Today's KNOWLEDGE Share : MFI, high shear viscosity and relaxation time spectrum.

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Today's KNOWLEDGE Share
MFI, high shear viscosity and relaxation time spectrum.

Let's look at these two Capillary viscosity curves depicting PC (blue) and HDPE (red).
These show very similar MFI (low shear viscosity) and also similar viscosity at processing rates in Injection Molding.


However there's more to it.

The Cox-Merz rule in rheology tells us that Dynamic Rheology (complex viscosity vs. frequency) would produce the same exact curves. But thanks to what we know in Dynamic Rheology we can bring additional critical information now.

The HDPE does not reach a Newtonian plateau within the shear rate testing range, which means there are still long relaxation present in this material that are longer than 1/gammadot at the lowest rate. Possibly 10's or 100's of seconds.

The PC is way more Newtonian and the green vertical line shows that there are no relaxation times longer than about 1/gammadot of that green line.

So this PC relaxes possibly 100 or 1000 times faster than the HDPE (do not forget the graph is a log-log scale).

This has profound consequences on :

- the final level of molecular orientation and related anisotropy in the part

- the ability to quickly re-entangle and create a strong weldline

This is what we refer to as visco-elasticity and relaxation time spectrum.
In coming posts, I will discuss in more details how these aspects will influence anisotropic shrinkage of semi-crystalline materiaks, still poorly predicted by commercial simulation software.

source :Vito leo

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