Stereo-lithography & Selective Laser Sintering

1. Stereo-lithography 

The present term Stereolithography was coined by Chuck Hull. According to the principles of Chuck’s Stereolithography, a 3D object is fabricated in a layered stepwise fashion from a material capable of solidification upon exposure to ultraviolet (U.V) radiation in the form of a laser beam. The process of Stereolithography uses STL (standard tessellation language) file as input. Thereafter, the 3D model is translated to 2D slices that contain the information of cross-sections. The platform on which the printing is performed is firstly located in a tank of liquid photopolymer with the required supports. The platform can be operated to move vertically as demonstrated in Figure 1. 

Figure 1. Sketch of Stereolithography process [Source: https://www.ijert.org/a-review-of-recent-developments-on-stereolithography]

          The U.V. light selectively strikes the liquid monomer layer and activates the initiators in the resin. These initiators, when activated eventually, create strong unbreakable bonds between the monomer Carbon chains. The UV light focuses on the predetermined path guided by a set of mirrors. After one layer is solidified, it moves downwards for the printing of the second layer with the thickness as processed by CAD modeling and the sweeper blade re-coats the surface, and the process is repeated until the whole object is fabricated. 

Mostly, thermoset polymers can be processed by SLA. Other materials that can be processed using SLA are castable resin, tough resin, ABS, PLA, and their blends. Although SLA printing takes a long time to print objects than newly developed printing methods; objects possessing complex geometrical design can be easily printed using SLA technique and have high accuracy, high surface finish. New generation SLA 3D printers like ProX800, ProX950, ProJet6000 HD, and ProJet 7000 HD from the company “3D Systems” can print objects up to 1.5 meters in length. These parts are claimed to have tolerances typically less than 0.05 mm as claimed by the company. 

 

2. Selective Laser Sintering 

The Selective Laser Sintering (SLS) process was originally developed at the University of Texas at Austin and then commercialized by the DTM Corporation (U.S.A). It is similar to Stereolithography but instead of liquid, the powder is solidified. The SLS also uses a laser beam to fuse the material into solid objects. The term sintering refers to a process that uses an atomic diffusion mechanism to transform powder to fused objects with the application of heat. The 3D model is processed into layers by the slicing software; the first layer which possesses geometric information of the cross-section is solidified with the help of a laser beam. The remaining powder which does not fuse with the object is thereafter used as support to the overhung parts of the component. As the gradual printing of the first layer is performed, the roller spreads the second layer of loose powder on the previously built layer and the process is repeated until the whole part is produced as illustrated in Figure 2. 

 

Figure 2. Schematic form of SLS System [Source: http://dx.doi.org/10.9790/1684-15008040453-57]

The most common materials used for SLS printing are wax, paraffin, polymer-metal powders, or various types of steel alloys, polymers, nylon, and carbonates. A number of systems and metal alloys (Fe-Cu, Fe-Sn, and Cu-Sn), metals (Al, Cr, Ti, Fe, and Cu), ceramics (Al2O3, FeO, NiO, ZrO2, SiO2, CuO), and other non-Ferrous alloys. 

SLS makes it possible to fabricate objects using material powders of Titanium and Nylon which are difficult to manufacture using traditional methods. Moreover, the support material is not needed for the overhanging parts as the unused powder provides support itself. The printers using the SLS technique for the fabrication of objects are claimed to have ±0.3% dimensional tolerances.

Brief Comparisons:

Stereolithography (SLA) is the oldest technique of 3D printing and it is still being used and researched upon. New generation machines developed functioning by SLA technique by a company named “3D systems” are constantly developing printers for better printing of overhanging structural designs with a wide range of materials. On the other hand, Selective Laser Sintering (SLS) which is developed on the working principles of SLA is known to be one of the most used rapid prototyping methods for printing objects. It can process and fabricate a wide variety of materials with a wide range of industrial applications.

A typical visual representation of SLA and SLS machines are shown below:

ProJet 7000 (SLS printer)

DTM Sinterstation 2500 Plus (SLS printer)