Understanding Shear

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Injection molded parts may have issues with Shear Rate and/or Shear Stress. Moldflow can predict both. What’s the difference?

Although you can’t see shear stress or shear rate when you’re designing or molding parts, Moldflow will accurately predict both. Simple, early analysis once the part design is established may save you lots of time and money later.

If it’s a cosmetic issue on the part – splay, streaks, changes in gloss – it’s shear stress. If there is evidence of burning downstream of the gate, then it’s shear rate.

In almost all cases, shear stress problems only occur in the part and not the feed system. It is directly proportional to cavity pressure. Figure out a way to reduce cavity pressure and you will solve your problem.

Materials have a maximum recommended shear stress value. Moldflow predicts the values in the part. Therefore, with analysis, you can determine how much of an improvement you need and if simple changes will get you there.

Here are ways to decrease cavity pressure: use a fill time that results in minimum pressure, profile injection for constant melt-front velocity, increase melt temperature, increase mold temperature, improve shear thinning of material in the runner system, use a lower viscosity material and, increase wall sections.

In almost all cases, shear rate is only an issue in the feed system. There is not a maximum recommended value, however, through analysis it is possible to see if the values spike.

The reason there is no maximum is that shear rate varies across the thickness of the runner and gate. Depending on what the maximum values are and the length of runner it may be high but still not cause issues. The most obvious indicator of excessive shear rate is burning downstream of the gate. A secondary indicator is significant shear heating in the runner and part. Again, something you can’t see at the machine but is easy to see with Moldflow.

High shear rate can be very good. One advantage of thermoplastics is as shear rate goes up viscosity goes down and it can go down significantly. Therefore, a smaller diameter runner may actually result in lower pressure due to reduced viscosity. This lower pressure will also reduce shear stress in the part because the pressure drop in the part will be lower.