A waterproof pen is a pen that can be submerged in water and remain functional without sustaining any damage. A waterproof pen can also, in many cases, write on damp or wet surfaces because of the nature of the ink in the pen. Waterproof pens can be designed in many ways and are typically constructed out of relatively inexpensive materials, such as plastic, rubber, or chrome. The ink used tends to be more important to its waterproof function, as many types of ink will not work well when exposed to water. The ink used in waterproof pens must be able to produce clear marks even when wet.
One tends to use a waterproof pen for one of two primary reasons. Either he works in a wet environment and his pen must function properly when wet, or he needs to write on moist surfaces without worrying about the ink running excessively. People who work outdoors in rainy or humid conditions often benefit from the use of waterproof pens.
A waterproof pen can be designed in many ways. Typically, there is no easy way to distinguish a waterproof pen from any other pen, as they are designed to look essentially the same. Waterproof pens can be designed as click pens or as pens with caps, just as other pens. It is, however, rare to see a waterproof fountain pen as the ink flows too freely and would likely diffuse into any body of water.
The materials used in making waterproof pens are often different than those used in regular pens. They are often made with or coated with rubber or plastic to keep water from reaching any parts that could rust or otherwise be damaged. Any metal parts, such as the body or any internal springs, are made out of metal alloys that will not rust. In some cases, especially with cap pens, there is some form of seal that prevents water from reaching the pen's ink source.
The ink in a waterproof pen is of particular importance as the ink specifically must be preserved and must remain usable in wet conditions. The ink, which is usually stored in a special pressurized cartridge, is generally high-quality pigmented ink. Most pens use simple dye ink, which dissolves easily in water and does not hold up in damp conditions. Pigmented ink, unlike dye ink, does not dissolve completely; parts of it remain solid and mix with the paper. This prevents it from dissolving or running when exposed to water.