Features and facts about the SLS Technology

Selective Laser Sintering 3D printing process

Selective laser sintering (SLS) is a powder-based fusion technology that uses a laser beam to locally sinter polymer powder to build parts layer by layer. SLS can be applied to produce uniquely shaped products and is an economical choice for small series production (up to a 1.000 parts!)

A bin of the powder material is heated to an elevated temperature. A recoating blade deposits a very thin layer of the powdered material (typically 0.1 mm) onto a build platform. After deposition of a layer, a laser beam starts to scan the surface. The laser beam selectively sinters the powder and solidifies a cross-section of the part. When the entire cross-section is scanned, the building platform moves down one layer thickness in height. Unsintered powder remains in place to support subsequent layers eliminating the need for support structures. The recoating blade deposits a new layer of powder on top of the scanned layer and the laser beam 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.

The result is a container filled with powder and consolidated products. Since multiple products can be produced simultaneously the process can be used for small series production. The placement and orientation of parts is optimised to maximise part occupancy in the powder box during each print.

When the printing process is complete and the powder container and product have cooled down, the powder container is unpacked. The solid products are parted from the unsintered powder and cleaned with compressed air and a blasting medium. The remaining (unsintered) powder is collected and reused. The parts are then ready to use or are post processed to improve mechanical properties or appearance.

SLS 3D printing materials

There are various materials available for SLS. Currently, the majority of the materials are based on polyamides. Polyamides are synthetic thermoplastic polymers. The most widely applied material is polyamide 12 (PA12) which has mechanical properties comparable to injection moulded polyamide, good dimensional stability, good wear resistance and high chemical resistance.

Designing for SLS

Like all manufacturing techniques there are several design recommendations that exist to improve the quality, surface finish and functionality of SLS parts. One of the most advantageous characteristics of designing and printing parts using SLS is that there is no need for support structures. The unsintered powder surrounding the part removes the need for support allowing highly complex and intricate designs to be printed.

Advantages and Design Rules SLS 3D printing

Design freedom

The main advantage of SLS is the powder bed. Since the unsintered powder supports the sintered products, there is no need for support structures. This means that nearly any geometry can be produced!

High product quality

Another main advantage is high product quality. Thanks to continuous research and development, the mechanical properties of printed products are very close or even similar to their injection moulded variant.

Applications for SLS 3D printing

For example:

• Casings
• Spareparts
• Machine parts
• Packaging (prototypes)
• Eyewear
• Jewellery
• Awards
• Maquettes
• Surgical guides
• Anatomical models
• Orthopedic appliances
• Parts for medical devices

Comparison with injection molding

Often in industry SLS parts are used as prototypes for determining form, function and fit of designs that will later be mass manufactured by injection molding. The main differences between designing parts for SLS compared to injection molding are:

• As an SLS part does not need to be removed from a die, SLS is able to easily produce undercuts, negative draft and interior features.
• SLS eliminates the need for costly tooling, which makes it an affordable choice for small (1 – 1000) series production.
• Perfectly sharp edges and corners are not able to be produced by SLS. The SLS process produces parts that have a radius of ± 0.4 mm at all edges and corners. A radius less than 0.4 mm on a design will be printed as 0.4 mm.
• The natural radius produced by SLS offers some stress relief. For areas of concern a larger radius (greater than 2 mm) should be added.

Shrinkage and warping

Due to the high temperatures experienced by SLS components during the printing stage some shrinkage and warping can occur. SLS parts are typically cooled slowly to limit the impact of warping and shrinkage.

Warping – Large flat surfaces are most at risk. Consider adding ribs to increase stiffness. Part orientation during the printing stage can also help reduce the likelihood of warping.
Shrinkage – Most designs for SLS printing have overall dimensions increased by 3 – 3.5% at the pre-print analysis and conversion stage to accommodate shrinkage. This does not affect the design of a part.

Limitations of SLS

Product size

The size a part is able to be printed at is limited by the size of the nylon container used in the SLS machines. Currently the maximum product size that can be produced is 192 mm x 240 mm x 315 mm up to 675 mm x 366 mm x 545 mm (depending on the type of material).

Consistency

Since every SLS printed part consists of hundreds (or even thousands!) of layers, small variations between products can occur (dimensions, surface quality). In addition, due to the uniqueness of the products most post processing steps are done manually. This also means that minor variations can occur (e.g. small colour or coating variations).

Surface finish

While SLS produces a consistent surface finish the surface appearance is a satin-like matte finish that is slightly grainy to the touch. If a shiny and smooth finish is desired post processing is recommended.

Post processing SLS 3D printing

3D printing is no longer just a prototyping tool. The high product quality combined with various post processing options enables the production of fully functional and ready-to-use products.

Vibro polish

For a smoother surface texture, Nylon SLS parts can be polished in our selection of Vibro machines. Each of these tumblers contains small ceramic chips that vibrate against the object and gradually erode the outer surface down to a polished finish. As a result this process does have an small effect on part dimensions and rounding sharp edges.

Dyeing

The fastest (and cheapest!) method to give your 3D print a beautiful colour is via a dye process. The 3D part is immersed in a hot colour bath. Using a colour bath ensures that even the most difficult areas are coloured. We currently offer 16 colours for our PA12 material.

Spray paint & Lacquering

If you want a more luxurious look, it is possible to spray paint your part in a specific RAL colour. It is also possible to lacquer the SLS parts. Via lacquering you can obtain various finishes, such as high gloss or a metallic sheen.

Functional coatings

Applying a functional coating can significantly improve the performance of the 3D printed parts. Examples of added functionalities are:

• UV protection
• Chemical resistance
• Decreased gas permeability
• Colour protection
• Increased wear resistance
• Oil- and (salt) water resistance

General conditions of SLS 3D printing

• SLS parts do not required support allowing for greater design freedom making SLS one of the easier 3D printing technologies to design for.
• SLS can be used to produce many functional features including axles, threads, tanks and hinges. This coupled with the range of engineering polymides result in SLS often being used to produce end use parts.
• The standard surface finish for SLS is a matte-like grainy surface. A range of post-processing options are available that can also help to improve part functionality.