Taken from the answer provided by @EricJohnson,
When should I use a raft, and when should I use a brim? What advantages does each have over the other?
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A raft will allow for better adhesion for the whole print as the raft attaches to the printing surface and the print attaches to the raft. Rafts go all the way under the print and consist of multiple layers, whereas a brim is only 1 layer and on the outside of the print. Rafts are normally harder to remove than brims because of the increased contact with the print.
From my own experiences, the brim does not help a lot with layer adhesion as it is only 1 layer. I normally use a raft when I need a nice looking 1st layer that is not on the bed or when there are not enough contact points.
A raft is used to prevent warping. Instead of printing directly on the build surface, parts are built on top of it. You remove and dispose of a raft post-print. The raft is larger than the part and so has more adhesion. Rafts are primarily used with ABS to help with bed adhesion. Rafts are also used to help stabilize models with small footprints (e.g. a pole), or to create a strong foundation on which to build the upper layers of a model. If your only concern is bed adhesion it is better to use a brim.
A brim is attached to a model and extends outward. Brims typically have several outlines and may be a few layers tall. Brims are often used to stabilize small parts of a model, such as legs of a table, because brims help these areas stay connected to the print bed.
The brim is only around a model and attached to its first layer, a raft is also under it.
A raft helps when the part has few points of contact with the print bed, and doesn't therefore adhere well at points within and without the part.
A brim helps when the part doesn't adhere well around the perimeter of the part.
There are very rare situations where you'll need both, but typically you'll only use one or the other.