Shear stress at wall result

The Shear stress at wall result shows the shear stress at the plastic frozen/molten interface at the time the result was written.

Mesh type:

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Analysis sequences that include:

• Fill

This result is an intermediate result, meaning that a certain number of results are written by default during fill, and relates to the guideline provided in the material database.

Using this result

While shear stress is not the actual residual stress in a 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 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.

Note: Thermoset materials do not have maximum recommended shear stress values in the material database.

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 part. A more precise prediction of fiber orientation can be obtained from a Fiber analysis.

Tip: To determine where and when high stress occurs for Midplane and Dual Domain results, you can set the scale with the minimum value to equal the shear stress limit, set the part to transparent, and deselect Nodal averaged in the Plot Properties-Optional Settings page. Only elements with a high shear stress are displayed.

Things to look for

• Check the wall shear stress is less than that recommended for the material.

  The shear stress in the part is important, but not in the feed system. Only when additives to the polymer are very shear sensitive does the shear stress in the gates and runners become important. The material shear stress limit can be found in the Material Database.

• Local thickening at the location of the high shear stress can be used to reduce shear stress.
• Slower flow rates may reduce the maximum shear stress.

  When the flow rate is at or near the end of fill, use an injection profile or switch to pressure control earlier to reduce the flow rate as the problem area is filling. If the high stress is near the gate or somewhere in the middle of the part, a longer injection time may reduce the shear stress. Use care when increasing the injection time as the melt temperature in the part may drop too much causing other filling and packing problems.

• Changing the material to a less viscous material or increasing the melt temperature will also reduce shear stress.