Design and Molding Simulation of a Plastic Part by Jiajun Shen An Engineering Project Submitted to the Graduate Faculty of Rensselaer Polytechnic Institute in Partial Fulfillment of the Requirements for the degree of MASTER OF ENGINEERING IN MECHANICAL ENGINEERING Approved: _________________________________________ Ernesto Gutierrez-Miravete, Engineering Project Adviser Rensselaer Polytechnic Institute Hartford, Connecticut Apr, 2010
30
Embed
Design and Molding Simulation of a Plastic Part€¦ · and SolidWorks. Examples of analytical software include systems such as Ansys, Comsol, Moldex3D, and Mold-Flow. ... parameters,
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
The values of all the other parameters used in the analysis are given in Table 2.
Mold temperature 90° C Melt temperature 200° C Coolant inlet temperature 25° C Material injection pressure 70-120MPa Injection speed medium-high Filling control automatic Velocity/pressure switch-over Automatic Tool open/close time 5s Pack/holding control 80% filling pressure VS time
Table 2 Parameters used in the analysis
3.4.2 Key Factors
There are nine combinations of the selected parameters. The maximum deflection is
the result used to find the relationship between the factors (Table 3). The maximum
deflection is not the dimension that we would like to be measured, but rather the most
easily identified (red colored areas in Figure 16) and we can take advantage of that in
order to identify the relationships between parameters by using the DOE.
Conditions Max deflection(mm)
A cycle time 10 seconds 0.4806 flow rate 10 liter/min
B cycle time 20 seconds 0.4734 flow rate 10 liter/min
C cycle time 30 seconds 0.4692 flow rate 10 liter/min
D cycle time 10 seconds 0.4694 flow rate 20 liter/min
E cycle time 20 seconds 0.4735 flow rate 15 liter/min
F cycle time 30 seconds 0.4692 flow rate 20 liter/min
G cycle time 20 seconds 0.4735 flow rate 20 liter/min
H cycle time 30 seconds 0.4694 flow rate 15 liter/min
I cycle time 10 seconds 0.4812 flow rate 15 liter/min
Table 3 Factorial design
20
From the factorial DOE analysis (Figure 15), the cycle time was identified as the
most significant contribution to the maximum deflection, whereas the flow rate is not as
important. Therefore the next step was to use a flow rate of 10 liters per minute in order
to get as much productivity as possible and in order to reduce the cycle time as much as
possible.
Figure 15 DOE analysis result
Since the deformation is not very sensitive to the flow rate, a flow rate of 10 liters
per minute was used as a fixed parameter for further analysis. Since the deformation is
very sensitive to the cycle time, the dimension 34±0.15 can meet design requirements
with a cycle time in the range of 10s~30s (Table 4), (Figure 16-21).
Conditions Max Deflection Dim 1(34±0.15) Dim 2(<=0.15)
A cycle time: 10s flow rate:
10L/Min
0.4806 33.95 0.16
B cycle time:20s 0.4734 33.96 0.12
C cycle time:30s 0.4692 33.98 0.10
Table 4 Single cavity analysis results
21
Figure 16 shows the computed warpage obtained by the Mold-Flow software using
a single cavity with a cycle time of 10s, coolant flow rate of 10 liter/min and a cold
runner system; all the other parameters are given in Table 2.
Figure 16 Warpage under condition A
Figure 17 shows the actual warpage obtained in a single cavity mold under the same
conditions as in the previous figure. Although the locations of largest warpage are well
predicted by the software, the actual magnitude in the real part is larger.
Figure 17 Actual part under condition A
22
Figure 18 shows the computed warpage obtained by the Mold-Flow software using
a single cavity with a cycle time of 20s, coolant flow rate of 10 liter/min and a cold
runner system; all the other parameters are given in Table 2. Figure 19 shows the actual
warpage. As seen in the previous case, the overall behavior is well predicted by the code.
Figure 18 Warpage under condition B
Figure 19 Actual part under condition B
23
Figure 20 is the computed warpage obtained by the Mold-Flow software, while
Figure 21 shows the actual warpage using a single cavity with cycle time 30s, coolant
flow rate 10 liter/min, colder runner system, all other parameters are given in Table 2.
The trend observed before still shows here, however, the warpage is slightly smaller as
the cycle time increases.
Figure 20 Warpage under condition C
Figure 21 Actual part under condition C
24
3.5 Multi-cavity Mold Design
The ideal injection molding system delivers molded parts of uniform density and
free from runners, flash, and gate stubs. To achieve this, a hot runner system, in contrast
to a cold runner system, is usually employed (Figure 22). The material in the hot runners
is maintained in a molten state and is not ejected with the molded part. Hot runner
systems are also referred to as hot-manifold systems, or runnerless molding.
Figure 22 Hot runner system
In a production environment the material is delivered within the mold by utilizing
an externally heated runner system that consists of a cartridge-heated manifold with
interior flow passages. The manifold is designed with various insulating features to
separate it from the rest of the mold, thus reducing heat transfer loss. In order to simulate
an actual production mold scenario, the parameters of a mold flow analysis must match
the mold design (Figure 23).
Figure 23 Multi-cavity mold designs
25
Figure 24 is the computed warpage obtained using the Mold-Flow software for 4
cavities with a cycle time 10s, coolant flow rate 10 liter/min, hot runner system; all other
parameters are given in Table 2. The maximum deflection calculated in this case is
0.4967mm. This is a little larger than the computed single cavity deflection of
0.4806mm (Table 4) due to the cold runner system losing heat resulting in a lower fluid
temperature within the cavity when compared to a hot runner system.
Figure 24 Warpage results for a multi-cavity mold
3.6 Additional Results and Discussion
Additional tests were done with fixed flow rate 10 liter/min and with different cycle
times from 10s to 30s at interval of 2s. Both computed warpage by Mold-Flow software
and measurements of actual molding parts were collected and compared (Figure 25, 26).
26
Figure 25 shows measured overall inner slider lengths as well as computed values
for both single-cavity (SC) and multi-cavity (MC) mold. There is a slight increase in
length with cycle time. However, the differences among all these lengths decrease as the
cycle time increases. Also the rate of the decrease is larger for the measured data than for
the predictions.
Figure 25 Comparison of length results
Figure 26 shows flatness data collected from both single-cavity (SC) and multi-
cavity (MC) mold. Computed flatness values are also shown for comparison. The
agreement between measurements and predictions is better for longer cycle time. The
warpage decreases with cycle time. This is more likely due to the longer time spent by
the part inside the mold at long cycle times.
Figure 26 Comparison of warpage results
27
The theoretical analysis produces results that are comparable to measured data,
although a slightly larger. Also, the parts obtained using the hot runner system deviate
more than those from the cold runner system.
Some additional observations include:
• The analysis does not simulate the ejection. A different ejection pin layout could
change the deflection which can change the residual stress.
• When the part reaches the ejection temperature, theoretically, this should be the
same as the mold temperature (~90°C). Because the tool temperature varies, and the
inside of the part is like a sandwich, it still retains a considerable amount of heat.
Therefore, it continues to warp even after it is ejected from the tool.
3.7 Another option to reduce warpage
Another way to minimize the warpage is to compensate in the design of the part. If
one designs the deformed area (top surface) with a compensation factor of 0.15mm
(Figure 27). The warpage results show that the top surface is straight which means the
final deflection is zero (0.15mm compensation minus 0.15mm warpage) (Figure 28).
Figure 27 Compensation design
Figure 28 Warpage result of opposite design
28
4. Conclusion
The result of this study using 3D design tools in conjunction with Mold-Flow warp
analysis produced an accurate representation of plastic part deflection.
In summary, the results of this paper have shown the following
• Cycle time is the most critical factor affecting warpage. The longer cycle time,
the less warpage.
• There are three contributors of warpage, the shrinkage due to differential cooling
shrinkage, differential shrinkage and orientation shrinkage. In most cases, the
differential cooling shrinkage and differential shrinkage cannot be avoided.
• A high quality mesh is critical to ensure accurate results.
• A simple DOE is helpful to identify key factors affecting warpage and to
minimize the amount of future work. Experimental evidence collected from
single cavity mold can be used to improve and verify the accuracy of the finite
element solution.
• Compensating the identified areas of potentially high deformation by using
opposite design can also reduce warpage.
Alternately, the study suggested another way of minimizing the deformation by
using a method of compensation in the design. However, if the part is complex, this will
require a substantial amount of time due to features being interactive with one another.
At present, this method has not been tested or verified within our company.
It has been shown in this study that the Mold-Flow analysis can have a significant
positive impact in the design and manufacturing processes. The using of Mold-Flow
could help shorten development time.
29
5. References
[1] Edward Chauncey Worden. Nitrocellulose industry. New York, Van Nostrand, 1911.
[2] Dominick V. Rosato, Donald V. Rosato, Marlene G. Rosato; Plastic Design Handbook; Kluwer Academic Publishers, Norwell, MA, 2001, p13. [3] L.M. Galantucci; Evaluation of Filling Conditions of Injection Molding by Integrating Numerical Simulations and Experimental Tests; Materials Processing Technology, July 11 2002. [4] Robert S. Brodkey, Harry C. Hershey; Transport Phenomena; Brodkey Publishing; Columbus, Ohio, 2001
[5] Seong Taek Lim, Woo II Lee; An Analysis of the Three-dimensional Resin-transfer Mold Filling Process; Composites Science and Technology; South Korea; Apr 13 1999.
[6] Auto desk Mold-Flow Insight material data warehouse.
[7] E. Alfredo Campo; The Complete Part Design Handbook for Injection Molding of Thermoplastics; Hanser; Cincinnati, Ohio, 2006.