MJF vs SLS: Which Process for PA12 Parts?

Disclosure: SLS Market operates an SLS system and does not offer MJF. The comparison below describes the two processes in general terms and applies to PA12 parts from any supplier.

Short answer

For most functional PA12 parts either process works. Choose SLS when the part should be white or dyed in a light or saturated color. Choose MJF when a gray or black part is acceptable and the supplier's pricing favors it for your quantity. For everything else, the supplier's process control and finishing matter more than the acronym.

How the two processes work

Both are powder bed fusion processes. A thin layer of PA12 powder is spread across a heated build area, the cross-section of the part is fused, and the next layer follows. Unfused powder supports the part, so neither process needs support structures.

The difference is how the energy reaches the powder.

• SLS: a CO₂ laser scans the cross-section point by point and fuses the powder directly. Nothing but powder goes into the part. Build time per layer depends on the area that has to be scanned.
• MJF: an inkjet print head deposits a dark, heat-absorbing fusing agent where the part should be, and a detailing agent around the contour to sharpen the edge. Infrared lamps then pass over the whole bed and the areas with fusing agent melt. Time per layer is roughly constant, whatever the number of parts in it.

The process description is the subject of the guide What is SLS 3D printing.

Comparison table

The values are typical for the processes in general. A given supplier may do better or worse depending on machine, parameters and quality control.

Color: the most visible difference

An SLS part is white because it consists only of white powder. An MJF part is gray because the dark fusing agent is mixed into the molten material while the outer skin picks up white powder. The gray is often uneven, so MJF parts are routinely dyed black to obtain a uniform appearance.

Dye is transparent: it adds color to the base color and cannot lighten it. A white SLS part can therefore be dyed yellow, red, blue or any other shade, and it can also stay white. A gray part can only become darker. If your part needs a brand color, color coding of variants or a clean white look, SLS is the direct route. If the part will be black anyway, this difference does not matter.

One more point for white parts: untreated white PA12 can yellow slightly over time under UV light. Where a stable appearance outdoors matters, discuss dyeing or testing with your supplier.

Surface and detail

Both processes produce a matte surface with a fine grain from the powder. After blasting the two feel similar. MJF surfaces are often described as slightly smoother and edges as slightly crisper, a result of the finer layer and the detailing agent. Large flat MJF surfaces can show differences in gray tone before dyeing. In both processes the final impression depends heavily on finishing: a properly surfaced part from one process looks better than a merely depowdered part from the other.

Accuracy and warping

Dimensional accuracy is in the same range. For SLS PA12 the usual convention is ±0.3 % with a lower limit of ±0.3 mm; figures quoted for MJF are of a similar order. Both processes share the same physical limits: shrinkage during cooling, and warping of long, thin or large flat parts. Design measures are identical for both, namely even wall thickness, ribs on flat areas and no massive sections.

Mechanical properties

Parts from both processes are dense, tough PA12 with near-isotropic behavior. Published values for tensile strength and stiffness are in the same range. MJF parts tend to show a smaller difference between the build direction and the layer plane, while elongation at break varies with parameters in both processes. For a typical housing, bracket or fixture the difference is not relevant. For a part that is loaded close to its limit, test samples from the actual supplier are more informative than any general table. Background on the material is in the PA12 nylon guide.

Cost and quantity

Neither process needs tooling, and in both the price of a part follows from the build volume it occupies, its material volume and the finishing. MJF machines have a constant layer time and larger build units, which favors densely packed production builds. SLS machines exist in many sizes; a compact system can be filled and started sooner with a mixed batch of smaller orders. Which one is cheaper for a specific part depends on the supplier's machine, utilization and pricing model, so compare actual quotes for your geometry and quantity.

How to choose

SLS at SLS Market

We print white Nylon PA 12 on an EOS FORMIGA P 110 Velocis, blast and surface the parts in a DyeMansion Powershot DUAL and, as a separate step, dye them on request with liquid dyes from the dye maker TCN; the shades are shown in the color catalog. Scope and design limits are on the SLS 3D printing service page.