Design for Additive
Manufacturability:
FDM Basics
Designers and engineers have always adhered to design
for manufacturability and assembly rules for traditional
manufacturing methods. And breaking these rules has
had its costs – increased production expenses, diminished
quality and poor aesthetics. But the biggest problem has
been suppressed innovation. Additive manufacturing frees
designers from the constraints of conventional manufacturing
methods that many have followed for decades and gives
them a chance to think differently about design. It allows
designers and engineers to create parts solely for desired
form, fit and function by eliminating tooling, machining,
casting and fabrication. Parts can be designed with complex
geometries. They can flow, twist and contort into organic
shapes with astonishing strength-to-weight ratios.
At the same time there are
also a few considerations
that are unique to 3D
printing. This article
will explain both the
additional design freedom
and the specific design
considerations for additive
manufacturing with Fused
Deposition Modeling (FDM)
to help design engineers make
the most of the technology and
motivate new design ideas.
TAKE ADVANTAGE OF DESIGN FREEDOM
WITH ADDITIVE MANUFACTURING
Manufacturability rules for molding
Conventional manufacturing methods have a great number of restrictions and limitations in order to
create parts that can be manufactured at a reasonable cost with acceptable quality. Some of these
rules for injection molded parts include:
• Draft angles must be included or parts won’t eject properly.
• Uniform wall thickness is required to minimize warpage and sink marks.
• Radiused corners are often needed to reduce stress concentrations and improve
plastic flow during machining.
• Shrink rates need to be taken into account to achieve dimensional accuracy.
• Wall thicknesses of bosses, ribs and gussets must be reduced to avoid sink marks.
• Part XY dimensions and volume must not exceed certain size limitations.
Additive manufacturing eliminates all of these restrictions. Since additive manufacturing builds a part from
the ground up, layer-by-layer with soluble or breakaway supports, you can achieve complex designs not
possible with injection molding. There is no need for a draft angle because there is no mold to remove
the part from—negative draft angles can even be used. Wall thickness can be varied throughout the part.
Sharp corners can also be achieved depending on the geometry. Shrink rates are automatically factored
in when the CAD file is analyzed by the FDM software. Sink marks rarely appear on parts built using FDM
technology because of the ability to add support structure for varying wall thicknesses. And finally, large
part designs can be split, built in sections and bonded together, eliminating size limitations.
Feature-laden parts can be produced with no effect on manufacturing cost or time. Since there isn’t a
need for tooling, additive manufacturing can significantly reduce part costs and the time it takes to build
tools. Without a large tooling investment, it also eliminates risk for design changes or errors.
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Manufacturability rules for subtractive manufacturing
Subtractive manufacturing methods, such as CNC machining, also require design for manufacturability
rules that limit design freedom. For example, it is impossible to produce internal channels because there
is no path for the tool to enter and leave the channel. Many types of undercuts and overhangs are also
impossible to produce for the same reason and if they can be produced, they may require expensive
hand tooling. In fact, each feature on a part produced by subtractive manufacturing requires one or more
operations which adds to its cost, so designers are often forced to reduce functionality in order to keep
the cost within limits. Holding fixtures also add to the cost of machining—if a part is round or doesn’t have
a sturdy base, fixtures are needed to secure the material.
The nature of the additive manufacturing process eliminates all of these restrictions, enabling designers
to create internal voids or channels and any type of undercuts or overhangs. There is no additional cost
associated with additional features or design complexity so designers can make each part as featurerich as their imagination allows without cost penalty. For example, voids can be constructed with soluble
support materials that can be easily washed away after the build is completed.
Design freedom
Additive manufacturing offers the advantage of
building very complex heterogeneous structures
that can sometimes be difficult to imagine because
they would be so expensive to produce with
conventional methods. Consider paper pulp tooling
produced with FDM. Where paper pulp molding
is a fairly simple process, producing the tool is
typically labor intensive. Traditional paper pulp
tooling is built from a metal tool in the shape of
the finished part with many holes drilled through
its surface. A vacuum is drawn through the holes to
form the pulp to the shape while a screen attached
to the surface of the mold prevents pulp from
clogging the holes.
This expensive-to-manufacture part can be replaced
by a much less costly one-piece FDM tool with
an internal crossbeam structure that provides
strength while enabling a vacuum to be pulled
through it. The part has a dense frame that mounts
to the molding machine and a porous outer shell
that prevents clogging. This extremely complex
structure is produced with additive manufacturing at
a fraction of the cost required in the past to provide
similar functionality.
FDM also offers the unique advantage among
additive manufacturing methods of enabling sparsefill which makes it possible to produce an internal
lattice structure with a solid skin. A sparse fill
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A BALL WITHIN A BALL WITH MULTIPLE HOLES
ILLUSTRATES ADDITIVE MANUFACTURING’S
ABILITY TO BUILD COMPLEX GEOMETRIES. USING
TRADITIONAL MANUFACTURING METHODS, THIS
WOULD REQUIRE SEVERAL DIFFERENT PROCESSES,
INCLUDING TWO MOLDS AND MACHINING,
MAKING IT IMPRACTICAL OR IMPOSSIBLE TO
MANUFACTURE. WITH ADDITIVE MANUFACTURING
IT CAN BE BUILT IN ONE OPERATION WITHOUT
TOOLING OR ASSEMBLY.
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provides exceptional strength-to-weight ratio so it can be used to reduce the weight of a part while also
reducing cost and construction time. In one example, BMW reduced the weight of a hand-held device
used for auto assembly by 72 percent with a sparse-fill build technique. Replacing the solid core with
internal ribs cut 1.3 kg (2.9 lbs) from the device. A worker uses the tool hundreds of times in a shift so this
weight savings makes it substantially easier to use.
FDM DESIGN REQUIREMENTS
On the other hand, the nature of the additive manufacturing build process gives rise to unique
considerations when designing parts. Support structures, stairstepping, surface finish and wall
thickness must be considered in part design. This section will focus on design for the FDM process
but other additive manufacturing methods have similar considerations which should be consulted
when designing parts.
STL file
An STL file is a format used by digital manufacturing software to generate information needed to produce
3D models. The STL format approximates the surfaces of a solid, surface or scanned model with triangles.
Several key settings need to be adjusted in creating the STL file in order to make sure you get the part
you want. The angle, deviation and chord height control the roughness and smoothness of the part.
When the part is too coarse you can see flat spots on curved surfaces. On the other hand, excessively fine
faceting can cause delays in processing and uploading because of the large file size. Faceting should be
just detailed enough so that features build to the file dimension while being simple enough to maintain a
manageable file size.
COARSE
EXCESSIVELY FINE
GOOD
When producing multiple shells, nested or tabbed parts, each individual piece should be saved
as a separate STL file. If files are not separated they may appear as one part, which slows the
production process.
STL files should be free of missing surfaces, surfaces that overlap, inversed normal or bad edges.
You can find errors in an STL file by using an STL viewing tool.
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Extrusion
FDM technology builds parts layer-by-layer by heating thermoplastic material to a semi-liquid state and
extruding it according to computer-controlled paths. FDM machines offer four different slice thicknesses:
0.005, 0.007, 0.010 and 0.013 inch.
The extrusion path is the layer height of the melted plastic laid down by the extrusion head. The
extrusion path width depends upon the diameter of the tip. Tips are available with diameters from
0.010 inch to 0.02 inch and the minimum extrusion path produced by a tip is typically the tip diameter
while the maximum extrusion path is two times tip diameter. For example, a T16 tip has an inside
diameter of 0.016 inch, a minimum extrusion path of 0.016 inch and a maximum extrusion path of
0.032 inch. As a general rule, smaller tips offer the advantage of being able to produce thinner walls
and other finer details while larger tips reduce build time.
HOW FDM TIP EXTRUSION WORKS. MATERIAL AND SUPPORT MATERIAL ARE HEATED,
LIQUEFIED AND EXTRUDED FROM SEPARATE TIPS.
Wall thickness
The minimum wall thickness for FDM parts depends on slice thickness that will be used to build the part.
The thickness of vertical walls should be at least twice the slice thickness in order to avoid creating walls
that are prone to buckle. Making walls four times the slice thickness or thicker will provide even stronger
walls. Another consideration is a vertical wall should be comprised of at least two filaments so wall
thickness should be at least twice the extrusion path.
Wall thickness is also important when constructing a part where the height of a horizontal surface must
be held to very close tolerances. A horizontal surface is constructed by laying down slices of a constant
thickness, so the greatest accuracy can be achieved when the height of the surface is a multiple of the
slice thickness. For example, a surface with a height of 0.995 inch could not be produced accurately with
a machine with a slice thickness of 0.010 inch. However the same machine could be used to accurately
generate a surface with a height of 0.990 or 1.000 inch.
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This chart defines the minimum acceptable and ideal wall thickness for each material and
layer height.
Overall wall thickness should be greater than or equal to the ideal amount, but thinner wall sections
greater than or equal to the acceptable minimum are achievable. Wall thickness is geometry and
application dependent. We recommend designing load bearing surfaces with ideal thickness or greater.
FDM
THERMOPLASTIC
LAYER HEIGHT (INCH)
WALL THICKNESS (INCH)
ACCEPTABLE MINIMUM
WALL THICKNESS (INCH)
IDEAL
ABS-M30
0.005
0.007
0.010
0.013
0.016
0.024
0.032
0.036
0.029
0.041
0.053
0.059
ABS-M30i
0.005
0.007
0.010
0.013
0.016
0.024
0.032
0.036
0.029
0.041
0.053
0.059
ABS
0.005
0.007
0.010
0.016
0.024
0.032
0.029
0.041
0.053
ABSi
0.005
0.007
0.010
0.016
0.024
0.032
0.029
0.041
0.053
ABS-ESD7
0.007
0.010
0.024
0.032
0.041
0.053
PC
0.005
0.007
0.010
0.013
0.016
0.024
0.032
0.036
0.029
0.041
0.053
0.059
Nylon 12
0.007
0.010
0.013
0.024
0.032
0.036
0.041
0.053
0.059
PC-ABS
0.005
0.007
0.010
0.013
0.016
0.024
0.032
0.036
0.029
0.041
0.053
0.059
PC-ISO
0.007
0.010
0.013
0.024
0.032
0.036
0.041
0.053
0.059
PPSF
0.010
0.013
0.032
0.036
0.06
0.07
ULTEM 9085
0.010
0.013
0.032
0.036
0.06
0.07
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Supports
Additive manufacturing cannot extrude material above open air, so special support material must be
extruded below any overhang such as a window, cutout, horizontal undercut, blind holes, etc. to act as
“scaffolding” until the plastic model material cures. Support material will automatically be provided during
the build process, but it increases the amount of material used and also the build time. The exception
to this rule is in the form a self-supporting angle, which varies depending on the material, but is usually
around 45 degrees. Designing an overhang at greater than the self-supporting angle, whenever possible,
will reduce material usage and build time. It’s also important to check whether any dimensions need to be
held to tighter tolerances than can be achieved with additive manufacturing. In that case, additional stock
should be designed into the part that can be removed with a secondary machining operation.
COIL PART WITH AND WITHOUT
BREAKWAY SUPPORT MATERIAL.
Threads
When designing built-in threads, avoid sharp edges and include a radius on the root to improve the
strength of the thread. An ACME thread design with rounded roots and crests has been found to work
well with FDM. Also, a dog point head of at least 1/32 inch makes starting a thread much easier. It may
be necessary to include a threaded insert when strength is a concern.
Assembly
When building an assembly, clearance between mating parts must be provided to prevent them from
fusing together. The standard guideline for creating clearances on assemblies is a minimum Z axis
clearance of the slice thickness. The X and Y axis clearance should be at least the default extrusion width.
The minimum clearance needed for mating parts when not producing the components fully assembled is
equal to the tolerance of the FDM machine itself.
In many applications, FDM makes it possible to consolidate subassemblies into a single component,
eliminating the assembly process and simplifying inventory management.
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Build orientation
Orientation of the part can significantly impact a part’s aesthetics or functionality. Various orientations can
maximize build time, strength or surface finish. The best orientation depends on the part’s intended use
and purpose. If aesthetics are critical to the final use of the part, reducing stepping and build layer lines
will dictate the orientation. If functionality and strength are critical, reducing supports and space between
layers will dictate orientation.
PART BUILT IN THREE DIFFERENT
ORIENTATIONS. VISIBLE LAYER LINES
VARY IN EACH BUILD ORIENTATION.
A: THIS ORIENTATION HAS THE LEAST
AMOUNT OF VISIBLE LAYER LINES
B: THERE ARE SOME VISIBLE LAYER LINES
ON THE DOWNWARD AND UPWARD
FACING SURFACES
C: THIS ORIENTATION BUILDS THE MOST
AMOUNT OF VISIBLE LAYER LINES
PART BUILT IN TWO DIFFERENT
ORIENTATIONS SHOWS THE EFFECT ON
STRENGTH AND FUNCTIONALITY.
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Large parts
Stratasys FDM systems have the capability of making single part builds as large as 36 inches by 24 inches
by 36 inches. But there’s virtually no size limit with FDM. Parts that are too big for the build chamber may
be produced by sectioning the CAD design, building the sections separately and bonding them together
after the build is completed. Sections can be bonded with the same exact model material and finished to
exact tolerances and requirements. Sectioning can also be used to reduce the amount of required support
structure by producing overhanging features in an orientation that requires less support structure than a
separate build.
OPEN THE DOOR TO NEW DESIGN POSSIBILITIES
Additive manufacturing helps free design engineers from the design for manufacturability rules of
traditional manufacturing methods. Production without tooling makes it possible to produce more
complex geometries, organic shapes, hollow interiors and negative draft, opening up many possibilities
and reducing cost. Embrace new ways of thinking to fully leverage the benefits of 3D printing. Take
advantage of additive manufacturing to increase functionality, combine multiple components into one
design, reduce weight and see your product’s full potential.
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©2014 RedEye. All rights reserved. RedEye is a business unit of Stratasys, Inc.
Stratasys, RedEye, FDM, FDM technology, Fused Deposition Modeling, ABS,
ABS-ESD7, ABS-M30, ABS-M30i, ABSi, PPSF, FDM Nylon 12, and PC-ISO are
trademarks or registered trademarks of Stratasys, Inc. and/or its subsidiaries
or affiliates and may be registered in some jurisdictions. PolyJet is a trademark
of Stratasys Ltd. and/or its subsidiaries or affiliates and may be registered in
certain jurisdictions.
ULTEM is a trademark of SABIC Innovative Plastics IP BV.
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