Either the brim has lifted off the plate along with the part, still attached to it, or the part has pulled free and left a perfectly bonded ring of brim behind.
Those two outcomes mean different things, and it is worth working out which one you had before adding more brim.
What a brim is actually doing
A brim is extra first-layer material printed around the part and bonded to it, so the part's effective footprint is larger than its own base. It is a lever: the peel force at a corner has to overcome the bond of the brim as well, and it has to do so at a greater distance from the point of attack.
Two things follow immediately.
A brim cannot improve a bad bond. It multiplies whatever adhesion exists per square millimetre. Multiply a poor bond by a larger area and you have a larger poor bond — which is why a brim on an unwashed plate lifts with the part in one piece, taking the ring with it.
A brim helps where it is, not where it is not. Contraction force concentrates at the corners of a footprint. A narrow brim spread evenly around a large rectangular base adds most of its area along the long straight edges, where it was never needed, and only a little at the corners, where the failure begins.
Read which failure you had
- The brim came up attached to the part. The bond to the plate failed. Wash the plate, check the bed temperature, and check the first layer is genuinely squashed. Adding brim will not help until that is fixed.
- The part detached and left the brim behind. The bond to the plate was fine; the join between brim and part failed, or the part simply peeled upward from inside the ring. This is the case where more and better-placed brim genuinely does help.
- The brim itself is only half stuck. The first layer is inconsistent across the plate — see uneven first layer, because a brim is a large, thin, single-layer object and it is the most sensitive test of plate flatness you own.
The settings that make a brim work
Separation gap of zero. Slicers offer a gap between brim and part so it snaps off easily. Any gap at all turns the brim into a decorative ring that is not mechanically connected to the thing it is supposed to hold down. If you have set a gap for easier removal, you have disabled the feature.
Width of at least 10 mm on high-contraction materials. The default of a few millimetres is sized for mild PLA cases.
Mouse ears at the corners. Small discs added to the model at each corner of the footprint, printed a couple of layers thick. They put holding area precisely where the peel starts, which a uniform brim does not, and they remove cleanly with a knife. On a large flat ABS part these outperform a wide brim of many times the material.
Consider a raft only as a last resort. A raft adds a printed base under the entire part. It uses considerably more material and time, it leaves a textured underside, and on a high-contraction material the raft itself can warp — which is why brims and mouse ears are the usual answer and rafts are reserved for surfaces that genuinely cannot be bonded to.
The honest limit
Some material and geometry combinations cannot be held down by any brim. A large flat ABS panel on an open-frame machine in a cool room generates more contraction force than the plate bond can resist, and the answer is to reduce the force rather than to keep increasing the anchor: an enclosure, a hotter plate held for the whole print, cooling turned off, and rounded corners on the model.
That is the same conclusion the warping page reaches from the other direction, and it is worth arriving at before printing a fourth brim.
Two habits that help more than brim width
Round or chamfer the base corners in CAD. A radius distributes the force a sharp corner concentrates. This is often the difference between a part that needs a brim and one that does not.
Print the part where the bed is hottest. Almost every heated plate is cooler at its edges, and a marginal part placed near a corner of a large bed is being held by the weakest part of the plate.
Materials where brims are routine
ABS, ASA, polycarbonate and the polyamides need one on anything with a sharp-cornered footprint. PETG rarely needs one but benefits on tall narrow parts. PLA usually does not need one at all, and a PLA part that requires a brim to stay down is telling you something about the plate rather than about the material.