Anisotropic Shrinkage Variance in Glass-Filled Polyamide Secondary Tooling Nests
Secondary tooling nests for glass-filled polyamide accommodate anisotropic shrinkage through floating datum pins aligned with transverse contraction vectors.
Post-Mould Moisture Swell
Meaning
Dimensional change occurs when a hygroscopic polymer absorbs water from the surrounding atmosphere after the moulding process is complete. Controlling post-mould moisture swell is necessary for parts made from polyamides or other polar resins. The material expands as water molecules penetrate the amorphous regions of the polymer matrix.
Hygroscopic Expansion
Physical volume increases in direct proportion to the mass of water absorbed by the part. Because post-mould moisture swell follows a predictable saturation curve, engineers can calculate the final dimensions of a component in its working environment. This expansion often compensates for initial mould shrinkage.
The rate of change depends on the ambient humidity and the thickness of the part wall.
Environmental Impact
Storage conditions influence the speed at which a part reaches equilibrium. When post-mould moisture swell is not accounted for during the design phase, assemblies may bind or fail. Dry as moulded parts are at their smallest and most brittle state.
Gradual moisture uptake increases ductility while decreasing tensile strength. This transition changes the functional performance of gears and snap fits. A part that fits perfectly in a dry assembly area might seize up after a week in a humid warehouse or shipping container.
Tolerance Management
Precise tooling must be cut slightly smaller to accommodate the eventual growth of the material. By understanding post-mould moisture swell, the manufacturer ensures the part remains within specification for its entire service life. Accurate measurement requires parts to be conditioned at a specific temperature.