Selective Laser Sintering By Rohan Malkar (132090008) Swapnil Pawar(132090006)

Definition of Rapid prototyping Rapid Prototyping (RP) can be defined as a group of techniques used to quickly fabricate a scale model of a part or assembly using three-dimensional computer aided design (CAD) data.

Why Rapid Prototyping The reasons of Rapid Prototyping are To increase effective communication. To decrease development time. To decrease costly mistakes. To minimize sustaining engineering changes. To extend product lifetime by adding necessary features and eliminating redundant features early in the design. 

Rapid Prototyping Goal is to fabricate 3-D models quickly and automatically directly from CAD models. Current RP system are based on Layered Manufacturing Technology. In this method solid model ( triangle polyhedral form ) is decomposed into cross sectional layer representation. The RP software prepares tool path to physically built these layers automatically to form the object in the machine.

RP process chain showing fundamental process steps

Liquid based processes Depending upon the form of the raw materials used RP systems are classified as follows: RP Systems Liquid based processes SLA SGC Solid based processes LOM FDM SLS Solid Liquid Powder

Selective Laser Sintering Selective laser sintering was originally developed by University of Texas at Austin. Commercialised by DTM Corporation USA. DTM’S SLS is presently owned by 3D systems. It is a solid based RP process. It is the first process to emerge at the commercial scale to make metallic parts and tools. Selective Laser Sintering (SLS) uses a laser to sinter powder based materials together, layer-by-layer, to form a solid model.

A SLS system has three major components: Laser Part Chamber Build Platform Levelling Roller Power cartridge Control System

Schematic of SLS apparatus

Methodology CAD Model Conversion to STL format 1 CAD Model 2 Conversion to STL format 3 Slicing model into layers (RP software). 4 Development of Laser Trajectory. 5 Building of Model in the Machine

Step 1 Step 2 Step 3 Step 4

Metal Powder SLS Machine ( Courtesy EOS ) Step 5 Metal Powder SLS Machine ( Courtesy EOS )

Working of SLS It is an additive RP techniques. Layer of powder is first deposited on part build cylinders. The laser ( CO2 ) traces a two-dimensional cross section of the part. During laser exposure, the powder temperature rises above the glass transition point after which adjacent particles flow together. This process is called sintering.  Platform descends down by an amount equal to the thickness. The roller pushes the material on the built platform and the process is repeated again an again.

Following video will clearly depict the working of the sls process.

Materials used for SLS. Metallic and non metallic powders are used typically of the size 50µm. Commonly used materials: Plastics Waxes (Investment wax casting) Low melting temperature metal alloys. Polymer coated metals. Ceramics (Green preforms). Nylon ( Duraform, Glass-Filled Nylon (Duraform GF), Flame Retardant Nylon and Durable Nylon.)

SLS Products Engine Manifold Valve Medical implant Electronic Packaging

Complexity!!! Not a Problem…In SLS

Strengths of SLS Any material that can be converted into powder having low melting temperature (350 to 500) can be used to make parts in this process. No support structure required. Parts obtained are tough. No post curing required. No tooling cost incurred. No wastage of material. Functional metal and ceramic parts can be obtained directly.

Weakness of SLS Surface finish is improper. Parts are porous in nature. Continuous monitoring of the building operation is required. Large amount of time is required to heat up material chamber before building part. Uniform cooling is difficult to maintain in order to avoid warpage. Parts obtained are brittle.

Steriolithoraphy (SLA) Selective Lasser Sintering (SLS) Limited to photosensitive resins making it brittle. Good surface finish Accuracy is very high. Less prone to residual stresses. SLA suffers from trapped volume problem (cups in the structure which hold hot fluid cause inaccuracies) Polymers can be used for sintering thereby approximating thermoplastics. Surface finish operation as required since part is powdery. Accuracy is one of the biggest disadvantage of this process. Higher residual stresses due to longer curing. SLS does not have this problem.

SLA SLS SLA products are difficult to machine due to their brittle nature. SLS and SLA objects can be made of the same size. SLA is rarely preferred ( “ Quick cast” is the one process in which investment casting is preferred developed by 3D systems using SLA.) SLS product are made from thermoplastic material which can be easily machined. SLS and SLA objects can be made of the same size. SLS is mostly used in investment casting also sometimes to prepare master patterns.

Areas employing SLS Aerospace. Automotive. Organ replacement. Haemotology. Surgical Tools. Medical Instruments.