Today's KNOWLEDGE Share: polymer melt shear viscosity.

Today's KNOWLEDGE Share

Some people have an over simplistic perception of polymer melt shear viscosity.

In a start-up flow, where you suddenly go from rest to a constant shear-rate (we do that every time when we fill a mold and the screw goes from rest to the prescribed velocity !), the polymer is initially fully entangled and unoriented.

As flow starts, within a time comparable to the polymer relaxation time (usually a fraction of a second for a molding grade), you reach a new equilibrium state with less entanglements and more orientation.

This lower entanglement state comes with a lower viscosity.

So, although our polymer is not "thixotropic", the viscosity is always a bit time dependent.

This is why in a capillary rheometer, we need a few seconds to get a stable reading after changing piston speed, by the way.

This excess viscosity at start-up means that your molding machine will feel this higher resistance to flow when the screw starts moving. In a hydraulic machine, the hydraulic system will supply more pressure for the screw to fight this and to reach its velocity target.

And the machine pressure reading would then invariably be higher than what you might attempt to predict with Flow Analysis, which typically doesn't cope with this "transient rheology effect".

Source:Vito leo

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