Plastics Design Principles: Maximum Shear Stress

By Jay Shoemaker

Related links: Moldflow Plastics Design Guide | Moldflow Communicator™ | Moldflow Design Guide: A Resource for Plastics Engineers

The maximum shear stress in the part should be below the material limit specified in the material database. The shear stress limit is approximately 1% of the tensile strength of the material and is also application-specific. For parts used in harsh environments such as elevated temperatures, under a high load during use, or exposed to chemical attack, the limit specified in the database may be too high. Alternatively, if the part is not used in a harsh environment the limit is conservative (low), and the stress can be significantly exceeded without any problems. However, when the shear stress does get above the limit, it should be kept as low as possible. Figure 5: Maximum shear stress shows the maximum shear stress in the part scaled from the material limit to the maximum shear stress value calculated in the analysis. Areas that are colored in the plot are therefore above the limit. In this case, the maximum shear stress is 0.45 MPa which is not too high. Most of the time, parts will have areas of high shear stress that will be 2 to 5 times the stress limit. In this case, it is only 1.5 times the limit. However, much of the part is slightly above the limit. The maximum shear stress in the cross section is at the frozen/molten layer interface, or “at the wall”.

Three main factors influence shear stress, including:

• Wall thickness - increase the wall thickness to reduce stress.
• Flow rate - lower the flow rate (locally or globally) to reduce stress.
• Melt temperature - increase the melt temperature to lower the shear stress.

Figure 5: Maximum Shear Stress

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