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Practical Rheology and Its Role in Polymer Processing. By Tim Womer. Outline . Definitions Basic Equipment Viscosity vs. Shear Rate Curves What they tell us How we can use them Melt Index: “What is it?” Moisture?. RHEOLOGY. The Science of Deformation and Flow of Matter.

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Practical rheology and its role in polymer processing

Practical Rheology and Its Role in Polymer Processing


Tim Womer


  • Definitions

  • Basic Equipment

  • Viscosity vs. Shear Rate Curves

  • What they tell us

  • How we can use them

  • Melt Index: “What is it?”

  • Moisture?

Practical rheology and its role in polymer processing


The Science of Deformation and Flow of Matter

from “ ”


Heracleitus, 5th Century B.C.

“All Things Flow”

Rheology is
Rheology is:

  • Science of material flow behavior

  • Uses very complicated and difficult to understand mathematical background (tensors, co-rotational or co-deformation curved space, etc.)

  • Can be presented in very complicated way which is hard to understand for non-specialists

Mathematics is complicated

 p

 vj

 tij

= 0


= 0

 xi

 xj

 xj



 T

 T

 T



+  Cp

vj = 2  eijeij

 xj

 xj

 xj


 tij


tij = tji



= 2  eij

 t


)e-(t-t’) dt’




K1(CI,CI-1) CI

+ K-1(CI,CI-1)CI-1)


Mathematics is complicated

Explanation of rheology
Explanation of Rheology

  • Mathematics is overemphasizedComplicated mathematical background leads sometimes to “scientific exhibitionism” of the presenter

  • Such an approach is useful for scientific people and conferences but usually useless for practical people

Explanation of rheology1
Explanation of Rheology

  • Understanding of the physics is emphasized

    • the mathematics is avoided as much as possible

    • the explanation is based on physical understanding what is happening

  • Such an approach cannot be used for development in rheology but may be useful for understanding material behavior

Practical rheology and its role in polymer processing



Viscosity [ Pa s ]

  • 10 20




  • 10 15

  • 10 10

3 mm

  • 105

polymer melts


Windows of the Cathedral of

Chartres in France have “Flowed”

since they were Produced 600

years ago.

  • 10 0




  • 10 - 5



Glass behaves like Fluids !!!

Tim s definition of rheology
Tim’s Definition of Rheology

  • The flow characteristic of a polymer at various shear and temperature conditions.

Crystallinity semi crystalline
Crystallinity (semi-crystalline)

Definition: A state of molecular structure in some resins attributed to the existence of solid crystal with a definite geometric form. Such structures are characterized by uniformity and compactness.

Semi-Crystalline Resins:

  • LDPE,HDPE, PP, FEP, Nylon, PET, PU

Amorphous non crystalline
Amorphous (non-Crystalline)

Definition: Devoid of crystallinity or stratification. Most plastics are amorphous at processing temperatures, many retaining this state under all normal conditions

Amorphous Resin:



  • A measure of the internal friction resulting when one layer of fluid is caused to move in relationship to another layer.

  • Example:

    The flow characteristic of 50 wt. motor oil at 50°F in the winter versus its flow characteristic when it is drained out of the oil pan after the motor has been warmed up to 200°F

Practical rheology and its role in polymer processing

n = 1

n = .3




n = .5

MFI region

Materials with different

Viscosity shear thinning

(Power-law Index Role)




Not shear sensitive but temperature sensitive

Screw Operating Range

Not Shear Sensitive but Temperature Sensitive


Not temperature sensitive but shear sensitive

Screw Operating Range

Not Temperature Sensitive, but Shear Sensitive

Melt density using a capillary rheometer
Melt Density using a Capillary Rheometer

  • Definition: The density of a polymer when it is in a molten state. The melt density is totally dependent to the temperature at which is it measure or processed.

  • ei, HDPE typically has a solid density of about .96 gm/cc but in the molten/process range it has a density of .75 gm/cc

Melt index mi mfr mfi
Melt Index (MI, MFR, MFI)

The terms “melt index” (MI), “melt flow rate” (MFR) and “melt flow index” (MFI) refer to the same test. MFR was introduced to replace MFI. The term MFI is used to refer to the flow rate of PE obtained under Condition 190/2.16 (formally know as Condition E). The use of such terms is not encouraged for other materials. It is suggested, by ASTM D 1238 that the term melt flow rate (MFR) be used for other plastic materials.

What does mfi mean not much


0 sec-1

10 sec-1



app = 2.4 MFI

What does MFI mean? Not Much!


  • Definition: Having the tendency to absorb moisture from the air. Some resins are hygroscopic, thus requiring drying prior to processing.

  • “Wet” resin will decrease the physical properties of the resin, typically embrittlement will occur.

Moisture content
Moisture Content

  • Definition: The amount of moisture in a material determined under prescribed conditions and expressed as a percentage of the weight of the moist specimen, that is, the original weight comprising the dry substance plus any moisture present.

  • Hygroscopic Resins

    • HIPS, ABS, Nylon, PET, PETG

  • In some processes, HDPE is dried.

Moisture analyzer
Moisture Analyzer

  • Moisture Analyzer is used to measure the moisture content of a hygroscopic material

  • Moisture content of resins can range from .005% to .18% measured in parts per million depending on the resin being processed

Resin drying
Resin Drying

Vacuum Dryer

Desiccant Dryer