Why Does an MJF Part's Z-Axis Tolerance Run Looser Than Its X-Y Tolerance on the Same Build?

Because the Z axis in MJF is built from stacked powder layers that each carry their own small thermal and fusion variation, while X-Y accuracy within a layer is controlled directly by the print head's positioning system. Height accumulates error layer over layer. A single layer's X-Y position doesn't.

What's actually different about the two axes

In Multi Jet Fusion, the print head deposits fusing and detailing agent across each layer with the same positioning precision the whole build uses, so X-Y dimensions within a single layer are governed mostly by that positioning accuracy and by how cleanly the powder fuses at the boundary of a feature.

Z height is a different problem. It's the sum of every individual layer's actual thickness, and each layer's thickness depends on powder bed leveling, local thermal gradients across the build, and how consistently the fusing energy penetrates that specific layer.

Small variation per layer doesn't matter much on its own. It compounds across however many hundred layers a tall part needs, and that accumulation is what shows up as a part running longer or shorter in Z than the model called for.

Why this gets worse on taller parts and denser builds

A short part has fewer layers to accumulate error across, so its Z tolerance looks tighter in practice even without any change to the process. A tall part run in a dense build, packed tight with other parts through the same thermal zone, sees more local heat variation between layers than an isolated short part would. If you need the tightest dimension on a design to land somewhere, that's the wrong axis to ask for precision from.

What to actually do about it

Orient the part so its most tolerance-sensitive dimension lands in X-Y rather than Z whenever the geometry allows it. If a dimension absolutely must be held tightly and it can only be measured along Z, a stack height or a snap-fit engagement depth, plan for it to land at the loose end of whatever tolerance band you're working with. Design the mating feature with enough clearance to absorb that.

Don't try to tighten it by printing hotter or slower. That doesn't change the fundamental layer-accumulation mechanism, it just shifts where in the band the part lands.

When MJF isn't the right call at all

If a part genuinely needs tight Z control across its full height, think a long shaft-like feature or a precision spacer stack, a printed process is the wrong tool regardless of which one you pick, and a machined part is the better answer for that dimension. That's not a knock on MJF. It's a geometry-dependent process limit, the same way you wouldn't ask a laser-cut sheet metal part to hold a bore tolerance a reamed hole gives you.

DigiForge runs industrial SLS and MJF with a tolerance of ±0.3 mm or ±0.3%, whichever is greater, across MJF and SLS nylon, glass-filled nylon, PA11, full color, polypropylene and TPU. If your design needs tighter control specifically along the build height, say so when you quote it and we'll tell you honestly whether printing or machining is the better fit for that feature.