Under Ember Prototypes’ MJF vs SLS vs SLA comparison, a viewer who buys from low-cost bureaus said the biggest problem was consistency: the same product came back with “wildly different results” from order to order. The channel’s answer was blunt. Those shops “often pack the builds in different orientations so batch to batch your parts can vary quite a bit.” Another viewer added that his screw holes were sometimes tight and sometimes barely gripped. That’s the MJF batch consistency problem in a nutshell, and it’s mostly about how the part is placed in the build.
Why does MJF batch consistency vary between orders?
Because orientation and position in the chamber change the part, and a bureau packing for density will put your file wherever it fits. HP’s own design guidelines (white paper 4AA7-0402ENW) say it directly: “For maximum repeatability, try to keep the same printing conditions.”
What changes when the orientation changes?
Accuracy, first. HP says accuracy is better in the XY plane than in Z, and that features needing the highest resolution should sit in XY. So a hole that printed horizontally in one order and vertically in the next can easily fit differently.
Mechanical behavior shifts too. The guidelines state that the best mechanical properties are achieved in the XY plane. HP’s PA12 datasheet lists the same 48 MPa tensile strength in XY and Z, but elongation at break drops from 20% in XY to 15% in Z. A snap fit that flexes happily in one orientation can crack in the other.
Does position in the bed matter?
Yes. HP recommends placing critical parts in the center of the bed, keeping solid, massive parts close to the center, and leaving at least 2 mm between parts. A part that lands near a chamber wall in one build and dead center in the next won’t see the same thermal history.
How do you get the same part every time?
HP’s advice for production is to run a small job first with the part in different orientations, pick the best one, and keep using it. The video’s creator made the same trade-off point: cheap services are fine if your design tolerates variation, otherwise pay a supplier that commits to printing the part the same way.
When you qualify a supplier, send a drawing that names the build orientation and asks for center placement on critical features. Then put a go/no-go gauge for your tightest hole in the incoming inspection, so a switched orientation shows up on day one instead of in assembly.
Related on the Solvi blog: MJF Warping: Why Large Flat Parts Warp; MJF Tolerances: Clearance for Holes and Nut Pockets; MJF Packing Density: How Full Should You Pack a Build?.
Sources: YouTube: Comparing MJF vs SLS vs SLA for our product (Ember Prototypes), viewer comments and replies; HP: Design and printing guidelines for HP Jet Fusion (4AA7-0402ENW); HP 3D High Reusability PA 12 datasheet.