SESSION TITLE – WILL BE COMPLETED BY MSC SOFTWARE
THE AFFECT OF TEMPERATURE DEPENDENT
MATERIALS IN AN EXPLICIT NASTRAN
ANALYSIS ON A PLASTIC CONTAINER
Travis Hunter (Graham Packaging Company, USA)
THEME
When performing an analysis on the hot-fill process, the simulation results did
not properly correlate to the lab results. Therefore, a new method needed to be
discovered to simulate the hot-fill process. This led to the use of a different
material model to better demonstrate the changes in the thermal characteristics
of PET.
SUMMARY
Most juices and isotonics are filled at a temperature of 185ºF then cooled to
room temperature. This change in the liquid has dramatic effects on the PET
container, because it is a thermally sensitive material. As the liquid inside the
bottle begins to cool, the deformation of the bottle needs to be controlled so the
container is presentable to the customer. Controlling the deformation requires
many design iterations as well as time to evaluate prototypes. However, using
simulation educated decisions can be made on which design will have the best
performance with minimal time and cost. This paper establishes the evolution
of the simulation process for the hot-fill analysis. Thus arriving at present day
where using the latest advances in software, the hot-fill process is accurately
simulated using the MATD 106 material card in an Explicit Nastran analysis.
KEYWORDS
Stretch Blow-Molding (SBM)
Hot-Fill
Nastran
SimXpert
Fluid Structure Interaction (FSI)
MATD 106
THE AFFECT OF TEMPERATURE DEPENDENT MATERIALS IN AN
EXPLICIT NASTRAN ANALYSIS ON A PLASTIC CONTAINER
1: Hot-Fill Process
In a typical fill process for juices and isotonics, the liquid needs to be heated to
near boiling temperatures to make the product shelf stable. Depending on the
type of product the temperature of the liquid is approximately 185º Fahrenheit.
Then the filled container is cooled over a period of time to room temperature.
Since the product is hot when filled it is less dense then when it is at room
temperature and the product contracts when it cools. As a consequence the
liquid reaches the required fill volume. Figure 1 shows the temperature versus
time probe data taken from a sample bottle and figure 2 shows the temperature
versus change in pressure.
180
Temperature (ºF)
160
140
120
100
80
60
0:00
0:04
0:08
0:12
0:17
0:21
0:25
Time (minutes)
Figure 1:
Temperature versus time probe data
15.6
15.4
15
14.8
14.6
14.4
14.2
14
180
160
140
120
100
Temperature (ºF)
Figure 2:
Temperature versus pressure probe data
80
60
Pressure (psi)
15.2
THE AFFECT OF TEMPERATURE DEPENDENT MATERIALS IN AN
EXPLICIT NASTRAN ANALYSIS ON A PLASTIC CONTAINER
This negative change in volume creates a vacuum, which needs to be
accounted for in the bottle design or the bottle will not pass the customer
requirements for labelling and customer handling. Consequently the need for
an accurate simulation of the hot-fill process to ensure the bottle will pass the
customer requirements.
2: The PET Material Model
PET is a thermally sensitive material. There is a significant difference in the
stress strain curve at 185ºF than at 72ºF. Figure 3 shows that difference in the
stress strain curve.
160
140
120
Stress (MPa)
100
72ºF
100ºF
80
128ºF
60
156ºF
40
185ºF
20
0
0
0.5
1
1.5
Strain (mm/mm)
Figure 3:
Temperature versus time probe data
Also the above figure shows how much weaker the material is at 185ºF, this is
what caused the need to use a different material model for simulating the hotfill process.
3: Simulating the Hot-Fill Process
Simulating the hot-fill process in the past was completed by applying a
negative pressure to the surfaces of the bottle. These results would help design
the package, however, there had to be a high safety factor applied to the
analysis. This was due to the fact that the results were within a 30% range of
actual lab samples. Then MSC introduced the fluid filled card for Dytran. The
THE AFFECT OF TEMPERATURE DEPENDENT MATERIALS IN AN
EXPLICIT NASTRAN ANALYSIS ON A PLASTIC CONTAINER
fluid filled card allowed the analysis to be run with conditions similar to actual
fill conditions, where the volume of the liquid and the temperatures would be
applied to simulate real fill conditions. The results improved to a safety factor
range of 20%. As innovations were made in the software and hardware the
fluid filled card got incorporated into MSC Nastran, and the speed of the
analysis improved. However, there still was some discrepancy in the simulated
results versus lab results. Therefore, additional improvements needed to be
made to the material card in the analysis to best represent the real world results.
4: The New Material Card
Before converting to the new material model, the original one only had one
stress-strain curve at one temperature, which accounted for a great deal of error
in the analysis. Consequently, the MATD106 card needed to be applied to the
simulation model to account for the change in temperature during the analysis.
Using the MATD106 card greatly improved the accuracy of the analysis, and
the simulation results were finally very similar to lab data.
5: Conclusion
With the latest advances in the material card and improvements made to the
material properties, the hot-fill simulation finally follows lab data within a
couple of percentage points. This improved accuracy in the analysis has led to
greater innovation in bottle design, faster speed to market, and material
savings.