Rafts, skirts and brims are structures created at the base of the bottom of a 3D print. These structures are temporary and used to improve print reliability and quality.
All three of these initially printed structures provide a starting point for prints. All three serve to prime the print head and provide a chance to observe any issues before the actual printed part begins.
Rafts provide a wider base than that of the print. The raft's wider base can help adhesion, prevent warping and act as a foundation of support to models that have a small footprint. The print of the model is gapped from the raft and while the raft adheres to the bed, it is easily separated from the print leaving a high-quality finish.
Skirts make a surrounding outline around the print before starting on its actual structure. Skirts do not touch the print nor do they provide support. They serve to prime the printer head and ensure proper flow of filament. Skirts are also used to check adhesion for any leveling issues that come up. Of the three structures, skirts take the least time and use the least amount of filament.
Brims are like a skirt that touches the print sides. They improve adhesion and provide support like a raft, but do not extend underneath the print. Brims use less filament and time than printing rafts. As with skirts, they can reveal initial problems before the print begins.
Rafts, skirts and brims do use up filament and these structures are often discarded. Despite this waste, they are justifiable since they save time and waste in printed parts while also improving quality. A whole wasted print uses more time and filament than a number of these preliminary structures.