Lecture # 05 Post processing of AM parts

BAHIR DAR UNIVERSITY
BAHIR DAR INSTITUTE OF TECHNOLOGY (BiT)
FACULTY OF MECHANICAL AND INDUSTRIAL
ENGINEERING
Rapid Prototyping & Reverse Engineering
[MEng6123]
Post processing of AM parts

Post processing techniques used to enhance components or overcome AM
limitations. These include:
1. Support Material Removal
2. Surface Texture Improvements
3. Accuracy Improvements
4. Aesthetic Improvements
5. Preparation for use as a Pattern
6. Property Enhancements using Non-Thermal Techniques
7. Property Enhancements using Thermal Techniques

Support Material Removal
Support material can be broadly classified into two categories:
• Material which surrounds the part as a naturally-occurring by-product of the
build process (natural supports)
• Rigid structures which are designed and built to support, restrain or attach
the part being built to a build platform (synthetic supports).
• Natural Support Post-Processing
• Synthetic Support Removal
• Supports Made from the Build Material
• Supports Made from Secondary Materials

Automated powder removal using vibratory and vacuum assist in a ZCorp 450
machine. (Courtesy Z Corporation)

Flat FDM-produced aerospace part. White build material is
ABS plastic and black material is the water-soluble support
material. (Courtesy of Shapeways. Design by Nathan Yo Han
Wheatley.)
Breakaway support removal for (a) an FDM part (courtesy of
Jim Flowers) and (b) an SLA part (Courtesy Worldwide Guide to
Rapid Prototyping web-site.

Surface Texture Improvements
• AM parts have common surface-texture features that may need to be modified
for aesthetic or performance reasons.
• Common surface textures are: stair-steps; powder adhesion; fill patterns from
extrusion or beam-based systems; and witness marks from support material
removal.
• The post-processing utilized for surface texture improvements is dependent upon
the desired surface finish outcome.
• If a matte surface finish is desired, a simple bead blasting of the surface can
help.

Surface Texture Improvements
• If a smooth or polished finish is desired, then wet or dry sanding and hand-
polishing are performed.
• It is desirable to paint the surface (e.g., with cyanoacrylate, or a sealant)
prior to sanding or polishing.
• Painting the surface has the dual benefit of sealing porosity and, by viscous
forces, smoothing the stair-step effect; thus making sanding and polishing
easier and more effective.

Accuracy Improvements
• There is a wide range of accuracy capabilities in AM.
• Some processes are capable of sub-micron tolerances, whereas others have
accuracies around 1 mm.
• Typically, the larger the build volume and the faster the build speed the
worse the
accuracy for a particular process.

Post processing of AM
parts Preparation for use as a Pattern Often
• The parts made using AM are intended as patterns for investment casting,
sand casting, room temperature vulcanization (RTV) molding, spray metal
deposition or other pattern replication processes.
• The accuracy and surface finish of an AM pattern will directly influence the
final part accuracy and surface finish.
• As a result, special care must be taken to ensure the pattern has the accuracy
and surface finish desired in the final part.
• In addition, the pattern must be scaled to compensate for any shrinkage that
takes place in the pattern replication steps.

importance for its end application.
• A difference in surface texture between
one region and another may be desired
(this is often the case in jewelry).
• Finishing of selected surfaces only is
required.
• In cases, where the color of the AM part
is not of sufficient quality, several
methods can be used to improve the part
aesthetics.
• Another aesthetic enhancement (which
also strengthens the part and improves
wear resistance) is chrome plating.
Aesthetic Improvements
• Aesthetics of the part is of
critical
SLA part (a) before and (b) after chrome plating.
(Courtesy of Artcraft Plating)

Preparation for use as a Pattern Often
Rings for investment casting, made using a ProJet1 CPX 3D Printer (Courtesy 3DSystems)

Property Enhancements using Non-thermal Techniques
• Powder-based and extrusion-based processes often create porous
structures.
• The porosity can be infiltrated by a higher-strength material, such as
cyanoacrylate (Super Glue). Newer, proprietary methods and materials have
also been developed to strengthen various AM parts.
• One of the best known is the RP Tempering process (PAR3 Technology,
USA).
• RP Tempering is a collection of materials and treatment operations used to
increase the strength, ductility, heat deflection, flammability resistance,
EMI shielding or other properties of AM parts
• using nano-composite reinforcements.

Post processing of AM
parts Property Enhancements using Thermal
Techniques
• Many parts are thermally processed to enhance their properties.
• In beam deposition and PBF techniques for metals, this thermal processing
is primarily heat treatment to form the desired microstructures and/or to
relieve residual stresses.
• Traditional heat treatment developed for the specific metal alloy being
employed are commonly used.
• In some cases, special heat treatment methods have been developed to
retain the fine-grained microstructure within the AM part while still
providing some stress relief and ductility enhancement.

Guidelines for process selection

According to Wohler's and Associates, parts from AM machines are used
for a number of purposes, including:
• Visual aids
• Presentation models
• Functional models
• Fit and assembly
• Patterns for prototype tooling
• Patterns for metal castings
• Tooling components
• Direct digital/rapid manufacturing

Selection Methods for a Part
Decision Theory
There are three elements of any decision
• Options – the items from which the decision maker is
selecting.
• Expectations – of possible outcomes for each option.
• Preferences – how the decision maker values each outcome.

Selection Methods for a Part
Approaches to Determining Feasibility
• Identifying suitable materials and AM machines to fabricate a part is
complex.
• For each application, one should consider the suitability of available
materials, fabrication cost and time, surface finish and accuracy
requirements, part size, feature sizes, mechanical properties, resistance to
chemicals, and other application-specific considerations.
• In order to solve AM machine and process chain selection problems, one
must navigate the wide variety of materials and machines, comparing one’s
needs to their capabilities, while ensuring that the most up-to-date
information is available.

Approaches to Selection
Most aid the selection in a qualitative manner.
• Several methods have been developed in academia are based on the large
literature on decision theory.
• The standard Selection Decision Support Problem (s-DSP) has been applied
to many engineering problems and has recently been applied to AM selection.
• The word formulation of the standard s-DSP as shown below;
•Given: Set of AM processes/machines and materials (alternatives).
•Identify: Set of evaluation attributes. Create scales and determine importance.
•Rate: Each alternative relative to each attribute.
•Rank: AM methods from most to least promising.

Selection Example
An example of a capital investment decision related to the application of metal
AM processes to the production manufacture of steel caster wheels
Scenario
• The caster wheel manufacturer is attempting to select an AM machine
that can be used for production of its small custom orders (steel caster
wheels).
• It is infeasible to stock all the combinations of wheels that they want to
offer, thus they need to produce these quickly, while keeping the price
down for the customer.

Selection Example
Scenario
The technologies under consideration are Direct Metal Deposition, Direct
Metal Laser Sintering, Electron Beam Melting, Laser Engineered Net
Shaping, Selective Laser Melting, and Selective Laser Sintering.
Readily available stainless steel material is used for this example.

Selection Example
Scenario
• Before beginning the selection process, the uncertainty involved in
the customization process was considered.
• Since these caster wheels will be customized, there is a degree of
geometric uncertainty involved.

Selection Example
Scenario
Model of steel caster wheel

Guidelines for process selection - Selection Example
• In this example, it is decided to allow customization of certain features.
• Only standard 12 mm diameter x 100 mm length bolts will be used for the inner
bore, therefore, these dimensions will be constrained.
• Customers will be allowed to customize all other features of the caster wheel
• within allowable ranges for this model wheel, as displayed in the table below.
Caster wheel dimensions

Guidelines for process selection - Selection Example
The alternative AM technologies will be evaluated based on 7 attributes that
span a typical range of requirements, as shown in the following section
• Ultimate Tensile Strength (UTS): UTS is the maximum stress reached before a
material fractures. Ratio scale [MPa].
• Rockwell Hardness C (Hard): Hardness is commonly defined as the resistance of a
material to indentation. Ratio scale [HRc].
• Density (Dens.): The density refers to the final density of the part after all
processing steps. This density is proportional to the amount of voids found at the
surface. These voids cause a rough surface finish. Ratio scale [%].

Selection Example
• The alternative AM technologies will be evaluated based on 7 attributes
that span a typical range of requirements, as shown in the following section
• Detail Capability (DC): The detail capability is the smallest feature size
the technology can make. Ratio scale [mm].
• Geometric Complexity (GC): The geometric complexity is the ability of the
technology to build complex parts. It is used to refer to the ability to
produce overhangs. Interval scale (1–10).
• Build Time (Time): The build time refers to the time required to fabricate a
part, not including post processing steps. Ratio scale [h].

Selection Example
• The alternative AM technologies will be evaluated based on 7 attributes
that span a typical range of requirements, as shown in the following section
• Part Cost (Cost): The part cost is the cost it takes to build one part with
all costs included. These costs include manufacturing cost, material
cost, machine cost, operation cost, etc. Ratio scale [$].

Selection Example
In this example, we examine two weighting scenarios (relative importance ratings).
Scenario 1
• Geometric complexity was most heavily weighted because of the significant
overhangs present in the build orientation of the casters.
• Build time and part cost were also heavily weighted because of their importance
to the business structure surrounding customization of caster wheels.
• Because of the environment of use of the caster wheels, UTS was also given a
high weighting.
• Detail capability was weighted least because of the lack of small, detailed
features in the geometry of the caster wheels.

Selection Example
• In this example, we examine two weighting scenarios (relative
importance ratings).
• Scenario 2
• All selection attributes were equally weighted.

Selection Example
• Table shows the results of the evaluation of the alternativeswith respect to
the attributes.
• Weights for the two scenarios, called Relative Importance, are included under
the attribute names.

Selection Example
• On the basis of these ratings, the overall merit for each alternative can be
computed.
• Merit values for each scenario are shown in the Table along with rankings.
• Slightly different rankings are evident from the different scenarios.
• This indicates the importance of accurately capturing decision maker
preferences.
• Process 4 is the top ranking process in both scenarios.
• Second choice could be Process 2, 3, or 6, depending upon preferences.

Guidelines for process selection Selection Example
Table: Merit values and rankings

Challenges in Selection
• The complex relationships among attributes, and the variations that can
arise when building a wide range of parts make it difficult to decouple
decision attributes and develop structured decision problems.
• With a proper understanding of technologies and attributes, and how to
relate them together, meaningful information can be gained.

When looking for advice about suitable selection methods or systems, it is
useful to consider the following points.
• The information in the system should be unbiased wherever possible.
• The method/system should provide support and advice rather than just a
quantified result.
• The method/system should provide an introduction to AM to equip the user
with background knowledge as well as advice on different AM technologies.
• A range of options should be given to the user in order to adjust requirements
and show how changes in requirements may affect the decision.

When looking for advice about suitable selection methods or systems, it is useful
to consider the following points.
• The system should be linked to a comprehensive and up-to-date database of
AM machines.
• Once the search process has completed, the system should give guidance on
where to look next for additional information.

Lecture # 05 Post processing of AM parts

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