The Zen of Rheological Data

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The Zen of Rheological Data A Path to Mold
Filling Enlightenment Compiled By Dave
Sterling
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Disclaimer

• No information supplied by RTP Company
  constitutes a warranty regarding product
  performance or use. Any information regarding
  performance or use is only offered as suggestion
  for investigation for use, based upon RTP Company
  or other customer experience. RTP Company makes
  no warranties, expressed or implied, concerning
  the suitability or fitness of any of its products
  for any particular purpose. It is the
  responsibility of the customer to determine that
  the product is safe, lawful and technically
  suitable for the intended use. The disclosure of
  information herein is not a license to operate
  under, or a recommendation to infringe any
  patents.

3
Rheology Moldflow?

• Why is rheological data important?
• Where do you obtain this data?
• How are rheological properties tested?
• What effect do rheological properties have on
  analysis?
• How do you identify bad data?
• What does the future of rheology hold?

4
Rheology Moldflow?

• Why is rheological data important?
• Where do you obtain this data?
• How are rheological properties tested?
• What effect do rheological properties have on
  analysis?
• How do you identify bad data?
• What does the future of rheology hold?

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Why is Rheological Data Important?
Note Graphic courtesy of Moldflow Corp.
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Moldflow Data Sensitivity Study

• A sensitivity study was conducted by Moldflow
  Corp. to determine which material properties were
  most critical for various types of analysis.
• In addition, the effect of experimental
  variability was examined.

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Moldflow Data Sensitivity Study
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Observations from Sensitivity Study

• Filling Pressure
• Primary Viscosity, Thermal Conductivity
• Secondary Specific Heat
• Frozen Layer
• Primary Transition Temperature
• Secondary Thermal Conductivity, Specific Heat
• Shrinkage/Warpage
• Primary Transition Temperature, pVT, Shrinkage
  Model
• Secondary Viscosity, Thermal Conductivity,
  Specific Heat
• Cavity Pressure Curve
• Primary Thermal Conductivity, Specific Heat
• Secondary Transition Temperature, pVT

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Rheology Moldflow?

• Why is rheological data important?
• Where do you obtain this data?
• How are rheological properties tested?
• What effect do rheological properties have on
  analysis?
• How do you identify bad data?
• What does the future of rheology hold?

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Where do you obtain data?

• Moldflow Database
• Materials Supplier
• In-House Testing
• Moldflow Plastics Labs
• http//www.moldflow.com/
• Datapoint Labs
• http//www.datapointlabs.com/

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Rheology Moldflow?

• Why is rheological data important?
• Where do you obtain this data?
• How are rheological properties tested?
• What effect do rheological properties have on
  analysis?
• How do you identify bad data?
• What does the future of rheology hold?

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Rheological Inputs for Moldflow

• Contains
• Viscosity Data
• Viscosity Model
• Transition Temperature
• Melt Flow Information

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Rheological Properties

• Contains
• Viscosity Data
• Viscosity Model
• Transition Temperature
• Melt Flow Information

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Viscosity Data

• Viscosity is a materials resistance to shear
  deformation.
• A higher viscosity indicates greater resistance
  to flow.
• Test Methods for Moldflow Data
• Capillary Rheology ASTM D-3835
• Injection Molding Rheology
• Slit Die Rheology

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Extrusion Based Rheology

• Material is extruded through a restriction of
  known geometry (e.g. capillary, slit-die,
  half-round, etc.).
• Pressure, temperature, and flow rate are either
  controlled or observed.
• The pressure drop across the restriction is used
  to determine the materials viscosity.

Note Image Courtesy Moldflow Corp
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Capillary Rheology

• Sample is placed in the melt reservoir.
• Piston is controlled to extrude material through
  the die at different rates of displacement.
• For non-Newtonian fluids two corrections are
  required.
• Bagley Correction
• Rabinowitsch-Mooney Correction

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Capillary Rheology

• Basic Equations for Capillary Flow
• tw - Shear stress at the capillary wall
• ?Pt Total pressure drop across the capillary
  die
• L Length of the capillary die
• D Diameter of the capillary die
• R Radius of the capillary die
• ?a Apparent shear rate
• ?R True shear rate
• Q Volumetric flow rate
• ?a Apparent shear viscosity
• ? True shear viscosity

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Capillary Rheology

• Bagley Correction
• A correction to account for the entrance/exit
  losses in the capillary due to extensional
  viscosity.
• May be negligible when very long capillaries are
  used (i.e. L/D gt 35).
• Usually run with two dies with different L/D
  ratios.
• Plot ?P versus L/R at constant stress with slope
  2tR.

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Capillary Rheology

• Weissenberg-Rabinowitsch Correction
• A correction to account for the fact that the
  viscosity decreases as the shear rate increases
  (Non-Newtonian).
• Converts apparent shear rate to true shear rate.
• Errors up to 10-20 in viscosity are common if
  this correction is not applied.

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Injection Molding Rheology

• Similar to Capillary, but uses a plasticating
  screw to quickly heat the material instead of a
  melt reservoir.
• Pros
• Quick plastication due to shear and pressure.
• Short dwell times and similar processing to
  injection molding.
• Mold verification studies at Moldflow have shown
  improved accuracy over capillary data.
• Cons
• Not a widely available test.
• Melt temperature is transient at the start of the
  test.
• Requires a large material sample.
• Cost

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Slit Die Rheology

• Uses the same principle as the previous two
  methods and is essentially a capillary test using
  a slit die in place of the capillary die.
• Many capillary rheometers have this capability as
  an option.
• Works well for characterizing long-glass fiber
  materials and highly filled materials.
• The slit die does not have the propensity for
  plugging that a capillary die does.

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Viscosity Models

• Viscosity models are required in injection
  molding flow analysis to account for the
  variation in polymer melt viscosity due to shear
  rate, temperature, and pressure.
• The goal of the model is to match experimentally
  observed behavior as close as possible.
• When fitting data for using in Moldflow, you can
  choose between three different viscosity models.
• Cross-WLF (Most Common)
• Second Order
• Matrix Model

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Matrix Model

• Simply a collection of triples (viscosity,
  temperature, shear rate) obtained by experiment.
• No functional dependence or curve fitting.
• Analysis program linearly interpolates between
  the data points closest to the existing set of
  conditions.
• Can be useful for materials with unusual
  viscosity characteristics such as LCP.

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Second Order Model

• An modified version of the Power Law model that
  improves viscosity modeling in the low shear rate
  region.
• There is some controversy over whether this model
  accurately describes polymer behavior.
• Most flow analysis software has migrated to the
  Cross-WLF viscosity model.

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Cross-WLF Model

• The Cross-WLF model accounts for the effect of
  temperature, shear rate, and pressure on the
  viscosity, over a wide temperature range.
• Zero-shear-rate viscosity is represented by a
  more extensive model based on the WLF functional
  form.
• The Cross-WLF model is more appropriate for
  packing analysis, because the temperature and
  pressure sensitivities of the zero-shear-rate
  viscosity are better represented.

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Rheological Properties

• Contains
• Viscosity Data Model
• Transition Temperature
• Melt Flow Information

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Transition Temperature (DSC)

• Similar concept to No-Flow Temperature.
• Obtained from a DSC cooling scan (except LCP).
• ASTM D-3418
• Typically -20C/min. cooling rate.
• Occasionally -10C/min. cooling rate.
• Faster rate is usually better due to
  super-cooling!

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Transition Temperature (DSC)
Note Image Courtesy Moldflow Corp
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Transition Temperature (DSC)
Note Image Courtesy Moldflow Corp
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No-Flow Temperature

• Originally developed by Moldflow in place of the
  Transition Temperature.
• Dropped when Moldflow acquired C-Mold
• The temperature at which the plastic stops
  flowing in a mold.
• Some still suggest this is best for amorphous
  materials and no-flow temperature is usually
  higher than the DSC Tg.
• Test Apparatus
• Capillary Rheometer
• Parallel Plate Rheometer (maybe!)

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No-Flow Temperature (Capillary)

• Tested using a capillary rheometer.
• Barrel is charged with material and a constant
  load/pressure is applied.
• Around 2,000 psi
• Die L/D of around 201
• Barrel heaters are turned off and the melt is
  allowed to cool.
• No-flow temperature is defined as the temperature
  at which extrudate flow lt2 mm/min.

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No-Flow Temperature (Parallel Plate)

• Some researchers have suggested an alternate
  method to obtain a No-Flow temperature using a
  parallel plate rheometer.
• Test performed at constant strain with decreasing
  temperature.
• A cross-over of G and G seems to correlate with
  the no-flow temperature.
• This method allows for easier characterization of
  blends vs. DSC.

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Rheological Properties

• Contains
• Viscosity Data Model
• Transition Temperature
• Melt Flow Information

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Melt Flow Information

• ASTM D-1238
• For information/comparison purposes only and is
  not used by the software.
• This is mainly here to allow people to easily
  compare grades of material as they are searching
  through the database.

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Rheology Moldflow?

• Why is rheological data important?
• Where do you obtain this data?
• How are rheological properties tested?
• What effect do rheological properties have on
  analysis?
• How do you identify bad data?
• What does the future of rheology hold?

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Example 1 Temperature Effect

• Simple example using a standard ASTM flex bar.
• Dimensions 6 x 0.5 x 0.125
• Materials
• Zytel 101
• Lexan 141
• Variables
• Melt Temperature
• Mold Temperature

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Example 1 Temperature Effect
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Example 1 Temperature Effect

• Temperature affects filling pressure by changing
  viscosity, frozen layer thickness, and freeze
  time.
• These results are using Zytel 101 molded at the
  extremes of the processing window.

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Example 1 Temperature Effect

• These results are using Lexan 141 molded at the
  extremes of the processing window.
• Temperature usually affects amorphous materials
  more than semi-crystalline materials.

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Example 2 Wall Thickness Variation

• Simple example using a UL FR bar and ASTM flex
  bar.
• Flex Bar Dimensions 6 x 0.5 x 0.125
• FR Bar Dimensions 6 x 0.5 x 0.0625
• Material
• Zytel 101
• Variable
• Wall Thickness

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Example 2 Wall Thickness Variation

• Here we compare to the filling pressure for two
  bars of varying thickness.
• The top bar is 0.125 thick and the bottom one is
  0.0625 thick.
• Processing conditions are the same for both bars.

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Example 2 Wall Thickness Variation

• Pressure varies with thickness due to the
  equations of fluid flow.
• Pressure can be reduced in thin sections by
  increasing shear rate.
• More information can be found in Peter Kennedys
  book Flow Analysis of Injection Molds.

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Example 3 Hesitation

• Hesitation occurs when wall thickness in a part
  varies significantly especially when it varies
  close to the gate.
• Plastic tends to flow in the path of least
  resistance.
• When the flow reaches a thinner section of the
  part, it hesitates until enough pressure builds
  to force the material into the thinner section.
• This can be minimized by reducing the viscosity
  of the material (heat, shear, etc.).

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Example 3 Hesitation
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Example 3 Hesitation
0.020
0.030
0.040
0.080
0.050
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Example 3 Hesitation
Thinnest section has the greatest frozen layer
fraction.
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Example 3 Hesitation
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Example 3 Hesitation
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Example 3 Hesitation
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Example 3 Hesitation
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Example 3 Hesitation
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Example 3 Hesitation
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Example 3 Hesitation
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Example 3 Hesitation
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Example 3 Hesitation
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Example 3 Hesitation
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Example 3 Hesitation
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Example 3 Hesitation
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Example 3 Hesitation
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Example 4 Racetracking

• Racetracking is the inverse of hesitation.
• It occurs when the flow in one area of the part
  leads the flow in other areas due to wall
  thickness variations.
• This is commonly seen in boxes or caps with thick
  rims and thin main walls.
• Racetracking can lead to backfilling and the
  formation of gas traps.

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Example 4 Racetracking
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Example 4 Racetracking
63
Rheology Moldflow?

• Why is rheological data important?
• Where do you obtain this data?
• How are rheological properties tested?
• What effect do rheological properties have on
  analysis?
• How do you identify bad data?
• What does the future of rheology hold?

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Identifying Bad Data

• Can be difficult without proper
  training/experience.
• Best to benchmark on a known geometry such as a
  tensile bar or flex bar.
• Shape of the viscosity curve can be a dead
  giveaway.

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Example Bad Data
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Example Good Data
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Rheology Moldflow?

• Why is rheological data important?
• Where do you obtain this data?
• How are rheological properties tested?
• What effect do rheological properties have on
  analysis?
• How do you identify bad data?
• What does the future of rheology hold?

68
What Does the Future Hold?

• Better measurement accuracy and repeatability?
• New viscosity models and better characterization
  for materials with odd behavior (LCP, PCPTFE,
  Long-Fiber, etc.)?
• Better understanding of the melt to solid
  transition in thermoplastics?
• Better understanding of thermoplastic behavior in
  thin walls?
• Lower cost equipment?

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Other Industry Uses for Rheology

• Identifying degraded material.
• Compounded pellets or molded parts.
• Comparing identical compounds.
• RTP Compound vs. Competitor
• Determining thermal stability of a material.
• Constant shear rate test.
• Evaluating the effect of blends on viscosity.

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Thanks

• Moldflow Corporation
• Cade Heiberg
• Robert Newman
• Russell Speight
• RTP Company
• Barb Matousek
• Bob Sherman

71
Disclaimer

• No information supplied by RTP Company
  constitutes a warranty regarding product
  performance or use. Any information regarding
  performance or use is only offered as suggestion
  for investigation for use, based upon RTP Company
  or other customer experience. RTP Company makes
  no warranties, expressed or implied, concerning
  the suitability or fitness of any of its products
  for any particular purpose. It is the
  responsibility of the customer to determine that
  the product is safe, lawful and technically
  suitable for the intended use. The disclosure of
  information herein is not a license to operate
  under, or a recommendation to infringe any
  patents.

72
Questions?