GlossaryKind of term: Manufacturing termMoisture absorption
What is moisture absorption?
Moisture absorption is the uptake of water by a plastic from air or liquid, which makes the material swell, so a dimension depends on its moisture condition.
Uptake continues until the material is in equilibrium with its surroundings. Stiffness and toughness change along with the dimensions. The process is reversible: in drier air the part gives the moisture back. Polyamide takes up a lot, POM, PEEK and PC little, PE and PTFE practically none.
What does moisture absorption depend on?
How much a part absorbs, and how fast, depends on the grade, any filler, temperature, relative humidity and wall thickness. Moisture diffuses from the surface inwards; the time to equilibrium grows roughly with the square of the wall thickness and ranges from days to months. Meanwhile the difference between a moist skin and a dry core can also distort the part. Glass and carbon fibres and mineral fillers absorb no water themselves, and so lower the uptake per kilo of material.
What does the standard say about moisture absorption?
Data-sheet values are usually determined to ISO 62, at saturation in water and at equilibrium in a standard atmosphere of 23 °C and 50 % relative humidity per ISO 291. A value without its condition is therefore not comparable. For a drawing dimension, ISO 1 fixes only the reference temperature; for a plastic that absorbs moisture, the moisture condition belongs on the drawing or in the agreement, for example straight after machining or conditioned in a standard atmosphere.
How does moisture absorption work in practice?
A tight tolerance in a plastic that absorbs moisture calls for a choice before you quote: a more stable material, machining in two stages with a rest between, a design that takes up the growth, or an agreement on the condition in which the part is measured. That choice affects material price, lead time or what you agree with the customer. Without an agreement the same dimension can measure right in your shop and wrong at the customer's.
What is moisture absorption often confused with?
Moisture absorption is not thermal expansion: temperature changes a dimension within minutes to hours and the dimension returns with the temperature, while moisture acts over days to months and only reverses in drier surroundings. Nor is it post-mould shrinkage, which is a moulded or cast part continuing to shrink after forming; moisture absorption is swelling from water.
Frequently asked questions
Which plastics absorb the most moisture?
Of the common engineering plastics, polyamide takes up by far the most. POM, PEEK and PC take up little, PE and PTFE practically none. For a tight dimension in humid conditions that is often the reason to change material.
How long before a plastic part is stable?
That depends on material, wall thickness, temperature and humidity, and ranges from days to months. So you agree the condition in which the dimension holds, rather than waiting for the part to settle.
Should the customer state the moisture condition on the drawing?
With polyamide and tight dimensions it is wise. If it is missing, ask before you quote; otherwise there is no way to establish when the part is right or wrong.