SELECTIVE LASER SINTERING (SLS)

Technology

SELECTIVE LASER SINTERING (SLS)

SLS 3D Printer
Test Bracket Printed in Gray SLS Nylon

In this method of 3D printing which is commonly used in industrial manufacturing, lasers fuse powdered materials together layer by layer. A high intensity infrared laser selectively melts powderized materials to create a layer. New powder is deposited above the previously formed layer and the laser melts the new powder to fuse the model together. SLS is usually only available to professional firms.

SLS prints are highly detailed and often mechanically superior to those produced by other technologies. SLS is used most often for fully functional prototypes in advanced materials. SLS printers can use a wide range of powderized materials including a number of standard thermoplastics like nylon, polycarbonate as well as metals like aluminium and steel. Metal parts that are fully functional can be produced by SLS. SLS machines are incredibly costly and reserved exclusively for professional printing, the machines require specialized trained personnel to operate,  the powder used in the machines can be dangerous if mishandled.

A key advantage of SLS is that it needs no support structures. The unsintered powder provides the part with all the necessary support. For this reason, SLS can be used to create freeform geometries that are impossible to manufacture with any other method.

In SLS, the bond strength between the layers is excellent. This means that SLS printed parts have almost isotropic mechanical properties.

SLS parts have excellent tensile strength and modulus, comparable to the bulk material, but are more brittle (their elongation at break is much lower). This is due to the internal porosity of the final part.

A thin layer of powder is first spread over the build platform. A CO2 laser then scans each cross-section, sintering the powder. The platform then moves downwards one layer and the process repeats until the job is complete. 

SLS parts are susceptible to shrinkage and warping as the newly sintered layer cools, its dimensions decrease and internal stresses buildup, pulling the underlying layer upwards

Since SLS requires no support material, parts with hollow sections can be printed easily and accurately.

Hollow sections reduce the weight and cost of a part, as less material is used. Escape holes are needed to remove the unsintered powder from the inner sections of the component. It is recommended to added to your design at least 2 escape holes with a minimum 5 mm diameter.

If high stiffness is required, parts must be printed fully solid. An alternative is to make a hollow design omitting the escape holes. This way tightly packed powder will be entrapped in the part, increasing its mass and providing some additional support against mechanical loads, without an effect on the build time. An internal honeycomb lattice structure can be added to the hollowed interior (similar to the infill patterns used in FDM) to further increase the stiffness of the component. Hollowing a part this way may also reduce warping.

If high stiffness is required, parts must be printed fully solid. An alternative is to make a hollow design omitting the escape holes. This way tightly packed powder will be entrapped in the part, increasing its mass and providing some additional support against mechanical loads, without an effect on the build time. An internal honeycomb lattice structure can be added to the hollowed interior (similar to the infill patterns used in FDM) to further increase the stiffness of the component. Hollowing a part this way may also reduce warping.

Post Processing - Selective Laser Sintering (SLS)​

SLS parts produces parts with a powdery, grainy surface finish that can be easily stained. Their appearance can be improved to a very high standard using various post processing methods, such as media polishing, dyeing, spray painting and lacquering. Their functionality can also be enhanced by applying a watertight coating or a metal plating.

Advantages of SLS

SLS parts have good, isotropic mechanical properties, making them ideal for functional parts and prototypes.

SLS requires no support, so designs with complex geometries can be easily produced.

The manufacturing capabilities of SLS is excellent for small to medium batch production.

Disadvantages of SLS

Only industrial SLS systems are currently widely available, so lead times are longer than other 3D printing technologies, such as FDM and SLA.

SLS parts have a grainy surface finish and internal porosity that may require post-processing, if a smooth surface or water tightness are required.

Large flat surfaces and small holes cannot be printed accurately with SLS, as they are susceptible to warping and oversitnering.