Effect of Peak Cavity Pressure on Unreinforced Polyamide Linear Shrinkage Rates
Elevating peak cavity pressure compresses unreinforced polyamide melt, suppressing specific volume and reducing post-moulding linear shrinkage rates.
Moisture Absorption
Meaning
The tendency of certain polymer resins to extract and hold water molecules from the surrounding atmosphere alters both processing behavior and molded part properties. When moisture absorption occurs in hygroscopic materials like polyamide or polycarbonate, the trapped water can cause polymer chain degradation during melting. This hydrolytic degradation reduces the molecular weight and weakens the mechanical strength of the plastic.
Desiccant dryers are required to extract this moisture before the resin enters the molding press.
Processing Defect
High water content in the melted plastic causes surface cosmetic defects such as splay or silver streaks. As the polymer enters the heated barrel, the moisture expands rapidly into steam bubbles that smear along the mold surface. These visual marks cannot be polished out and usually require scrapping the affected parts.
In addition, steam bubbles disrupt melt density and lead to erratic part weights.
Material Behavior
Hygroscopic resins must be dried to specific limits defined by the raw material manufacturer. If the moisture level is above these limits, viscosity drops drastically, which leads to mold flashing. Conversely, over-drying the polymer can increase viscosity and cause short shots.
Post-Molding Effect
Molded polyamide parts will continue to undergo moisture absorption after they have been ejected from the tool. This post-molding hydration causes the polymer to swell and increases its impact strength while reducing its tensile modulus. Parts must be allowed to reach equilibrium before dimensional inspections are completed to ensure accurate measurement.
Failure to account for this change can result in assembly misalignment during final production.