Selective Laser Sintering (SLS) 3D Printing Technology
What is Selective Laser Sintering (SLS)?
Selective Laser Sintering is a plastic 3D printing or additive manufacturing process. The technical term for the process is Powder Bed Fusion of Polymers.
ASTM has defined a set of standards that encompass all of additive manufacturing. What many people refer to as “3D Printing” is a collection of 8 different printing technologies. Within these technology subsets are many different terms, trade names, branded terms and more that people use to refer to the different printing technologies or methods. SLS in its purest form is considered Powder Bed Fusion of Polymers, which are thermoplastics. There is also laser-based powder bed fusion of metals.
How does Selective Laser Sintering work?
This technology uses very fine plastic powder with an average particle size of around 50 microns – about the diameter of a human hair. Essentially, the powder is spread over a platform in layers that are 50 to a 100 microns thick, then is sintered or fused together by a low-wattage laser. The laser scans a raster trajectory back and forth across the powder bed, in the profile of the desired part at every layer. It is a typical additive application in that the powder is spread, the laser scans, more powder is spread, and the laser scans again, and on and on continuing until the part is complete.
It’s a very recyclable process as well, as any powder that is not used can be recycled. The powder that surrounds and supports the finished part can also be brushed away and used again. An adding bonus is that this surrounding powder cake prevents parts from curling up or drooping down due to gravity or resistance.
What materials are used in Selective Laser Sintering?
Selective Laser Sintering is limited to thermo-plastic resins, as opposed to thermo-set resins. Basically, the particles in these polymers are melted together or fused by the laser.
Typically for SLS, nylon-based resins are used, commonly including:
- Composite nylons
- Carbon-filled nylon
- Glass-filled nylon
- Aluminum-filled nylon
- Glass fiber-filled nylon
There are a broad spectrum of material properties within this grouping. Some other materials that can be used include polypropylene, which is a great engineering plastic, as well as urethanes or TPU.
Can metal be sintered using SLS?
In the early days of SLS, everyone tried to get SLS to be able to fuse metal particles together. Sintering does not refer to melting particles down, but rather fusing parts together.
If you take a walk down the beach along Lake Michigan, you can find large clumps of sand that are stuck together and dried out. If you grab a lump of sand like this, you’ll notice it is a hard lump – you can pick it up as one piece, but if you put pressure on it it will crumble. That sand is actually sintered together. It has a small molecular bond between the sand particles, but it can be easily broken up.
25 years ago, when metals were sintered, fundamentally the same issue occurred. The bonds created between the metal particles using SLS were much stronger than the bonds between the sand particles in the lump from the beach, but they were still not strong enough. The parts that resulted were too fragile right out of the 3D printer. Using metal materials in SLS requires too much post-processing to be a productive application. The particles would be fused together using SLS, but the parts would have to be submerged in a liquid bronze solution. The bronze would wick into the part, infiltrating the pores and spaces between the metal particles. Then, the infiltrated part needed to be put into a sintering furnace, further fusing all of these materials and particles together.
Today, we have Direct Metal Laser Melting, or Laser Powder Bed Fusion of Metals. With the high strength and precision of lasers available today, we are able to get a full weld between the metal particles used. Whereas previously the metal parts created using SLS were too fragile, today using DMLM we can get nearly full-density metal parts right out of the printer.
Can ceramics be used in SLS?
There are a limited number of metal application printers available that can also process ceramics. The very fine powders used in SLS applications are created in a variety of ways. The most common powder production for metals is gas atomized production. Molten metal is sprayed through an atomizer into a cyclonic tunnel, where the particles cool in the air before dropping down. The particles then go through a series of sieves and filters to ensure the particles are the right size.
Another way to create powders is the mechanical creation of the particles – think grinding, pulverizing or chopping. Most plastic powders are atomized, but ceramics have such a high heat tolerance that they are generally mechanically formed. This results in the ceramic particles being geometrically different from the atomized particles. Mechanically created particles are more jagged or rough compared to the spherical particles created using gas atomization.
Picture ground pepper or coffee. That’s sort of how the ceramic particles come out. This results in the ceramic powders being much more difficult to spread evenly and accurately, as the powder doesn’t flow as smoothly as a powder with spherical particles would. This makes ceramics very difficult to use in SLS printing.
Can waxes be used in SLS?
We service and support types of wax printers, but generally these are Multi-Jet Fusion type printers. There are some plastic resins, such as polystyrene, with similar melting points to waxes. Some of those materials are used for lost wax casting or investment casting in the SLS process.
A stainless steel part like this would conventionally be cast using lost wax or investment casting. An investment of this part would be made in wax or a low-melting point plastic. It would then be coating in a high-temp ceramic, very similarly to how jewelry is made. The ceramic-coated investment would then be put in a high-temp furnace around 2000 degrees, fusing the ceramic material together. The wax or plastic filled pattern would then melt away.
After the mold is formed, it can be backfilled with molten metal in place of where the wax or plastic has been. That is how many investment cast and high-detailed metal cast parts are made. These wax or plastic investments can be made using SLS printing processes. Anything smaller than this is probably more suited for Multi-Jet Fusion printing.
What does an SLS machine or SLS printer look like?
An SLS machine just looks like a big metal box, with a front-facing viewing window through which the sintering process can be monitored. Typically, there are two powder feed pistons which supply the powder to the printer. There is a compression roller which spreads the powder back and forth across the build platform. Above that there is a laser window, for a Co2 laser, which is a low-wattage laser (30-100 watts) used to fuse the particles.
The build volumes on these machines are on average around 12” x 12” x 15”. They can go all the way up to around 22” by 22” by 30”. The machines are significantly larger than the build volume, since a lot of mechatronics and insulation is required. The selective laser sintering process is very hot. The melting point of nylon is 350 degrees Fahrenheit. The powder bed itself gets up to 320 degrees. The laser pushes the material over the melting point to fuse the particles together.
How long has SLS been around?
Selective Laser Sintering has been around for around 30 years. It began in the 1990s.
What are the applications of SLS?
It is widely popular in the automotive and aerospace industries, and has been used increasingly in the medical field. Functional prototyping using SLS is used in both automotive and aerospace applications, especially for part consolidation purposes.
In the automotive field, because SLS uses true thermoplastic, SLS is used quite often for under-the-hood applications. Anywhere a glass-filled Nylon would or could be used for injection molding, SLS can be used. SLS is used for functional prototyping for the automotive industry regularly.
We’ve also seen a large increase in demand for robot arm end effectors – the part of the robot arm that actually handles components or parts in automotive and general industry. These are custom pick-and-place devices for moving and placing parts, or handling delicate parts that are coming out of injection mold machines.
We’re also seeing growth in jigs and check fixtures for automotive components and other high-tolerance components. Many fixtures can be very time consuming and difficult to produce reductively out of a block of material. SLS is being used to create these near-net geometries and challenging surfaces and textures in a lightweight material. The lead time is much faster when you’re not cutting away from a big block of material, but rather building the part from the ground up using powder bed fusion or SLS.
In the aerospace industry SLS is used to create many parts. The main application for the aerospace market is part consolidation. It is used for environmental controls or ductwork used to keep the interiors and cabins cool. SLS enables manufacturers to consolidate their parts into many fewer parts. For example, if there are 8 or 9 pieces of environmental controls and ductwork used around the cabin of a helicopter, SLS can be used to print these all together as one piece. This saves not only time, but also materials, assembly costs, tooling costs and more.
In the medical field, it is amazing what can be done using SLS printing. For example, for a patient needing a knee replacement, doctors can use a CT scan of the patient’s knee to make surgical cutting or drilling guides that conform exactly to the specific anatomy of the patient. Surgeries like knee replacements can be customized or personalized for an individual patient using these methods, resulting in faster surgeries and reduced turnaround times, lessening the risk of infection and exposure during surgery. This in turn results in dramatically faster healing, less patient downtime, and overall improved surgical outcomes.
Generally, medical device manufacturers have their own 3D printing machines and capabilities. They can create custom sets of cutting guides, drilling guides, and more and send them to orthopedic surgeons and hospitals. 3D Printed Parts supports the medical field through machine specification and SLS support rather than directly building these medical parts.
3D Printed Parts provides parts and prototypes for general industry every day. Producing production-level parts using SLS is a main focus of 3D Printed Parts. If you are looking for SLS support for SLS machine specifications, equipment sales, or 3D printing of prototypes or parts, we can support you and guide you through the process. If you have a critical need and are implementing SLS technology on-site, we can help you integrate SLS into your processes as well.
What is SLS rapid prototyping?
SLS is an ideal technology for many rapid prototyping or rapid tooling applications. Since the throughput and speed of SLS is dramatically quicker than some other forms of additive manufacturing, it allows for rapid prototyping and testing of novel custom parts or elements. There is a lot of variability in rapid prototyping requests, including materials to be used. Some SLS machines are well-suited to changing out material, while others are better suited to use the same material. Usually these are dedicated to one application, such as medical devices or surgical cutting guides. It’s much easier to recycle unused material in a dedicated machine.
What is the difference between SLS and FDM?
Many people are more familiar with FDM style 3D printing, in which plastic filament or thread is extruded through a print head. SLS is on average about 8 times faster than an FDM printer, and in some instances, up to 20 times faster. SLS is a very high-throughput, high-productivity form of 3D printing.
FDM extruders dispense heated filament through a nozzle very similarly to a hot glue gun. When you use a hot glue gun and pull the trigger, the glue is extruded where your hand leads it. The FDM extruder is mounted on a gantry system, so very similarly to a glue gun, the rate and speed of the movement of the nozzle is limited by the rate of the extrusion of the heated filament. SLS is not limited in this manner since the material is not being extruded, but being fused by a laser moving along a raster path at speeds up to 5000 meters per second. This makes SLS an extremely fast and precise 3D Printing method.
If you needed to make a simple part like this using a FDM printer, it may take 5 hours. If you needed to make two of them, it would double, and take 10 hours, since the extruding head needs to run the exact same path again after the first print.
Using SLS printing, with the speed of the laser, production time scales much better. You wouldn’t see a doubling of production time until maybe 20 parts or so.
What is the cost of SLS?
Using SLS for parts is very cost-effective and competitive. It has a tendency to be less expensive than FDM part-for-part, especially with higher volumes of parts. It’s also a very flexible style of additive manufacturing, as you don’t have to produce one type of part at once.
You can include numerous shapes or geometries within one build using SLS. Many softwares used can even add a new part into a build that is already started. There are a lot of great tools for maximizing build output, or fitting as many parts into one build as you can.
There is opportunity for customization of SLS machines if you are building a lot of the same parts, so it builds them faster and more accurately. It is a very high throughput, very flexible, relatively easy-to-work-with technology.
Are there any disadvantages of SLS?
Clear plastics are not able to be used in SLS production. Because of this, SLS is not able to be used for any type of lens or window application.
High Temperature Applications
Because SLS uses thermoplastics, the materials used have melting points that prevent SLS from being used for very high-temperature applications. For these applications, Composite Stereolithography material or Metal materials are better suited.
Very Large Parts
Extremely large parts are outside of the scope of SLS. The build envelope for SLS is around 30 inches tall. Any single-piece part that is larger than this is not a suitable use for SLS.