GlossaryKind of term: Manufacturing termPolyamide (PA6, PA66, nylon)
What is polyamide?
Polyamide (PA) is a tough, wear-resistant engineering plastic that absorbs moisture, so its dimensions can still change after machining.
Polyamide (PA), commonly called nylon, is a family of engineering plastics; the common grades are semi-crystalline. PA6 and PA66 are the usual grades in rod, plate and tube; cast PA6 comes in larger sizes, and PA12 absorbs noticeably less moisture. PA is tough, wear-resistant and takes knocks well, and is used for plain bearings, rollers, gears and guides. It takes up moisture from the air, which changes its size, stiffness and toughness.
What does dimensional change in polyamide depend on?
How much a dimension moves depends on the grade, the moisture content when machined, the humidity and temperature of the surroundings, the wall thickness and time. On a free part every dimension grows, a bore included. A bush pressed into a metal housing cannot grow outwards, so its bore closes in; that is how a running fit on a shaft can start to bind. Polyamide also expands far more with temperature than metal. When machined it gives long, tough chips and can leave a burr; sharp tools and good chip evacuation keep the heat down.
How does polyamide work in practice?
A tight dimension in polyamide is only unambiguous with an agreed condition in which it is measured, for example as received and straight after machining, or conditioned in a standard atmosphere. To hold it you can rough, rest and finish, adapt the design or tolerance to the growth, or choose a more stable material. The options differ in route, lead time and material price; which makes sense depends on what the dimension does and where the part operates.
What is polyamide often confused with?
Polyamide is not POM: POM takes up far less moisture and is therefore more stable; conditioned polyamide is also less stiff than POM, but tougher under impact. Nor is cast PA6 the same as extruded rod: the cast grade comes in larger sizes with somewhat different properties; compare the data sheets.
Frequently asked questions
Why does a dimension in polyamide change after machining?
Polyamide absorbs moisture from the air and swells, over days to months. A free part grows in every dimension; a pressed-in bush cannot grow outwards, so its bore gets smaller. Agree the condition in which the dimension applies.
When do you choose polyamide over POM?
When toughness under impact and wear resistance matter more than dimensional stability, as with a roller or a sprocket. If a dimension must stay tight under changing humidity, POM is often the safer choice.
How do you hold a fit in polyamide to size?
By machining in two stages with a rest between, by designing the fit for the growth, or by agreeing the moisture condition with the customer. That choice decides route and lead time.