Interpreting Viscosity Curves

As a response to the Fill Time, Viscosity & Molding Economics blog I received this question…

With respect to a in-mold rheology test, why should we choose to process on the right-hand side of the curve?
My Response
As the shear rate, or flow rate, of the polymer increases… the viscosity decreases. This rheological behavior is unique to polymers and is called ‘shear thinning’.
When graphing this, viscosity is plotted on the vertical, ‘Y axis’ and shear rate is plotted on the horizontal, ‘X axis’. Shear thinning will appear as a steep decline in the viscosity of the polymer as the shear rate increases.
Once most of the shear thinning occurs the polymer’s viscosity starts to level out. After this point, the viscosity will remain relatively consistent – resulting in a more stable process. For this reason, you should process on the right hand side of the curve.
Additional Thoughts
You can learn more about this test and other aspects of scientific molding if you participate in our free online webinars.
  1. MJ left a comment on 2009/09/02 at 11:32 am

    If I receive same type of material with different viscosity(variation during making material in company material supplier), will fast injection (chosen according rheology test) decrease this material variation?

  2. Andy Routsis left a comment on 2009/09/02 at 11:40 am

    Yes, the likelihood that you are still processing in the right hand region is high.
    You should still check the melt temperature with your new material to ensure the melting characteristics (and morphology) are similar.

  3. MJ left a comment on 2009/09/02 at 2:17 pm

    Thanks for explanation, but I have next question.
    I know, how to measure melt temperature, but you said in some seminar, that measuring melt temperature depends on: thickness of probe, amount and type of material. Where I can find right procedure how measure melt temperature, when I want to avoid deviations in measuring?

  4. Andy Routsis left a comment on 2009/09/02 at 2:25 pm

    We cover this in many of our online courses including out intelligent molding series, but the basics are covered in our webinar entitled ’10 procedures to fine tune your process’ at about 8 minutes into the presentation.
    The webinar can be found at

  5. MJ left a comment on 2009/09/02 at 4:01 pm

    thanks Andy.

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