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3.1 SELECTIVE LASER SINTERING

SELECTIVE LASER SINTERING

Selective laser sintering (SLS) is an additive manufacturing (AM) technique that uses
a laser to sinter powdered material

1. SLS technology is in wide use around the world due to variety of raw materials (plastics,
glass, ceramics, or metals) and its ability to easily make very complex geometries by
using digital CAD data. it is being used to produce end-use parts.
2. The Selective Laser Sintering process begins with the conversion of customer generated
3D CAD data into a sliced STL file using software. Once created the STL file is then sent
to print on the Selective Laser Sintering machine.
3. The first layer is then traced out by a CO2 laser which melts and fuses the LS material
upon contact. Once the first layer has completed the build platform/part bed drops by
a pre-set amount.
4. The SLS® process creates three-dimensional objects, layer by layer, from CAD-data
generated in a CAD software using powdered materials with heat generated by a CO2
laser within the Vanguard TM system. CAD data files in the STL file format are first
transferred to the Vanguard TM system where they are sliced. From this point, the SLS®
process (see Figure 5.2) starts and operates as follows:
5. A thin layer of heat-fusible powder is deposited onto the part building chamber.
6. The bottom-most cross-sectional slice of the CAD part under fabrication is selectively
“drawn” (or scanned) on the layer of powder by a heat-generating CO2 laser. The
interaction of the laser beam with the powder elevates the temperature to the point of
melting, fusing the powder particles to form a solid mass. The intensity of the laser beam
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is modulated to melt the powder only in areas defined by the part’s geometry. Surrounding
powder remains a loose compact and serves as supports.
7. When the cross-section is completely drawn, an additional layer of powder is deposited
via a roller mechanism on top of the previously scanned layer. This prepares the next layer
for scanning.
8. Steps 2 and 3 are repeated, with each layer fusing to the layer below it. Successive layers
of powder are deposited and the process is repeated until the part is completed.
9. Materials are in powdered form, the powder not melted or fused during processing serves
as a customized, built-in support
10.Selectively fusing powdered materials is achieved by scanning cross-sections, which are
controlled by CAD data. After each section has been scanned, the base of the machine is
lowered, in order to apply the next layer. This additive manufacturing technique also
permits a wide use of materials. Like polymers, polystyrene, nylon, titanium, alloys and
steel mixtures
11.The feeder bed rises and a fresh layer of powder is swept along the build platform. The
next layer is then traced out and the process repeats layer by layer until the model has
“fully grown”. Upon completion the model is left to cool before being removed and any
loose material brushed away (as the Selective Laser Sintering process requires no
support structures)
12. The machine warms up (with LS material heated to just below melting point) prior to the
feed bed rises (typically by <0.1mm) and the levelling roller pushes fresh powder across
the build platform/part bed.
13.Laser automatically scanned at points defined by a 3D model make possible binding the
material together to create a solid structure.
14.Selective laser sintering [SLS] involves the use of a high power laser ( carbon dioxide
laser) to fuse small particles of plastic, metal, ceramic, or glass powders into a mass that
has a desired three-dimensional shape.
15.The laser selectively fuses the powdered material by scanning based on the CAD file on
the surface of a powder bed.
16. After each cross-section is scanned, the powder bed is lowered by one layer thickness, a
new layer of material is applied on top, and the process is repeated until the part is
completed.
17.Because finished part density and developed layer thickness is depends on the laser
power intensity
18.The some SLS machine preheats the bulk powder material in the powder bed somewhat
below its melting point, to make it easier for the laser to raise the temperature from the room
temperature to melting point
19.Stereo lithography (SLA) and fused deposition modeling (FDM) Process requires support to
fabricate overhanging designs SLS does not need a separate support for manufacturing over
hanging parts because the part being constructed is surrounded by un sintered powder at all
times, this allows for the construction of previously impossible geometries,
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20.Compared with other methods of additive manufacturing, SLS can produce parts from a
relatively wide range of commercially available powder materials. These include nylon ,
polystyrene, metals including steel, titanium, alloy mixtures, and composites and green
sand
21.The physical process can be full melting, partial melting, or liquid-phase sintering.
Depending
22. SLS Made parts are comparable to those from conventional manufacturing methods.
23.In many cases large numbers of parts can be packed within the powder bed, allowing very
high productivity.
24.SLS has been increasingly utilized in industry in situations where small quantities of high
quality parts are needed, such as in the aerospace industry, where SLS is being used more
often to create prototypes for aircraft. Aircraft are often built in small quantities and stay in
service for decades,
ADVANTAGES OF SLS
1. A main advantage of the SLS process is it does not need a separate support for
manufacturing over hanging parts because the part being constructed is surrounded by
un sintered powder, because it is fully self-supporting, Parts possess high strength and
stiffness
2. Various finishing possibilities (e.g., metallization, stove enameling, vibratory grinding,
tub coloring, bonding, powder, coating, flocking)
3. Complex parts with interior components can be built without trapping the material inside
and without the help of supports
4.Fastest additive manufacturing process for printing functional, durable, prototypes or end
user parts.
5. SLS offers huge variety of materials and characteristics of Strength, durability, and
functionality,
6. SLS modeled parts can be used in load bearing applications and at higher temperatures

7. Build prototypes and end-use parts without tooling

8. Increase market opportunities through enhanced properties

9 Rapidly test new designed parts in their applications to know the actual performance

DIS ADVANTAGES OF SLS

1. Large flat surfaces and small holes cannot be printed accurately with SLS
2. SLS parts have a grainy surface finish and internal porosity that may require post
processing
3. SLS parts have at risk to warping and over sintering.
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4. Consistency Since every SLS printed part consists of hundreds of layers, small
variations between products can occur In addition, most post processing steps are done
manually. This also leads to minor variations can occur

SLS WORK WORKING PROCEDURE

Here is how the SLS fabrication process works:

I. The powder build area is first heated just below the melting temperature of the polymer
and a recoating blade spreads a thin layer of powder over the build platform.
II. A CO2 laser then scans the shape of the next layer and selectively sinters (fuses together)
the particles of the polymer powder.
III. The entire cross section of the component is scanned, so the part is built solid.
IV. When the layer is complete, the build platform moves downwards and the blade re-coats
the surface.
V. The process then repeats until the whole part is complete.
VI. After printing, the parts are fully packed into un sintered powder,
VII. and the powder bin has to cool down before the parts can be unpacked. This can take a
considerable amount of time up to 12 hours
VIII. The parts are then cleaned with compressed air or other blasting media and are ready to
use or further post process.
IX. The remaining un sintered powder is collected and can be reused Schematic of an SLS
printer

CHARACTERISTICS OF SLS
 1. In SLS almost all process parameters are preset by the machine manufacturer. The
default layer height used is 100-120 microns. The typical build volume of an SLS system is
300 x 300 x 300 mm.

2. A key advantage of SLS is that it needs no support structures.

3. In SLS, the bond strength between the layers is excellent. This means that SLS printed
parts have almost isotropic mechanical properties.

4. SLS parts have excellent tensile strength and modulus, comparable to the bulk material, but
are more brittle (their elongation at break is much lower). This is due to the internal porosity of
the final part.

5. SLS parts are subject to shrinkage and warping as the newly sintered layer cools, its
dimensions decrease and internal stresses buildup,

6. 3 to 3.5% shrinkage is typical in SLS, but machine operators take this into account during
the build preparation phase and adjust the size of the design accordingly.
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7. Large flat surfaces are most likely to warping. The issue can be reduced somewhat by
8. Since SLS requires no support material, parts with hollow sections can be printed easily
and accurately. Hollow sections reduce the weight and cost of a part, as less material is used.
9.Tolerances Typical tolerances for SLS parts are ± 0.3 mm or ± 0.5 %, whichever is greater

POST PROCESSING

SLS parts produces parts with a powdery, grainy surface finish that can be easily stained. The
appearance SLS printed parts can be improved to a very high standard using various post
processing methods, such as media polishing, dyeing, spray painting and a protective coating
consisting of a resin, cellulose ester, or both, dissolved in a volatile solvent,

APPLICATIONS FOR SLS 3D PRINTING

 Casings , Spare parts ,Machine parts ,Packaging ,Eyewear, Jewellery, Awards

 Surgical guides ,Anatomical models, Orthopedic appliances, Parts for medical devices