Glass Filled Polymer Injection Tool Wear Mechanisms
Glass filled polymer injection tool wear stems from fiber micro-ploughing and matrix erosion, requiring hard powder metallurgy steels or duplex PVD coatings.
Runner Wall Washing
Meaning
The high-velocity flow of molten polymer that removes or displaces the solidified skin layer along the channel walls of an injection mold runner system during the filling phase influences the distribution of additives and pigments. During plastication and molding, runner wall washing occurs when the shear forces generated by the flowing core exceed the adhesion strength of the frozen layer against the steel wall. This behavior alters the thermal profile of the entering melt by mixing colder polymer from the walls back into the main stream.
The phenomenon is most pronounced in high-flow, low-viscosity resins during high-speed injection.
Flow Dynamics
Velocities at the center of the flow channel are high, while the velocity at the runner wall is zero, creating a steep velocity gradient. This gradient generates shear stresses that act on the frozen skin layer of the polymer. When injection speeds are too high, the shear stresses overcome the boundary layer, dragging the solidified material into the cavities.
Shear Heating
Frictional heating occurs within the high-shear region next to the wall, which reduces melt viscosity. This viscosity reduction makes the skin layer more susceptible to washing and alters the subsequent flow behavior.
Surface Quality
Component surface defects like jetting or gate blush can arise when the washed skin enters the mold cavity. Maintaining correct melt temperatures prevents these aesthetic failures and improves part consistency.