Additive Manufacturing General

Vat Photopolymerization

A vat of liquid photopolymer resin is cured through selective exposure to light (via a laser uv light) which coverts the exposed areas to a solid part

Powder Bed Fusion (PBF)

Powdered materials is selectively consolidated by melting it together using a heat source such as a laser or electron beam. the powder surrounding the consolidated part acts as support material for overhanging features

Binder Jetting

Liquid bonding agents are selectively applied onto thin layers of powdered material to build up parts layer by layer. The binders include organic and inorganic materials. metal or ceramic powdered parts are typically fired in a furnace after they are printed

Material Jetting

Droplets of material are deposited layer by layer to make parts. Jetting a photocurable resin and curing with uv light, as well as jetting thermally moltan materials that then solidify in ambient temperatures

Sheet Lamination

Sheets of material are stacked and laminated together to form an object. The lamination method can be adhesives or chemical (paper/plastics), ultrasonic welding, or brazing (metals). Unneeded regoins are cut out layer by layer and removed after the object is built

Material Extrusion

Material is extruded through a nozzle or orifice in tracks or beads, which are then combined into multi-layer models. Common varieties include heated thermoplastic extrusion (similar to a hot glue g) and syringe dispensing

Directed Energy Deposition (DED)

Powder or wire is fed into a melt pool which has been generated on the surface of the part where it adheres to the underlying part or layers by using an energy sources such as a laser or electron beam. This is essentially a form of automated build-up welding

Vat Photopolymerization

• Build material

• Material distribution method

• Binding technique

• Photopolymer (thermoset)

• Vat/reservoir

• Light (UV laser)

Steps for Vat Photopolymerization

1) The build platform is lowered from the top of the resin vat downwards by the layer thicknes

2) UV light/laser cures the resin layer by layer. The platform continues to move downwards and additional layers are built on top the previous layer

3) After completion, vat is drained of resin and the object removed

4) Post-processing includes curing the resin through heat treatment and removing supports

SLA

Stereolithography

DLP

Digital Light Processing

3SP

Scan, Spin & Selectively Photocure

FTI

Film Transfer Imaging

CLIP

Continuous Liquid Interface Production

Vat Photopolymerization strengths

• excelent resolution

• high accuracy

• excelent surface finish

• range of technologies

• relatively quick process

Vat Photopolymerization disadvantages

• brittle parts

• support removal required

• solvents required

• post-curing required

• parts can warp & twist

• moisture & UV sensitive

Resolution

The smallest movement the printer can make. Divided into X-Y plane movements and Z direction movement

Vat Photopolymerization layer thickness

50 to 125 um

Digital Light Processing

• parts are built upside down (build plate moves up)

• uses standard UV light source

• layer thickness 25 um

CLIP (Continuous Liquid Interface) Process

• a special tray bottom allows UV light and oxygen to pass through

• oxygen inhibits resin from solidifying and sticking to the window

• dead zone is persistent liquid interface→ allows the build plate to continuosly be pulled up

• FASTEST AM PROCESS

Two-Photon Vat Photopolymerization

Photoinitiator requires two photons to strike it before it decomposes to form a free radical that can initiate polymerization.

• greatly increases the resolution

• very small parts (0.2 um)

What was the first commercialized AM process?

Powder Bed Fusion (PBF)

PBF

• Materials

• Binding

• Distribution method

• plastic, metal, ceramic, and composite powders

• thermal source

• mechanisms for adding and smoothing powder layers (hoppers, blades/rollers)

What are the steps of Selective Laser Sintering (SLS)

1) Powder layers have been spread across the buld area using a roller

2) Laser is moved using galvanometers on required area

3) Chamber is heated to 80% melting temp

4) Steps repeated layer by layer

5) Part is removed from the bed, loose powder is cleaned off and further finishing operations can be carried out

What is the inside condition of the chamber in SLS printing?

• chambr is filled with nitrogen gas to minimize oxidation and degradation of the powdered material

• the chamber is held at an elevated temperature just below the melting point (80%)

Why is a pre-heat stage applied to SLS and EBM?

Powder is pre-heated prior to spreading over the build area to prevent warping

Sintering

Fusion of powder particles without melting (ie their solid state) at elevated temperatures

As fusion occurs in SSS, total surface area decreases and the surface energy…

decreases

What can be done to achieve very low porosity levels in SSS printing?

• Long sintering times

• high sintering temperatures

Smaller particles experience a greater driving force for necking and consolidation, so smaller particles sinter more…

rapidly and initiate sintering at lower temperatures than larger particles

For AM, the shorter the time it takes to form a layer…

the more economically competitive the process becomes

Few AM processes use sintering as a primary fusion mechanism because…

the time it takes for fusion by sintering is typically much longer than for fusion by melting

What is Chemically Induced Sintering?

Thermally activated chemical reactions between two types of powder or powder and atmospheric gases forms a by-product which binds the powders together

Chemically Induced Sintering (CIS)

• material

• post-processing

• limitation

• high temperature structural ceramic and/or intermetallic precursor materials

• infiltration or high-temperature furnace sintering to reduce the porosity

• cost and time required are high

Separate Particles

Well-mixed combination of binder and structural powder particles are used

• binder particles are smaller than structural material

Composite Particle

Particles contain both the binder and structural material within each powder particle

• higher density green parts with better surface finish

Coated Particle

Best type

• better absorbtion or laser energy, more effective binding of the structural particles, and better flow properties

Any powder delivery system for powder bed fusion must meet at least 4 characteristics

1) Powder must be spread to form a smooth, thin, repeatable layer

2) Powder spreading must not create excessive shear forces that disturb the previous layers

3) Correct volume of powder must be applied

4) Powder reservior must have sufficient volume to enable the process to build to the maximum build height without pause

Any powder delievery system must be able to deal with these universal characteristics of powder feeding (3)

• Inter-particle friction and electrostatic forces (powder size dec → friction inc)

• Surface energy and reactiveness (powder size dec → reactiveness inc)

• Airbone and float

What is an approach to opimizing these powder handling issues?

• Use a counter-rotating roller

• doctor blade

In multi-material powder bed processing, the only effective method of powder handling is?

Using multiple hoppers with separate materials

What is Selective Laser Melting (SLM)?

• Uses a high-power laser to melt and fuse metallic powders together

• Has the ability to fully melt the metal material into a solid three-dimensional part

What are the differences between EBM and SLM?

EBM

• vacuum, process temp: 680-720 C

• powder spreading on building platform

• sintering

• melting

• platform goes down

• process repeats

SLM

• same except for NO sintering

What are common applications for SLS?

• Functional parts

• Low run part production

• Complex ducting (hollow sections)

When using plastic in PBF

• no supports are required

• loose powder bed is sufficient enough

• this saves time during post-processing

• ability to nest polymer part in 3-dimensions

When using metal in PBF

• supports are required

• post-processing of metal parts after AM can be expensive and time consuming

• BUT accuracy and surface finish are much better

  • finer particle sizes → smoother finish

Polymer-based laser sintering is commonly used for

prototyping and end-use applications in many industries

What is Binder Jetting?

Process of depositing a binding agent onto a powder bed to form a part, one layer at a time.

• these layers bind to one another to form a solid part

• the powder bed is lowered and a new layer of powder is spread over the recently printed layer

Types of Binder Jet printing

• sand

• metal

How can BJ make full color prints?

• a printhead jets the binding agent

• a secondary print head jets in color allowing the colored model to be made

  • DISADVANTAGE: full color BJ parts are very brittle and not reccmended for application even after being exposed to infiltrant material and coating the part in epoxy

What are the advantages for sandcasted BJ parts?

• excelent for mold and core making

• elaborate pattern and designs

• saves time

• low cost

• high speed

What kind of binding agent is used in metal BJ?

Polymer binding agent

Infiltration

• after printing the part is placed in furnace where the binder is burned out

• at this point, the part is 60% porous

• Bronze is then used to infiltrate the voids via capillary action, decreasing the porosity and increasing the strength

What are the rates of shrinkage in BJ green part? (25-75mm) (larger)

• 0.8-2.5%

• 3%

Sintering (BJ)

• parts are placed in a high temperature furnace

• binder burnt out

• remaining metal particles are sintered (bonded) together, parts have low porosity

After infiltration

90% dense part

After sintering

97% dense part

What are the advantages of metal BJ?

• 10x more economical than SLS or SLM

• build size of BJ is much larger

• no support structures needed

• minimum surface roughness

• complex geometries

What are the disadvantages of metal BJ?

• less mechanical properties

• not suitable for high end applications

Printing parameters of BJ?

• powder layer hight → accuracy and surface finish

• no support required

• materials: metals and ceramics (limited options)