All plastics are non-Newtonian. This means that their viscosity does not remain
constant over a given range of shear rates. In the strict sense, the
rheological behavior of a plastic is a combination of non – Newtonian and
Newtonian behavior. At lower shear rates, the plastic is non-Newtonian but as
the shear rate increases, the plastic tends to exhibit a Newtonian behavior.
This happens because with increasing shear rate, the polymer molecules start to
untangle from each other and start to align themselves in the direction of
flow. Refer to the picture on the next page.
Importance to Injection Molding:
During Injection Molding, the material is subjected to large amount of shear
forces during the cavity filling stage. The shear rate is proportional to the
injection speed. If the shear rates are in the non-Newtonian region of the
curve, then small variations in the shear rate will cause a large shift in the
viscosity. This will make the mold filling inconsistent resulting in shot to
shot inconsistency. It is therefore important to find the Newtonian region of
the curve and set the injection speed (therefore shear rate) in this region.
The viscosity curve can be generated at the molding machine for any given mold.
The term ‘In-Mold Rheology’ is another term used for developing the viscosity
curve. The effect of shear rate on viscosity is far greater than that of
temperature. Therefore as long as the actual melt temperature is in the within
the recommended range you will get a similar profile during the development.