Meaning
Dimensional contraction that occurs in a molded plastic part after it has been ejected from the cavity and cooled to ambient temperature. In semi-crystalline resins like polyamide or polyoxymethylene, post mold shrinkage continues for hours or even days as the polymer chains slowly reorganize into more stable crystalline structures. This ongoing change can cause the part to shrink below the specified design limits, leading to fitment issues in multi-component assemblies.
This phenomenon represents a major challenge for high-precision molding applications where tight dimensional tolerances must be maintained over the lifespan of the product.
Material Variable
The magnitude of this dimensional drift is highly dependent on the crystallinity of the resin and the presence of fillers. Semi-crystalline resins exhibit much higher levels of post-mould contraction than amorphous resins because of the ongoing crystallization process after cooling. Glass fibers and mineral fillers can significantly reduce this effect by acting as physical barriers that restrict the movement and contraction of the polymer chains.
However, this reinforcement can also introduce anisotropic shrinkage, which causes the part to warp if the fiber orientation is not uniform.
Process Influence
Mould temperature and cooling time are the primary process variables that determine the extent of this subsequent contraction. Running a cold mould freezes the polymer chains in a highly disordered, amorphous state, which minimizes the shrinkage that occurs within the tool but increases the potential for subsequent contraction when the part is exposed to heat. Conversely, a hot mould promotes more complete crystallization during the molding cycle, resulting in higher initial shrinkage but much greater dimensional stability over time.
This trade-off requires process engineers to balance the cost of longer cycle times against the benefit of long-term dimensional consistency.
Environmental Effect
Subsequent exposure of the finished part to elevated temperatures or moisture can accelerate the relaxation of internal stresses and the completion of the crystallization process. This environmental exposure causes the part to undergo rapid dimensional changes that can lead to assembly failures or mechanical stress. For hygroscopic materials like polyamide, moisture absorption can actually counteract some of the contraction by causing the polymer to swell, illustrating the complex interactions that must be managed to ensure long-term dimensional control.
