The RepRap is a 3D printer that can print plastic objects. There are different type of plastic material (PLA, ABS, HDPE) and it can be granular or in filament, that is the most used.
Here the plastic is melted and then push out through the nozzle. The filament is then pressed between the bed and the nozzle, it stick itself to the bed because of the temperature and solidify quickly.
The RepRap builds object putting plastic levels one upon the other. Once put down the first level, the extruder pass over it again in the following levels, superimposing at the first solid level, another plastic layer and so on, giving in the end a solid object.
Because of this layer technique, you have to think well about the orientation to give to your object. In fact the objects will have different mechanical and aesthetic features depending on the print direction. The levels could be a weak point if pressed constantly by a perpendicular force.
For what we said before it’s clear that isn’t possible to print objects with overhangs. In fact the level, at one point, couldn’t be pressed over the previous level. To bypass this inconvenient you can use support material, that you can remove at print ended. So you can print any object, even if it is hollow.
Another problem that you could meet during the print is warping. This is when the objects angles raise up from the bed, deforming the item. This happen because the angles cool off more quickly than the rest and incline to detach from the bed.
So to have a right print, the pieces have to remain attached to the bed. The different type of plastic behave in different ways. PLA is little subject to warping, and to resolve the problem it’s enough to use a particular scotch. For ABS the best method is the heated bed. With both material it can be also used the raft, a sort of base printed before the object and were the object will lean on.