Today's KNOWLEDGE Share: Shear Rate

Today's KNOWLEDGE Share:

Shear Rate

Sorry to disappoint some of you, but there is NO such a thing as Maximum admissible Shear Rate for a material in Injection Molding.

 

30+ years back, Colin Austin (Moldflow founder) wanted to put a recommendation in the code documentation and, having no clear answers from suppliers ,he decided to list some values that were loosely based on a wild guess exercise. Actually an estimated typical shear rate (at some standard T) for a critical Shear Stress equal to an arbitrary fraction (abt 10%) of a Stress at break in the solid state. How wild is that ??

Shear rate does not destroy a polymer.

If chains break, that is due to Shear Stress, not shear rate. If they degrade thermally, it is the result of accumulated shear-heating along the flow. In both cases, shear rates alone CANNOT resolve the risk of damaging the material.

At low temperature (high viscosity hence higher stress) a lower shear rate can be more dangerous than a higher rate at a higher temperature (lower viscosity so potentially much lower stress).

So if you want to say something about the injection rate being too fast, check the stresses or the melt temperature increase. A high shear rate will be a warning signal, at best.

Source:VITO LEO

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