The Shear stress at wall (overmolding) result shows the shear stress, in the overmolded part, at the plastic frozen/molten interface at the time the result was written.
- Overmolding Fill
This result is an intermediate result, meaning that a certain number of results are written by default during overmolding fill, and it relates to the guideline provided in the material database.
Using this result
While shear stress is not the actual residual stress in the overmolded part, it is related to it. It is a measure of the factors effecting the degree of orientation of the melt next to the frozen layer. Orientated materials tend to shrink more than unoriented materials, so a large amount of orientation near the edge of the melt compared to near the center will lead to higher residual stress. Higher residual stresses can result in overmolded parts stress–cracking during ejection or in service.
Wall-shear stress is the shear force at the frozen/molten interface, per unit area, and is proportional to the pressure gradient at each location (If the polymer cross-section is completely molten, the frozen/molten interface is at the mold wall). Under the viscous flow formulation, shear stress is zero at the center of the cross–section and linearly increases to the frozen/molten interface. Thus, the wall-shear stress could be at its maximum value at any portion of the cross-section.
The shear stress should be less than the maximum recommended for the material in the material database. The shear stress can be compared directly with the values stored in the material database. Regions above this limit could be subject due to stress-cracking during ejection or in service.
Shear stress is also an indirect indication of the degree of molecular or fiber orientation. Higher shear stress would induce higher orientation, especially near the surface of the overmolded part. A more precise prediction of fiber orientation can be obtained from a Fiber analysis.
Things to look for
- Check the wall shear stress is less than that recommended for the material.
- Slower flow rates may reduce the maximum shear stress.
- Changing the material to a less viscous material or increasing the melt temperature will also reduce shear stress.