What Is SLS 3D Printing? A Guide to Selective Laser Sintering

SLS printing in one paragraph

Selective laser sintering is a powder bed fusion process. A machine spreads a thin layer of polymer powder, a laser fuses the cross-section of the part into that layer, and the cycle repeats until the part is complete. The unfused powder stays in place and supports the part, so no support structures have to be designed, printed or removed. The result is a solid thermoplastic part, most often in polyamide 12 (PA 12, nylon 12), that can be used as a functional prototype or as an end-use component.

How selective laser sintering works

1. Preparation and nesting

The 3D models of all parts in a job are arranged in three dimensions inside the build volume. This is called nesting. Because parts can be stacked above each other, one build usually contains many different parts from several orders. How densely a build is packed has a direct effect on cost per part.

2. Heating

The build chamber and the powder are heated to a temperature just below the melting point of the polymer. The laser then only has to add a small amount of energy to fuse the powder, which reduces thermal stress and warping.

3. Layer-by-layer sintering

A recoater spreads a layer of powder, typically around 0.1 mm thick. A CO₂ laser, steered by scanning mirrors, traces the cross-section of every part in that layer. The platform lowers by one layer and the next layer of powder is applied. A full build runs for many hours without an operator.

4. Cooling

The block of powder with the parts inside, often called the cake, has to cool slowly and evenly before it is opened. Cooling can take as long as the build itself. Opening the cake too early causes warping and dimensional errors, which is one reason SLS lead times cannot be compressed indefinitely.

5. Unpacking and finishing

The parts are removed from the cake and loose powder is recovered; part of it is mixed with fresh powder and reused. The parts are then blasted to remove powder that adheres to the surface. Optional steps include surfacing, dyeing and other treatments.

Materials

The standard SLS material is PA 12. It sinters reliably, absorbs little moisture for a polyamide and gives a balanced combination of stiffness, strength and ductility. Other powders exist: PA 11 for higher ductility, glass-filled or aluminum-filled PA 12 for higher stiffness, and TPU for flexible parts. Each needs its own machine parameters, and many service providers run only one or two of them.

SLS Market prints white Nylon PA 12 only. The PA12 nylon guide describes the material and its limits.

Accuracy and surface

The usual general tolerance quoted for SLS PA 12 is ±0.3 %, with a lower limit of ±0.3 mm. A 100 mm dimension can therefore be expected within about ±0.3 mm, a 200 mm dimension within about ±0.6 mm. Shrinkage during cooling is the main source of deviation, and it depends on geometry, wall thickness and the position of the part in the build. Large flat areas are the most likely to warp.

The surface is matte and slightly grainy because powder particles adhere to the molten outline of each layer. Layer lines are far less visible than on filament-printed parts. Blasting evens the surface out; further surfacing steps make it smoother to the touch. An SLS part does not have the gloss of an injection-molded part unless it is treated additionally.

Mechanical properties are close to isotropic. Strength in the build direction is somewhat lower than within a layer, but the difference is small compared with filament printing.

Typical design rules

These are common industry guidelines for laser-sintered PA 12. Exact limits depend on the machine, the parameters and the geometry.

Because there are no supports, features that are expensive elsewhere cost nothing extra in SLS: undercuts, internal channels, lattice structures, integrated hinges and several functions consolidated into one part.

What drives the cost of an SLS part

SLS has no tooling cost, so the price of a part is set by how much of a build it consumes.

• Part volume. The amount of sintered material.
• Bounding box and packing. A bulky, hollow shape occupies build space that cannot be used for other parts. Shapes that nest well are cheaper.
• Height in the build. Build time grows with the number of layers.
• Quantity. Fixed handling effort per order is spread over more parts, and a full build is more economical than a sparse one.
• Finishing. Blasting is standard; dyeing and additional surfacing add steps.

Two practical consequences follow. Hollowing a thick part and adding escape holes reduces material and distortion at the same time. And the unit price of 50 parts is usually well below the unit price of 5, even though no mold is involved.

SLS compared with other processes

When to use SLS

SLS is a good fit when the part has to work, not only look right. Typical applications:

• functional prototypes that are tested under load;
• housings, covers and brackets for machines and devices;
• jigs, fixtures and gripper fingers for production lines;
• ducts, manifolds and parts with internal channels;
• small series and spare parts where a mold does not pay off.

It is the wrong choice for transparent parts, for parts that need a glossy cosmetic surface without further treatment, for very large parts that exceed the build volume of the machine, and for high volumes of a simple geometry where injection molding is cheaper per piece.

What to ask an SLS printing service

• Which machine and which powder will be used, and is there a datasheet?
• What general tolerance applies and how are critical dimensions handled?
• How are parts finished as standard, and which options exist?
• Is the part made in-house or passed on to a network of suppliers?

At SLS Market the answers are an EOS FORMIGA P 110 Velocis with a 200 x 250 x 330 mm build volume, white Nylon PA 12, and blasting in a DyeMansion Powershot DUAL, all in the Czech Republic. Details are on the SLS 3D printing service page; our earlier article on SLS printing at SLS Market describes the equipment.