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Eight-Cavity LinkedIn Mold Answer and Explanation

Original Question: Assuming steel is uniform which cavities will fill first?

Understanding Rheology

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.
    picture5 picture6

    Mold Number: LinkedIn
    Current Conditions:

  • 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.picture7


  • 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


Answer to

LinkedIn Mold:

Cavities 2,4,5,and 7

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