Up to this point our discussion of dimensional stability has focused on those influences that cause parts to get smaller. But there are environmental factors that also cause parts to increase in size over time. The best example of this is the dimensional growth that occurs when parts molded in nylon absorb moisture from the atmosphere. Many polymers are hygroscopic; they absorb water. Since water vapor is always present in the atmosphere, this is the usual source of the water that becomes absorbed into the polymer. Most hygroscopic materials under normal atmospheric conditions can absorb 0.1-0.2% water over an extended period. But nylon, because it contains hydrogen bonding, attracts water to a much greater degree. At room temperature in a "normal" environment where the relative humidity is in the range of 35-65%, the equilibrium moisture content for an unfilled nylon will hover around 1.5-2% by weight. If continually immersed this can increase to 5-8%.
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