The biggest improvements to an MJF part happen before it is printed: in how it is designed and in what you tell us about it. This note shares what we see every day as a production bureau, and how a few simple choices on your side let us orient, place and finish your parts for the best result.
By Jerry Sutton, founder and engineer, Incremental-AM.
The short version
- Tell us your A-surfaces so we can orient the part to give them the best finish.
- Avoid shallow angles on show faces. Faces under 30° to the horizontal show stair-stepping.
- Give us a hidden face to point upwards. The up-skin is always slightly rougher.
- Add fillets or chamfers to near-flat upward edges to reduce capillarity.
- Spread the mass evenly and design to resist warp.
- Use realistic tolerances and flag only the dimensions that really matter.
1. Tell us your A-surfaces
Parts are rarely printed flat. We angle them in the build to balance finish, accuracy and warp. If we know which faces are cosmetic (your A-surfaces), we can orient the part so those faces get the best possible finish, and put any compromise on faces nobody will see.
Either way works: mark them with text on the drawing, or colour them in the 3D model. You can also simply list them in your enquiry.
2. Stair-stepping: keep show faces above 30°
MJF builds parts in layers about 80 microns (0.08 mm) thick. On any surface sitting at less than about 30° to the horizontal in the build, those layers show as fine steps, known as stair-stepping.
- Where you can, design cosmetic curves and slopes so they can sit steeper than 30° in at least one orientation.
- Tell us where steps would matter, and we will orient to avoid them.
- We also have tricks to hide them. For example, we can orient a part to hold a little more heat in an area, which causes slight micro over-fusion and blurs the layer lines.
- Vapour polishing smooths stair-stepping further. See TN-005.
3. Choose your up-skin
Some area of every part has to face upwards in the build. That upward-facing surface, the up-skin, has a slightly rougher finish than the rest. If your design has a face that will be hidden in use, such as the inside of a cover or a mounting face, let us make that the up-skin. It is a free improvement to the faces that show.
4. Capillarity on near-flat up-skins
Where an up-skin sits at less than about 10° to the horizontal, the edges can show capillarity. The molten nylon is effectively sucked up into the loose powder above, leaving a slightly raised, rough edge.
Fix it in the design: add a fillet or chamfer to those edges. Even a small one makes a visible difference, and it usually looks better anyway.
5. Spread the mass evenly
Imagine your part tilted at around 45°, as it often will be in the build. Ideally the mass should be evenly distributed in that position. Big changes from thick to thin sections heat and cool at different rates, which affects both finish and warp. Where you can, keep wall thicknesses even, core out heavy sections, and use ribs rather than solid blocks.
6. Warp: the biggest issue
Warp is the most common problem with MJF parts, and it is mostly down to physics. In thin parts, the thermal centre of mass is often offset from the geometric centre. As the part cools, one side shrinks before the other, causing buckling in one or two directions. Long, flat, thin parts are the classic case.
What you can do in the design:
- Avoid large, thin, flat plates where you can. Add ribs, flanges or a slight curve for stiffness.
- Keep wall thickness even and avoid one heavy side.
- Tell us which dimensions or faces must stay flat or straight.
What we do in the build:
- Place warp-prone parts carefully in the middle of the build volume, where cooling is most even.
- Orient parts so the lighter side cools first.
- Digitally pin two identical parts together, opposing each other, so that as one cools and pulls, the other resists. The parts are joined with 1 mm diameter sprues, which we cut off only after every process involving heat (including dyeing and vapour polishing) is finished. Each sprue leaves a small witness mark, so tell us where a mark is acceptable.
This is one reason we ask what a part does and which features matter. The more we know, the more of these tools we can use.
7. Tolerances: specify what matters
HP states a typical dimensional accuracy for MJF PA12 parts of ±0.2 mm up to 100 mm, and ±0.2% above 100 mm. This is a sensible general tolerance for MJF drawings.
- Don't over-tolerance. Tight tolerances on every dimension add cost and rarely add value. Flag the few that really matter and we'll orient and check for them.
- Allow for finishing. Dyeing, vapour polishing and Cerakote all change the surface slightly. Tell us the finish when you specify a fit.
- For precise fits, such as bearing bores or press fits, design for a post-machining or reaming step, or ask us about test parts.
Drawing notes you can copy
Adapt these notes for your drawing or purchase order:
1. Process: HP Multi Jet Fusion (MJF). Material: HP 3D High Reusability PA 12.
2. General tolerance unless stated: ±0.2 mm up to 100 mm, ±0.2% above 100 mm.
3. Faces marked "A" on the drawing (or coloured in the 3D model) are cosmetic. Supplier to orient for best finish on these faces.
4. Faces marked "U" (or coloured in the 3D model) may be used as the up-skin and are non-cosmetic.
5. Witness marks from joining sprues are acceptable on faces marked "U" only.
6. Dimensions marked as critical to be inspected and reported.
7. Finish: bead blasted / black dyed / vapour polished (delete as appropriate).
8. Parts must be free of loose powder, including internal channels.
What to send with your file
- A STEP file (preferred), or a high-resolution STL or 3MF
- Your A-surfaces and any acceptable up-skin faces, marked in text or colour
- Where witness marks would be acceptable
- Critical dimensions and features that must stay flat
- The finish you want, and the quantity
- A sentence on what the part does
Not sure about any of this? Send us the file and we'll look at it with you. Our design advice is free, and you can get prices straight away from our instant quote.
Related notes: TN-001: Thread call-outs and file quality · TN-002: 3D modelled threads · TN-003: Threaded inserts · TN-005: Vapour polishing · MJF PA12 material