Guide to Selective Laser Sintering (SLS) 3D Printing

Selective laser sintering (SLS) was one of the first additive manufacturing techniques, developed in the mid-1980s by Dr. Carl Deckard and Dr. Joe Beaman at the University of Texas at Austin. Their method has since been adapted to work with a range of materials, including plastics, metals, glass, ceramics, and various composite material powders. Today, these technologies are collectively categorized as powder bed fusion—additive manufacturing processes by which thermal energy selectively fuses regions of a powder bed.

The two most common powder bed fusion systems today are plastic-based, commonly referred to as SLS, and metal-based, known as direct metal laser sintering (DMLS) or selective laser melting (SLM). Until recently, both of these systems have been prohibitively expensive and complex, limiting their use to small quantities of high value or custom parts, such as aerospace components or medical devices.

Innovation in the field has surged recently, and plastic-based SLS is now poised to follow other 3D printing technologies like stereolithography (SLA) and fused deposition modeling (FDM) to gain widespread adoption with accessible, compact systems.

SLS 3D printers use a high power laser to fuse small particles of polymer powder.

1. Powder is dispersed in a thin layer on top of a platform inside of the build chamber.
2. The printer preheats the powder to a temperature just below the melting point of the raw material. This makes it easier for the laser beam to raise the temperature of specific regions of the powder bed as it traces the model to solidify a part.
3. The laser scans a cross-section of the 3D model, heating the powder to just below or right at the melting point of the material. This fuses the particles together mechanically to create one solid part. The unfused powder supports the part during printing and eliminates the need for dedicated support structures.
4. The build platform lowers by one layer into the build chamber, typically between 50 to 200 microns, and a recoater applies a new layer of powder material on top. The laser then scans the next cross-section of the build.
5. This process repeats for each layer until parts are complete, and the finished parts are left to cool down gradually inside the printer.
6. Once parts have cooled, the operator removes the build chamber from the printer and transfers it to a cleaning station, separating the printed parts and cleaning of the excess powder.

Selective laser sintering post-processing requires minimal time and labor, and leads to consistent results for batches of many parts.

After a print job is complete, the finished parts need to be removed from the build chamber, separated, and cleaned of excess powder. This process is typically completed manually at a cleaning station using compressed air or a media blaster.

SLS parts have a slightly rough, grainy surface finish right out of the printer similar to a medium grit sandpaper. Nylon provides a range possibilities for post-processing, such as tumbling, dyeing, painting, stove enameling, metal coating, bonding, powder coating, and flocking.

Any excess powder remaining after part recovery is filtered to remove larger particles and can be recycled. Unfused powder degrades slightly with exposure to high temperatures, so it should be refreshed with new material for subsequent print jobs. This ability to re-use material for subsequent jobs makes SLS one of the least wasteful manufacturing methods.

All selective laser sintering systems are built around the process described above. The main differentiators are the type of laser and the size of the build volume. Different systems employ different solutions for temperature control, powder dispensing, and layer deposition.

Selective laser sintering requires a high level of precision and tight control. The temperature of the powder along with the (incomplete) parts must be controlled within 2 °C during the three stages of preheating, sintering, and storing before removal to minimize warping, stresses, and heat-induced distortion.

Industrial SLS 3D printing has been in use for numerous applications, from spare parts for the world's biggest truck maker to custom-printed clothing. The largest systems can print 1-meter-long parts.

Industrial SLS 3D printing systems use a single or multiple high-power carbon dioxide lasers. The larger the build volume, the more complex the system. Industrial SLS requires an inert environment–nitrogen or other gases–to prevent powder from oxidizing and degrading. Thus, industrial selective laser sintering calls for specialized air handling equipment. These systems also require industrial power; even the smallest industrial machines need at least 10 m² installation space.

Benchtop SLS 3D printers (like Formlabs’ own Fuse 1) achieve output comparable to industrial systems in a more compact, manageable form.

Benchtop SLS 3D printing uses a diode or fiber laser instead of the CO2 lasers used by industrial systems to provide equal beam quality at a lower cost.

A benchtop machine's smaller build area requires less heating. As the powder gets exposed to elevated temperatures for a shorter period of time, there is no need for inert gases and specialized air handling equipment. The overall less energy consumption allows benchtop systems to run on standard AC power with no specialized infrastructure.

Overall, benchtop SLS 3D printers offer a slightly reduced build volume and slower speed compared to the smallest industrial SLS systems, in return for a substantially smaller footprint and lower cost.

The comparison is based on the Formlabs Fuse 1 benchtop SLS 3D printer and industrial SLS systems by EOS and 3D Systems.

The most common material for selective laser sintering is nylon, a popular engineering thermoplastic beloved for its lightweight, strong, and flexible properties. Nylon is stable against impact, chemicals, heat, UV light, water, and dirt, making it ideal for both rapid prototyping and production.

Nylon is a synthetic thermoplastic polymer that belongs to the family of polyamides. Its two versions commonly used for selective laser sintering are Nylon 11 and 12, or PA11 and PA12.

PA is the abbreviation of polyamide, and the numbers represent the number of carbon atoms in the material. Both are similar in material properties, PA11 is slightly more flexible and impact resistant, whereas PA12 is stronger, more abrasion resistant, and biocompatible.

Nylon 11 and 12 are both single component powders, but SLS 3D printers can also use two-component powders, such as coated powders or powder mixtures. Nylon composites with aluminide, carbon, or glass are developed to optimize parts for higher strength, stiffness, or flexibility. With these two-component powders, only the component with the lower glass transition point is sintered, binding both components.

Engineers choose selective laser sintering for its design freedom, high productivity and throughput, low cost per part, and proven track record.

Most additive manufacturing processes, such as stereolithography (SLA) and fused deposition modeling (FDM), require specialized support structures to fabricate designs with overhanging features.

Selective laser sintering does not require support structures because unsintered powder surrounds the parts during printing. SLS printing can produce previously impossible complex geometries, such as interlocking or moving parts, parts with interior components or channels, and other highly complex designs.

Engineers generally design parts with the capabilities of the final manufacturing process in mind, also known as design for manufacturing (DFM). When additive manufacturing is used for prototyping alone, it is limited to parts and designs that conventional manufacturing tools can ultimately reproduce during production.

As selective laser sintering becomes a viable rapid manufacturing method for an increasing number of end-use applications, it has the potential to unleash new possibilities for design and engineering. SLS can print complex designs in a single print that would normally require multiple parts. This helps alleviate weak joints and cuts down on assembly time.

Selective laser sintering can take generative design to its full potential by enabling lightweight designs that employ complex lattice structures impossible to manufacture with traditional methods.

SLS printing 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 like industrial FDM.

Multiple parts can be tightly arranged during printing to maximize the available build space in each machine. Operators use software to optimize each build for the highest productivity leaving only minimal clearance between parts.

Parts can be added to the build while printing is already in progress. This provides the opportunity for last minute design changes or to add consecutive iterations of a prototype.

The key to SLS 3D printing’s functionality and versatility is the material. Nylon and its composites are proven, high-quality thermoplastics. Laser-sintered nylon parts have close to 100 percent density with mechanical properties comparable to those created with conventional manufacturing methods like injection molding.

SLS nylon is a great substitute for common injection molding plastics. It offers superior living hinges, snap fits, and other mechanical joints compared to any other additive manufacturing technology. It is ideal for functional applications requiring plastic parts that will last where parts produced with other AM methods would degrade and become brittle over time.

Calculating cost per part usually requires accounting for equipment ownership, material, and labor costs:

• Equipment ownership: The more parts a machine can produce over its lifetime, the lower the costs attributable to each individual part. Consequently, higher productivity leads to lower equipment ownership cost on a per-part basis. Given the fast scanning speed of the laser, the nesting of parts to maximize build capacity, and simple post-processing, SLS 3D printing offers the highest productivity and throughput of all plastic additive manufacturing techniques.

• Material: While most 3D printing technologies use proprietary materials, nylon is a common thermoplastic produced in large quantities for industrial purposes, making it one of the least expensive raw materials for additive manufacturing. Moreover, the lack of support structures and reusable powder mean that SLS printing produces minimal waste.

• Labor: The Achilles heel of many 3D printing solutions is labor. Most processes have complex workflows that are hard to automate, which can substantially influence cost per part. The simple post-processing workflow of SLS printing means less labor is required.

Selective laser sintering enables engineers to prototype parts early in the design cycle, then use the same machine and material to produce end-use parts. SLS 3D printing does not require the same expensive and time-consuming tooling as traditional manufacturing, so prototypes of parts and assemblies can be tested and easily modified over the course of a few days. This drastically reduces product development time.

Given its low cost per part and capable, lasting materials, SLS printing is an economical way to produce complex, custom parts, or a series of small components for end products. In many cases, laser sintering is a cost-effective alternative to injection molding for limited-run or bridge manufacturing.

Until now, SLS 3D printers have been prohibitively costly for most businesses, with a single machine running over $200,000.

Introducing the Fuse 1, the first benchtop SLS system by Formlabs.

With the Fuse 1, Formlabs is bringing the industrial power of selective laser sintering (SLS) to the benchtop, helping to put the means of production in the hands of more businesses.