Skirts are generally the most common support use. A 3D print skirt is essentially a brim that doesn’t touch the edges of the object that you’re printing.
Skirts have a couple of uses and are usually just a couple of layers thin. They are a great way to get a preview of how your equipment is working and allows for necessary calibration before an actual print.
To use the skirt to your best advantage, simply print one that’s only a few outlines wide at a relatively low print speed. This will allow you to get a good look at how the material is flowing. If the skirt prints cleanly, then you let the print continue. If it doesn’t, you can abort the print and make the adjustments accordingly
Skirts can also be useful as thermal barriers. One way is printing them wide over the bed, to insulate it and keep the temp up where your print it.
A 3D printed raft is essentially a disposable horizontal surface that sits under your object. It’s made up of a pre-programmed number of layers, with a specific infill percentage, that only extends a specific distance away from the sides of your print.
The raft is there to help with bed adhesion. The raft is printed first to the specified layer depth, then the 3D print is printed on top of the raft.
Due to its larger surface area, the raft significantly improves adhesion. It also reduces warping in the 3D print itself.
A raft can also be used to provide greater stability to 3D prints that smaller. It can also give a 3D print a solid foundation. This becomes important when printing larger objects or objects that may tend to be top heavy.
Rafts can also be used to compensate for any tiny inaccuracies in the build plate calibration or even-out warped heat beds, scratches or dents.
A brim is very similar to a raft.
What separates the two is that a brim only extends outward from the perimeter of an object whereas the raft has complete contact with the prints underside.
Both rafts and brims help with adhesion and both can be used to stabilize objects that have very small contact points with the print surface.
Some materials require brims more than others. For example, ABS can warp if you’re not using the correct printing settings or bed surface (or just using cheap ABS). So, it may be worth it to 3D print brim around ABS parts to assist with adhesion.