Facts about plastic flow that help explain shear imbalances:
- Plastic viscosity is affected by shear rate
- As shear rate increases, viscosity decreases (non-Newtonian)
- Plastic viscosity is also affected by temperature
- As temperature increases, viscosity decreases
- Plastic has a laminar flow. No turbulence!! Reynolds numbers are often less than 10. Full turbulence requires a Reynolds number of 4000. Transition zone 2300-4000. Highest Shear Rate is just inside the frozen layer
- Highest Shear Rate is just inside the frozen layer. Shear-thinning and shear-heating reduces viscosity in these laminates.
- Due to the laminar flow, when plastic encounters a feature such as a intersection in a runner system, the hotter, less viscous plastic hugs the corner and travels along the outside wall, creating a cavity to cavity imbalance.
- Cross sections of runner channels showing position of higher sheared, hotter, less viscous material in red. Material that has experienced less shear heating is marked with blue.
Mold Number: LinkedIn
- The red lines again represent the higher sheared, less viscous material; the blue lines shows the less sheared more viscous material. Various melt properties will be distributed to and across the cavities. The various melt properties will result in 2 Flow Groups (FG) as shown below.
- Cavities that receive the high sheared, less viscous material will fill before the cavities that will receive the low sheared, more viscous material.
- Dimensional variations, due to volumetric and orientation induced shrinkage differences, may result between the cavities as a result in the variation the melt distribution properties