Contents1. Introduction:.....................................................................................................22. Literature review:..............................................................................................3

2.1. Recent developments :..............................................................................52.1.1. 3D printing in rapid prototyping:........................................................52.1.2. Creating electrical devices based on RP:.............................................62.1.3. Sls:........................................................................................................7

2.2. Future scope:.............................................................................................72.2.1. Medical Industry:.................................................................................92.2.2. Aerospace & aviation industries:.......................................................102.2.3. Automotive industry:..........................................................................102.2.4. Industrial printing:..............................................................................102.2.5. Architecture:.......................................................................................11

3. Characteristics of selective laser sintering produced parts:............................114. Highlight of selective laser sintering:.............................................................125. Technology of the selective laser sintering:..................................................136 . Working of selective laser sintering process:................................................14

6.1. Laser:........................................................................................................166.2. Powder Feeder:.......................................................................................176.3. Enclosed Chamber for controlling the environment:..............................17

7. Process parameters:........................................................................................188. Material used in selective laser sintering:.......................................................219. Applications:..................................................................................................2710. Advantages of selective laser sintereing:......................................................3011.Disadvantages of selective laser sintering:....................................................3212.Limitation of selective laser sintering:..........................................................3513. Conclusion:...................................................................................................3614. References:...................................................................................................37

1

1. Introduction:

Rapid Prototyping (RP) is a technology that produces models and prototype

parts from 3-Dimensionl (3D), Computer Aided Design (CAD) model data,

Computer Tomography (CT), Magnetic Resonance Imaging scan data, and

model data created from 3D object digitizing systems . In additive fabrication,

the machine reads the data from a CAD drawing and lays down successive

layers of liquid, powder, or sheet material and builds up the model from a series

of cross sections. Rapid prototyping is mainly classified in four types.

Stereo lithography (SL), Selective laser sintering (SLS),Fused deposition

modelling (FDM), Laminated object sintering (LMS).But in this chapter we

focus on the selective laser sintering. In the SLS we use the laser term , the laser

term "light amplification by stimulated emission of radiation" . A laser differs

from other sources of light in that it emits light coherently. And the term

Sintering is the process of compacting and forming a solid mass of material by

heat or pressure without melting it to the point of liquefaction.

SLS is reactive when using a chemical reaction of mixing components in the

presence of a laser and a selective laser melting (SLM), a direct metal laser

sintering (DMLS) or direct metal laser re-melting, when the complete melting

of powders is pervasive over the solid state dust sintering. The materials used

in Rapid Prototyping are photo sensitive polymer resins, plastics, paper,

ceramics Aluminium, Copper, Stainless steel ,Titanium . Based on the type of

material used RP systems can be categorized into liquid-based, solid-based and

powder-based. This paper focuses on selective laser sintering (SLS) method,

which is a powder based RP system. SLS is a method of sintering and building

up RP models by sintering the powder grains using CO2 laser beam. Rapid

Prototyping technology is applied in engineering and medical field. It is helpful

in engineering for product design, tooling and manufacturing moulds, rapid

2

handling of electrodes manufactured, polymer moulds, die casting, die casting

of titanium zirconium, pieces of zirconium-titanium (PZT) and sheet metal

parts etc. RP medical applications are designing and development of medical

devices, instruments, tissue engineering, teaching purposes, forensic pathology,

prosthetics and implantation. It is also helpful in planning of complex surgical

operation. This process was also used in Currently, the experiments in the

laboratory of INCDMTM and dedicated magazines and articles draw attention

to a new comprehensive, integrated approach, of the SLS process.[19]

2. Literature review:

This literature review will describe the development of rapid prototyping,

present scope of the selective laser sintering etc. Finally, applications of rapid

prototyping (RP) and its recent development are discussed.

Rapid Prototyping (RP) is defined as the layer-by-layer fabrication of three-

dimensional models from a computer-aided design (CAD). The first phase

of prototyping (Manual or hard prototyping) began many centuries ago.

Prototypes in this phase are typically not very sophisticated and their

fabrications take on average about four weeks. The techniques used in making

these prototypes are extremely labour intensive and craft-based. This process

was also used in manufacturing moulds, rapid handling of electrodes

manufactured, polymer moulds, die casting, die casting of titanium zirconium,

bio-medical applications, pieces of zirconium-titanium (PZT) and sheet metal

parts. Currently, the experiments in the laboratory of INCDMTM and

dedicated magazines and articles draw attention to a new comprehensive,

integrated approach, of the SLS process.[6]

3

In the future, the SLS activity may continue in several areas. Materials can be

tested for SLS with low volume production, with specific properties. However,

it is envisaged that the size of pre-sintered parts will become more than a critical

value, the minimum thickness of the layer will decrease and, consequently,

the surface roughness decreases. It is therefore necessary to study the

microstructure properties to be developed in future research studies. Another

area of future research is to manufacture various parts of the same product using

different materials. Desired properties of parts can be achieved by using

different materials for different layers, resulting in new features metallurgical

and mechanical parts. Various process parameters can be optimized either by

modelling or by a subsequent experimental strategy, so that laser energy

transfers to the surface to make the anterior surface near the interface.

Optimizing these parameters is also necessary to anticipate resistance, strength

and hardness in a SLS product.[27] Because the product post-processing is

inevitable and time-consuming and pre-processing of powders takes time, an

improved version of the SLS can be developed with post-processing techniques

for better surface finish, track resistance, dimensional accuracy to achieve a and

a reduced working time. It is envisaged that the metals with a special

performance will be tested in future for complex products of good quality. In

addition, to achieve this goal, the melting will be used for chemical reactions

between phases. In recent years, new technologies have been successfully

developed and applied in the field of components for research and biomedical

devices, implants, 3D scanning, CT scan, MRI scan, handling DICOM files, 3D

design, e-Manufacturing (Rapid Prototyping, Rapid Manufacturing, Rapid

Tooling). They open a new era of collaboration between the engineer, the

physician and the IT expert.[14]

Selective laser sintering (SLS) was developed and patented by Dr. Carl

Deckard and academic adviser, Dr. Joe Beaman at the University of Texas at

4

Austin in the mid-1980s. Deckard and Beaman were involved in the resulting

start up company DTM, established to design and build the SLS machines. In

2001, 3D Systems the biggest competitor of DTM and SLS technology acquired

DTM. The most recent patent regarding Deckard's SLS technology was issued

28 January 1997 and expired 28 Jan 2014.A similar process was patented

without being commercialized by R. F. Housholder in 1979.[3]

2.1. Recent developments :

2.1.1. 3D printing in rapid prototyping:

Three-dimensional printing is a process for the manufacture of tooling and

functional prototype parts directly from the computer models. Three

dimensional printing functions by the deposition of powdered material in layers

and the selective binding of the powder by ink-jet printing of a binder material.

This process can be applied to the production of metal, ceramic, and metal

ceramic composite part. Colour RP prototypes can also be created with the ink-

jet mechanism applied on the existing SLS process. It was successfully proven

that SLS prototypes with colour at particular and desired positions could be

created. This would form the basis of a hybrid RP system that produces

functionally graded components with complex internal structure as well as

external detailing can be manufactured by SLS . The coloured binders can be

replaced by appropriate materials and prototypes with various material

properties can be made. Besides, showed that dielectric RF filters could be

manufactured by combining modern ceramic processing with solid freeform

fabrication. Also succeeded in creating low volume fraction ceramic green parts

5

by using hot wax ink-jet printing mechanism. Viscosity, fluid density and

surface tension were found to affect the ink-jet deposition process.[6]

Also in the medical field work id that Current drug fabrication methods are

imprecise because there are a large variation in drug sizes, drug loadings and

release kinetics. Micro Fab Technologies has applied the use of ink-jet

technology to create highly mono dispersed micro-spheres that can be loaded

with drug or biological material. It has showed successful results that the ink-jet

technology can be applied to biocompatible materials such as PLG and

cholesterol . Embedding the therapeutic agents (or drugs), such as proteins,

peptides, genetic materials, etc., in the polymer, this allows

controllable release of drugs with the polymer formulation and the geometry for

controlling the release profile .

2.1.2. Creating electrical devices based on RP:

It was feasible to create SLS components with electrically conductive channels .

Electrically conductive channels were created by depositing electrically

conductive materials, silver paint and carbon cement, on polymeric substrate

using a non-contact deposition method. The electrical properties of the

conductive channels were found to be affected by the carbon/solvent ratio,speed

of deposition head and sintering condition of the base material. A direct-write

approach using MicroPen system can fabricate high precision resistors.

MicroPen system consists of design system with a CAD interface and writing

or dispensing system, and it produces patterns directly from the computer file.

Also showed positive results that the commercial MicroPen system could be

used to deposit precise patterns in manufacturing resistors.

6

2.1.3. Sls:

With selective laser sintering, by controlling the porosity of the part of

the matrix, into which drug is filled, a well controllable drug delivery can be

fabricated. However, limited studies have been done on the manufacture of drug

delivery devices by the SLS process. There was only one research in

which showed that a porous drug delivery device can be built by the

SLS process. With the incorporation of methylene blue within the pores of the

already sintered medical-grade Fine Nylon samples, a drug delivery device

could be created. Vacuum environment was used for forcing trapped air out of

the pores and drawing the liquid dye inside them. Also, there was a

comprehensive understanding of the relationship between laser power, scan

speed, scan space, porosity, pore characteristics, structural integrity, quantity of

dye released and duration of release. All these showed the feasibility of building

drug delivery devices by sls.[9]

2.2. Future scope:

Rapid prototyping is starting to change the way companies design and build

products. On the horizon, though, are several developments that will help to

revolutionize manufacturing as we know it. One such improvement is increased

speed. "Rapid" prototyping machines are still slow by some standards. By using

faster computers, more complex control systems, and improved materials, RP

manufacturers are dramatically reducing build time. For example, Stratasys

recently (January 1998) introduced its FDM Quantum machine, which can

produce ABS plastic models 2.5-5 times faster than previous FDM machines.

7

Continued reductions in build time will make rapid manufacturing economical

for a wider variety of products. Another future development is improved

accuracy and surface finish. Today’s commercially available machines are

accurate to ~0.08 millimetre in the x-y plane, but less in the z (vertical)

direction. Improvements in laser optics and motor control should increase

accuracy in all three directions. In addition, RP companies are developing new

polymers that will be less prone to curing and temperature-induced war page.

The introduction of non-polymeric materials, including metals, ceramics, and

composites, represents another much anticipated development. These materials

would allow RP users to produce functional parts. Today’s plastic prototypes

work well for visualization and fit tests, but they are often too weak for function

testing. More rugged materials would yield prototypes that could be subjected to

actual service conditions. In addition, metal and composite materials will

greatly expand the range of products that can be made by rapid manufacturing.

Many RP companies and research labs are working to develop new materials.

For example, the University of Dayton is working with Helisys to produce

ceramic matrix composites by laminated object manufacturing. An Advanced

Research Projects Agency / Office of Naval Research sponsored project is

investigating ways to make ceramics using fused deposition modelling. As

mentioned earlier, Sandia/Stanford’s LENS system can create solid metal parts.

These three groups are just a few of the many working on new RP materials.

Another important development is increased size capacity. Currently most RP

machines are limited to objects 0.125 cubic meters or less. Larger parts must be

built in sections and joined by hand. To remedy this situation, several "large

prototype" techniques are in the works. The most fully developed is

Topographic Shell Fabrication from Forums in San Jose, CA. In this process, a

temporary mould is built from layers of silica powder (high quality sand) bound

together with paraffin wax. The mould is then used to produce fibre glass,

epoxy, foam, or concrete models up to 3.3 m x 2 m x 1.2 m in size. At the

8

University of Utah, Professor Charles Thomas is developing systems to cut

intricate shapes into 1.2 m x 2.4 m sections of foam or paper. Researchers at

Penn State’s Applied Research Lab (ARL) are aiming even higher: to directly

build large metal parts such as tank turrets using robotically guided lasers.

Group leader Henry Watson states that product size is limited only by the size

of the robot holding the laser. All the above improvements will help the rapid

prototyping industry continue to grow, both worldwide and at home. The United

States currently dominates the field, but Germany, Japan, and Israel are making

inroads. In time RP will spread to less technologically developed countries as

well. With more people and countries in the field, RP’s growth will accelerate

further. One future application is Distance Manufacturing on Demand, a

combination of RP and the Internet that will allow designers to remotely submit

designs for immediate manufacture. Researchers at UC-Berkeley, among others,

are developing such a system. RP enthusiasts believe that RP will even spread

to the home, lending new meaning to the term "cottage industry." Three-

dimensional home printers may seem far-fetched, but the same could be said for

colour laser printing just fifteen years ago. 3D  printing or additive

manufacturing is a process for making a physical object from a three-

dimensional digital model, typically by laying down many successive thin

layers of a material. Each of these layers can be seen as a thinly sliced

horizontal cross-section of the eventual object.[17,10]

Some application of 3D painting by RP technique are:

2.2.1. Medical Industry:

Medical science is exploiting this technology at an extremely rapid pace. With

the advent of this technology, patients around the world are able to experience

improved quality of 3D printed implants and prosthetics like never before.3D

bio printing of human tissue has been around since 1990s. But of late, the

9

development and the implementation of this technology in medical sciences

scientists are making a shift from printing tiny sheets of tissue to entire 3D

organs. Layers of living cells are deposited onto a gel medium and slowly built

up to form three dimensional structures by using inkjet techniques.[9]

2.2.2. Aerospace & aviation industries:

The developments in the metal additive manufacturing sector has largely

boosted the utilization of 3D printing technology in the aerospace and aviation

industries. NASA for example prints combustion chamber liners using selective

laser melting and as of march 2015 the FAA cleared GE Aviation’s first 3D

printed jet engine part to fly: a laser sintered housing for a compressor inlet

temperature sensor.

2.2.3. Automotive industry:Despite the fact that  the automotive industry was among the earliest adopters of

3D printing, it has for decades relegated 3d printing technology to low volume

prototyping applications. These days, the use of 3D printing in automotive is

evolving from relatively simple concept models for fit and finish checks and

design verification, to functional parts that are used in test vehicles, engines,

and platforms.

2.2.4. Industrial printing:

This technology is used to create prototype designs for traditional

manufacturing and research purposes, which is called Rapid Prototyping.3D

printing allows ideas to develop faster than ever. Being able to 3D print a

concept, shrinks the development process, helping companies stay one step

ahead of the competition. Prototyping injection mold tools and production runs

are expensive investments. The 3D printing process allows the creation of parts

and/or tools through additive manufacturing at rates much lower than traditional

machining. Similarly, Rapid Manufacturing is a method of manufacturing

10

where companies use 3D printers for short run custom manufacturing. In this

process of manufacturing, the printed objects are not prototypes but the actual

end user product.

2.2.5. Architecture:

3D printing holds immense potential to translate imagination into reality. Given

the boom in digital art and design, we can now 3D print almost anything we

imagine after drawing it up virtually. Hobbyists and enthusiasts can exploit the

technology to add multiple dimensions to their idea and concepts.

3. Characteristics of selective laser sintering produced parts:

The measurable properties of sintered parts are yield strength, elongation,

Young’s modulus, hardness, surface roughness, line width, layer thickness,

shrinkage, porosity, wear rate, density, tensile strength, sintering depth and

scanning speed . [12,16]

The surface of an SLS part is powdery, like the base material whose

particles are fused together without complete melting.

The temperature dependence of the SLS process can sometimes result in

excess material fusing to the surface of the model, and the thicker layers

and variation of the process can result in more inaccuracy.

SLS parts, because of the supporting powder, sometimes do not need any

support. SLS parts can be easily machined compared to SLA and FDM

parts. SLS parts are ideally used for form, fit and function and direct

manufacturing applications.

SLS parts are consistent, stable and durable and provide excellent

prototypes and end-use parts for industries and applications that apply

11

demanding and functional testing, giving you the flexibility to modify,

optimize and evolve designs on the fly.

4. Highlight of selective laser sintering:

It is patented in 1989.Considerably stronger than stereo lithography.

Laser beam selectively fuses powder materials; nylon, elastomers and so

on metals.

Process is simple. There is no milling or masking steps required.

Powdering, porous surface unless sealant is used. Sealant also strengthens

the part. Uncured materials are easily removed after a build by brushing

off.

Ideal for durable, functional parts with a variety of applications.

Capable of producing snap fits and living hinges.

Recommended for chemically resistant application.

High strength and stiffness.

Fully functional, high quality plastic parts.

Excellent mechanical properties, ideal substitute for common injection

moulding plastics.

High abrasion resistance, ideal for use in development of parts with

movable components.

Excellent long-term stability.

High selectivity and detail resolution.

Biocompatible according to EN ISO 10993-1 and USP/level VI/121 °c,

allowing its use in medical applications e.g. for prostheses.[11,14]

12

5. Technology of the selective laser sintering:

An additive manufacturing layer technology, SLS involves the use of a high

power laser (for example,  carbon dioxide laser) to fuse small particles

of plastic, metal, ceramic, or glass powders into a mass that has a desired three-

dimensional shape. The laser selectively fuses powdered material by scanning

cross-sections generated from a 3-D digital description of the part (for example

from a CAD file or scan data) on the surface of a powder bed. 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.[21,23]

Because finished part density depends on peak laser power, rather than

laser duration, a SLS machine typically uses a pulsed laser. The 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 of the

selected regions the rest of the way to the melting point.

In contrast with some other additive manufacturing processes, such as stereo

lithography (SLA) and fused deposition modelling (FDM), which most often

require special support structures to fabricate overhanging designs, SLS does

not need a separate feeder for support material because the part being

constructed is surrounded by unsintered powder at all times, this allows for the

construction of previously impossible geometries. Also, since the machine's

chamber is always filled with powder material the fabrication of multiple parts

13

has a far lower impact on the overall difficulty and price of the design because

through a technique known as 'Nesting' multiple parts can be positioned to fit

within the boundaries of the machine. One design aspect which should be

observed however is that with SLS it is 'impossible' to fabricate a hollow but

fully enclosed element. This is because the unsintered powder within the

element can't be drained.

Since patents have started to expire, affordable home printers have become

possible, but the heating process is still an obstacle, with a power consumption

of up to 5 kW and temperatures having to be controlled within 2 °C for the three

Stages of preheating , melting and storing before removal.

6 . Working of selective laser sintering process:

The term "sintering" refers to a process by which objects are created from powders using the mechanism of atomic diffusion. Although atomic diffusion occurs in any material above absolute zero, the process occurs much faster at higher temperatures which is why sintering involves heating a powder. Sintering is different from melting in that the materials never reach a liquid phase during the sintering process. It (SLS) is a powder-based layer-additive manufacturing process generally meant for rapid prototyping and rapid tooling. Laser beams

14

either in continuous or pulse mode are used as a heat source for scanning and joining powders in predetermined sizes and shapes of layers. The geometry of the scanned layers corresponds to the various cross sections of thecomputer-aided design (CAD) models or (STL) files of the object. After the first layer is scanned, a second layer of loose powder is deposited over it, and the process is repeated from bottom to top until the part is complete.[16,20] Inthis process a high power laser beam selectively melts and fuses powdered material spread on a layer. The powder is metered in precise amounts and is spread by a counter-rotating roller on the table. A laser beam is used to fuse the powder within the section boundary through a cross-hatching motion. The table is lowered through a distance corresponding to the layer thickness (usually 0.01 mm) before the roller spreads the next layer of powder on the previously built layer. The unsintered powder serves as the support for overhanging portions, if any in the subsequent layers .[25,26]

15

Figure 1 . SLS system

Figure 2 . Laser sintering nomenclature

16

6.1. Laser:

LASER is the main source to carry out the process which develops energy to sinter the powder particles. The energy source laser is either pulsed or continuous wave laser. Laser power along with the scanning speed is one of the important parameter in the laser sintering process as it affects the overall mechanical properties of the produced part for example hardness, strength, porosity. CO2 Laser, Fibre Laser or Nd YAG laser can be used. Now a days. Fibre lasers are becoming most popular due to their high power, excellent control, less maintenance and reliability. The power requirements will depend on the raw material, speed of scanning etc. R. M. Miranda used 8KW fibre laser to sinter titanium based alloy

6.2. Powder Feeder:

SLS uses a powder feeder mechanism which lays a powder bed of a predetermined thickness over the base plate on which part is to be built. It is one of the essential component of any SLS based rapid prototyping machine. The layer of powder has to be very accurate and consistent over the complete process assuring the uniformity of the part throughout the geometry. Researchers have used various techniques to deposit a layer for their experiments such as roller arrangements, scrapper blades etc. There are some techniques like

Classical deposition

17

Pressure gradient deposition Ultrasound deposition Spread method

6.3. Enclosed Chamber for controlling the environment:

SLS process involves diffusion of the atoms of one particle into atoms of adjacent particle. The heat energy involved is very high which will be near the melting point of the materials. At this high temperature, there are lot of chances of oxidation of surface which is in contact of the air. To control the oxidation, an inert gas atmosphere is provided. K.Murali used argon gas to drive out the air from reactive zone. Vacuum gives the

additional mechanical stability to the part if provided.

7. Process parameters:

Process parameters are the defined variables that influence and control the SLS

process. A number of parameters can affect the quality of parts fabricated using

SLS. The parameters that vary in SLS include powder size, scan speed, powder

density, pulse frequency, fill laser power, scan size, scan spacing, part-bed

temperature, layer thickness, pulse size, laser power, laser energy, spot size,

powder size distribution, ratio of the powders of the mixture . Design of

18

experiment is necessary to find the significant parameters and the effect of those

parameters on the physical and mechanical characteristics of the parts.

The amount of energy that can be maintained at the surface for a given point

during the laser irradiation-materials interaction period is dependent on the

period of single exposure, the number of total exposures, the time between each

exposure and also the intensity of the laser irradiation . Some parameter are:

[2,7]

7.1. Duration of a pulse :

The duration of a pulse is a function of the laser beam spot size, D, and linear

velocity, V, of the laser .The exposure period of the laser irradiation is defined

as

τ = D/V ... (1)

where τ is the duration of a pulse.

7.2. Overlap :

The number of the exposures seen by a point on a surface is related to the

overlap, O. The function of overlap is described by the ratio of hatch spacing,

HS, to spot size, D, that

O=1-D/HS ...(2)

A single point on the powder surface is exposed to laser irradiation multiple

times due to overlapping scan lines. Overlapping scanning patterns are achieved

19

as the distance between scan lines, hatch spacing, is always less than the laser

beam radius.

7.3. Number of exposures :

The number of total exposures, Ne, is defined as

Ne=D/HS-1 ..(3)

The amount of energy stored at the sintered surface is determined on the

time between exposures. Laser power, laser beam spot size, hatching spacing,

laser beam speed (or step speed) and the scan line vector length are independent

process parameters which govern the intensity and method of energy delivered

to the powder surface.

7.4. Powder :

Intensity of the laser irradiation is related to power and spot size by integrating

the radial intensity of the spot area such that

p= ∫0

Rb

I (r). 2π r dr

..(4)

where I(r) is the radial intensity distribution, r is the radial distance from the

center of the spot and Rb is the spot radius.

7.5. Dwell period :

During the SLS process, energy is lost due to conduction into the powder

bed and radiation and convection at the surface during the dwell period. The

delay period between successive irradiation exposures is determined by the

20

linear velocity, V, of the laser beam and also the scan line length, L. The delay

period, td, is defined as

td=L/V ..(5)The energy stored at the surface during the SLS process is determined by

considering the intensity of laser radiation, number of exposures, the length of

each exposure and the delay between successive exposures.

8. Material used in selective laser sintering:

The SLS process flexibility allows a variety of materials. Some of these

materials make the SLS process superior to other rapid prototyping techniques,

21

where the material properties depend on the process. Among these materials,

the most common are: wax, paraffin, polymer-metal powders, or various types

of alloys, polymers, nylon and carbonates. Polycarbonate powders were initially

used as starting materials for both experimentation and modelling in the SLS

process.

For example, a number of systems and metal alloys (Fe-Cu, Fe-Sn, Cu-Sn),

metals (Al, Cr, Ti, Fe, Cu), ceramics (Al2O3, FeO, NiO, ZrO2, SiO2, CuO) and

other alloys (bronze, nickel, Inconel 625) were tested for laser sintering. The

results demonstrated that any material could be combined with another material

with a low melting point and acts as an adhesive. INCDMTM researchers tested

the use of bonding a protective polymer, commonly used in conventional SLS

sintering, thus revealing that a wide range of laser sintered materials can be

bonded without protection, which is an advantage compared with other rapid

prototyping techniques[20].

It appears that the use of special materials for rapid prototyping is growing and

the quality of products is visibly higher. The sintering achieves higher

performance if you use a powder mixture consisting of two groups of materials:

(i)Thermoplastic materials (nylon, polyesters, waxes, some nylon or

polycarbonate mixtures especially).

(ii)Completion materials whose mechanical properties and thermal properties

determined decisive use of new products (metal, non-metallic and composite).

With such networks, some remarkable performances have been achieved.

(a) Some sustainable forms made from a special polycarbonate or a polyamide

completing ceramics are used frequently (currently, fine casting workshops of

metal parts in serial production conditions).

22

(b) The direct implementation of successive layers of metallic parts in the future

lead to a replacement casting processes for a satisfactory performance in terms

of materials such as:

Copper alloy Titanium alloys, Tool steel Fire-resistant steel.

They were made by selective sintering that takes place in a controlled atmosphere, a melting temperature, but their cost is very high and, for the time, it is not being implemented in the industry.

After , a great success and achievement is the metal part made from different metals in better economic conditions:

Ambient temperature No protective atmosphere.

Laser sintering powders used may have different components depending on the purpose for which the final product will be used. For example we show somebasic features and processes that occur when working with DM20 (DirectMetal 20) Powder for EOSINT M270 from EOS GmbH - Electro Optical Systems Germany. Photo ME, Co-Cr powder mixtures are shown in figure.

23

figure 3. Co-Cr powder mixtures .

Sampling batches purity and their repeatability are paramount conditions closely followed by manufacturers of metal powder mixtures recipes ofbiocompatible metallic powders, any abnormality leading to product rejection (both in terms of physics, mechanics and biomedicine).The particle size diagram is shown in figure .

24

Figure 4 : particle size diagram

The procedures of selective sintering (SLS – Selective Laser Sintering) developed after 1992 are based on the experience gained in designing and manufacturing stereo lithography equipment (STL) and the expansionof technological research on other groups of materials with mechanical properties closer to the technological needs of functional assemblies engineering (ceramics, ferrous and nonferrous metals).In this way, we managed to demonstrate that a thin layer of certain mixtures of powders under the action of the laser beam can reach the local level, function and duration of exposure, which marks the transition of the melting temperature of the powder layer in the liquid.[15]

Based on the physical properties of the powders used, immediately after the laser beam action ceases, the local solidification takes place, achieving a compact drawstring made after the directions of molecular chains, surrounded by a

25

volume of powders unexposed to the beams mentioned above.A view with emphasis on individual layers of the solidified powder bed is shown in figure .

figure 5. individual layers of the solidified powder.

The explanation is more complex because the solidification range of materials is especially diverse. It is essentially based on stereo lithography processes exposed to the same mechanism: the installation of chemical bonds that form macromolecular linear chains, or three-dimensional tree.

For these situations state transitions, which involve a significant local heat input, they can be accelerated by inhibiting initialized and controlled substances, and energy intake may be given by concentrated heat sources on the desktop, laser radiation etc.

26

Micrographic appearance of pure metal powder is shown in figure 6.These sources must be adapted and adjusted on the fly, so as to give extra heat necessary to achieve the melting temperature, which provides thermo-kineticconditions favorable for the development process by establishing the macromolecular chains and a partially crystalline structure, with the transition of the liquid state to the solid state, reinforced, which marks the sintering product.

figure 6. Micrographic appearance of pure metal powder.

In terms of energy, the powders used in the in powdery have a wide range of melting temperatures which require a different input of heat from the concentrated source of energy. Choosing the necessary activation energy is possible by selecting rapid heating schemes in accordance with the dynamics of the sintering process. The diversity of these schemes has attracted a final designation of laser sintering.

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In conclusion, this phase is superior to the anterior period when the stereo lithographic technologies and equipments have been researched, designed andhomologated.Selective laser sintering processes are based on a variety of materials which will result in superior products at a stage performance, is the physical and mechanical properties close to the loads of the usual parts of a machine.Rapid prototyping and manufacturing technologies using materials and processes have been developed in recent years in several directions, depending on the material used and the technology of solidification of the material.[15]

9. Applications:

SLS materials are suited for direct functional applications where robust

performance characteristics are required. Chemical resistance to alkalines,

hydro carbonates, fuels and solvents, High temperature applications, Wear and

abrasion, Flexibility, thin walls, internal/external surface pressures.Applications

in the various fields are:[21,25]

(i) Rapid Manufacturing :

Aerospace 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.

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(ii) Tooling and Patterns :

Rapid Tooling, Injection Mould Inserts Tooling and Manufacturing Estimating

Visual Aid, Investment Casting Patterns Jigs and Fixtures, Foundry Patterns,

Sand Casting.

(iii)Parts for mechanical and thermal tests:

The polyamides material allows the production of strong, durable parts that can

be used tor extensive functional testing. Sintered products have mechanical

properties comparable to those of injection molded PA 12 parts, typical

applications are snap fits but it is also possible to produce working hinges.

Polyamide parts with glass filling have a much higher thermal resistance and

are perfectly suited for lighting elements and ventilation systems or products

that require high thermal loads. Apart from their use as test products, the

functional SLS parts often also need to be used at the same time for a

visual/csthetical control or dimensional check.

(iv)Series of small plastic parts:

SLS is an interesting and cost-effective alternative to injection molding (Rapid

Tooling). With the P 700 machine which has a large build area, a series of small

pieces can be built in one single laser sintering process. This dramatically

decreases the price, as the cost of an SLS part depends on its volume. Or in

other words, the cost is defined by the amount of powder it takes to build it and

not by an initial investment in an injection molding tool. Moreover, series of

SLS parts are available in a few days. So no need for high start-up investments,

no long lead times to produce a mould and injection mould the parts, no

difficulties in case the parts are complex.

Figure 7.sls fabricated part

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(v)Large and complex functional parts:

The machines can build large, complex geometries in one piece( shown in

figure), up to 700x380x580mm. The number of layers to be built is significantly

reduced as large parts can be built horizontally, which considerably shortens the

building process. Parts exceeding the P 700' maximum dimensions .can be built

in multiple pieces and put together afterwards. The process of gluing sub-parts

and assembling components can be done in the most accurate and secure way

using the Rapid Fit system. Rapid Fit allows to firmly position the parts on a

unique support system with individualized fixtures, supporting the part on well

positioned points.

(vi) Agricultural applications :

Since the introduction of plastic films in the 1930s and 1940s for greenhouse

coverings, fumigation and mulching, agricultural applications of polymers have

grown at an enormous rate. All classes of polymers such as plastics, coatings,

elastomers, fibres and water-soluble polymers utilize the applications with the

control release of pesticides and nutrients, soil conditioning, seed coatings, gel

plantings and plant protection. Biodegradable polymers could also be used in

the field of agriculture as soil-improving materials[9].

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10. Advantages of selective laser sintereing:

This research was mainly focused on the SLS process as it offers the widest range of materials compared to any other RP systems. SLS is capable of handling a range of materials such as polycarbonate, nylon, fine nylon, medical grade fine nylon, glass-filled nylon, sand, rapid steel, copper, polyamide and formulations called trueform, veriform, protoform and duraform. There are already attempts to use new and untested materials to fabricate objects . The primary advantage of SLS is the flexibility of selection of material systems compared to other Solid Freedom Fabrication techniques . SLS is also ideal for applications that require functional properties.Specific attributes of SLS include investment casting, multiple materials (to meet a variety of applications requirements) and durable materials (especially composite and fine nylon for functional prototyping applications). Another advantage of SLS is its close relationship with three dimensionalcomputer-aided design modeling systems. In fact, regard it as a general prerequisite in any process. SLS also provides for an ideal inert sterilized environment for the building of drug delivery devices which is one of the applications of the

31

manufacture of functionally graded materials. This is mainly due to its enclosed build chamber, the build materials that come in a dry powder medium and the nitrogen gas-permeated heated chamber. These features can be exploited to prevent the growth of bacteria when building drug delivery devices.Complex prototypes can be created by SLS in the design phase or forgenerating small quantities of production parts. It is also an ideal technology for the iterative design process. SLS process is so effective that it can prototype many more parts compared that can be done by traditional methods. Since no tooling is required, most of the patterns can be created and ready for casting within 24 hours. Multiple patterns can be created in a single build which allows for multiple iterations of the same part for testing or different parts for multiple designers and teams within a company. As a result, more parts can be created by using SLS compared with the traditional methods. The amount of money and time saved by using SLS to create patterns is enormous. The SLS process is powder based which means that there are no support structures to create during the process or to remove afterwards. And this would improve build rates, minimize pre-processing and post-processing labor and also create distinct advantage over many other rapid prototyping processes. Parts also can be nested inside one another and stacked vertically in the build chamber for maximum productivity.[13]

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SLS also provides itself an ability to turn patterns around quickly. Jobs with sample patterns and parts in hand can be quoted. Design changes and modifications can be easily accommodated once a job has been started. This would give SLS users a very competitive advantage.

11.Disadvantages of selective laser sintering:

Common problems such as bonus z, clumping, curling, growth, cracking

of powder bed, etc occur during the selective laser sintering. Some of them are

due to inappropriate settings, handling, processing or maintenance of the

Sinterstation system. This would result in prototypes of poor quality.

Sometimes, serious damages are introduced to the components. The details of

the problems would be described as follows .

33

(i)Bonus z : is the growth of a part in a downward z direction. This occurs when

the laser beam penetrates through to the unsintered powder below the part

boundary during the scanning stage of the first layer. As a result, the growth of

the part in the z-direction occurred. And this would cause out of tolerance of the

part .[22]

No table of figures entries found.Figure 1

Figure 8. Schematic diagram of bonus z in the SLS process

(ii) Clumping : Also occurs in the SLS process. It refers to agglomerated

powder in front of the roller on the powder surface. The roller pushes clumps

across the part bed and it may cause streaks to appear after the rolling process.

Clumping is caused by improperly sifted and recycled powder and also

overheating of the powder in the feed cartridges. Clumping would result in part

of poor quality as powder cannot be fed properly. It also causes some side

effects due to improper powder feeding such as uneven powder thickness,

which may cause growth or inadequate melting. Streaks may be apparent on

upward and downward facing surfaces of parts.

34

Figure 9.Schematic diagram of clumping in the SLS process

(iii)Curling (or in-build): is the rise of edges or corners of the part above the

powder bed surface after a layer of powder is scanned by the laser beam. This

would make the parts thinner in the z-axis and not flat in shape as designed. The

temperature differences that exist between various regions of the part during the

selective laser sintering process is one of the reasons which causes the

problems. These temperature differences cause uneven shrinkage and thus

curing is resulted. In-build curl usually occurs when the part temperature dips

too low after a layer of powder is added. In-build curl can also occur when

the part bed temperature is too low .[22]

Figure 10. Schematic diagram of curling (or built in) in the SLS process

35

(iv)Growth: Growth occurs when the detailed features of the part blur. It is

particularly apparent with small features or small holes. This would result in

part of oversize and increase the difficulty in breakout of the part. The main

difference between growth and bonus z is that growth may occur on any part

edge while bonus z occurs only on downward facing surfaces .[22]

Figure 11. Schematic diagram of growth

(v)Cracking : of powder bed also occurs in the SLS process when the roller

moves across it. This is caused by excessive heating rate or temperature from

the heaters, which causes partial melting of the powder bed surface. Cracking

also causes other problems like clumping and streaking .

Figure.12 Schematic diagram of cracking of part bed

36

12.Limitation of selective laser sintering:

(i). 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.

(ii). Dimensional accuracy: SLA is more accurate immediately after completion

of the model, but SLS is prone to residual stresses that are caused by long term

curing and environmental stresses. Both SLS and SLA suiler from inaccuracy,

but SLS is less predictable because of the variety of materials and process

parameters[1].

13. Conclusion:

Over the last decade Rapid Prototyping techniques gained a wide acceptance.

Among the different RP techniques, Selective Laser Sintering had the advantage

of being able to process a wide range of materials. This large variety of

materials gradually allowed to extend the field of applications, from simple

visual prototyping to functional prototyping, and even Rapid Tooling and Rapid

Manufacturing applications.

In recent years, the idea of full melting metal powders was explored, supported

37

by the continuously improving process parameters (smaller layer thickness,

smaller spot size, etc.), resulting in mechanical properties being much better

than those of early time selective laser sintered parts and comparable with bulk

metal properties. Rapid Tooling and specially Rapid Manufacturing

technologies are still limited in use, but they are steadily growing. It can be

expected that in the next decade they will achieve the same level of acceptance

as Rapid Prototyping, by continuous improvement of material properties and

accuracy and by a decrease of the costs. It can be concluded that, among various

techniques of rapid prototyping Selective Laser Sintering is the most flexible

process that accommodates large variety of materials being processed. Though

the technique used is difficult to control and automate as the large number of

parameters included, having its superior qualities, it is the most important and

useful process for various industrial applications.

14. References:

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AlSi10Mg alloy: process optimisation and mechanical properties development.

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[2] T. Pereira, et al., (2012) , Effect of process parameters on the properties of

selective lasersintered poly (3-hydroxybutyrate) scaffolds for bone tissue

engineering: this analyzes how laser scan spacing and powder layer thickness

affect the morphology and mechanical properties of SLS-made scaffolds by

38

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[3] Lou, Alex and Grosvenor, (December 07, 2012. Retrieved on March 22,

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[18] H.C.H. Ho, W.L. Cheung, I. Gibson, (2002)Rapid Prototyping 8 , 233–242.

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