Teoria di Additive Manufacturing

STL:

It works by removing any construction data and model history, approssimating the surfaces

of the model with triangular facets ( a triangular mesh which covers the whole part and it’s

used to describe the entire shape ).

Every triangle is described by coordinates of their vertices and directional cosine of the

facet normal.

Its accuracy is represented by the maximum offset between the designed object and the

STL approximated geometry, and it affects the quality of the parts; as a rule we can ensure

that the minimun triangle is smaller than the resoluton of the AM machine.

Every mesh needs to be water tight, wich means with no holes in it.

Part orientation:

The orientation of the part could be chosen automatically by the machine or manually by the

desginer; different orientations affects:

1. Building time: to have a shorter production time we should orientate the part in a way to

minimize the Z height, because less layers takes less time to be created.

1. Mechanical properties: Depending on the orientation the part could be stronger in one

direction than the others.

2. Appereance: staircase effect, it is worst for horizontal surfaces than for vertical ones and

you can’t get rid of it.

3. Supports material uasge: the best orientation is the one that minimizes supports; not

every material needs supports and there is a self supporting angle of inclination of the

surface wich is 45 degres.

For each characteristic there is a specific orientation that maximises or mnimises it.

Placement:

To build more parts during the same process it is necessary to place them efficently so that

the building platform is fully exploited; nesting possibilities depend on the AM technology,

for example, in Metal L-PBF it is impossible to build parts over another part due to heat

transfer and distorsion issue; it became possible in Plastic L-PBF and EBM.

Support structure:

Support structure may be needed for overhangs and they affect the build process and the

final result:

• Increase build time and material usage

• Require subsequent removal operations

• Have an impact on surface finish

• A maximum overhang angle for each process/material is identified

Slicing:

Software operation by which the object geometry is cut in layers with same thickness so

that it can be built layer by layer; minimum layer thickness il limited by the technology and

tthe characteristics of the AM machine. Thicker layers lead to faster and cheaper builds but

with lower part surface quality.

Classification:

Additive manufacturing technologies:

Vat photopolymerization: AM process in which liquid photopolymer in a vat is selectively

cured by light-activated polymerization.

• Cured with laser — Stereolithography (SLA) — Plastic

• Cured with projector — Digital Light Processing (DLP) — Plastic

Material extrusion: AM process in which material is selectively dispensed through a nozzle

or orifice.

• Fused Deposition Modeling (FDM) — Composite / Plastic

Material Jetting: AM process in which droplets of build material are selectively deposited.

• Cured with UV light — Material Jetting (MJ) — Plastic

• Milled to form — Drop On Demand (DOD) — Wax

Binder Jetting: AM process in which a liquid bonding agent is selectively deposited to join

powder materials.

• Joined with bonding agent — Binder Jetting (BJ) — Sand / Metal

Powder Bed Fusion: AM process in which thermal energy selectively fuses regions of a

powder bed.

• Fused with agent and energy — Multi Jet Fusion (MJF) — Plastic

• Fused with laser — Selective Laser Sintering (SLS) — Plastic

• Fused with laser — Selective Laser Melting (SLM, DMLS) — Metal

• Fused with electron beam — Electron Beam Melting (EBM) — Metal

Directed Energy Deposition: AM process in which focused thermal energy is used to fuse

materials by melting as they are being deposited.

Sheet Lamination: AM process in which sheets of material are bonded to form an object.

AM for polymers:

Powder Bed Fusion (PBF):

• Fused with agent and energy — Multi Jet Fusion (MJF): Tiny liquid droplets are applied

locally to a layer of polymer powder; they can increase or decrease the heat absorption of

the powder. The material is melted using an infrared source.

• Fused with laser — Selective laser sintering (SLS): Using a laser beam, a polymer powder

is selectively sintered locally layer by layer creating the cross section of the component. It

is present an inert gas to create a controlled atmosphere for the process.

Material Extrusion (MEX):

• Material extrusion filaments — Fused Deposition Modeling (FDM): Wire-shaped plastic, or

filament, is plasticised in a nozzle unit and selectively deposited locally layer by layer.

• Material extrusion granulate — Arburg Plastic Freeforming (APF): Plastic granulate is

plasticised in a nozzle unit and selectively deposited locally layer by layer.

Material Jetting (MJT):

• Cured with UV light — Material Jetting (MJ): Small droplets of photopolymer are applied

locally and layer by layer through many nozzles; the photopolymer is then cured instantly

by UV lights.

Vat Photopolymerization (VPP):

• Cured with laser — Stereo Lithography (SLA): Using a movable laser beam, a viscous

photopolymer is selectively cured locally in layers and then solidifies.

• Cured with projector — Direct Light Processing (DLP): A photopolymer is exposed layer

by layer to a projector; the photopolymer is polymerized and then solidifies.

AM for metals:

Powder Bed Fusion (PBF):

• Fused with laser — Selective Laser Melting (SLM): By means of a movable laser beam,

powder is selectively melted locally layer by layer. It is present an inert gas to create a

controlled atmosphere

• Fused with electron beam — Electron Beam Melting (EBM): Using a movable electron

beam the powder is selectively melted locally; the process is realised with vacuum.

Direct Energy Deposition (DED):

• Fused with laser — Laser Engineering Net Shape (LENS): Material is applied and melted

simultaneously by a laser beam. The process continues layer by layer.

• Cold contact welding — Metal Powder Applciation (MPA): Metal is applied in layers using

a very high kinetic energy. Material combination are possible.

• Fused with electric arc — Wire and Arc Additive Manufacturing (WAAM): Metal wire is

melted by arc welding and applied locally layer by layer to create the near net shape of

the product.

Material Extrusion (MEX):

• Green part is printed to be sintered afterwards — Fused Deposition Modeling (FDM): wire

shaped metal containing plastic is plasticised in a nozzle unit and selectively deposited

layer by layer.

Binder Jetting (BJT):

• Joined with bonding agent to be sintered afterwards — Binder Jetting (BJ): Tiny binder

droplets are selectively applied locally through many nozzles and into metal powder; the

droplets sticks the metal powder together.

Material Jetting (MJT):

• Cured with heat to be sintered afterwards — Nano Particle Jetting (NPJ): A metal particle

solvent fluid is selectively deposited by a nozzle unit; the solvent evaporates and the

particles bond together.

AM for other materials:

Binder Jetting (3D Printing):

It uses a printer head containing several

ejection nozzles to deposit binder droplets that

form spherical agglomerates of binder liquid

and powder particles to provide bonding the

previously printed layer. Once a layer is printed,

the powder bed is lowered and a new layer of

powder is spread onto it. This process does

not require high energy and is relaively

inexpensive and fast. This process does not

involve changes in the powder microstructure.

Layer thickness: 0.09 - 0.20 mm

Materials:

• Chalk

• Metal powders

• Ceramic powders

Binders:

• Furan

• Silicate

• Aqueous based

• Polymer metal

Also added with colored ink

Post processing:

• Remove the powder from the building table

• Remove powder particles from the part with air or a brush

• Infiltrate the part to consolidate the powder.

For metal and ceramics powders, full densification can be reached; the part built is called

green and must be handled with care. The green is subjected to thermal treatment to

remove the binder and the the temperature in the furnace is increased until the sintering

temperature is reached. The full densification could also be reached through infiltration with

metals such as copper or bronze.

Advantages:

• Low energy required

• No toxic materials involved

• Relatively inexpensive and fast

• Best AM technique to produce colored models

• Complex shapes are possible

• No supports needed

Cons:

• Low dimension stability of metal parts due to the densification process

• Not suitable for final parts, it is preferred for models and molds.

Material Extrusion (MEX):

This class is only connected with Fused Deposition Modeling

(FDM). The part is produced by extruding small flattened

strings of molten material to form layers, then the material

hardens immediately after the extrusion from the nozzle. The

nozzle is heated to melt the thermoplastic material past its

glass transition temperature. After a layer is deposited, the

extruder or the building platform, moves along the z direction

and a new layer is added.

This process creates anisotrophy due to the little contact

surface between layers.

Some machine have two nozzles to deposit two different materials, one for build material

and one for support structures (soluble material).

Supports are needed for angles under overhanging surfaces or less than 45 degrees. The

only exception is for bridging (under 5mm of lenght, over the surface finish is not good and

needs supports). Support tipes are lines, grid or trees. The supports can be removed

manually or with chemical dissolution.

Every part is composed of a shell and the core of the part called infill; the filament is first

deposited in the perimeter. The infill can be made by different pattern and different material/

void ratios.

Materials:

• PLA: Biodegradable, easily printable, low

properties.

• ABS: Better mechanical performances, toxic, higher temp needed.

• PC: For transparent parts.

• TPU: Rubber like behavior

• NYLON: high elongation at fracture

• Filled filaments: Short fibers in polymeric matrix, high stiffness, high

wear of the extruder nozzle.

• PEEK: Very high mechanical properties, heat resistant, very expensive.

• Metals: Requires a debinding process.

Advantages:

• Not localized heat source

• No loose powder

• Low energy

• Suitable for