Sharp Transition In Rheology Curve…

After performing an In-Mold Rheology Test, one blog reader had this question…

When performing the in-mold rheology test, I noticed a sharp drop when reaching a specific shear rate. After this point, the viscosity stabilizes at the lower value for the remainder of the higher shear rates. What is the cause for this abnormal viscosity curve.
My Response
Although this seems strange, it is not uncommon. Since changing the injection rate causes a difference in the amount of material entering the mold during fill, this might occur when molding a part where the highest pressure losses occur near the end of fill. You might see a drop in pressure if the part thickness increases, and a rise in pressure when the thickness decreases at the end of fill.
Additional Thoughts
In most cases, you will see a smooth in-mold rheology curve.

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