheel, image © John Hart
2. CAD rendering of a Yo-Yo assembly © MIT
3. FDM process diagram © John Hart
4. FDM filament path, Figures 6.3 and 6.5 from "Additive Manufacturing
Technologies (2nd Edition)" by Gibson, et al. (2015) © Springer International
Publishing AG, part of Springer Science+Business Media
5. Amorphous polymer network, Figure 1.2 from "Plastic Part Design Injection
Molding (2nd edition)" by Malloy (1994). © Hanser Publishers (1994)
6. Photo of FDM printed part on a Solidoodle printer © John Hart
7. Injection molding machine diagram, Figure 13.21 from Title: Fundamentals of
Modern Manufacturing; Author: Mikell P. Groover; © Wiley; (2010);
8. Tensile stress and strain as a function of temperature for an amorphous
thermoplastic, Figure 2.26 from "International Plastics Handbook" by Osswald et.
al. © Hanser Publishers (2006).
9. Extrusion force versus feed rate, Image by Jamison Go © MIT. All Rights
10. Photograph of dual nozzle extruder © John Hart
11. Diagram of filament material flow, made by Jamison Go © MIT. All Rights
12. Quote from "Manufacturing: engineering and technology (7th Edition). Kalpakjian,
S., Schmid, S. R., & Sekar, K. S. V. (2016). © Pearson Australia.
Process Basics
Photopolymerization - often called sterolithography or SLA; A 3D printing process that
involves scanning a laser across a bath of photocurable polymer to build up a part.
Process Diagram:
A diagram of a typical SLA machine is shown below. In an SLA machine, a laser beam
passes though a series of lenses, and then is positioned with x-y scanning mirrors to
shine on a precise location of a liquid photopolymer bath in a vat. This builds a part in
layers on a build platform. After each layer is formed, the elevator increments the build
platform down into the bath and the sweeper runs across the surface of the bath to
evenly distribute the liquid photopolymer before the next layer of part is formed.
Image Citation: [1]
Support Materials:
The cured resin is denser than the liquid resin, so a large support structure is often
needed to support the part. Since SLA requires a single resin material in the bath,
support materials on the part must be manually removed instead of chemically
removed. Therefore, the part orientation must minimize the surface area of attachment
between the part and the support. The software for SLA printers, including the Form 1,
often says that the best orientation for the build is at an angle.
An example of an SLA printed hamster for one of the 2.008 yoyos is shown below. The
idea was to have a little plastic hamster that ran around the inside of the yo-yo as the
yo-yo spun. And in the prototyping process small hamsters were made by
stereolithography to demonstrate what the hamster would look like. As you can see, it
was printed in a tipped configuration and had a large support structure that needed to
be removed before the part was usable.
Image Citation: [2]
Comparison of SLA to FDM:
Typically the layers thicknesses of SLA can be smaller than FDM
The SLA process is slower if you have thinner layers, but the thinner layers result in
higher detail and better surface quality than you can get in FDM.
In both SLA and FDM, an infill trajectory and a perimeter trajectory are needed to give
the best possible smooth, fine edge.
Process Limitations
One limitating factor of sterolithography is laser intensity. When the laser beam is
scanning across the surface of the resin, there is a distribution of intensity across the
diameter of the laser. If you measured the number of photons hitting the surface of the
resin, they would be maximum in the center of the laser spot and decaying as you go
farther out. This intensity distribution on the surface combined with the fact that the resin
absorbs light means that each laser pass cures a parabolic cross-section of the resin. If
you imagine you're sitting on the surface of the resonance and you measured the
intensity of light and measured the amount of polymer that was photo cured, you would
find that you form a parabolic noodle of cured resin each time the laser passes.
Image Citations: [3], [4], and [5]
You can relate this cure depth, i.e. the depth of the parabola, to parameters of the photo
resin that can be measured and calibrated. There is a log linear relationship between
the cure depth of the resin and the exposure or the amount of energy in millijoules per
unit area. When a certain critical minimum intensity to cure the resin is surpassed, there
is this long linear relationship. It's the slope and the intercept of this line that let you take
the reported parameters of a photo resin and match them with the
performance characteristics of a particular stereolithography system. This relationship
between the intensity of the laser and the cure depth limits the layer thicknesses that
SLA can achieve which impacts production rate.
A second limitation is Looking at the cross-section of a stereolithography part
shows that the laser is not printing isolated parabolas, but rather parabolas that overlap.
To build a contiguous full density part, you need to have good adhesion between
the layers laterally and the layers vertically. An important factor is that polymers shrink
as they cure, and stress is developed as you go from the fluid molecules with a higher
free volume than their counterparts. So the intelligence inside stereolithography
machines actually executes a zigzag scan pattern that's designed to give a uniform
contiguous solid part, and to try to reduce the amount of shrinkage and stress that you
get. The chemistry of the resin is also important to that. In the cross section shown
below, roughly each horizontal line represents a layer, and each vertical column
represents the laser spot size.
Image Citation: [6]
You can see that the laser is going back and forth with every consecutive laser, and
you're actually curing into the previous layer a little bit more to make sure the next layer
is well adhered to the part itself. This layer pattern has important impacts on the part
quality produced by an SLA machine.
Another limitation of SLA is that the support material must be removed manually. This is
often a laborious and painstaking processes that reduces the amount of finished parts
that can be produced per hour.
Design Considerations
Consider how the material will be supported and how the support material will be
removed. Removal of the support material may leave marks and remnants which may
require post-processing to clean up.
Consider the tolerance that the machine can deliver. From Manufacturing Engineering
and Technology 7th edition, Kalpakjian: " The smallest tolerance that can be achieved in
sterolithography depends on the sharpness of the focus of the laser; typically, it is
around 0.0125 mm (0.0005 in)." [7]
Understand material properties of the resin used and the mechanics and durability that
the part produced will have.
1. Sterolithography process diagram, image copyright © 2009 CustomPartNet. All Rights
3D printed prototype hamster and hamster wheel, image © John Hart
3. Exposure at a point in SLA, © MIT. All Rights Reserved.
4. Cured resin cross-section, figure 1 (b) from Title: Epoxy and Acrylate Stereolithography
Resins: In-Situ Property Measurements; Authors: T. R. Guess, R. S. Chambers, T. D.
Hinnerichs; Year: January 1996; Journal: Sandia Report SAND95-2871. This work is in the
public domain.
5. Cure depth in photopolymerization, Figures 4.6 and 4.7 from "Additive Manufacturing
Technologies (2nd Edition)" by Gibson, et al. (2015). © Springer International Publishing
AG, Part of Springer Science+Business Media
6. Laser scanning patterns, Figures 4.10 b) and 4.12 from "Additive Manufacturing
Technologies (2nd Edition)" by Gibson, et al. (2015). © Springer International Publishing
AG, Part of Springer Science+Business Media
7. Quote from "Manufacturing: engineering and technology (7th Edition). Kalpakjian, S.,
Schmid, S. R., & Sekar, K. S. V. (2016). © Pearson Australia.