Laser Powder Bed Fusion – SLS | DMLS

Powder Bed Fusion technologies use a thermal source (e.g. a laser) to cause fusion between powder particles. Most Powder Bed Fusion technologies use mechanisms for applying and smoothing powder as a part is constructed, resulting in the final component being covered in powder.

TL;DR

• Powder Bed Fusion Technologies
  • SLS – Selective Laser Sintering
    • SLS Industry Applications
  • SLM – Selective Laser Melting
    • DMLS Industry Applications
  • Differences between SLS & SLM
• SLS Printer Characteristics
• SLS & DMLS Materials
• Benefits and limitations of SLS 3D printing
• Common applications of SLS 3D printing

Within the 3D printing industry, using Powder Bed Fusion technology with powdered material (typically nylon or polyamide) to produce parts is generally referred to as Selective Laser Sintering (SLS) or just Laser Sintering (LS).

Powder Bed Fusion Technologies

SLS – Selective Laser Sintering

The SLS process begins with a bin of the polymer powder being heated to a temperature just below the melting point of the nylon (polyamide) or polymer. A recoating blade deposits a very thin layer of the powdered material (typically 0.1 mm) onto a build platform. A CO² laser beam is then aimed at points defined by the 3D model being built. The laser sinters the nylon or polymide powder, binding the material together, solidifying a cross-section of the part.

When the entire cross section is sintered, the building platform moves down one layer thickness in height. The recoating blade deposits a new layer of powder on top of the recently scanned layer and the laser starts to sinter the successive cross section of the part onto the previously solidified cross-sections.

This process is repeated until all parts are fully manufactured.

Unsintered powder remains in place to support the part as it is built, eliminating the need for support structures. This is one of the major advantages of SLS.

The placement and orientation of parts is optimized to maximize part occupancy in the powder bin during each print.

The result is a bin filled with powder and built parts. Since multiple parts can be produced simultaneously, the process can be used for batch manufacturing.

When the printing process is complete and the powder bin and parts have cooled down, the powder bin is unpacked. The solid products are parted from the unsintered powder and cleaned with compressed air and a blasting medium. 50% of the unsintered powder is collected and reused. The parts are then ready to use or are further post processed to improve their appearance.

SLS Industry Applications

SLS technology is in wide use at many industries around the world due to its ability to easily make complex geometries with little to no added manufacturing effort.

Its most common application is in prototype parts early in the design cycle such as for investment casting patterns, automotive hardware, and wind tunnel models. SLS is also increasingly being used in limited-run manufacturing to produce end-use parts for aerospace, military, medical, and electronics hardware. On a shop floor, SLS can be used for rapid manufacturing of tooling, jigs, and fixtures.

Selective Laser Melting (SLM)

 Selective laser melting (SLM) uses a comparable concept to SLS, but in SLM the material is fully melted rather than sintered, allowing different properties (crystal structure, porosity, and so on).

Another name for selective laser melting is direct metal laser sintering (DMLS), a name used by the EOS brand.

This additive manufacturing (AM) technique uses a high power-density laser to melt and fuse the (atomised) powders together. As the powder particles are being melted during the production, not sintered, the part is fully dense.

This technology is used with metal powders.

Materials used in this technology include copper, aluminium, stainless steel, tool steel, cobalt chrome, titanium and tungsten. SLM is especially useful for producing tungsten parts because of the high melting point and high ductile-brittle transition temperature of this metal.

Currently, the available alloys used in the process include 17-4 and 15-5 stainless steel, maraging steel, cobaltchromium, inconel 625 and 718, aluminum AlSi10Mg, and titanium Ti6Al4V.

The mechanical properties of parts produced using direct metal laser sintering differ from those manufactured using casting.

The types of applications most suited to the selective laser melting process are complex geometries & structures with thin walls and hidden voids or channels. Low lot sizes parts are also a common use case.

Producing hybrid forms where solid and partially formed or lattice type geometries. Hip stems, acetabular cups or other orthopaedic implants where oseointegration is enhanced by the surface geometry.

Much of the pioneering work with selective laser melting technologies is on lightweight parts for aerospace where traditional manufacturing constraints, such as tooling and physical access to surfaces for machining, restrict the design of components.

This technology is also used to manufacture direct parts for a variety of industries including aerospace, dental, medical and other industries that have small to medium size, highly complex parts and the tooling industry to make direct tooling inserts.

DMLS is a very cost and time effective technology. The technology is used both for rapid prototyping, as it decreases development time for new products, and production manufacturing as a cost saving method to simplify assemblies and complex geometries. With a typical build envelope (e.g., for EOS’s EOSINT M280) of 250 x 250 x 325 mm, and the ability to ‘grow’ multiple parts at one time

DMLS Industry Applications

• Aerospace – Air ducts, fixtures or mountings holding specific aeronautic instruments, laser-sintering fits both the needs of commercial and military aerospace
• Manufacturing – Laser-sintering can serve niche markets with low volumes at competitive costs. Laser-sintering is independent of economies of scale, this liberates you from focusing on batch size optimization.
• Medical – Medical devices are complex, high value products. They have to meet customer requirements exactly. These requirements do not only stem from the operator’s personal preferences: legal requirements or norms that differ widely between regions also have to be complied with. This leads to a multitude of varieties and thus small volumes of the variants offered.
• Prototyping – Laser-sintering can help by making design and functional prototypes available. As a result, functional testing can be initiated quickly and flexibly. At the same time, these prototypes can be used to gauge potential customer acceptance.
• Tooling – The direct process eliminates tool-path generation and multiple machining processes such as EDM. Tool inserts are built overnight or even in just a few hours. Also the freedom of design can be used to optimize tool performance, for example by integrating conformal cooling channels into the tool.

Other DMLS Applications

• Parts with cavities, undercuts, draft angles
• Fit, form, and function models
• Tooling, fixtures, and jigs
• Conformal cooling channels
• Rotors and impellers
• Complex bracketing

A similar process is electron beam melting (EBM), which uses an electron beam as the energy source. This technology is primarily used for medical implant parts.

Difference between Selective Laser Sintering (SLS) & Selective Laser Melting (SLM)

The use of SLS refers to the process as applied to a variety of materials such as plastics, glass, and ceramics, as well as metals. What sets SLM apart from other 3D printing process is the ability to fully melt the powder, rather than heating it up to a specific point where the powder grains can fuse together, allowing the porosity of the material to be controlled

SLS Printer Characteristics

Printer parameters

There are a range of parameters that govern how well a part will print on a SLS machine.

Laser spot size and layer height generally define the accuracy and surface finish of a printed part. Most SLS parts are printed with a default layer height of 100 microns (0.1 mm).

Powder particle geometry and size also play a large role in defining the properties of a part. Finer powders will result in a smoother part surface, but present issues with handling and spreading during the recoating stage of the print. Coarser powders, while simpler to handle, will have a detrimental effect on surface finish and achievable feature sizes.

The surface finish of SLS parts is typically matte and grainy to the touch. The downward facing side of a print will generally have the best surface finish.

Optimal machine settings are typically set up by the printer manufacturer. This results in machines automatically adjusting parameters based upon the build material input by the operator. SLS machines are autonomous during the heat up, printing and cool down phases with, operator interaction only being required for the loading and unloading of the powder bins and print monitoring.

Layer adhesion

Adhesion between layers is important to achieve a robust, cohesive part.

Initial heating of the build powder followed by exposure to the sintering laser causes the powder particles to fuse together in multiple directions. This results in parts that are essentially homogeneous.

While isotropy is a strength of single material SLS parts, the addition of composite particles (like glass or carbon) results in parts that are anisotropic (sometimes as much as 40% weaker in the build direction). This should be considered when deciding on SLS materials for a specific application.

Dimensional accuracy

SLS parts are also susceptible to shrinkage and warping during printing.

As each layer is sintered, it fuses with the layer below as it cools. This cooling causes the newly printed layer to shrink, pulling up the underlying layer. It is best to orientate large flat parts at an angle or vertically to reduce the cross sectional area of each layer.

To restrict the likelihood of parts warping or shrinking during printing, SLS printers use heated build chambers that raise the temperature of the powder to just below the sintering temperature. However, this can still result in temperature gradients in large SLS parts where the bottom of the part has cooled, while the recently printed top layers remain at an elevated temperature.

One of the most crucial steps in the SLS process is the cooling stage. To further mitigate the likelihood of warping occurring, parts are left in the powder bin to cool slowly (sometimes up to 50% of the total build time) before handling.

SLS & DMLS Materials

Materials with a low thermal conductivity are best suited for Powder Bed Fusion, as they exhibit more stable behavior during the sintering phase.

The polymer side of Laser Sintering almost exclusively uses one type of thermoplastic polymer known as polyamide (PA) or Nylon to produce parts. Polyamide parts have excellent long-term stability and good chemical resistance with the most common commercial polyamide being nylon.

SLS powder can vary in price depending on material, with standard PA 12 nylon costing approximately ZAR750 – ZAR900 per kg.

While SLS powders generally only come in white, grey or black, parts can be dyed in a range of colours.

To further enhance the mechanical properties, heat/chemical resistance of SLS parts, or to obtain a different appearance, nylon can be mixed with other materials like aluminium, glass, carbon and graphite to form a composite powder.

Benefits and limitations of SLS 3D printing

SLS is best suited for producing strong functional parts with complex geometries. This coupled with the isotropic nature and high level of accuracy (although not as good as Vat Polymerization or Material Jetting) sees the technology often adopted for the production of end use parts.

The other big advantage of the SLS process is that parts do not require any support material. This means support does not need to be removed after printing and also results in a consistent overall surface finish, as there is no negative effect from supports being in contact with a surface like FFF and SLA.

The biggest downside to SLS printing is that the technology is an industrial process with machines costing around $250,000 that require highly skilled operators and advanced material handling procedures. Because of this, lead times can be longer than other 3D printing technologies.

One of the main contributors to SLS lead time is the heating and cooling stages required during printing, resulting in prints for a full 300 x 300 x 300 mm bin taking around 20 – 24 hours plus another 12 hours of cooling time before parts can be handled for post processing. Most machines now allow for removal of powder bins to be heated/cooled while out of the machine improving efficiency.

SLS parts also have a grainy, matte like surface unless post processed.

Common Applications of SLS 3D Printing

The versatility of the SLS process sees it used for a large range of applications.

Functional parts. The biggest strength of SLS printing is that it offers a range of strong, functional materials. Because of this, SLS is often used for the production of parts that will be under load when placed in service. SLS allows for complex geometries that can be easily printed from well-known materials like PA 12.

Low run part production. SLS allows cost effective, low run production of function parts to provide feedback on the design and performance of parts. Because SLS always prints a full powder bin, multiple parts can be manufactured in a single build, offering viable economies of scale at certain build sizes (“smaller than a fist”).

Complex ducting (hollow sections). The powder based nature of SLS means that it can create parts with hollow sections, something other support dependent technologies are unable to do. SLS is ideally suited for the low run production of complex ducting and piping. By removing traditional design constraints, SLS is capable of printing parts that are optimized for application rather than manufacture.