MJF and SLS both build nylon parts in a bed of powder with no support structures, and both are priced as a share of a build. The difference is how the powder is fused: SLS traces each layer with a laser, MJF jets agents onto the powder and fuses the whole layer under infrared lamps. In practice MJF is often cheaper per part in production quantities and comes out grey, usually dyed black, while SLS offers more materials and colours and is available from more suppliers and on smaller machines.
Key points
- Both processes are priced by build share: the part pays its fraction of a whole build plus its own powder and handling.
- MJF fuses whole layers at a time, so time per layer depends less on how much is in the build.
- MJF adds agents and printhead wear as consumables; SLS has laser time that grows with scanned area.
- Many MJF powders are specified for lower refresh ratios than typical SLS PA12, which cuts powder cost.
- SLS offers a wider range of materials and natural white parts; MJF parts are grey and usually dyed black.
- For one-offs, the choice matters less than whether your part can share a build.
How do MJF and SLS work?
SLS (selective laser sintering) spreads a thin layer of polymer powder, heated close to its melting point, and a laser traces the cross-section of every part in that layer. The bed drops, the next layer is spread, and so on. Unfused powder supports the parts, so no support structures are needed.
MJF (Multi Jet Fusion, from HP) spreads powder the same way, then an inkjet head prints a fusing agent where the part should be and a detailing agent around its edges. Infrared lamps pass over the layer; the areas with fusing agent absorb more energy and fuse. SAF (Selective Absorption Fusion, from Stratasys) works on a similar principle. For pricing purposes, SAF behaves like MJF.
How do they compare?
| SLS | MJF | |
|---|---|---|
| Fusing | Laser traces each part’s cross-section | Agents jetted, whole layer fused by lamps |
| Time per layer | Recoat plus laser time, grows with area to scan | Recoat plus a fixed lamp and print pass |
| Common materials | PA12, PA11, glass, carbon and mineral-filled grades, TPU, PP; high-temperature polymers on specialised machines | PA12, PA11, PA12 glass beads, TPU, PP |
| Native colour | White or off-white | Grey, usually dyed black |
| Surface | Slightly grainy, matte | Slightly grainy, matte; often described as a little finer |
| Fine detail | Good; depends on laser spot and powder | Good; detailing agent sharpens edges |
| Mechanical uniformity | Some anisotropy, varies with machine and settings | Often described as more uniform across directions |
| Machine range | Desktop to large industrial | Industrial, fixed-size build units |
| Consumables beyond powder | Laser and optics maintenance, inert gas | Fusing and detailing agents, printheads |
| Cooling | Slow in the cake; in the machine or a separate unit | Natural or fast cooling, often in a separate build unit |
Dimensional accuracy is similar for both and depends heavily on part size, wall thickness and how well the supplier controls cooling. Ask for the supplier’s own tolerance statement and treat generic figures with care.
What drives the cost of each?
Both processes use the same cost model, explained step by step in SLS pricing and nesting: cost the whole build, then charge each part its build share by the space its cell takes, plus its own powder and per-part labour. The differences are in what the build costs.
Time. On SLS the laser has to trace every cross-section, so a densely packed layer takes longer than a sparse one. On MJF the print and lamp passes cover the whole bed regardless, so time per layer is closer to fixed and builds are usually run to the full height of the build unit. That is a reason MJF tends to win on dense production builds.
Powder. Unsintered powder ages and has to be blended with virgin at a powder refresh rate. For SLS PA12 that is often in the 30-50 % range. Some MJF powders are specified lower: HP’s data sheet for HP 3D High Reusability PA 12 gives a refresh ratio of 20 % for stable performance (data sheet). Other MJF powders have different figures, so use the one for your material.
Consumables. MJF uses fusing and detailing agents, which you can charge per cubic centimetre of part and per square centimetre of layer area, and printheads that wear out per build. SLS has no agents, but lasers, optics and filters need maintenance, and the chamber is usually filled with nitrogen.
Cooling. Both need a slow cool to avoid warping. MJF systems commonly offer natural or fast cooling in a separate unit; fast cooling shortens lead time but may affect dimensional stability. It is reasonable to price the two as different lead-time options.
A worked example: what the refresh ratio is worth
Worked example: powder written off per build at two refresh ratios (example figures, same powder price for comparison)
| Line | Calculation | Amount |
|---|---|---|
| Unsintered powder in the cake | Example build | 9 kg |
| Written off at 30 % refresh | 9 kg × 30 % × €55.00/kg | €148.50 |
| Written off at 20 % refresh | 9 kg × 20 % × €55.00/kg | €99.00 |
| Difference per build | €148.50 - €99.00 | €49.50 |
| Over 100 builds a year | €49.50 × 100 | €4,950.00 |
Real powder prices differ between MJF and SLS suppliers, and the refresh ratio you can run depends on your packing density and quality requirements. The point is that refresh is a line in the build cost, and comparing processes without it compares the wrong numbers.
Do the design rules differ?
Less than you might expect. Both processes print without supports, so overhangs and internal features are free, and both share the same main hazards:
- Trapped powder. A closed hollow traps unfused powder inside. Add escape holes, usually at least two, large enough for the powder to be blown or blasted out.
- Minimum walls. Thin walls can warp or break during de-powdering. Suppliers often quote a minimum in the region of 0.7-1 mm for nylon, with more for long unsupported panels.
- Clearance. Moving parts printed in place need a gap so they do not fuse together. MJF’s detailing agent can help keep edges sharp, but follow the supplier’s figure for either process.
- Thick sections. Large solid blocks hold heat, shrink more and can sink or warp. Shelling or ribbing them saves material and improves accuracy.
A design that suits one usually suits the other. Our design for 3D printing guide covers these rules with the reasons behind them.
What about smaller SLS machines?
SLS is available on compact and benchtop machines as well as large industrial systems, which MJF is not. For a shop with modest powder volumes, a smaller SLS machine can be easier to keep full, and a full machine is what makes build-share pricing work. The cost model is the same; the build cost and the envelope are simply smaller.
Which should you choose?
| Choose | When |
|---|---|
| MJF | Black PA12 or PA11 parts in volume; dense builds of production parts; uniform properties matter |
| SLS | You need a material or colour MJF does not offer; white or brightly dyed parts; a larger part than the MJF build unit; a desktop or mid-size machine fits your volume |
| Either | Functional prototypes, housings, brackets, clips and snap fits in PA12 |
| Neither | One very large, simple part (FDM may be cheaper); a smooth cosmetic surface (SLA or finishing needed) |
For a buyer, the honest answer is often that both are fine for a functional nylon part, and the price and lead time decide. For a shop, the decision is about utilisation: a powder machine earns its rate when its builds are full, so the process you can keep busy is the one that should get the work. If the part could be made another way, FDM vs SLA vs SLS compares the wider choice, and how to calculate 3D printing cost shows where the build cost sits in the full price.
Quotometry prices both SLS and MJF on the same build-share core, with agents, printhead wear and a fast or natural cooling choice for MJF. The MJF process page and the SLS process page list what a shop configures for each.
Questions people ask
Is MJF cheaper than SLS?
Often per part in production volumes, because MJF fuses a whole layer in one pass and many MJF powders are run at a lower refresh ratio. For one-offs and small batches the difference narrows or reverses, since both processes cost a build and a part that cannot share one carries most of it. Compare the build cost and nesting you actually achieve on each, not list prices.
Are MJF parts stronger than SLS parts?
In the same material, such as PA12, the two give broadly comparable strength. MJF parts are often described as more uniform between the horizontal and vertical directions, while SLS results depend more on the machine and its settings. For a critical part, test coupons from the actual supplier and machine rather than relying on a generic comparison.
Why are MJF parts grey?
MJF jets a dark fusing agent onto the powder so that the lamps heat it. The agent stays in the part, which comes out grey. Most suppliers dye MJF parts black as standard. SLS parts come out white or off-white and can be dyed in a wider range of colours.
When should I choose SLS over MJF?
Choose SLS when you need a material MJF does not offer, such as some filled grades or high-temperature polymers on specialised machines, when you want white or coloured parts, when a large part needs a bigger build envelope than the MJF machine has, or when your supplier only runs SLS. For black nylon PA12 or PA11 parts in volume, MJF is often the cheaper route.
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