The ball joint is the polymer industry's gift to Transformers. It is simple in concept; a ball rotates in a socket, held in position by friction, and when used in a Transformer toy's limb or other flexible area it can allow a great range of motion, as long as parts don't get in the way, because it can both hinge on any axis and rotate.
Use in toys
Ball joints may be made of many materials, but it was the use of strong yet tough plastics that allowed their economical application to toys: the ball and socket can each be cast in one piece, then snapped together. The Cyberjets of Generation 2 were an early example of ball-jointed limbs, and by the Beast Wars era it was a rare toy that didn't have at least one ball joint.
Ball joints are cheap and flexible, but they are completely dependent on friction. Since friction depends on the force squeezing two parts together, and this force may not be increased arbitrarily without making the joints impossible to assemble, ball joints are hard to make with enough friction to support the heavy parts of a large toy. In these cases more elaborate mechanisms such as ratchets must be used.
Furthermore, if a ball joint wears or stretches even a little, it quickly becomes loose, and some plastics simply aren't tough enough to make a firm joint even when new. Knockoffs frequently have loose ball joints. Joints loosened in this way can sometimes be "repaired" by applying a thin layer of epoxy or glue to the ball and/or socket.