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There are two types of roller ball pens: those that use a liquid ink and those that use a gel containing ink. A roller ball has two advantages over a ball point: first, less pressure needs to be applied to the pen to have it write cleanly. This permits holding the pen with less stress on the hand. Second, the ink is brighter. Gel roller ball pens use a jelly-like ink: the ink sets just after it is applied to the paper and is no longer being "rolled" around. Gel inks do not sink into the paper as much as liquid ink. Also, gels carry color better that liquid ink, so gel based pens are available in a brighter and wider range of colors than liquid ink pens.
Disadvantages
There are two disadvantages inherent to roller ball pens: first, the ink is more liable to smudge than a ball-point pen's ink because the ink dries more slowly, and second, the ink may seep through the paper. Thicker paper must be used with a roller ball pen than with a ball point, because the oil based ink does not penetrate deeply into the paper. The problem with gel inks is that they do not flow as readily as a liquid ink. This increases the likelihood of intermittent inking, where the flow of ink will randomly cease.