4.selective laser sintering (by Hari Prasad)

SELECTIVE LASER
SINTERING TECHNOLOGY
SLS is a rapid prototyping (RP) process
that builds models from a wide variety of
materials using an additive fabrication
method.
The build media for SLS comes in powder
form, which is fused together by a
powerful carbon dioxide laser to form the
final product

HISTORY
The Selective Laser Sintering (SLS)
process was developed by The University
of Texas in Austin, and was
commercialized by DTM, Corporation out
of Austin, TX in 1987 with support from
B.F. Goodrich.
The first SLS system was shipped in
1992, and there are currently several
systems in use worldwide.

The build media for SLS comes in powder
form, which is fused together by a
powerful carbon dioxide laser to form the
final product
SLS currently has 10 different build
materials that can be used within the
same machine for a wide variety of
applications
The SLS process begins, like most other
RP processes, with the standard .STL
CAD file format, which is exported now by
most 3D CAD packages

SLS – MECHANISM
Moving laser beam sinters heat-fusible powders in
areas corresponding to the CAD geometry model
one layer at a time to build the solid part
After each layer is completed, a new layer of loose
powders is spread across the surface
Layer by layer, the powders are gradually bonded
by the laser beam into a solid mass that forms the
3-D part geometry
In areas not sintered, the powders are loose and
can be poured out of completed part

Powder bed on
Build position
Excess powder
take-up
Powder-levelling roller
Built
part
Powder feed
CO2 Laser
Scanning system
Temperature-controlled Build
Chamber with Nitrogen atmosphere
SLS –
Mechanis
m

4.selective laser sintering (by Hari Prasad)

MATERIAL THAT CAN BE USED:
Plastic powders
Metal powders (direct metal laser
sintering),
Ceramic powders
Glass powders

 Patented in 1989.
 Considerably stronger than SLA; sometimes structurally
functional parts are possible.
 Advantage over SLA: Variety of materials and ability to
approximate common engineering plastic materials.
 Process is simple: There is no milling or masking steps
required
 Living hinges are possible with the thermoplastic-like
materials
 Powdery, porous surface unless sealant is used
 Sealant also strengthens part
 Uncured material is easily removed after a build by
brushing or blowing it off.

SLA SLS A SUMMARIZED
COMPARISON
Material Properties:
The SLA process is limited to photosensitive
resins which are typically brittle.
The SLS process can utilize polymer powders
that, when sintered, approximate
thermoplastics quite well.

Surface Finish:
 The surface of an SLS part is powdery, like the
base material whose particles are fused together
without complete melting.
 The smoother surface of an SLA part typically
wins over SLS when an appearance model is
desired.
 In addition, if the temperature of uncured SLS
powder gets too high, excess fused material can
collect on the part surface.
 This can be difficult to control since there are so
many variables in the SLS process.
 In general, SLA is a better process where fine,
accurate detail is required. However, a varnish-
like coating can be applied to SLS parts to seal
and strengthen them

Dimensional Accuracy:
SLA is more accurate immediately after
completion of the model, but SLS is less
prone to residual stresses that are caused by
long-term curing and environmental
stresses.
The temperature dependence of the SLS
process can sometimes result in excess
material fusing to the surface of the model
SLA parts suffer from the "trapped volume"
problem in which cups in the structure that
hold fluid cause inaccuracies.
SLS parts do not have this problem

Support Structures:
SLA parts typically need support
structures during the build.
SLS parts, because of the supporting
powder, sometimes do not need any
support, but this depends upon part
configuration.
Marks left after removal of support
structures for parts cause dimensional
inaccuracies and cosmetic blemishes.

DATA PREPARATION FOR SLS
 The SLS technology is housed in the Sinterstation line of systems by
DTM.
 The current model is the Sinterstation 2500, which has various
improvements over its predecessor, the Sinterstation 2000.
 Figure shows the Sinterstation 2500
Sinterstation 2500

The SLS process begins, like most other RP
processes, with the standard .STL CAD file
format, which is exported now by most 3D CAD
packages
The DTMView software can import one or
several.STL files, and allows you to orient or
scale the parts as you see necessary
The 2500 systems have "auto-nesting"
capabilities, which will place multiple parts
optimally in the build chamber for the best
processing speed and results.
Once the .STL files are placed and processing
parameters are set, the models are built
directly from the file

With the build piston at the top, a thin layer
of powder is spread across the build area by a
roller/sweeper from one of the feed pistons.
The laser then cures in a raster sweep motion
the cross-sectional area of the parts being
built

PROCESS PARAMETERS
1. Part bed temperature
2. Layer thickness
3. Laser power The longer the laser dwells in a
particular location, the deeper the fusion depth and
the larger the melt pool diameter. Typical layer
thicknesses range from 0.1 to 0.15 mm
4. Laser scan speed
5. Energy density
6. Powder shape

Process parameters can be lumped into four
categories:
• (1) laser-related parameters (laser power, spot
size, pulse duration, pulse frequency, etc.),
• (2) scan-related parameters (scan speed, scan
spacing, and scan pattern),
• (3) powder-related parameters (particle shape,
size and distribution, powder bed density, layer
thickness, material properties, etc.), and
• (4) temperature-related parameters (powder
bed temperature, powder feeder temperature,
temperature uniformity, etc.)

Part bed temperature:
• The part bed is the central region of the SLS
machine (DTM Sinterstation 2000) where the
part is built
• The part bed temperature is controlled
primarily by the heater underneath the build
area
• the temperature should be lower than melting
temperature of the powder
• The higher the temperature is set, the less the
incident energy is required during the SLS
process

Layer thickness:
• Layer thickness is a measure of the
thickness of each layer during the SLS
process
• It is also the depth by which the part
piston is lowered after the laser scanning
of each layer
• A thicker layer requires greater incident
energy
• Thicker layers will not give us very good
surface finish because of ‘stair stepping’

Energy Density:
• Energy density is defined as the amount of
energy input per unit area
• It is dependent upon laser power, scan
speed and scan spacing and is determined
by the following equation
• ED = LP / (BS x SS)
• where ED is the energy density, LP is the
laser power, BS is the beam scan speed and
SS is the scan spacing. The laser power,
scan speed and scan spacing need to be
optimized according to the amount of input
energy required to fuse the particles in the
layer

Selective Laser Sintering Applications:
• Rapid Manufacturing
• Aerospace Hardware
• UAS, UAV, UUV, UGV Hardware
• Medical and Healthcare
• Electronics; Packaging, Connectors
• Homeland Security
• Military Hardware
• Rapid Prototypes:
• Functional Proof of Concept Prototypes
• Design Evaluation Models (Form, Fit & Function)
• Product Performance & Testing
• Engineering Design Verification
• Wind-Tunnel Test Models

Tooling and Patterns:
Rapid Tooling (concept development & bridge
tools)
Injection Mould Inserts
Tooling and Manufacturing Estimating Visual
Aid
Investment Casting Patterns
Jigs and Fixtures
Foundry Patterns - Sand Casting

ADVANTAGES AND
DISADVANTAGES
 Stereolithography offers a lot of advantage to a variety of business
situations.
 SLA parts have probably the best surface quality of
 all other RP systems, and are also highly competitive in dimensional
accuracy.
 Also, the latest SLA systems have significantly increased the speed
at which parts can be produced, which is ultimately the goal of RP.
 Finely detailed features, like thin vertical walls, sharp corners, and
tall columns can be fabricated with ease even on older SLA systems,
and the growing list of available resins are pushing the envelope on
temperature and strength characteristics as well

The main disadvantage of the SLA process is
most likely the Postprocessing requirements.
Although significant advances have been made to
make working materials safer and easier to work
with, procedures to handle raw materials for the
SLA still require careful and aware practices.
Another disadvantage, which may decrease as
resin competition increases, is the relatively high
cost of photocurable resins, weighing in at
around $600 to $800 per gallon.

4.selective laser sintering (by Hari Prasad)

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