SLS or Selective Laser Sintering Technology : Explained Briefly

Selective Laser Sintering also known as Laser Sintering is an Additive Manufacturing method that uses CO₂ Laser Beam as a power source to fuse or bind plastic (mostly Nylon material) powdered material into 3D model. The technology is new and it is mainly used for rapid prototyping and companies where low volume and high-quality products are needed.

SLS 3D Printing. Image Source: EOS 3D Printer

Similarly, Powder Bed Fusion is one of the 3D printing family that is widely accepted by various manufacturing industry. The SLS or Selective Laser Sintering is one of the most important method of Powder Bed Fusion technology.

SLS technology was invented and developed in 1980 by a student Dr. Carl Deckard and his academic adviser Dr. Joe Beaman of University of Texas at Austin. Later, Carl and Joe started a company named Desk Top Manufacturing (DTM) Corp. to manufacture and sell SLS 3D printer. 

Back then, 3D Systems was the leader in 3d printing technology and competitor of DTM Corp. as the SLS and SLA both are similar in some aspects. So, in 2001, 3D Systems acquired DTM Corp. and the last patent issued to Deckard on SLS technology in 28 January 1997 expired in 28 January 2014. 

Today, there are mainly 4 companies who are manufacturing high quality SLS 3D printer machine like EOS, 3D Systems, SINTERIT and Sintratec. Machines manufactured by these companies are expensive so, there is less use of SLS machines in small scale companies. Some Chinese and Korean companies have successfully manufactured low cost SLS machines but the quality of the product is way too much weak then the former.

Pre-Printing Process

Every 3D Printing process starts by developing the 3D model in the 3D design software called the CAD (Computer Aided Design) file. This file is then converted into .STL file also known as Stereolithography or Surface Tessellation Language file. 

This STL file is then repaired using software like Materialise Magics or Autodesk Netfabb which is recommended in every 3D Printing processes. For example, in some cases the file is damaged or have empty shells than its recommended to repair or fill the empty part of the file else that part will not get printed in the 3D printer and there are 90% chance of printing failure. 

Generally, after repairing the 3D model supports are added to help build the part properly as the part which are overhang or has weak structure needs good support from the base to hold. But in SLS technology supports are not mandatory as the part is being printed on the powder bed so the unsintered powder will act as a support to the overhang and weak structure. 

The STL file is then uploaded into the printer program where it is cut into slices as per the micron we set. For example, if we set 50 microns in printing parameter than the program will divide 1 mm height into 20 slices, which means the thickness of each slice will be 0.05 mm.

SLS 3D Printing Process

The SLS 3D printing is a Powder Bed Fusion technology which uses high power CO₂ laser to bind or fuse small granules of raw materials like plastic, ceramic, metal or even glass into a 3D model. The CO₂ laser selectively sinter the raw material layer by layer according to the design made in CAD file.

SLS 3D Printing Diagram

(Refer Image for better understanding)

SLS 3D printing starts by filling the powder delivery chamber with raw material or material in powder form. The printer preheats the powder material to a temperature just below the melting point of the raw material which makes it easier for the laser to perform sintering process on the powder bed to trace the cross section of the 3D model.

The roller spreads the powder material on the heated built platform. The CO₂ laser scans the cross section of one layer of 3D model on the powder spread on the built platform and perform the sintering process. The small granules of material on the platform fuses with each other as the temperature is raised by the laser. 

Laser beam is navigated on the platform through ‘galvanometer’. The laser beam is passed through scanning system where galvanometer is fixed to direct the laser beam on the surface of the built platform to a specific point. 

The scanned part gets solidify and gains mechanical strength. Based on the micron configuration set by the operator the built platform moves one layer downward and the roller disperse the powder onto the built platform for laser to solidify the next cross section of the layer. 

The whole process is repeated for each layer until the whole 3D part is printed. The unsintered powder acts as a support structure to the 3D model which holds the overhangs and thin part of the model. So, there is no requirement to add the support before printing. 

Once the printing process is completed the operator removes platform and transfer it to the post processing area.

Post Processing

Unlike other 3D Printing methods like FDM, SLA and DLP technology the post processing of SLS technology requires very less time to clean the parts. 

After the printing process is done the built plate is transferred to the cleaning station where extra powder material is removed with the help of compressed air or hand brush. 

The excess powder can be reused after filtering it for large particles. But unsintered powder material degrades in high temperature environment so, it is mandatory to refresh the powder material at regular interval. 

The surface quality of parts made by SLS have grainy texture and are slightly rough so, to make it more attractive one can used tumbling, dyeing, painting, stove enamelling, metal coating, bonding, powder coating can be done.

Common SLS Materials

The most commonly used material is the Polyamide (PA) due to its ideal sintering semi-crystalline thermoplastic behavior it is a type of nylon plastic. Here PA is a short form of polyamide and the number represents the number of carbon atoms present in the material. 

Nylon is the most versatile material as it composites with many different fibers like carbon fibers, glass fibers and aluminium to improve the mechanical and thermal properties of the part made in SLS printer. 

Other thermoplastic such as Polystyrene (PS), Thermoplastic Elastomers (TPE), and Polyaryletherketones (PAEK) are also used for SLS 3D Printing.

Facts About SLS 3D Printing

The SLS technology uses high power CO₂ laser and heated beds which needs more electric energy to make it work so, it is not only expensive but also very dangerous to use it in the home. There some cheap SLS printers available in the market which are desktop and you can use it in home because they don’t use heated beds and also the laser is not to powerful. This cheap SLS are used for DIY purposes or where there is no need of high-quality products. 

Parts made by SLS 3D Printing. Image Source: Formlabs

SLS 3D printing process uses sintered powder as a raw material so there is no need of adding supports structures to the overhangs in the 3D model like we do in FDM, SLA and DLP 3D Printing. The unsintered powder acts as a support structure for the sintered part. 

The material chamber is always filled with raw powdered material so by the technique called ‘Nesting’ multiple parts can be printed at a time which saves printing time and cost.

Laser Sintering technology can print solid complex parts with functional interior components that can be built without material getting trapped inside. It is the fastest 3D printing technology which can make functional, durable, and complex geometry prototypes or end user parts. 

Part made by SLS. Image source: Formlabs

Many industries around the world has adapted the use of SLS 3D printing due to its ability to print complex geometry with less manufacturing efforts. SLS is most commonly used for prototyping in investment casting patterns, automotive hardware manufacturing and many more. 

Companies who takes small production jobs uses SLS 3D printing for making end user products for aerospace, medical, electronic hardware, military and defense industries.

Disadvantages

High end SLS 3D printers are very much expensive compared to other 3D printing machines like the FDM or SLA due to its high-end laser technology and heating bed which needs more power consumption. So, one cannot use it at home as a desktop 3D printer. 

Parts made by SLS technology has porous surface as the sintering process does not melt the granules but it binds or fuses them with each other. But with the help of cyanoacrylate coatings and hot isostatic pressing this porous surface can be sealed. 

Designs made by sintering technology are solid as it is impossible to print hollow and fully enclosed parts. The unsintered powder material will always get trapped inside the hollow material and there is nothing we can do to drain the powder.