Simulation of Injection Molding

Features

Injection molding analysis is the analysis the filling behavior of resin injected into a mold from an injection molding machine.
It is a common analysis for plastic products in product design and mold specification design, and can predict flow patterns, resin pressure distribution, resin temperature distribution, weld line generation locations, etc. Based on the obtained results, it is possible to predict whether molding is possible or not, and to perform preliminary studies when fabricating molds.

Advantages/disadvantages of injection molding analysis — Fig. 1 Advantages/Disadvantages 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

Case studies-1

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. Resin flowed along the flange to increase fiber orientation and improve rigidity. Comparison of the warp deformation obtained from the analysis with the actual warp deformation showed a close match.

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

Example of oil pan injection molding analysis — Fig. 4 Validation of optimal gate position and warpage prediction accuracy

Case studies-2

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. 3) 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. 4. 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. 5 Rear member model used

Molding
Conditions — Fig. 6 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 Figure 5 (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 Figure 5 (right).

Measurement position (left) and Z-direction displacement output result (right) — Fig. 7 Measurement points (left) and z-direction displacement output result (right)

Fig. 6 shows the experimental results and analysis results together. The amount of displacement matches well, indicating that the analysis was able to predict the actual warpage.

Fig. 8 Comparison between experimental and analytical results

Accuracy validation of glass fiber orientation

For fiber-reinforced resins containing glass fibers, such as the material used in this verification, fiber orientation can have a significant impact on product performance, and it is important to take this orientation into account. In order to know the fiber orientation information, the fiber orientation tensor is output by the injection molding analysis. The fiber orientation tensor was evaluated at a measurement position set slightly below the center of the product (Fig. 7 left).

Example of measurement points and cross-sectional images observed with an optical microscope — Fig. 9 Measurement points and Cross-sectional image observed with an optical microscope
(Numbers on the right of the image indicate the measurement position (%) in the cross-sectional depth direction)

The fiber orientation tensor is a probability distribution (0 to 1) of fiber orientations; in Autodesk's Moldflow, the fiber orientation tensor is evaluated in three axes: flow direction, flow orthogonal direction, and thickness direction, which are called a11 direction, a22 direction, and a33 direction, respectively.
The actual value of the fiber orientation tensor was calculated from the image of the cross section of the product at the measurement position observed by optical microscope as shown in Fig. 7 (right) using an original method. In this case, the fiber orientation tensor was calculated by dividing it by 5% in the direction of the molded product thickness. As shown in Fig. 9, the obtained measured values (solid line) and the analyzed values (dashed line) are well matched, indicating that the actual fiber orientation tensor could be predicted by the analysis. As can be seen from Fig.7 (right) and Fig. 9, there are more glass fibers aligned in the flow direction (a11) on the molded product surface (measurement points: 70-90%), whereas in the center (measurement points: 40-60%), there are more glass fibers oriented in the orthogonal direction (a22), indicating a scattering.