ISO 25178 Areal Surface Texture Parameters for Optical Inspection
ISO 25178 defines 3D areal surface parameters like Sa and Sdr, replacing 2D profiles to qualify optical non-contact inspection on textured tool steel and mouldings.
Polymer Viscous Flow
Meaning
Non-Newtonian melt deformation during the injection stroke describes how macromolecular fluids yield and flow under applied thermal energy and shear stress inside mold cavities. When filling complex mold geometries, polymer viscous flow controls pressure drop and shear heating throughout the runner system. The rheological model stops applying once melt temperature drops below the glass transition or crystallization point where solid elastic behavior takes over.
Shear Thinning
Molten polymers exhibit pseudoplastic behavior where viscosity drops dramatically under increasing shear rates. During high-speed injection, polymer viscous flow allows high molecular weight resins to pass through narrow gates without requiring excessive hydraulic pressures. Entangled polymer chains untangle and align along flow streamlines, reducing internal friction within the moving melt front.
Mold filling simulation software models this non-linear viscosity response using Carreau-Yasuda or Cross-WMS equations. Excessively high shear rates induce thermal degradation and material discoloration.
Pressure Loss
Melt channel resistance creates substantial hydraulic pressure drops between nozzle and cavity end. Characterizing polymer viscous flow enables tooling engineers to size runners correctly to prevent premature gate freeze-off.
Processing Window
Moulders adjust barrel temperatures and injection velocities to maintain stable viscous flow across production runs. Variations in lot-to-lot melt flow index disrupt polymer viscous flow, leading to short shots or flash defects in molded components. Incorporating post-consumer regrind alters shear response, necessitating real-time pressure compensation adjustments on advanced injection molding machines.
Controlling melt rheology ensures dimensional consistency in high-volume production.