Ultimate Guide to Selective Laser Sintering SLS 3D Printing

Selective laser sintering, popularly abbreviated as SLS is a relatively new technology that so far has been mainly used for rapid prototyping and for low-volume production of component parts. This additive manufacturing technique is rapidly gaining its popularity among large scale manufacturing units too. Lately this form of 3D printing has gained humongous trust among engineers and manufacturers across several industries for its ability to produce strong, functional parts.

Some properties of SLS 3d Printing, such as low cost per part, high productivity, and established materials make this 3D printing technology ideal for a range of applications from rapid prototyping to small-batch, bridge, or custom manufacturing. Global companies like General Electric, Boeing, Ford, Nike, American Pearl, DIY Rockets and et al are already using this technology successfully over the past few years, with great results and applications.

Brief History of SLS

In mid-1980s, SLS was developed and patented by Dr Carl Deckard and his mechanical engineering professor, Dr Joe Beaman at the University of Texas at Austin, under sponsorship of DARPA (Defence Advanced Research Projects Agency). After several years of trial and error, Deckard’s machine was capable of manufacturing real parts. In 1989, Deckard and Beaman were involved in the founding of one of the first 3D printing Startups, Desk Top Manufacturing (DTM) Corp. In 2001, DTM was sold to 3D Systems, a company that had previously developed its own method of 3D printing known as stereolithography.

What is SLS 3D Printing?

Selective laser sintering is a powder based 3D printing technology that uses a laser to fuse material layers into its final shape. After the laser traces a cross-section of the CAD design onto a material layer, the build platform lowers and another layer is fused on top. The build platform continues to lower until every layer is built and the part is complete.

During this additive manufacturing process, 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.  Compared to other additive manufacturing processes such as stereolithography (SLA) and Fused Deposition Modeling (FDM) or Fused Filament Fabrication (FFF), Selective Laser Sintering (SLS) does not require support structures since the powder acts as self-supporting material. This allows intricate and complex geometries to be constructed at ease.

How Selective Laser Sintering works?

3D Printing is considered to be an industrial revolution that will and is definitely changing the way manufacturing sector operated. SLS operates on the principle of powder sintering with the help of infrared laser in an elevated temperature which helps the grains of the powder to consolidate before being fused together with the laser beam. 

In the conventional SLS printer there is something called “bed” on which the roller spreads a thin layer of powder followed by sintering according to the layers sliced from a 3D model file that is derived from CAD like designing software.

Later the platform moves down by a little bit and the process repeats until the last layer is formed. After this, the post-processing part happens which requires removing the model from the un-sintered powder suspension and then sandblasting is done to it. Unlike FDM, SLS is available to 3D print without any support structures for models with complex geometry, as they are suspended in powder. Moreover, one can easily print moveable objects almost instantly.

SLS 3D Printing Materials

While the popularity of SLS printing is on the rise, the current technology only allows one material to be sintered at a time, limiting its ability to fabricate graded alloys and multi-material polymer parts. Even though researches are on to develop new method of multi-material printing, commercial usage of the same is yet to happen.

The most common material for SLS Printing is nylon, a highly capable engineering thermoplastic for both functional prototyping and end use production. Nylon is ideal for complex assemblies, light weight properties, durability and high environmental stability.

However, commercially available materials used in SLS Printing comes in powder form and include, but are not limited to, polymers such as polyamides (PA), polystyrenes (PS), thermoplastic elastomers (TPE), Polyether ether ketone (PEEK), Polyetherketoneketone (PEKK), and Polyaryletherketones (PAEK). Basically, materials that is stable against impact, chemicals, heat, UV light, water and dirt, makes an ideal one for both rapid prototyping and production through SLS 3D Printing.

SLS 3D Printing Workflow

SLS printing is the perfect example of “Powder to Product” technology and how that happens step-by-step is a fascinating thing to know. The workflow of the SLS 3D printing process is pretty simple and is given below :

1. Designing and Preparation of the file: the very first step of any printing process is to create a design, whose three-dimensional structure needs to be generated. Any CAD software can be used or 3D scan data can go in for designing the model which needs to be printed. Next export the design in STL or OBJ file format. Each SLS printer is powered by a software to specify printing settings, orientation and arrange models, estimates print time, and slice the digital model into layers for printing. Once the entire setup is complete, the software sends out instructions to the printer via a wireless or cable connection.

2. Make the Printer ready – next step is to prepare the printer, so that the design can be executed into a 3D model, quite easily. Preparing the printer varies from company to company, but most traditional SLS systems require extensive training, tools, and some efforts to prepare and maintain. Loading the powder using powder cartridge and removing the build chamber to run another print function while the previous one is cooling, are some of the essential functions that needs to be executed in order to make a printer ready.

3. Printing function – once the previous functions and checks are in place, the SLS machine is ready to print. These 3D prints can take anywhere from a few hours to multiple days depending on the size and complexity of the object, as well as its density. On completion of the printing task, the build chamber needs to slightly cool down in the print enclosure before moving to the next step. The build chamber is then removed and a new one inserted to run another print. Before conducting post-processing, the build chamber needs to cool down to ensure optimal mechanical properties of the object and avoid warping in parts. This may take up to half of the print time.

4. Part Recovery and post-processing – Post-processing in SLS 3D printing is a must and mostly done in batches, due to lack of support structures. After completion of the print function, remove the finished parts from the build chamber, separate them, and clean them of excess powder. This is typically done manually at a cleaning station using compressed air or a media blaster. Any excess powder left over after part recovery is filtered to remove larger particles and is recycled. The ability to re-use the material for subsequent jobs makes SLS one of the least wasteful manufacturing process and hence cost efficient. A common theme in the SLS industry is to offer separate devices for reclaiming, storing, and mixing powder. 

5. Miscellaneous processing – There are few other post-processing activity that needs to be done before the final product is ready. The printed object comes with a grainy finish, as a result media blasting or tumbling is suggested for a smooth finish. Printed parts may be spray painted, lacquered, electroplated, and coated to achieve different colours, finishes, and properties.

Why choose SLS over other 3D Printing methods?

Undoubtedly there are certain advantageous factors for which designers and engineers prefer SLS over other additive manufacturing methods. It provides design freedom, high productivity and throughput, cost effective, and proven end-use materials.

• Most of the additive manufacturing require specialized support structures to fabricate designs, unlike SLS where un-sintered powder surrounds the parts during printing process. Impossible intricate geometrics, moving parts and other complex forms can be easily created through SLS printing. SLS can perform generative design to its full potential by enabling lightweight designs that employs complex lattice structures impossible to manufacture with traditional methods.

• SLS is the fastest additive manufacturing technology for functional, durable prototypes and end-use parts. The lasers that fuse the powder have a much faster scanning speed and are more accurate than the layer deposition methods used in other processes of 3D printing. Multiple parts can be tightly arranged during printing process to maximize the available build-space in each machine. As a result, high productivity and throughput can be obtained through SLS.

• The material used in SLS 3D printing is mostly nylon and its composites, which is proven high grade thermoplastics. It has 100% density and produces high quality printed objects as compared to other additive manufacturing methods. 

• SLS method is also cost effective as compared to other forms of 3D printing. Maintenance cost, material cost, labour cost are all lower than other types of additive manufacturing.

SLS 3D Printing Applications

A wide range of industries, including engineering, manufacturing, and healthcare have been hugely benefitted by the application of SLS 3D Printing. This printing technology is known to accelerate innovations and supports businesses. SLS 3D Printing applications are largely used in rapid manufacturing, prototyping and creating tooling patterns. 

• Rapid Manufacturing – through SLS 3D printing several hardware for sectors like aerospace, healthcare, electronics packaging, military and security are manufactured. UAS, UAV, UUV, UGV hardware are also created through this.
• Rapid Prototyping – prototypes such as design evaluation models (Form, Fit & Function), Product Performance & Testing, Engineering Design Verification, Wind-Tunnel Test Models are manufactured without any hassle.
• Tooling patterns – various tooling units can be easily made through SLS 3D printers. Jigs and fixtures, foundry patterns, injection mould inserts, casting patterns etc are created with ease.

Advantages & Disadvantages of SLS

Some of the benefits that SLS 3D printing offers over other form of additive manufacturing are listed below:

• Durable, functional parts with intricate geometries
• Ideal for parts with high heat requirements or chemical resistance
• Capable of producing parts with mechanical joints, snap fits or living hinges
• Wide variety of materials and post processing options
• Short lead times
• No requirement of any support structures
• Ideal for dyeing

Despite the merits of SLS printing, it is far from being an all-encompassing solution for all rapid prototyping needs. SLS printing on a desktop scale is even more problematic, as the complexities and costs associated with it often turns out to be problematic. Some of the limitations of SLS printing are listed below.

• The output is porous and brittle
• Prone to shrinkage and warping
• Cleaning process is full of hassle

Even though SLS printing has gained fast popularity, it still has a long way to go before it can be adopted by the market of desktop-scale 3D printing professionals or hobbyists. The capital investment for an SLS printer is simply too high, as are the associated costs for a powdered raw material that cannot be recycled fully. The messy and complex nature of post-processing SLS prints is also off-putting for a lot of people. As of now, it may be enough that SLS printing technology remains relevant at an industrial level.