Exam Questions

Question 1

Describe the generic steps involved in additive manufacturing, applicable for most 3D printing processes.

resit 2018

Answer 1

1. Conceptualization and CAD
2. Conversion to STL
3. Transfer and manipulation of STL on AM machine
4. Machine setup
5. Build
6. Part removal and clean up
7. Post-processing of the built part
8. Application

Question 2

Which additive manufacturing processes use powder as feedstock?

2018 1a

Answer 2

1. Powder bed fusion (PBF)
   - selective laser sintering (SLS)
   - selective laser melting (SLM)
   - direct metal laser sintering (DMLS)
   - electron beam melting (EBM)
2. Binder Jetting
3. Directed energy deposition (DED)
   - Laser-engineered net shaping (LENS)
4. Material Extrusion?

Question 3

What are the main differences between these processes (which use powder as feedstock) in the way of converting loose powder particles into 3D printed parts?

2018 1b

Answer 3

1. Powder bed fusion:
   an energy source sinters or melts the desired layer within the powder bed. The build platform is then lowered and a new powder layer is distributed over the previously formed layer by a roller, sweeper or blade. The laser source is either a laser beam or an electron beam.
2. Binder jetting:
   Rather than melting or sintering powdered particles, in binder jetting a molten liquid is printed onto a powder bed, binding the powder to it.
3. Directed energy deposition:
   Here, the powder is molten upon extrusion by the nozzle, after deposition it solidifies. The feedstock can be in powder or wire form, hence it belongs to one of the processes that uses powder.

Question 4

What are the characteristics of powder that may influence the quality of 3D printed part (dimensional accuracy, residual porosity and mechanical properties)?

2018 1c

Answer 4

1. particles shape
2. size and size distribution
3. powder bed density
4. powder bed thermal conductivity
5. powder spreading and powder flowability

Question 5

Which additive manufacturing processes are capable of making multi-material parts?

2018 2a

Answer 5

1. Material extrusion
2. Material jetting
3. Directed energy deposition

Question 6

Describe an exemplary AM process during which two materials are added and tell the advantages that dual-material parts may offer.

2018 2b

Answer 6

With material jetting one can use multiple nozzles. One nozzle can extrude build material and the other could extrude support material. The support material could be chosen so that the support is easily removable/breakable, water/chemically solvable or thermally removable.

Question 7

Which additive manufacturing processes are capable of making multi-colour parts?

2018 2c

Answer 7

1. Material extrusion
2. Material jetting
3. Binder jetting

Question 8

Give one example of a 3D printed multi-colour part and tell the benefits of multiple colors of this part.

2018 2d

Answer 8

1. Surgeons reported that having multicolored, complex models of the head or abdomen areas were invaluable in planning the surgeries, which can take 12–24 hours and involve large teams of surgeons and support staff.
2. A coloured part could help indicate/discern support structures from the part for easy removal and recognition.

Question 9

Within which sintering techniques are melting of particles involved, and in which way?

2018 3

Answer 9

Liquid phase sintering with distinct and indistinct binders:
The binders are either other materials(distinct binders) that have a lower melting point temperature, or the binders are smaller particles of the same material which will absorb more laser energy. In both cases the binders will melt earlier than the main particles.

Question 10

Name one additive manufacturing process in each of the 4 categories of additive manufacturing according to the “classic classification of additive manufacturing”:

resit 2018 4a

Answer 10

1. Liquid polymer systems:
   Answer:
   SLA, Dual beam SLA
2. Discrete particle systems:
   Answer:
   SLS, LST, LENS, SDM, 3D printing
3. Molten materials systems:
   Answer:
   FDM, MDDM, Atomic Diffusion Additive Manufacturing, MEX, MJ
4. Solid-sheet systems:
   Answer:
   Paper Lamination technology (PLT), LOM, UAM

Question 11

Name three additive manufacturing processes out of 7 process categories according to the ASTM classification of additive manufacturing. And describe how these three processes work, tell the type of materials that may be suitable for each of the three processes.

resit 2018 4b

Answer 11

1. Material extrusion
   processes that deposit a material by extruding it through a nozzle, typically while scanning the nozzle in a pattern that produces a part cross-section.
   Materials:
   ABS, PLA, PCL or hydrogel
2. Powder bed fusion
   processes that utilize a container filled with powder that is processed selectively using an energy source, most commonly a scanning laser or electron beam.
   Materials:
   Thermoplastic (nylon) or thermoset polymers. Metals (weldable) and composites. Ceramics and composites.
3. vat photopolymerization
   processes that utilize a liquid photopolymer that is contained in a vat and processed by selectively delivering energy to cure specific regions of a part cross-section.
   Materials:
   liquid photopolymer / Photo curable resins, e.g epoxy
4. material jetting
   ink-jet printing processes.
   Materials:
   Photopolymers, Ceramics, Metals
5. binder jetting
   processes where a binder is printed into a powder bed in order to form part cross-sections.
   Materials:
   Starch and plaster based powders, ceramics, metals, polymer and sand powder.
6. Sheet lamination
   processes that deposit a layer of material at a time, where the material is in sheet form.
   Materials:
   Paper, Metals, Ceramics, Polymer
7. Directed energy deposition
   processes that simultaneously deposit a material (usually powder or wire) and provide energy to process that material through a single deposition device.
   Materials:
   Any powder material or powder mixture which is stable in a molten pool can be used for construction of parts (No reflective materials).

Question 12

Describe hybrid manufacturing and its possible benefits.

resit 2018 4c

Answer 12

Hybrid system is a combination of additive manufacturing with subtractive manufacturing. SM is used after or during AM to increase surface finish, dimensional accuracy, dedicated features (like threads) and to remove support. Benefits of hybrid manufacturing are: Including a subtractive component can assist in making the process more precise.

Question 13

Name three additive manufacturing processes out of seven process categories according to the ASTM classification of additive manufacturing, tell the type(s) of materials that may NOT be suitable for each of these three additive manufacturing processes, and explain the reasons.

2021/2023 5a

Answer 13

1. Material Extrusion:
   Materials with high melting points or low flowability
2. Material Jetting:
   Materials that cannot be cured by light or have a high viscosity.
3. Vat Photopolymerization:
   Materials that cannot be cured by light or are not photoreactive
4. Powder Bed Fusion:
   Materials with high reactivity(not weldable) or high melting point & thermal conductivity
5. Directed Energy Deposition: Materials with high reactivity (not weldable) or high melting point & thermal conductivity.
6. Binder Jetting:
   Materials that do not bind well with the binder or are not in powder form, like certain polymers.
7. Sheet Lamination:
   Materials that cannot be laminated effectively, such as some ceramics or brittle materials.

Question 14

Name two additive manufacturing processes out of seven process categories according to the ASTM classification of additive manufacturing, which do NOT need support for overhanging structures during 3D printing, and explain the reasons.

2021/2023 5b

Answer 14

1. Sheet Lamination
   As each sheet layer is bonded together with the previous layer, therefore only part of the surface area is needed from the previous layer to position the next layer.
2. Powder bed fusion (Only Selective Laser Sintering)
   - No support is needed for polymers.
   - Support IS needed for metals, this is mainly to avoid shrinkage and part warping.
3. Binder jetting
   As the printed parts are encapsulated by loose powder which takes the role of support (just as for powder bed fusion).

Question 15

Identify the ways to achieve a high packing density before the start of a 3D printing process in the powder-bed fusion system and tell how to achieve a high packing density.

2021/2023 6a

Answer 15

• A desirable characteristic of powders is a high packing density so that printed parts have a high volume fraction of powder and are strong enough to survive depowdering and cleanup operations.
• High packing densities can be achieved by tailoring powder particle shape or by including a range of particle sizes so that small particles fill in gaps between larger particles.

Question 16

Identify the ways to achieve a high relative density during a 3D printing in the powder-bed fusion system and tell how to achieve a high relative density of a 3D printed part.

2021/2023 6b

Answer 16

1. Using coated or composite binders will lead to higher density due to their homogeneity.
2. Furnacing: high-density parts can be achieved by a post-process called furnacing,
   - (Debinding of polymer)
   - Infiltration: filling the gaps with lower melting point metal or polymer to increase density .

Question 17

Describe the main differences between the material jetting and binder jetting processes in
working principle, capabilities, limitations, suitable materials and possible applications.

resit 2021 4a

Answer 17

1. Material jetting
   Working principle:
   works by jetting droplets of molten feedstock material(mainly uv-curable polymers, but also low melting point metals and ceramics) onto a build platform which is solidified layer by layer(by UV-light).

Materials:
- Photopolymers (UV curable)
- Ceramics (with solvent to make suspensions with powder loading)
- Metals (with low melting point)

Capabilities:
- producing at high speeds & easily scalable.
- Multi-material/color parts.
- Low investment in printing machine.
- Easy to scale/speed up by adding nozzles.
- High surface finishing.

Limitations:
- Poor strength.
- Limited to mainly uv-curable polymers.
- Low accuracy.
- support needed.

Applications:
- Complex anatomical models

VS

2. Binder jetting
   Working principle: selectively jets adhesive droplets onto a powder bed(mainly metals, ceramics and sand), adhering the previous layer of powder to the next layer at the location of the binder.

Materials:
- Starch and plaster based powders
- Ceramic powder
- Polymer powder
- Metal powder
- Sand powder

Capabilities:
- Producing at high speeds.
- Wide range of materials.
- multiple material/color printing.
- Does not require high energy.
- Does not involve toxic materials.
- Relatively inexpensive

Limitations:
- Porous parts. infiltration is needed.
- Post processing required.
- slow (to let the binder cure).
- poor accuracy

Applications:
- Used as a master pattern for casting

Question 18

Describe the main differences between the powder bed fusion processes and directed energy deposition processes in working principle, capabilities, limitations, suitable materials and possible applications.

resit 2021 4a

Answer 18

1. Powder bed fusion
   Working principle & materials:
   a power source(laser or EB) melts or sinters powder particles in a container(most often weldable metals) together layer by layer.

Material:
- Polymers (low melting point)
- Metals (weldable)
- Ceramics (Difficult)

Capabilities:
- Complex geometries.
- No support needed for SLS.
- Wide range of polymers and weldable metals and ceramics.
- No support needed for polymers.

Limitations:
- Has low accuracies.
- Slow printing process.
- Powder handling.
- Post-processing.
- Residual stresses
- shrinkage + part distortion

Applications:
- Dental industry
- Spare parts
- Jewelry
- Medical prosthesis

VS

2. Directed energy deposition
   Working principle & materials:
   a deposition head(consisting of laser, nozzle and gas tubing) melts powder particles or a filament (most often weldable metals) on deposition.

Capabilities:
- Producing high-strength parts.
- Repair capabilities.
- Large scale printing.
- Repairing and refurbishing parts.
- Multi material parts.
- Producing fully dense parts.

Limitations:
- Has low accuracies.
- Slow.
- Requires supports.
- Low complexity.
- Controlled environment needed.
- Post-processing:
a. Poor ductility (fast heating and cooling, very strong parts)
b. Poor surface finish

Applications:
- Expensive and sturdy part: a F1 car part

Question 19

Some metal powders are a perfect choice for SLM but are not available as powders, others metal alloys don’t have good properties but are available as powders for SLM. Describe which properties are important to consider for powders in SLM.

2023 8

Answer 19

1. Reflectivity
2. Thermal conductivity
3. Melting point
4. Weldability

Question 20

What are the major differences between additive manufacturing and subtractive manufacturing with respect to product’s characteristics and mechanical properties?

2021/2023 2a

Answer 20

1. AM products can have more complex geometries, but are anisotropic and have porosity (un)intentionally. Also, AM has lower accuracy and surface finish.

VS

2. SM can create homogenous products that are solid, but less complex in geometry. Also, the accuracy is higher and it has a better surface finish.

Question 21

Elaborate on the post-processing step: which methods may be used to serve what purposes?

2021/2023 2b

Answer 21

A. Form: Improval of surface quality
- Support removal.
- Surface texture improvement.

B. Fit: improving dimensional deviations
- Accuracy improvement:
- Machining strategy

C. Function: improving mechanical properties
- Property Enhancement Using Non-Thermal Techniques.
– Property Enhancement Using Thermal Techniques.

D. Preparation for mass pattern: Make part ready for mold used for mass production using casting

Question 22

List the limitations of additive manufacturing in comparison with conventional subtractive manufacturing in product features and material properties.

resit 2021 3a

Answer 22

Anisotropic
more porous
poorer surface finish
Lower accuracy
Lower strength

Question 23

Explain the reasons why additive manufacturing cannot reach the level of subtractive manufacturing in each of these aspects (i.e., the limitations listed above)?

resit 2021 3b

Answer 23

1. Anisotropic: Layer by layer buildup
2. More porous: because it is additive so it will always be more pores than when material is removed
3. Poorer surface finish: due to layer by layer nature with limited resolution
4. Lower accuracy: due to factors like: shrinkage, limited resolution layer by layer build up
5. Lower strength: Again due to layer by layer buildup or due to for instance boundaries of adhesion or gradients in heat affected zones, which are absent within subtractive manufacturing of bulk material

Question 24

What can be done to minimize each of these limitations? Consider the starting material, part design, and 3D printing process parameters, but not post-processing.

resit 2021 3c

Answer 24

A. Anisotropic:
Orientate the part in the slicer such that the anisotropy direction is least critical for the envisioned loadcase.

B. More porous:
- Proper combination of speed and temperature parameters.
- In some techniques choose appropriate binders.

C. Poorer surface finish:
- Minimize layer height.
- Slow printing speed.

D. Lower accuracy:
- Minimize layer height.
- Slow printing speed.
- Optimize part orientation & shape.

E. Lower strength:
- Part redesign: use complex geometries to advantage.
- high performance materials.
- AM techniques with high cooling rates to manipulate microstructure.

Question 25

What are the 3 Fs that are the essential criteria in the design and manufacturing of a part?
Take a part in a bicycle as an example to elaborate on these 3 Fs.

resit 2021 4a

Answer 25

For instance the luggage carrier.

Form:
Need to have a flat part onto which luggage can be placed

Fit:
Attachment holes need to be at the right position relative to each other.

Function:
Needs to be strong enough to withstand the loads from luggage or maybe even a person or be corrosion resistant

Question 26

Describe the definitions of design for manufacturing and design for additive manufacturing.

resit 2018/21/23 1a

Answer 26

1. Design for manufacturing (DFM):
   Can be defined as the practice of designing products to reduce, and hopefully minimise, manufacturing and assembly difficulties and costs.
2. Design for additive manufacturing (DFAM):
   Maximize product performance using the AM capabilities which are shape complexity (virtually any shape), hierarchical complexity (different kind of scales), functional complexity (e.g. porous implants) and material complexity (multiple materials in one part).

Question 27

What are the essential differences between these two practices (DFM vs DFAM)?

resit 2018/21/23 1b

Answer 27

1. DFM:
   - To minimize the complexity of the parts.
   - To minimize manufacturing time and costs.
2. DFAM:
   - Utilizing the design freedom(complex shapes at no extra costs)
   - Customization
   - Weight saving
   - Performance
   - Combing/integrating parts

Question 28

How can a medical doctor make use of the additive manufacturing technology to help himself
/ herself and his / her patients? Give three examples.

resit 2021 2a

Answer 28

1. Anatomical models:
   AM enables medical doctors to produce high-fidelity anatomical models that replicate patient-specific anatomy for surgical simulation and training purposes.
2. Prosthetics:
   AM technology enables medical doctors to design and fabricate customized medical devices, prosthetics, and orthoses tailored to the unique needs and preferences of individual patients. By leveraging AM’s design flexibility and rapid prototyping capabilities, doctors can develop personalized solutions that improve patient comfort, functionality, and quality of life.
3. Tissue Engineering and Organ Printing:
   With AM technology, there is the possibility to directly fabricate replacement body parts. Materials are deposited as living cells, proteins, and other materials that assist in the generation of integrated tissue structures.

Question 29

What are the limitations of the state-of-the-art additive manufacturing technology for medical
applications?

resit 2021 2b

Answer 29

• FDA approval
• Acceptance by insurance companies
• Location in hospital of AM machines
• Engineering training in hospitals

Question 30

Give an example of a currently used medical device (i.e., an implant, or an external
prosthetic device, or an orthotic device) that may be better made by using 3D printing.
What will be the advantages, in your view?

resit 2021 2c

Answer 30

Implantable hip joint: mainly the acetabular cup.

Advantages:
1. Customization: the possibility to tailor the acetabular cup to patient specific anatomy, possibly by using imaging techniques such as CT or MRI.

2. Reduced costs.
3. Fast turnover → patient can quickly be supplied with implant.
4. Easily replaceable (from a manufacturing pov) due to digital database of parts.
5. Gradual or abrupt material transitions could offer effective differences mechanical properties where needed.