Intro

Fused Deposition Modeling (FDM), or Fused Filament Fabrication (FFF), is an

additive manufacturing process that belongs to the material extrusion family. In FDM,
an object is built by selectively depositing melted material in a pre-determined path
layer-by-layer. The materials used are thermoplastic polymers and come in a
filament form.
FDM is the most widely used 3D Printing technology: it represents the largest
installed base of 3D printers globally and is often the first technology people are
exposed to. In this article, the basic principles and the key aspects of the technology
are presented.

A designer should keep in mind the capabilities and limitations of the technology
when fabricating a part with FDM, as this will help him achieve the best result.

Fused deposition modeling (FDM) is a technology where the melt extrusion method is

Similar to
used to deposit filaments of thermal plastics according to a specific pattern.
3DP, the layout for FDM consists of a printhead able to move along
X and Y directions above a build platform. The polymer is extruded
through the heated nozzle and laid down as filaments according to
the CAD design. The build platform is then lowered and another
layer can be built, until the scaffold is completed.

FDM is a production method used for fabrication, production

applications, and mechanical system modeling [208]. The technique
produces a tissue scaffold by the melt extrusion method that is
making use of a layer-by-layer thermoplastic polymer [209]. FDM
uses a moving nozzle to extrude a fiber of polymeric material (x-
and y-axis control) from which the physical model is built layer-by-
layer [167]. FDM technique has some advantages such as there is
no unbound loose powder and there is no solvent removal required
in FDM differently 3D printing, it provides flexibility to the material in
processing and handling [210,211]. FDM technique requires
preformed fibers that have a consistent size and material properties
for feed through the rollers and nozzle, and it is the main difficulty of
this technique
Adv - the main advantage of FDM over 3DP is that it does not
require any organic solvent, and there is no need to remove
excessive polymer powder.
Working- The nozzle has a programmed mechanism that allows the
flow of the melted material to be turned on and off. The result of the
solidified material laminating to the preceding layer is a plastic 3D
model built one strand at a time. The system operates in x, y,
and z axes, drawing the model one layer at a time. The filament is
extruded in a thin ribbon form and confirms the bond of filaments at
each layer (Liou, 2008). The extruded filament deposited onto the
platform is recognized as a “road” (Grimm, 2003; Bellini et al.,
2004). A road is quickly solidified after being stacked by another
layer of road on the platform. The road that was deposited
previously, which will be stacked by the latter road, is called a
substrate. Support structures are automatically generated for
overhanging geometries and are later removed by breaking them
away from the object

FDM, also known as material extrusion, is currently the most

popular AM technology on the market [103]. It allows the fabrication
of durable components made of high-strength thermoplastics such
as ULTEM, polycarbonate, polyphenylsulfone, polylactic acid, and
acrylonitrile butadiene styrene (ABS) [104]. FDM systems are
widely versatile in applications, ranging from quick and inexpensive
rapid prototyping to tough and rigid parts suitable for end-use.

FDM, being a layer-by-layer manufacturing technique, can be used

to manufacture prototypes in which each layer has different road-
width, road-gap and angle between successive layers. These
processing parameters can be easily altered to create structural
tissue engineering scaffolds with desired pore size, geometry,
interconnectivity and biomechanical performance. This novel
concept of designing and developing gradient-controlled porosity
structures with complex internal architectures can be used to
fabricate bone implants similar to human bone, especially for
cancellous bone graft application.

Fused deposition modeling (FDM) is another of the latest advances

in manufacturing technology [68,69]. Similar to 3D printing, this
technology also involves a layer-by-layer concept of manufacturing.
The process takes in data from a CAD model and virtually draws
layer-by-layer. The source of the CAD data could be a geometric
conceptual model that may be derived from MRI/CT data [70]. The
FDM extrusion head moves in X and Y axes, while the platform is
lowered in the Z direction. The FDM extrusion head feeds on
the thermoplastic polymer filaments which are fed in a temperature-
controlled fashion. The layer deposition is done ultrathin and the
direction of the heat is very critical for placing the material directly
into place. After a layer is completed, the platform is lowered, and
another layer is constructed. In this manner, layer-by-layer, the
entire 3D model is generated. The materials used for FDM have
increased in number over the last few years. Biocompatible
products with good mechanical properties and interconnectivity of
pores with high porosity can be achieved.

Characteristics of FDM

Printer Parameters

Most FDM systems allow the adjustment of several process parameters, including
the temperature of both the nozzle and the build platform, the build speed, the layer
height and the speed of the cooling fan. These are generally set by the operator, so
they should be of little concern to the designer.

What is important from a designer's perspective is build size and layer height:

The available build size of a desktop 3D printer is commonly 200 x 200 x 200 mm,
while for industrial machines this can be as big as 1000 x 1000 x 1000 mm. If a
desktop machine is prefered (for example for reducing the cost) a big model can
be broken into smaller parts and then assembled.

The typical layer height used in FDM varies between 50 and 400 microns and can
be determined upon placing an order. A smaller layer height produces smoother
parts and captures curved geometries more accurately, while a larger height
produces parts faster and at a lower cost. A layer height of 200 microns is most
commonly used. An article discussing the impact of layer height in a 3D printed part
can be found here.

Warping

Warping is one of the most common defects in FDM. When the extruded material
cools during solidification, its dimensions decrease. As different sections of the print
cool at different rates, their dimensions also change at different speeds. Differential
cooling causes the buildup of internal stresses that pull the underlying layer upwards,
causing it to warp, as seen in figure 3. From a technology standpoint, warping can be
prevented by closer monitoring of the temperature of the FDM system (e.g. of the
build platform and the chamber) and by increasing the adhesion between the part
and the build platform.

The choices of the designer can also reduce the probability of warping:

 Large flat areas (think of a rectangular box) are more prone to warping and should
be avoided when possible.
 Thin protruding features (think of the prongs of a fork) are also prone to warping. In
this case, warping can be avoided by adding some sacrificial material at the edge of
the thin feature (for example a 200 microns thick rectangle) to increase the area that
touches the build platform.
 Sharp corners are warping more often than rounded shapes, so adding fillets to
your design is a good practice.
 Different materials are more susceptible to warping: ABS is generally more
sensitive to warping compared to PLA or PETG, due to its higher glass transition
temperature and relatively high coefficient of thermal expansion.

Layer Adhesion

Good adhesion between the deposited layers is very important for an FDM part.
When the molten thermoplastic is extruded through the nozzle, it is pressed against
the previous layer. The high temperature and the pressure re-melts the surface of
the previous layer and enables the bonding of the new layer with the previously
printed part.

The bond strength between the different layers is always lower than the base
strength of the material.
This means that FDM parts are inherently anisotropic: their strength in the Z-axis is
always smaller than their strength in the XY-plane. For this reason, it is important to
keep part orientation mind when designing parts for FDM.
For example, tensile test pieces printed horizontally in ABS at 50% infill were
compared to test pieces printed vertically and were found to have almost 4 times
greater tensile strength in the X,Y print direction compared to the Z direction (17.0
MPa compared to 4.4 Mpa) and elongated almost 10 times more before breaking
(4.8% compared to 0.5%).
Moreover, since the molten material is pressed against the previous layer, its shape
is deformed to an oval. This means that FDM parts will always have a wavy surface,
even for low layer height, and that small features, such as small
holes or threads may need to be post processed after printing.

Support Structure

Support structure is essential for creating geomentries with overhangs in FDM. The

melted thermoplastic cannot be deposited on thin air. For this reason, some
geometries require support structure. A detailed article explaining the use of support
structure can be found here.

Surfaces printed on support will generally be of lower surface quality than the rest of
the part. For this reason, it is recommended that the part is designed in such a way
to minimize the need for support.

Support is usually printed in the same material as the part. Support materials that
dissolve in liquid also exist, but they are used mainly in high-end desktop or
industrial FDM 3D printers. Printing on dissolvable supports improves significantly
the surface quality of the part, but increases the overall cost of a print, as specialist
machine (with dual extrusion) are required and because the cost of the dissolvable
material is relatively high.

Infill & Shell Thickness

FDM parts are usually not printed solid to reduce the print time and save material.
Instead, the outer perimeter is traced using several passes, called the shell, and the
interior is filled with an internal, low-density structure, called the infill.
Infill and shell thickness affect greatly the strength of a part. A guide for choosing the
best shell and infill parameters for FDM 3D Printing can be found here. For desktop
FDM printers, the default setting is 25% infill density and 1 mm shell thickness, which
is a good compromise between strength and speed for quick prints.

Common FDM Materials

Material Characteristics
Good strength
ABS Good temperature resistance
More susceptible to warping
Excellent visual quality
PLA Easy to print with
Low impact strength
High strength
Nylon (PA) Excellent wear and chemical resistance
Low humidity resistance
Food Safe*
PETG Good strength
Easy to print with
Very flexible
TPU
Difficult to print accurately
Excellent strength to weight
PEI Excellent fire and chemical resistance
High cost

Benefits & Limitations of FDM

The key advantages and disadvantages of the technology are summarised below:

+ve
FDM is the most cost-effective way of producing custom thermoplastic parts and prototypes.
The lead times of FDM are short (as fast as next-day-delivery), due to the high availability of
the technology.
A wide range of thermoplastic materials is available, suitable for both prototyping and some
non-commercial functional applications.
-ve
FDM has the lowest dimensional accuracy and resolution compared to other 3D printing
technologies, so it is not suitable for parts with intricate details.
FDM parts are likely to have visible layer lines, so post processing is required for a smooth
finish.
The layer adhesion mechanism makes FDM parts inherently anisotropic.

Working of FDM 3D Printers

Fused Deposition Modeling (FDM) is a 3D printing technique pioneered in the 1990s by
Stratasys. In fact, the term ‘FDM’ is the trademark of Stratasys. The company continues to be a
leader in manufacturing 3D printers all over the world, including India. 
Alternatively, the 3D printers that are based on this technology are also called as Fused Filament
Fabrication (FFF), Plastic Jet Printing (PJP) or material extruding printers, which is the generic
name for these 3D printers.
The 3D printers that work on FDM technology consist of the printer platform, a nozzle (also
called as printer head) and the raw material in the form of a filament.
The Printer Platform
The printer platform or the bed is typically made of some metal, ceramic or hard plastic, and each
successive layer is deposited on this platform. 
The Nozzle / Printer Head
The nozzle of FDM printers is attached to a mechanical chassis which uses belt and / or lead
screw systems to move it. The entire extrusion assembly is allowed to move in X, Y and Z
dimensions by a motorized system. A fourth motor called as the stepper motor is used to advance
the thermoplastic material into the nozzle. All the movements of the head and the raw material
are controlled by a computer.
The Raw Material
The raw material is typically production grade thermoplastics, though sometimes metal is used as
well. The thermoplastic material is capable of being repeatedly melted when exposed to heat and
re-solidified when the heat is withdrawn. The thermoplastic filament or metal wire is wound as a
coil on a mounted spool. It is then fed through the printer nozzle. The better class of 3D FDM
printers allows the temperature of the nozzle to be maintained just close to the glass transition
temperature of the material being extruded. This allows the material to be extruded in a semi-
liquid state, but return to solid state immediately. This results in a better dimensional accuracy.
In principle, any thermoplastic can be used as raw material for FDM printers. Commercially, a
few of the popular choices of raw material include nylon, Acrylonitrile Butadiene Styrene (ABS)
and its variations, polycarbonates, ply-lactic acid, polystyrene and thermoplastic urethane.
MED610, a raw material that Stratasys provides is bio-compatible. Their ULTEM material too is
certified by the aerospace industry. 
The FDM 3D Printing Process
When the FDM printer begins printing, the raw material is extruded as a thin filament through the
heated nozzle. It is deposited at the bottom of the printer platform, where it solidifies. The next
layer that is extruded fuses with the layer below, building the object from the bottom up layer by
layer. 
Most FDM printers first print the outer edges, the interior edges next and lastly the interior of the
layer as either a solid layer or as a fill in matrix.
In some objects / models, there are fragile ‘overhangs’ that will droop unless they are given some
support. FDM printers incorporate a mechanism whereby these support structures (called struts)
are printed along with the object. They are later removed once the build is complete. These struts
are usually of the same material as the object. Some printers have a second extruder to
specifically deposit soluble thermoplastic struts when there is a need to prevent the overhangs
from drooping. These struts may be of a different composition than the thermoplastic used for the
3D model. They are later dissolved by an appropriate solvent.

How accurate are 3D FDM Printers?

Remember that a 3D printer works by depositing raw material layer by layer along the X, Y and Z axis.
The accuracy of the 3D printer therefore depends upon the minimum distance the nozzle can travel
vertically (the Z axis). Minimum the distance it can move, more the points along the sinusoid that it can
capture, and better the accuracy.For Stratasys 3D printers, which are the pioneers of the FDM printers,
the current best possible dimensional accuracy is about 0.127 mm. Of course, the choice of raw material
too plays an important part in achieving dimensional stability. It should also be remembered that the
accuracy comes at the cost of printing time required.
A few advantages of FDM 3D printers include:
a wide range of FDM printers are available in the market today
the raw material is inexpensive, durable and maintains dimensional integrity
there is a wide choice of raw material
they are affordable
low turnaround time
One disadvantage is that if the desired level of accuracy is extremely high, then the FDM printers
may be found wanting.
FDM 3D Printers find application in:
creating prototypes for Fit, Form and Function testing
rapid tooling patterns and mould inserts
creating and testing any parts that work under thermal loads
production of precise and complex end-use parts e.g. jigs & fixtures
Sectors that use FDM 3D Printers include:
 Automotive
 Aerospace
 Manufacturing
 Industrial
 Medical
 Architecture
 Consumer Goods 
 Fashion
 Education & Research
Overall, FDM 3D printers give a very high value for money and are very popular in India and 
Other countries.