Rheology vs Shear Rate

Rheology vs shear rate and ideal processing parameters relating to shear

Plastics are non-Newtonian, meaning they change viscosity with changes in shear rate. For some plastics, viscosity is influenced more by shear rate than by temperature. When a polymer is forced to flow quickly by large amounts of stress (pressure), the molecules are forced to align themselves in a more parallel fashion which allows them to flow more easily. This alignment results in a significant drop in the viscosity of the resin. As the viscosity becomes lower, it also becomes more stable.

In injection molding, shear rate translates to injection rate or fill time which is one of the most important process parameter to establish for an injection molding process. Fill time affects how much shear heating and shear thinning the plastic experiences which in turn affects the materials viscosity, the pressure and temperature of the plastic inside the cavities and overall part quality. Any change in fill time may adversely affect the molded part. So, once the ideal fill time has been established, that fill time should stay with the mold forever regardless of which machine it may be run on.

Using an injection molding machine, you can calculate this effective critical shear rate by generating a viscosity curve for a particular process. This critical shear rate will provide the processor the minimum speed allowable to produce consistent parts. We first start by setting the hold phase to zero, injection pressure to maximum available and the cooling time to safe value. Filling the part 90% with a cushion, we then run a series of parts at different fill speeds without changing any other processing parameters. Fill speeds should range from fast to slow. You can then use the injection pressure at transfer and fill speed to develop the viscosity curve. 

The critical shear rate is defined as the point where the slope changes from shallow to steep. For process stability, the plastic likes the flat section of the curve. Looking at the curve, note that the viscosity stays fairly constant after about 60% of the machines injection speed. Therefore, setting the injection speed to 70% would ensure that the filling stage of the process will stay consistent. Any small natural variations (i.e. lot to lot MFI values) will not cause large changes in viscosity which could result in shot to shot variation.

Although being on the flat section of the curve is ideal, it is not always possible and compromises may need to be made based upon part geometry, mold, machine and/or thermal limitations of certain materials (i.e. PVC). Excessive shear rates can lead to degradation of the polymer resulting in aesthetic and mechanical property deterioration. If you suspect a process may be introducing excessive shear into the polymer, the following equations can be used to calculate shear rate through specific geometry or sections of the mold (i.e. sprue, runner, gate).

The resultant value may be then cross referenced to published shear rate values to determine if the current process exceeds shear rate limitations of the material. This data can then be used to mathematically determine ideal flow rate and/or mold geometry.

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