Standard Uniaxial Testing Deficiencies in Amorphous Polymer Characterization
Uniaxial tensile tests misrepresent multi-axial yield, physical aging, and strain-rate sensitivity in amorphous polymers, causing structural failure in real parts.
Mold Filling Flow Orientation
Meaning
Hydrodynamic alignment of polymer chains and reinforcement fibers occurs along flow streamlines during cavity filling in injection molding. Flow profiles established during mold filling flow orientation dictate the mechanical strength, thermal expansion, and volumetric shrinkage directions of the finished part. The effect governs structural performance in thin-walled molded components and fiber-filled resins, but dissipates in thick sections with long thermal relaxation times.
Velocity Gradient
Fountain flow mechanics push hot melt from the core center out to cold mold surfaces. High shear rates near the mold wall align polymer molecules parallel to the flow direction before frozen skin layers form. The central core cools more slowly, allowing aligned chains to relax back into random coils before solidification.
Higher injection speeds increase skin orientation while reducing core relaxation time.
Anisotropic Shrinkage
Aligned polymer chains shrink more parallel to the flow direction than perpendicular to it during cooling. This differential contraction causes severe part warpage, twisting, and dimensional instability after mold ejection. Fiber-filled materials exacerbate this effect because rigid glass fibers align along flow lines, increasing parallel stiffness while reducing transverse strength.
Molders adjust gate locations and fill rates to balance flow pathways and minimize differential warpage.
Processing Control
Adjusting injection speed modifies orientation thickness across the part wall.