The Zen of Rheological Data - PowerPoint PPT Presentation

1 / 72
About This Presentation
Title:

The Zen of Rheological Data

Description:

Works well for characterizing long-glass fiber materials and highly filled materials. ... method allows for easier characterization of blends vs. DSC. ... – PowerPoint PPT presentation

Number of Views:87
Avg rating:3.0/5.0

less

Transcript and Presenter's Notes

Title: The Zen of Rheological Data


1

The Zen of Rheological Data A Path to Mold
Filling Enlightenment Compiled By Dave
Sterling
2
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?

5
Why is Rheological Data Important?
Note Graphic courtesy of Moldflow Corp.
6
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.

7
Moldflow Data Sensitivity Study
8
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

9
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?

10
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/

11
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?

12
Rheological Inputs for Moldflow
  • Contains
  • Viscosity Data
  • Viscosity Model
  • Transition Temperature
  • Melt Flow Information

13
Rheological Properties
  • Contains
  • Viscosity Data
  • Viscosity Model
  • Transition Temperature
  • Melt Flow Information

14
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

15
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
16
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

17
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

18
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.

19
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.

20
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

21
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.

22
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

23
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.

24
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.

25
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.

26
Rheological Properties
  • Contains
  • Viscosity Data Model
  • Transition Temperature
  • Melt Flow Information

27
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!

28
Transition Temperature (DSC)
Note Image Courtesy Moldflow Corp
29
Transition Temperature (DSC)
Note Image Courtesy Moldflow Corp
30
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!)

31
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.

32
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.

33
Rheological Properties
  • Contains
  • Viscosity Data Model
  • Transition Temperature
  • Melt Flow Information

34
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.

35
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?

36
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

37
Example 1 Temperature Effect
38
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.

39
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.

40
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

41
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.

42
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.

43
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.).

44
Example 3 Hesitation
45
Example 3 Hesitation
0.020
0.030
0.040
0.080
0.050
46
Example 3 Hesitation
Thinnest section has the greatest frozen layer
fraction.
47
Example 3 Hesitation
48
Example 3 Hesitation
49
Example 3 Hesitation
50
Example 3 Hesitation
51
Example 3 Hesitation
52
Example 3 Hesitation
53
Example 3 Hesitation
54
Example 3 Hesitation
55
Example 3 Hesitation
56
Example 3 Hesitation
57
Example 3 Hesitation
58
Example 3 Hesitation
59
Example 3 Hesitation
60
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.

61
Example 4 Racetracking
62
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?

64
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.

65
Example Bad Data
66
Example Good Data
67
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?

69
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.

70
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?
Write a Comment
User Comments (0)
About PowerShow.com