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Design for Additive Manufacturing

Abstract and Figures

Additive Manufacturing has become a buzz word in today’s manufacturing world. It has gone through tremendous improvements over the past few decades and has matured from simple prototyping to actual manufacturing and tooling. Various methods have emerged like Fused Deposition Modeling (FDM), Stereolithography (SLA), PolyJet (3DP), Selective Laser Sintering (SLS), Direct Metal Laser Sintering (DMLS), etc. All these methods manufacture the part through addition of materials layer by layer and hence this technology is referred as additive manufacturing. There are many other synonymous terms used like rapid prototyping, rapid manufacturing, 3D printing, etc. Applications of additive manufacturing cover a vast variety of industries like automotive, consumer goods, medical devices, aerospace, defense, etc. Though in principle, any component can be manufactured by either subtractive manufacturing or additive manufacturing techniques, various design features pose completely different challenges in both methods. With a growing number of parts manufactured directly by additive manufacturing techniques, it is important to lay down design principles suitable for such manufacturing processes and to ensure parts are designed for additive manufacturing. There are several factors that are to be considered at the design stage for effective manufacturing of parts using additive manufacturing. Few such design issues in additive manufacturing are discussed in this paper.
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W H I T E P A P E R
Nesting An essential aid for
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Version 1.1
June, 2010
Design for Additive Manufacturing
Version 1.0
July, 2013
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Contents
Introduction ............................................................................................................ 4
Maximum part size ................................................................................................. 4
Faces requiring support .......................................................................................... 4
Minimum wall thickness and rigidity ...................................................................... 5
Minimum feature size and manufacturing quality ................................................. 6
Geometric DFX ........................................................................................................ 7
DFX Rules for Additive Manufacturing ................................................................... 7
Conclusion ............................................................................................................... 9
About the Author .................................................................................................... 9
References .............................................................................................................. 9
About Geometric .................................................................................................. 10
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Introduction
Additive Manufacturing has become a buzz word in today’s manufacturing world. It has gone
through tremendous improvements over the past few decades and has matured from simple
prototyping to actual manufacturing and tooling. Various methods have emerged like Fused
Deposition Modeling (FDM), Stereolithography (SLA), PolyJet (3DP), Selective Laser Sintering
(SLS), Direct Metal Laser Sintering (DMLS), etc. All these methods manufacture the part through
addition of materials layer by layer and hence this technology is referred as additive
manufacturing. There are many other synonymous terms used like rapid prototyping, rapid
manufacturing, 3D printing, etc.
Applications of additive manufacturing cover a vast variety of industries like automotive,
consumer goods, medical devices, aerospace, defense, etc. Though in principle, any component
can be manufactured by either subtractive manufacturing or additive manufacturing techniques,
various design features pose completely different challenges in both methods. With a growing
number of parts manufactured directly by additive manufacturing techniques, it is important to
lay down design principles suitable for such manufacturing processes and to ensure parts are
designed for additive manufacturing. There are several factors that are to be considered at the
design stage for effective manufacturing of parts using additive manufacturing. Few such design
issues in additive manufacturing are discussed in this paper.
Maximum part size
Parts can be either manufactured with in-house additive manufacturing machines or can be
outsourced. The maximum size of parts is generally constrained by the additive machines
available for manufacturing. If the part is of bigger size than the maximum machine capacity,
then the following methods can be adopted to tackle this. (a) If the end use is only to create a
prototype, then the part can be scaled such that the maximum dimensions fit the machine. But
due to scaling, some finer details could be lost depending on the scaling factor an d the feature
dimensions. This could require some editing or clean up in the scaled 3D model. (b) The part can
be redesigned to fit the machine if the mismatch is small in dimension or the part can be
redesigned in to multi-piece assembly. (c) If the model is too large then the model could also be
spilt into two or more pieces to be glued or welded later. This could also require some
modifications or addition of few features to increase the bond strength.
Faces requiring support
As additive manufacturing methods build the part layer by layer, some designs might require
additional support due to the nature of additive manufacturing. Features such as negative drafts,
overhangs and undercut features as shown in figure 1 require support in FDM/SLA/3DP. Such
features can be avoided wherever possible, as they require supports which increases the part
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creation time and in turn cost of manufacturing. Parts requiring supports also might require
secondary processes like removal of support, cleaning and sanding of the part at support joints.
Figure 1. Faces requiring support
Minimum wall thickness and rigidity
The minimum wall thickness of a part is generally constrained by the additive manufacturing
method, machine resolution, etc. Very thin walls could make the part very fragile and hence a
minimum wall thickness has to be maintained to provide sufficient strength and rigidity to the
part. Apart from this, the part has to be strong enough to withstand the stress caused while
removing the support material. This necessitates a minimum wall thickness to be maintained for
faces requiring supports such as negative drafts, overhangs and undercuts.
Rigidity of a part can also be increased by adding ribs. Ribs are added as protrusions, which can
stiffen and strengthen the part and in turn facilitate having thinner walls. However, tall ribs or
long ribs can also create some problems. Ribs need to be designed in correct proportions of
length, height and thickness to provide the required strength. Rib thickness should be large
enough to be manufactured by additive manufacturing. If ribs are too long or too tall, supporting
ribs may be required. It is better to use a number of smaller ribs instead of one large rib . For
large surfaces, it is advisable to add rib networks. For additive manufacturing, it is generally
recommended to increase the wall thickness rather than adding ribs to avoid thin walled
structures. But thick walled structures increase the weight of the part as well as cost of additive
manufacturing, hence it is generally a tradeoff between design requirement and cost.
Generally part designs may have bosses, which serve as points for attachment and assembly. The
most common boss designs consist of cylindrical projections with or without holes. Holes in
bosses are designed to receive screws, threaded inserts, or other types of fastening hardware.
Under service conditions, bosses are often subjected to stresses not encountered in other
sections of a component. So, bosses are generally designed with a draft to increase the strength
at the bottom. In injection molding, drafts in bosses also facilitate easy removal of parts from
core and cavity of the mold. In additive manufacturing, drafts in boss outer surface serve as boss
stiffeners. A fillet of certain radius is also provided at the base of boss to reduce stress. The
Undercuts
Negative
Draft
Overhang
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radius at the base of boss should be larger than a certain minimum value depending on the
additive manufacturing machine. Tall and slender bosses should be avoided. While designing the
bosses, correct proportions of height, outer radius, hole radius and hole depth should be
provided to achieve the required strength.
Minimum feature size and manufacturing quality
The minimum feature size of various features like holes (blind or through), pockets (depression
texts or symbols, cutouts), islands (protrusion texts or symbols, bosses, pins) in a part is generally
constrained by the additive manufacturing method, machine resolution, wall thickness, whether
the feature is in vertical or horizontal wall, etc. The minimum feature size is constrained by the
bead width in FDM and laser in SLS. Machine manufacturers recommend that the minimum
feature size in any section i.e. XY plane should be greater than or equal to four times the
resolution, whereas the minimum feature size in Z direction should be greater than or equal to
the resolution. So, features should be designed to have any dimension greater than the minimum
feature size to get more accurate parts from additive manufacturing. Also all sharp corners in the
XY section plane should be filleted or chamfered to accommodate the natural radius inherent to
the manufacturing process and to reduce the stresses. Fillet radii should be greater than the
minimum natural radius, which is generally four times the resolution. Similarly any knife edges
with zero thickness at the edge will get manufactured with some thickness hence it is advisable
to flatten such knife edges to a minimum thickness.
There are specific design considerations to be taken into account while designing various
features.
Quality of holes depends on the wall thickness and the diameter of the hole. Minimum
diameter that can be used increases with the wall thickness. In other words, ratio of
hole diameter to hole depth should be higher than a minimum specified value.
The quality of pockets or cutouts depends on the wall thickness and the feature
dimensions. Quality is generally better in thin walls for dimensions, which are
perpendicular to the wall thickness. Minimum value of such dimensions increase with
the wall thickness. In other words, ratio of min dimension perpendicular to wall
thickness to pocket depth or wall thickness should be higher than a minimum specified
value. While designing such features, a minimum inter-feature distance and a minimum
feature to edge distance have to be always maintained.
Texts are similar to pockets and islands. Texts should be larger than a minimum font size
and the manufacturing quality is generally better when they are placed in vertical walls
than in horizontal walls.
Bosses or pins should be larger than a minimum diameter depending on the additive
manufacturing machine.
Bosses or pins should be larger than a minimum diameter depending on the machine.
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Generally 3D models are converted to STL file format before manufacturing. STL files store the
surface geometry of a 3D model by tessellating into triangulated surfaces, which introduces an
approximation error for curved surfaces. Such errors are less prominent with finer tessellation.
Hence it should be ensured that the tessellation quality is set as per the allowable deviation as it
plays a major role in quality of parts manufactured.
Geometric DFX
For additive manufacturing, though all the design rules might appear simple, verifying all these
rules manually in 3D models is very difficult and time consuming. Manual verification can be
error prone as there are chances of missing few checks. An automated system can largely help in
speeding up this process and bringing a standard way of verifying parts to avoid
manufacturability issues.
Geometric DFX is a design for manufacturing product that takes 3D parts as input and
automatically analyses all the manufacturability issues based on certain predefined rules. DFX
highlights the rule failures directly in the 3D model and also generates xml and xl reports. The
rules are configurable based on the machine and the type of additive manufacturing. A variety of
CAD formats can be handled like Pro/E, NX, CATIA, SolidWorks, Inventor, STEP, IGES, Parasolid,
ACIS, etc. DFX checks for additive manufacturing reduces multiple design iterations and helps to
ensure right designs to be submitted for manufacturing. DFX reduces design to manufacturing
lead time, reduces multiple trials and in turn cost of manufacturing. Apart from additive
manufacturing module, there are many other modules in DFX like milling, turning, sheet metal,
injection molding, casting and assembly.
DFX Rules for Additive Manufacturing
General design rules that should be ensured before sending a part for additive manufacturing are
given below.
1. Maximum part size check Compares part size with allowable maximum part size and
shows a failure if the part is larger.
2. Minimum wall thickness check - Compares wall thickness of the part and highlights the
regions where thickness is lesser than the allowable minimum thickness. This rule also
helps to check for minimum distance between generic pockets (Hole/cutout/pocket)
and minimum distance from edge to generic pockets.
3. Faces requiring support rule (negative draft/ overhang/ undercut recognition)
Recognizes faces requiring support and highlights those faces.
4. Minimum thickness of faces requiring support rule (negative draft/ overhang/ undercut
recognition) Compares the thickness of faces requiring support with that of the
allowable minimum thickness and highlights the faces that fail.
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5. Minimum feature size (Pocket/Island/Text) Compares the feature sizes with that of
the allowable minimum feature size and highlights the features that fail.
6. Recommended rib parameters Recognizes ribs and compares the ratios of (a) rib-base
thickness to nominal wall thickness and (b) rib height to nominal wall thickness with that
of the maximum allowable ratio.
7. Rib reinforcement check - Compares the ratios of (a) rib area to nominal wall thickness
and (b) rib width to nominal wall thickness with that of the allowable maximum ratio
and highlights the features that fail.
Figure 2. Various feature parameters
8. Boss ID to OD ratio - Recognizes bosses and compares the ratio of inner diameter to
outer diameter with that of the allowable minimum ratio and highlights the features
that fail.
9. Boss height to OD ratio - Compares the ratio of boss height to outer diameter with that
of the maximum allowable ratio and highlights the features that fail.
10. Minimum hole diameter to thickness or depth ratio Recognizes holes and compares
actual diameter to thickness (depth) ratio with that of the allowable minimum ratio and
highlights the features that fail.
11. Knife edge - Recognizes knife edges and highlights them.
12. Recommended corner radius Recognizes fillets and compares actual diameter with
that of the allowable minimum radius and highlights the features that fail. Also
recognizes sharp edges and highlights them.
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13. XYZ slice dimensions Checks whether all XY dimensions are exact multiples of 4 times
resolution and Z dimensions are exact multiples of resolution and highlights the regions
that fail.
Conclusion
To stay competitive in the era of globalization, it is very important to look for all means to reduce
manufacturing lead time, cost and time to market. It is generally agreed that design change
requests that come at various stages affect cost and lead time. Changes made in the early design
stage are less costly and hence concurrent engineering concepts got in place where experts from
different teams analyze to reduce known issues in the design. Even this process requires multiple
iterations and for additive manufacturing processes, the availability of expertise is rare. By
incorporating Geometric DFX, organizations can reap considerable benefits to achieve first time
right designs for additive manufacturing which in turn will reduce manufacturing cost and time.
About the Author
Dr. Kannan has over 20 years of R&D experience in CAD/CAM, engineering
software development, and manufacturing automation. He has a Ph.D. in
computer integrated manufacturing and process planning. He has published
multiple research papers in renowned international journals and conferences in
related areas. His area of expertise includes product management and R&D for
next generation CAD/CAM software products. He is currently responsible for
research and development of various products like DFMPro, Feature Recognition and Nestlib. He
can be contacted at TR.Kannan@geometricglobal.com.
References
1. http://www.shapeways.com/tutorials/design_rules_for_3d_printing
2. http://www.solidconcepts.com/resources/dg/selective-laser-sintering-sls-design-
guidelines/
3. www.3dsystems.com
4. www.eos.info/en
5. www.stratasys.com/
6. www.makerbot.com/
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About Geometric
Geometric (www.geometricglobal.com) is a specialist in the domain of engineering solutions,
services and technologies. Its portfolio of Global Engineering services and Digital Technology
solutions for Product Lifecycle Management (PLM) enables companies to formulate, implement,
and execute global engineering and manufacturing strategies aimed at achieving greater
efficiencies in the product realization lifecycle.
Headquartered in Mumbai, India, Geometric was incorporated in 1994 and is listed on the
Bombay and National Stock Exchanges. The company recorded consolidated revenues of Rupees
5.12 billion (US Dollars 108.1 million) for the year ended March 2010. It employs close to 3000
people across 11 global delivery locations in the US, France, Romania, India, and China.
Geometric is assessed at SEI CMMI Level 5 for its software services and ISO 9001:2000 certified
for engineering operations.
Geometric’s Desktop Products and Technologies (DPT) business unit develops cutting-edge point
productivity solutions that enhance design and improve manufacturing operations. The end-user
products from Geometric include CAMWorks®, eDrawings® Publisher, DFMPro, GeomCaliper®
and 3DPaintBrush™. The key technologies from Geometric are NestLib®, Feature Recognition (FR),
GeomDiff and 3DSearchIT®. Geometric licenses these technologies to OEM partners and also
designs and implements customized process solutions using these technologies for industrial
customers.
For further details about Geometric’s DPT business unit, please visit
www.geometricglobal.com/products or call +1.480.222.2255
The copyright/ trademarks of all products referenced herein are held by their respective companies.
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