Recent Developments in the Process of Selective Laser Sintering 5Figure 3: EOSINT P 800 (Source: EOS 4)An SLS machine developed by EOS, one of the leading SLS setup manufacturers in theworld is shown in Fig. 3. It is designed to achieve high-temperature for laser sintering ofhigh-performance polymers.3 Process ParametersThe process parameters involved in SLS have to be optimal in order to achieve a productwith minimal defects. These parameters include, but not limited to, laser power, scan speed,spot size, powder quality, layer thickness. Some of these parameters and their effects arediscussed below.3.1 Laser powerLaser power plays an important role in determining laser energy density which regulates themaximum temperature involved in the process. The desired temperature mainly dependson the material properties. Laser power for the SLS machines typically starts at 30 W andcan go up to 400 W in high-temperature sintering. Laser energy density is calculated as inEq.1, according to 5Laser energy desnity =Laser powerSpot size Scan speed(1)Recent Developments in the Process of Selective Laser Sintering 63.2 Scan speedAs can be seen in the Eq.1, laser energy density also depends on scan speed. Energy density,and hence the maximum temperature, goes down if the scanning is too fast. Insufficientheating may lead to smaller melt volume. It also leads to an adverse effect called balling.However, if the scan speed is too slow, the melt-width as well as melt-depth are too large,which may not be desirable.3.3 Spot sizeThe laser spot distribution can be a Gaussian and a flat-top. However, a Gaussian distributionis the widely used one for SLS (and also for most of the other laser operations inmanufacturing). The power is highest at the beam center and reduces with distance as goneaway from the center. If a spot size is too small, it will lead to localized heating in lateraldirections, and the melt-dimensions in these directions may not very large. This effectivelymakes the process slower. However, if a spot size is too large, the peak power density at thecenter may not be large enough to melt or heat a material to the desired temperature 8.3.4 Optimization of process parametersThe defects in the final components can be significantly decreased by optimizing processparameters. As a result, a wide area of research in SLS is devoted to process optimization.In a recent development, Kumar et al. ? uses Taguchi method to investigate and optimizeprocess parameters that affect the quality of prototype in SLS process. A statistical measurecalled signal-to-noise ratio is used to measure and compare the effect of variation inprocess parameters along with some external disturbances. The parameters varied in theexperiments are laser power, temperature and part orientation, and their effect on part qualityis observed. An optimal set of parameters is found out for by integrating the Taguchimethod . It is concluded that the effect of other parameters which were assumed to be notsignificant can also be found out using this technique. It can also be extended to optimizeparameters for different materials.4 Recent DevelopmentThere has been a significant increase in the research devoted to additive manufacturing intwo few decades. As a result, SLS is being used in several fields where it can replaceRecent Developments in the Process of Selective Laser Sintering 7Figure 4: Some of the complex scaffolds used in tissue engineering. 3D CAD model (left)and corresponding SLS fabricated models (right) 7traditional manufacturing methods. Medical engineering is one of the fields that is investingmore in the SLS process development because of its ability to process bio-compatiblematerials (such as polymers) with ease. The scope of this paper limits the study of SLSapplications to 1) tissue engineering, 2) anatomical models and 2) implantable devices.4.1 Tissue EngineeringTissue engineering is one of the sub-fields of medical engineering that makes use of SLSfor parts development. The SLS process is mainly used for making scaffolds for restoringdamaged tissues. What makes SLS the best option for making scaffolds in this field isits ability produce porous and lightweight parts. Large sized pores in the final scaffoldingallow tissues to grow. The interconnection between the pores promotes nutrient and wasteexchange by cells 8. Some of these porous scaffolds used in tissue engineering are shownFig. 4.Recent Developments in the Process of Selective Laser Sintering 8(a) Non-restorable incisor is removed and a SLSprocessed RAI is placed in position(b) A provisional restoration attached on the RAI,this is a step immediately after the surgery(c) Three months after the surgery when the RAIis well adapted, a permanent crown is delivered(not shown)(d) Permanent crown at one year after the surgeryFigure 5: Installation of RAI and the restotarion process 10Recent Developments in the Process of Selective Laser Sintering 94.2 Anatomical modelsModeling of human anatomy from medical scans is a widely known technique in the medicalfield. It is also referred to as biomodeling. it involves processing data from the medicalscan and developing a 3 dimensional model in computer software. A model of equivalentdimensions is then fabricated using the process of SLS. These models are primarily usedfor surgical planning and practice 9. It is also used for determining the extent of a surgery,operating time, etc. to optimize results.4.3 Implantable devicesDuring last few years, SLS has emerged to be a replacement for conventional methodsused for manufacturing medical implants. SLS is known for its ability to produce highlycomplex models without any intermediate steps, unlike traditional processes that involvecasting, followed by machining. It is also difficult to machine complex 3D parts. So, SLShas been a common method for fabricating bone implants in recent times. However, it isonly recently that researchers have started making use of this method to fabricate smallscaleimplants for dental applications10. The process of SLS is used for making RootAnalogue Implant (RAI) in dentistry.Fig. ?? shows the process of installation of an RAI. As can be seen in Fig. ?? (a), a nonrestorableincisor is removed and an SLS processed RAI is placed in position. A 3D scan isperformed and with the help of 3D CAD model of similar dimension, an RAI is producedwith the help of SLS process. The RAI is shown next to the actual removed incisor for thesake of displaying the dimensional resemblance. Immediately after the surgery, in order toprotect the RAI, a provisional restoration is attached on the RAI. It can be seen in the Fig.??(b). Three months after the surgery, when the RAI is well adapted, a permanent ceramiccrown is delivered onto the RAI. It can be seen in the scanned image in Fig. ??(c). Thepermanent crown is well adapted and restored as well as functional at one year after thesurgery. It can be seen in Fig. ??(d).SLS has recently become the first choice for RAIs since it makes an immediate implantationfeasible. The materials involved here in dental prosthetic implants are mostly metalsor ceramics. The SLS-made products at this scale are fairly light-weight due to porosity,and stiffness of a final product is also within limits for these applications.Recent Developments in the Process of Selective Laser Sintering 105 Summary and ConclusionThe working principle and mechanism involved in the selective laser sintering (SLS), anadditive manufacturing process are discussed in the paper. The process of SLS makesused of laser power source to heat a layer powder to allow particles to bond with adjacentparticles. It is a layer-by-layer manufacturing method and thus is capable of producinghighly complex products. It has a potential to process several biocompatible materialsincluding polymers, ceramics, and metals.The parameters that significantly affect the process are also discussed. It is followed byan optimization technique that makes use of Taguchi method to compare and optimize a setof process parameters for optimal product quality.The recent developments in the process of SLS that have led to its application for makingsmall-scale products in the field of medicine are then discussed. One of the majorapplications falls in the sub-field of tissue engineering, which includes making scaffoldsfor the restoration of tissues. The porous nature of scaffolds made by SLS enhances thetissue growth over time. Another application is fabricating human anatomy from medicalscans. it is widely used by surgeons for studying and practicing surgeries. The last majorapplication listed in this study is for making implantable devices. Again, the ability ofSLS to fabricate complex parts makes it a good choice to replace traditional manufacturingmethods. Bone and dental prosthetic implants are the common ones that make use of SLS.