How do you design a raft in CAD instead of the slicer?
Rafts are useful. They protect the first layer, soak up a bed that isn’t quite level, and hold the part down without a brim to trim. Slicer rafts are another story. A custom raft for 3D printing, modeled in CAD, prints the same on your machine, a print farm, or a blind print service, because it’s part of the file.
Applies to
- Process
- FDM / filament
- When
- Bed adhesion and first-layer protection, especially on warp-prone parts
- Materials
- Especially high-warp plastics like ABS
- Slicer
- Slicer raft OFF; the raft is in the model
Why it matters
Slicer rafts are vague. How thick is the top layer? The first layer? How far apart are the lines? Every slicer answers differently, and so does every machine’s cooling. A part you proved on your printer can come out different somewhere else.
That gets worse when nobody’s watching. Send a file through an API or an online print service and it’s sliced and printed blind. You want the result locked in the file.
The raft itself is often built wrong, too. Many slicers lay the base as fat rastered beads. In high-warp plastic like ABS, a fat bead shrinks and peels the raft off the bed. The upper layers are often spaced too far apart, so they’re stringy, leave junk on the part, or let the part come loose from the raft altogether.
Model it and you decide all of that once.
What good looks like
- The raft is a thin plate in CAD, not a slicer setting
- It sits about one layer height below the part so it bonds but releases
- Outer edges are rounded; a disc under the part is ideal
- The part’s own vertical edges are filleted, with no sharp corners on the first layer
How to apply it
- Fillet the vertical edges of the part. Sharp corners are where first layers warp.
- Model a thin plate under the part.
- Space it one layer height below the part: 0.2 mm at a 0.2 mm layer. Go a little less for more grip.
- Round the raft’s outer edges. If the footprint allows, make it a circle.
- Turn the slicer raft off, print, and check the part stays down and releases clean.
- If corners still lift off the raft, tie them down with struts. If the raft warps off the bed, add relief cuts.
Test it on your setup
- Print the same part with the modeled raft at a gap of one layer height, then at slightly less. Keep the gap that holds the part flat through the print and still pops off by hand.
Exceptions & tradeoffs
Mind you, a plain plate fixes consistency, not every raft problem. The part can still peel off the plate at its corners, and the plate itself can still warp off the bed. Tie-down struts fix the first. Relief cuts fix the second.
PLA on a good bed rarely needs a raft at all. This is for warp-prone material, odd footprints and production runs where every part has to come out the same.
Common mistakes
- Proving a part with your slicer’s raft, then sending it to a farm or service with different settingsModel the raft so it travels with the file
- A square raft with sharp corners under ABSRound the edges; a circular raft has no corners to lift
- Leaving the slicer raft on as well as the modeled oneTurn the slicer raft off; you only want one
Frequently asked questions
- How do you make a custom raft in CAD?Model a thin plate under the part, one layer height below it (0.2 mm at a 0.2 mm layer), with rounded or circular edges. Turn the slicer’s raft off.
- What gap should a modeled raft have?About one layer height. Slant 3D uses 0.2 mm at a 0.2 mm layer height and goes a bit less when they want more adhesion.
- Why are slicer rafts bad?They vary between slicers and machines, the fat base beads can shrink and peel in high-warp plastics, and loose upper layers can leave residue or let the part come free.
- Should a raft be round?Round is ideal. Sharp corners are where a first layer warps, so a circular raft has nowhere to start lifting.
What rafts do: protect the base layer, absorb bed variation, hold tolerances, hold the part down — Design Rafts in CAD | Stop Using Slicer Settings ↗ at 00:00.
Slicer rafts are ambiguous and machine-dependent, a problem for print farms and blind API/plug-in orders — Design Rafts in CAD | Stop Using Slicer Settings ↗ at 00:19.
Large rastered base beads shrink and peel in high-warp plastic; upper layers spaced too far; part can disconnect from the raft — Design Rafts in CAD | Stop Using Slicer Settings ↗ at 01:27.
Design the raft into the part as a plane, about one layer height below; fillet the part, round or circular raft edges — Design Rafts in CAD | Stop Using Slicer Settings ↗ at 02:24.
The modeled raft plate, the one-layer gap and the round edges are Slant 3D’s. They take it further with tie-down struts, relief cuts and a hemisphere interface, which have their own pages. The gap test and the note about PLA are mine.