All About SLS Printing: Advantages, Disadvantages, History, and more
Of the technologies that have been developed for 3D printing, Fused Deposition Modeling (FDM) is by far the most popular. Stereolithography (SLA) comes as a distant second. Identifying which technology comes third is far from definite, but a strong case could be made for Selective Laser Sintering (SLS). What is SLS and how does it work? Does SLS have the potential to be the next big thing in the world of 3D printing?
The History of SLS
SLS refers to selective laser sintering, which was developed in the mid-1980s by Dr. Carl Deckard and Dr. Joe Beaman. Much like other 3D printing technologies, SLS is an additive manufacturing method that uses a high-powered laser to “sinter” or solidify a powdered plastic material. The same concept has since been modified to work with various materials including metals, glass, and ceramics. Nowadays, SLS along with direct metal laser sintering (DMLS) and selective laser melting (SLM) are some of the most significant examples of a technology collectively known as powder bed fusion.
Although SLS technology has been around for a long time, it is still currently restricted for use in rapid prototyping and low-volume production of custom parts. The use of high-powered lasers and handling of powder material have proven to be either too dangerous or too expensive to be scaled down with practical costs. However, recent developments and innovation in the field of powder bed fusion has revived interest in the technology, and SLS seems poised to fall right behind FDM and SLS in terms of popularity.
How does SLS work?
SLS relies on the principle of “sintering” a material to form a solid mass. Sintering refers to the process of compacting a loose material, such as a plastic powder, by the application of heat or pressure. Sintering does not melt the material, but rather provides just enough energy for the atoms of separate objects to diffuse across the material boundaries. Since the discovery of sintering, it has been widely used for crafting ceramic and material products. In the field of 3D printing, SLS by default refers to the process of creating custom prints using a nylon or polyamide powder.
The process of SLS printing starts by filling the powder bin with the nylon powder. Inside this powder bin is the build platform, which has a home position that is typically near the top edge of the bin. The powder bin is equipped with a recoating blade that spreads a thin layer of powder over the build platform every time a layer is printed.
Another necessary feature in the powder bin of an SLS printer is the ability to pre-heat its contents. The powder is heated to just below its melting temperature so that a momentary application of the high-powered laser will be enough to push it over the edge, so to speak.
Much like any other 3D printing technology, an SLS printer is guided by a slicer software that separates a 3D model into very thin “slices”. By using the cross-section area of each slice, the slicer software directs the laser to hit the top layer of powder in the bin. This solidifies the powder according to the model being printed, after which the build platform moves down, and the recoating blade applies a new layer of un-sintered powder.
The process repeats until all layers have been printed, at which point the part needs to cool down inside the powder bin. The build chamber has to be removed and cleaned to separate the finished print from the rest of the unused powder. Most SLS printers come with specially designed cleaning stations that can make this process easier and less messy.
The advantages of SLS
SLS has been one of the go-to methods for rapid prototyping, especially for batch production of custom parts. In industrial-scale applications, it is a reliable, precise, and fast method that has stood the test of time.
1. No need for support structures
One of the chief benefits of using SLS printing is that it designs do not need any support structure. As the print is being built, all hollow spaces are automatically filled with unused powder, thus making SLS prints self-supporting. This offers modelers and product designers a huge degree of design freedom. Models with large hollow spaces, overhanging features, and very thin features will no longer be a problem when printing with SLS. Printing with SLS can also be a viable solution for printing complex designs that would otherwise require printing in multiple parts with FDM or SLA.
Since the nylon powder used in SLS only requires very brief exposure to the laser to be sintered, SLS printing has the potential to be one of the fastest 3D printing technologies – even faster than the impressively fast SLA. Of course, speed has to be quantified in the context of print resolution, as even an FDM printer can print incredibly fast if the software was set to print very thick layer lines. With SLS, there is hardly any tradeoff between printing speed and quality.
Much like SLA, printing a small layer and a wide layer takes just as much time in SLS. For this reason, it is often more practical and economical for commercial SLS printing businesses to print a batch that fills the entire volume of the powder bin. Multiple objects can be spaced closely together to maximize the build space. For this reason, many businesses that offer SLA printing services typically wait to fill a whole batch before they start printing.
3. Excellent layer adhesion
SLS printing has been known to result in prints with very strong layer adhesion. Because of this characteristic, SLS prints have virtually isotropic mechanical properties. This means that the tensile strength, hardness, and elongation of an object printed using SLS are almost equal in all directions.
4. Ideal for dyeing
SLS prints come out with a naturally porous surface, which makes them very prone to intake by moisture and other liquids. This can be excellent for the coloring of parts post-printing, as dyeing using a hot bath process becomes much more effective. However, SLS prints that will be exposed to high moisture conditions will likely require a waterproof finishing.
The limitations of SLS
Despite the benefits 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 SLS are hardly attractive for the casual 3D printing enthusiast.
1. Porous and brittle
The same porosity that makes SLS prints excellent for dyeing also compromises their structural integrity. Although SLS prints have a comparable tensile strength compared to SLA prints, they are much less flexible and can undergo less deformation before failure. For this reason, SLS prints are best used as proof-of-concept prototypes and not as functional parts.
2. Prone to shrinkage and warping
Much like SLA and FDM, SLS prints are also prone to warping and shrinkage. The nylon powder needs to be subjected to elevated temperatures to enable sintering, which means that the printed object will undergo a cooling process almost immediately after the solid layer has formed. As the print cools, it contracts or shrinks in all directions which can lead to a dimensionally inaccurate product. The stress due to the contraction can also accumulate in certain parts of the print, particularly in sharp edges and corners, resulting in these parts getting warped or distorted.
Unlike in FDM printing, there is no intervention or modification of the SLS printing process that can be done to reduce warping. Shrinkage will inevitably happen at a rate between 3% to 4%. This needs to be considered in the design phase by adjusting the volume of the model accordingly.
Warping can be reduced by avoiding or reducing the volume of flat areas in the design. Cutouts can also be integrated into the design that can absorb some of the stress caused by absorption without distorting the rest of the print.
One of the most bothersome parts of the SLS printing process comes when the part has been built and cooled – the cleaning process. When you retrieve the printed part from the powder fin, it will typically be hidden inside a firm block of the unused powder. You will have to break apart this block of powder and clean off the excess powder from the printed part using compressed air.
This is not much of a problem for industrial-scale SLS printers that typically come with specially designed cleaning chambers that have airtight enclosures and an inlet for compressed air. Desktop-scale SLS printers may not be as fortunate. As you can imagine, having such a cleaning chamber will likely translate to increasing the cost of an already expensive printer. However, trying to post-process an SLS print without a cleaning chamber is a nightmare. The powder is very hard to control and will just get everywhere. Aside from the mess in your garage or workshop, the loose nylon powder is also a respiratory hazard which you’ll need to protect yourself from.
4. Produces a lot of waste
In some ways, SLA printing looks similar to SLS printing. Instead of nylon powder, SLA printing uses a liquid resin solution that is polymerized by a beam of ultraviolet light. Although the liquid resin that SLA uses is a bit expensive, the silver lining is that any remaining resin can still be reused.
This is not the case for SLS. As we have mentioned, the powder inside the build chamber needs to be pre-heated so that it can be sintered with just the slightest exposure to the laser. This preheating step is enough to compromise the quality of the powder. Based on experience, an SLS print made using 100% recycled powder comes out so brittle that it can fall apart with just the slightest pressure.
Most SLS printing experts recommend using only a maximum of 50% recycled powder. Since you will always end up with a portion of unused powder with every single print, you will inevitably accumulate so much unused nylon powder that you’ll need to throw some out. With this single detail, SLS printing already generates much more waste compared to FDM and SLA.
Ultimately, the most significant factor that is preventing SLS printing from becoming mainstream technology is its cost. While it’s possible to get a high-quality FDM printer for less than $500 and a beginner SLA printer for less than $1000, a desktop-scale SLS printer will set you back at least $5,000.
The SIntratec Kit, which sells for a little more than $5,300, is a very basic SLS printer that is capable of a modest 100 mm x 100 mm x 100 mm build volume. Scaling up will bring you closer to the $10,000-mark, which coincides with the price of the popular Formlabs Fuse 1. Premium desktop-scale SLS printers can even cost as much as $40,000.
With such a price range, it’s no wonder that SLS printing has not picked up among the desktop 3D printing community. SLS printing is simply not the kind of thing that you can go into out of pure curiosity or as a hobby.
Is SLS the future of 3D printing?
3D printing using SLS technology is a long-established method that is still heavily used in the field of rapid prototyping. It’s fast, reliable, and has a good economy of scale. It’s also quite flexible – it can be used to create custom parts made from metals, plastics, ceramics, and several composite materials. Although our discussion has focused on using nylon powder, other plastic powders such as TPU and nylon composites can also be used. However, the use of SLS printing technology has remained limited to industrial or commercial scenarios.
There have been a lot of efforts in recent years to bring SLS printing technology to the desktop or benchtop-scale market. Although the results of these efforts have been quite impressive, the SLS printers are still a bit too expensive to be viable for casual 3D printing hobbyists.
SLS printing is also inherently more complex than FDM and SLA. Working with the powder raw material is messy and can even be hazardous if the proper precautions are not put in place. The limited recyclability of the powder is problematic, as it makes SLS technology a more wasteful alternative. This has become a more relevant issue in today’s market where consumers put a premium towards products that sustainable and more environmentally friendly.
In conclusion, we think that SLS printing 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.
For now, it may be enough that SLS printing technology remains relevant at an industrial scale. It certainly fills a unique niche as an extremely versatile method that can work for a huge variety of materials. We look forward to an age when small-scale SLS printers become more easily accessible and affordable – you can bet we will be first in line for them.