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Introduction
Moldflow Corporation's predictive analysis products, Moldflow Plastics
Insight (MPI) and Moldflow Plastics Advisers (MPA), are used to
simulate the complex behavior of injection molded polymer melts.
As Moldflow is constantly striving to assure that our analysis results
are as accurate as possible, one of our highest priorities is to
provide analysis solver algorithms and material data to achieve
that goal. Regarding material data, Moldflow has developed some
of the world's most innovative testing methods to assure that the
material characteristics used for analysis represent those of injection
molded polymer melts. For example, Moldflow uses instrumented injection
molding machines to determine rheological data, which is basic to
all flow analyses.
This article covers the thermoplastic material properties required
for Moldflow analysis, the test methods used to derive the data,
and some of the unique features which contribute to making the Moldflow
Material Database the most comprehensive of its kind in the world.
We follow internationally recognized standard test methods as well
as fully documented, internal standard methods, which have allowed
us to achieve significant simulation improvements. We provide a
large database on as wide a range of polymer materials as possible,
including data obtained from resin manufacturers and third-party
sources. Also introduced are the key features and enhancements of
the Moldflow Plastics Insight 3.0 (MPI 3.0) Material Database.
Viscosity test methods
Moldflow's injection molding simulation software is used to predict
the flow of polymer melts through mold sprues, runners, gates, and
part cavities. As such, the most accurate analyses that can be performed
start with the most accurate material rheological data. Moldflow
studies have clearly shown that rheological characteristics measured
on instrumented injection molding machines can yield better simulation
results for both filled and unfilled materials. Therefore, Moldflow's
standard and recommended test method employs instrumented injection
molding machine rheometers.
However, standard capillary rheology data is widely available and
it may be the only data available to many of our users. Other testing
techniques are also applied at times due to unique material behavior
or limited access to preferred techniques. Moldflow will continue
to provide such data upon request, and will offer such data within
our standard databases.
A major drawback of having such a variety of testing techniques
is that Moldflow users were previously unaware of which testing
technique was used for each data set. Therefore, a new feature of
the MPI 3.0 Material Database is that each data set within a given
material grade now includes test method information, including the
source, date of testing, and test method used, if the information
is available.
Viscosity data models
In order to provide a more consistent database, the Cross/WLF
model has been applied as the default model to the materials in
Moldflow's database. The Moldflow second-order model is still available
and will be provided for many materials.
Viscosity indexes
Users have expressed a desire to have a simple, single-point comparison
of viscosity for different grades. While such values do not show
the relevant effects of shear and temperature upon viscosity, they
often can be useful for comparing very similar materials. Melt Flow
Rate (MFR) values are listed in the Moldflow Material Database when
available. To avoid any confusion in comparisons, only complete
MFR data sets, with appropriate test temperature and plunger mass
details, are listed.
However, it is important to note that there are some limitations
to MFR data: it is not available for all materials, and it is a
measurement performed at fairly low shear rates relative to those
that occur during injection molding. To address these issues, the
Moldflow Material Database lists a Moldflow Viscosity Index value.
This single-point viscosity value is generated from a material’s
actual viscosity model at a typical melt processing temperature
and a shear rate of 1000 1/sec. To further aid in the ease of comparing
materials of the same generic type, the Moldflow Material Database
now has Moldflow Viscosity Index values generated at the same temperature.
This makes it very easy to search the database to identify materials
of the same generic type with similar Moldflow Viscosity Index values.
Transition temperature data
A single-point transition temperature is used to define the material's
state transition from melt to solid. This data is commonly measured
by a differential scanning calorimeter in a cooling mode. However,
various methods may be used by different data sources. The MPI 3.0
Material Database now provides the method used to measure or estimate
the transition temperature, if the information is available.
Thermal conductivity and specific heat
data
The thermal properties of polymers change with temperature. In
order to improve the accuracy of simulation, the ability to store
and use temperature-dependent thermal data is required. A new feature
of MPI 3.0 is that tabulated thermal conductivity and specific heat
data as a function of temperature can be used for simulations. Such
data is available for over 2,000 grades. Because thermal properties
data may be generated by a variety of test methods, the MPI 3.0
Material Database now provides the test method used, if the information
is available.
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Pressure-volume-temperature data
The effect of temperature and pressure on material density
is an important property for simulations. Data can be gathered
by either indirect or direct dilatometry techniques.
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pvT data model
The two-domain, modified Tait model currently is used for all data
in MPI 3.0. In order to assure consistency within a data set, all
single-point density values are derived from the pvT model in use.
The MPI 3.0 Material Database now provides the source of pvT data,
date of testing, and test method used, if the information is available.
Shrinkage data
MPI 3.0 allows the use of three types of shrinkage data and models:
- Residual Stress model (mechanical properties data required).
- Residual Strain model (Moldflow shrinkage data required).
- Corrected In-Mold Residual Stress (CRIMS) model (Moldflow
shrinkage data required).
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Moldflow strongly recommends using the CRIMS model to achieve the
best simulation results. The CRIMS technique uses the predicted residual
in-mold stress from the theoretical model and corrects it to achieve
unprecedented accuracy in the prediction of shrinkage and warpage.
The result is anisotropic residual stress distributions that account
for morphology and structure in both amorphous and semi-crystalline
materials.
The CRIMS method combines a theoretical model for residual stress,
a model for morphology development, and correction of errors due
to the use of material data that are obtained under laboratory conditions
rather than those experienced by the material in injection molding.
Because CRIMS data must be correlated with the remaining data set
and to the software build, the MPI build number has been added to
the data set.
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Conclusion
Accurate material properties data is critical to assure the accuracy
of your analysis predictions. For this reason, Moldflow spends considerable
development resources to maintain and update the Material Database,
as well as to develop and improve test methodology.
The Moldflow Material Database for MPI 3.0 includes data on over
7,500 resins and the following enhancements:
- Additional information regarding data source, last modified
date, and test method will be provided if it is available.
- Viscosity index data has been improved by listing MFR
data only when complete with test conditions and by improving
the consistency of Moldflow Viscosity Index data within
generic families.
- The Cross/WLF viscosity model is applied universally throughout
the database.
- Multi-point specific heat and thermal conductivity data
as a function of temperature now can be stored.
- pvT data is fit to the two-domain, modified Tait Model.
- CRIMS data is provide with a build ID number to assure
appropriate data is used with MPI releases.
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For information on the MPI 3.0 Material Database, Moldflow's injection
molding rheology, or CRIMS shrinkage modeling, contact Moldflow Plastics
Labs at: mpl@moldflow.com.
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