Dimensional stability refers to the ability of polymers to maintain their size, even under varying environmental conditions. As a result, a dimensionally stable plastic shows low water absorption with low thermal expansion.
Examples of dimensionally stable plastics include polymers such as PEEK, PPS, PSU, PPSU, PEI and PET.
Moisture absorption (also known as water absorption) is the capacity of a material to absorb moisture from its environment. Plastics absorb water to a limited degree. The degree of moisture absorption depends on the type of plastic and the ambient conditions such as temperature, humidity and contact time.
Not only can dimensions change due to moisture absorption, but also material properties, such as mechanical strength, electrical conductivity and the dielectric loss factor, can be also affected.
The only polymer with zero water absorption is PTFE. Plastics with very low water absorption are polymers such as PEEK,PPS, PSU, PPSU, PEI, PVDF, PET, PPE, PP and PE. Furthermore, low water absorption is exhibited by POM, PA12, PC and ABS.
Polyamides (nylons) generally show higher water absorption than other engineering plastics. This leads to dimensional changes to finished parts, a reduction in strength, and also to changes in electrical insulating characteristics.
The coefficient of linear thermal expansion specifies the extent of the change in length of a material due to increasing or decreasing temperature. Due to their chemical structure, plastics generally demonstrate a significantly higher coefficient of linear thermal expansion than metals. This characteristic should considered in instances of:
The coefficient of linear thermal expansion of plastics can be significantly reduced by adding reinforcing fibres. By doing so, values comparable to aluminium can be achieved.
Plastics with low thermal expansion and water absorption and therefore dimensionally stable include:
