3D printed rafts: when should you use them?

A skirt is a loop of plastic printed around the part with no contact to it. It primes the nozzle and shows you that the bed is level, then you throw it away. A brim is the same loop, widened until it touches the part outline and adds roughly 5–10 mm of flat surface area to the base. Neither one sits between the part and the bed.

The raft goes underneath. The slicer builds a flat pad of plastic, usually 3–5 layers, and the part is printed on top of that pad with a small vertical air gap. Contact area between part and bed drops to nearly zero, and the pad takes the job of holding onto the plate.

That is the whole mechanical difference, and it explains both the benefit and the cost. You gain a base that grips the bed hard and spreads peel stress over a wider footprint. You pay in material, print time, and a post-process step that can scratch or dimple the bottom face.

Brim and raft are not interchangeable settings you pick by habit. Pick the one that solves the failure you actually have.

Tall, narrow parts are the classic case. A 4 mm wide, 80 mm tall column printed with no base has a tiny footprint and a high center of mass. The nozzle drag pushes at the top of that column with a long lever arm against the bed contact. Once it breaks loose, the whole print is scrap. A raft widens the footprint and the part survives the trip.

Warping materials come next. ABS, ASA, PC and large flat nylon parts shrink as they cool, and the bottom corners lift first. A raft acts as a compliant buffer: the pad shrinks with the part instead of fighting it, so the corner stays down. On parts over roughly 150 mm in the X direction, the difference is usually visible in the first hour of the print.

Thin, flat parts with sharp corners are another candidate. Nameplates, gaskets, thin brackets, and anything under about 3 mm thick with a square corner tends to curl. The pad holds the corner flat while the part cools below its glass transition temperature.

Then there is the worn or poorly leveled bed. If the first layer is inconsistent across the plate and you cannot re-level before the job ships, a 4-layer raft forgives more variation than a single-layer first layer does. That is a workaround, not a fix. Re-level when you can.

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  Tall and thinHeight-to-width ratio above about 5:1, no other support at the base.
• 2
  Warp-prone resinABS, ASA, PC, large nylon and PP parts with long straight edges.
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  Sharp bottom cornersParts under 3 mm thick where a curl ruins flatness.
• 4
  Unknown bed conditionOld build surface or a plate you cannot re-level before the deadline.

If the bottom face is a datum, skip the raft. The release surface leaves a rough, matte texture and the air gap often prints a visible step at the part edge. On a mating face with a flatness or seal requirement, that texture means a facing operation before the part is usable. Printing on a clean PEI sheet or a glass plate gives a much flatter bottom face.

Small parts with a lot of detail on the underside are also poor raft candidates. Removing the pad around fine features takes time with a deburring knife, and it is easy to snap a 1 mm post or a thin rib. Print those on a textured sheet with a brim instead, or add a breakaway interface of soluble material if the printer has two extruders.

Tight tolerance work is another mismatch. Rafts add z-height variation and can shift the part slightly as the pad cools, so a ±0.1 mm feature across the base is not reliable. For anything held to ±0.05 mm, printing is a roughing step and CNC finishing sets the final dimension. We hold ±0.005 mm on 5-axis work for that reason.

Finally, consider the material cost on long runs. A raft on a 100 × 100 mm part typically adds 15–30 g of plastic and 5–15 minutes of print time. Across 200 units, that is real money for a base you cut off and throw away.

The air gap is the one setting that decides whether the raft works. It is the vertical distance between the top of the pad and the bottom of the part. Too small and the two layers fuse; you will pull chunks out of the base trying to separate them. Too large and the part's first layer has nothing to sit on, so it droops or peels off mid-print.

Start at 0.2 mm for a 0.2 mm layer height, then adjust. PLA tolerates a slightly larger gap because it bonds less aggressively to itself when cool. ABS wants a smaller gap, around 0.15–0.18 mm, because it needs the extra grip to resist warp. PETG sits between the two and releases well when the gap is right.

Pad thickness matters too. Three layers is the minimum for a stable base; four to five layers resist curling better on large prints. Below three layers the pad curls with the part and stops helping.

Keep the pad dense. A 75–90% infill on the raft gives the part a solid surface to bond to. A sparse raft flexes under the nozzle and the first layer of the part ends up uneven.

• 1
  PLAAir gap 0.20–0.25 mm, 3 layers, releases with light hand pressure.
• 2
  PETGAir gap 0.18–0.22 mm, 3–4 layers, cut with a knife at the edge.
• 3
  ABS / ASAAir gap 0.15–0.18 mm, 4–5 layers, grip matters more than easy release.
• 4
  Pad infill75–90% to keep the top surface flat under nozzle load.