Simulation of Injection Molding

Features

injection molding analysis is the analysis of the filling behavior of resin injected into a mold by an injection molding machine. (Click here for more information on injection molding.)
This is a common analysis for plastic products in product design and mold specification design, and it is possible to predict flow patterns, resin pressure distribution, resin temperature distribution, weld line occurrence positions, etc. From the obtained results, it is possible to predict whether molding is possible or not, and to carry out advance considerations when manufacturing molds.

Fig. 1 Advantages of Injection molding analysis

● For information on data acquisition for injection molding analysis, please click here.

Injection molding analysis details

In injection molding analysis, filling and packing analysis and warpage analysis are important simulations in the injection molding process.

Filling and Packing Analysis

Filling and packing analysis is an analysis that simulates how the resin flows into the mold (filling process) and the subsequent packing stage (applying holding pressure to suppress resin shrinkage). Specifically, it focuses on the following points:

Filling analysis: This simulates how the resin is injected and fills the cavity inside the mold (the space that determines the shape of the molded product). It determines whether molding is possible or not and optimal molding conditions based on the resin flow, flow speed, temperature distribution, pressure distribution, etc.
Packing analysis: After filling is complete, the process of applying packing pressure to compensate for the resin cooling and shrinkage is simulated. Check for sink marks, voids, and short shots.
Cooling analysis: Simulates the cooling behavior inside the mold due to heat conduction and heat transfer. Calculates the mold surface temperature and resin temperature distribution, and considers the layout of the cooling circuit and the temperature of the coolant. Can be coupled with a filling and packing analysis.

Pressure at V/P switching — Fig. 2 Example of pressure at V/P switchover

Warpage analysis

In addition to the above-mentioned Filling and Packing analysis, warpage analysis analyzes the warping that occurs during the cooling process of the molded product. It simulates the warping and deformation that occurs due to uneven shrinkage when the resin cools and solidifies after injection molding.

Warpage analysis: Predicts shrinkage and warpage of molded products after release from the mold based on the results of filling, packing, and cooling analysis. The causes of warpage can be classified into three factors: cooling difference, shrinkage difference, and orientation difference. Once the cause is identified, solutions such as changing the gate position or molding conditions can be considered.

Warp deformation — Fig. 3 Example of warp deformation

Evaluation of fiber orientation

In the case of fiber-reinforced resins containing glass-fiber, it is important to take into account the fiber orientation, as this can have a significant effect on product performance.
To know fiber orientation information, the fiber orientation tensor is output from injection molding analysis.
Here we show how an evaluation is performed using a flat test piece (Figure 4).
The fiber orientation tensor is a probability distribution (0 to 1) of the fiber orientation. Here, the fiber orientation tensor is evaluated in three directions: x-axis, y-axis, and z-axis. As the tensor value increases (approaching 1), it means that the fibers are more aligned in each direction.
The measured fiber orientation tensor value was calculated using a unique method from the image of the product cross section at the evaluation position observed with an optical microscope. The fiber orientation tensor was calculated at 5% intervals in the thickness direction of the molded product. The rod-shaped and circular objects in the enlarged image of Figure 4 are glass-fiber.
Figure 5 shows the fiber orientation tensor measurements and the analytical results obtained from injection molding analysis.
It can be said that the analysis was able to predict the actual fiber orientation tensor.

評価位置と光学顕微鏡による断面観察像 — Fig. 4 Evaluation position and cross-sectional observation image using an optical microscope

Comparison between experimental and analytical results — Fig. 5 Fiber orientation tensor comparison results

Case studies-1

Verification of prediction accuracy of "warping" and "glass-fiber orientation"

The prediction accuracy of warpage and glass-fiber orientation was verified using a model (Fig. 6) that mimics the rear member of an automobile part. LEONA™ 14G35 (PA66, GF35%) was used in this verification. The actual molding conditions are shown in Fig. 7. These conditions were also entered in injection molding analysis.
By incorporating the actual molding conditions into the analysis, a better analysis can be performed.

Model used — Fig. 6 Rear member model used

Molding
Conditions — Fig. 7 Molding conditions

Accuracy validation of warpage

The warpage was evaluated using the displacement in the Z-axis direction at measurement positions on the product. As shown in Fig. 8 (left), 20 measurement positions were set on the outer rib, and anchor points necessary for setting the reference plane were set at numbers 6, 13, and 19. The results of the actual product were measured using a three-dimensional measuring device. For the analysis results, the Z-direction displacement output from Autodesk's Moldflow was used for comparison, as shown in Fig. 8 (right).

Measurement position (left) and Z-direction displacement output result (right) — Fig. 8 Measurement position (left) and Z-direction displacement output (right)

Fig. 9 shows the experimental and analytical results together. The displacements match well, indicating that the analysis was able to predict the actual warpage.

Accuracy validation of glass fiber orientation

The method described in ”Evaluation of fiber orientation” was used to verify the accuracy of the fiber orientation prediction for this geometry.
As an example, Fig. 10 shows the fiber orientation tensor result in the x-axis direction.
As can be seen from Fig. 11, there are many glass-fiber aligned in the x-axis direction on the surface of the molded product (measurement position: 70-90%), whereas in the center (measurement position: 40-60%), there are more glass-fiber aligned in the y-axis direction. This shows that the degree of orientation increases because the surface of the molded product is quickly cooled by the mold.

Fig. 10 Fiber orientation tensor output results

評価位置と繊維配向テンソル比較結果 — Fig. 11 Evaluation position and fiber orientation tensor comparison results

Case studies-2

Gate position optimization

An oil pan of an automotive part was modeled, and the gate position was optimized to reduce warping deformation of the flange.
By allowing the resin to flow along the flange, the fiber orientation is improved, increasing stiffness.
It was confirmed that the fiber orientation could be predicted, and the warpage deformation obtained by the analysis was almost consistent with the actual warpage deformation. Even for complex shapes such as oil pans, the actual behavior can be reproduced in the analysis by taking fiber orientation into account.

For details, please see CAE Case Study "Accuracy Validation of Warpage Analysis".