Abstract: Taking injection plastic cages in high speed precision Ｒoller bearings as research object，the position and quantity of sprue are simulated and analyzed by using analysis software Moldflow. The reasonable sprue scheme is designed，and the position and quantity of sprue for mold and structure of cages are optimized through assembly test and performance detection． The strength and load carrying capacity of injection plastic cages are improved，and the problem is effectively solved about fracture of injection plastic cages．
Key words: rolling bearing; injection plastic cage; sprue; simulation analysis; Moldflow
In injection molds, the sprue is a short and narrow channel connecting the runner and the mold cavity, which plays a role in regulating and controlling the flow rate, replenishment time, and preventing backflow. The position and quantity of gates are important mold structural parameters that directly affect the quality of injection molded parts. For injection molded retainers with complex structures, the selection of gate positions and quantities based on experience is often not well-designed. By using CAE software to analyze and optimize the position and quantity of the sprues in the injection molding cage, changing the distribution of weld lines can improve the tensile strength of the injection molding cage and avoid the impact of weld lines on the appearance and performance of the cage; This provides a scientific basis for the design of injection cage molds and enables timely optimization of injection cage structure and mold design.
Under high-speed operation conditions, the injection molded cage of high-speed precision bearings is subjected to extremely uneven normal loads on each rolling element, and the motion of the rolling elements is also uneven. When the angular velocity of the rolling elements is not consistent with that of the cage, collisions will occur between the rolling elements and the cage pockets. In severe cases, it can cause the cage crossbeam to break and the bearing vibration to increase rapidly, resulting in bearing failure. Therefore, it is required that the high-speed precision injection molded cage should have good mechanical strength.
1. Retaining frame structure
The injection molded cage structure used in high-speed precision cylindrical roller bearings is shown in Figure 1, with 10 square window holes evenly distributed around the circumference. Each crossbeam of the cage has a convex platform in the middle of the inner side, and the end face of the convex platform has a certain angle with the crossbeam, making the structure more complex. The dimensional tolerance and geometric tolerance accuracy of the retaining frame require high precision, and the rollers are required to roll flexibly when installed in the window holes.
Figure 1 Schematic diagram of cage structure
2. Moldflow analysis of retaining frame
Create a 3D solid model of the cage using Pro/E, with dimensions of outer diameter 47.7 mm, inner diameter 42.8 mm, inner diameter 40.8 mm, and width 18.3 mm. Convert the cage model into an STL file and import it into Moldflow for injection molding simulation analysis. After modifying the surface mesh type, a mesh model with 3106 triangles was obtained. The injection molded parts are made of carbon fiber reinforced polyether ether ketone, with the following parameters: mold surface temperature of 170 ℃, injection temperature of 380 ℃, and default settings for filling control, speed/pressure switching, and holding pressure control.
2.1 Pre analysis of Gate Position
When designing the pouring system, considering the type of mold, the pouring point should be placed as close as possible to the optimal pouring gate position. The optimal gate position given by Moldflow in image form is shown in the dark area of Figure 2, which means that the gate is more suitable to be set in the middle of the cage crossbeam.
Figure 2: Optimal Gate Position
2.2 Location and quantity of gates
Injection molded retaining frames generally use concealed gates or point gates, and the gate size should be taken as large as possible according to the width of the retaining frame ring beam and crossbeam.
The optimal gate position obtained from software analysis is an important reference information for gate setting, but it may not necessarily be the actual gate position for mold design. According to the structural characteristics of the injection molding cage, the gate position can be set in the middle of the crossbeam or on the ring beam. The multi gate setting facilitates smooth mold filling and ensures accuracy, therefore four schemes were set up for comprehensive simulation analysis of the cage pouring system.
(1) The sprue is set in the middle of the crossbeam, with 5 sprues. The distribution of weld marks after filling analysis is shown in Figure 3. As can be seen from the figure, 5 weld marks are formed in the middle of the crossbeam without sprues, and 5 weld marks are distributed above and below the boundary between the ring beam and the crossbeam.
Figure 3 Distribution of weld marks with 5 gates on the crossbeam
(2) The sprue is set in the middle of the crossbeam, with 10 sprues. The distribution of weld marks after filling analysis is shown in Figure 4. It can be seen from the figure that the weld marks are distributed in the middle of the window ring beam, reaching 20 weld marks.
Figure 4 Distribution of 10 sprue weld marks on the crossbeam
(3) The sprue is set on the ring beam, with one sprue. The distribution of fusion marks after filling analysis is shown in Figure 5. It can be seen from the figure that there are a total of 9 fusion marks distributed at different positions on the crossbeam of the cage, and only 2 fusion marks are distributed on the ring beam.
Figure 5 Distribution of weld marks with one gate set on the ring beam
(4) The sprue is set on the ring beam, with 3 sprues. The distribution of fusion marks after filling analysis is shown in Figure 6. It can be seen from the figure that there are a total of 5 fusion marks distributed on the crossbeam, and the fusion marks on the ring beam at the sprue are partially distributed at the window holes of the cage.
Figure 6 Distribution of weld marks with 3 gates on the ring beam
2.3 Mold pouring system setup and testing
The positions and quantities of the four gate designs mentioned above all form weld marks on the retaining frame crossbeam, with Scheme 2 and Scheme 3 having significantly more weld marks than the other two schemes; The weld marks on the ring beam at the gate of the retaining frame in Scheme 4 are partially distributed at the window holes of the retaining frame, while in Scheme 1, all weld marks on the ring beam of the retaining frame are distributed at the junction of the ring beam and the crossbeam; Scheme 1 symmetrical injection molding can ensure the accuracy of the retaining frame. The structural diagram of the cage pouring system for option 1 is shown in Figure 7, and the mold is designed according to the pouring scheme.
Figure 7 Structure diagram of cage pouring system
After pouring, the surface of the cage is flat and smooth, without cracks or concave convex defects. The radial tensile strength of the cage was tested to be 111.6 MPa. At the initial stage of the installation test, the bearings operated normally, with no abnormal fluctuations in test parameters such as bearing temperature rise, host current, and host vibration, and no abnormal operating noise; As the load increases, the vibration of the main body of the testing machine suddenly increases, and there is no significant change in the current of the main body and the temperature rise of the target bearing. After shutdown inspection, it was found that the crossbeam of the test holder was partially broken, and the broken part happened to be located at the fusion mark of the injection molded holder. During the installation test of the retaining frame, the crossbeam broke due to the action of rollers, and the magnitude of the force perpendicular to the crossbeam direction needs to be detected. Due to the diameter of the window hole being 11.4 mm, a tensile fixture with a width of 6 mm and a height of 5.6 mm was designed to conduct tensile tests on the retaining frame window hole crossbeam.
According to the test results, the maximum tensile force at the time of fracture of the retaining frame crossbeam is 566 N, and the minimum tensile force is 353 N, with a large degree of variability, indicating that there is a significant difference in the strength of the weld marks on the retaining frame crossbeam.
3. Optimization and analysis of retaining frame structure
Due to the unchanged size and quantity of bearing rings and rollers, while ensuring appropriate guide clearance of the cage and not affecting the lubrication of the bearing, the radial and axial space of the bearing should be fully utilized. The thickness and width of the cage ring beam should be reasonably increased to improve the strength of the cage. The optimized cage structure is shown in Figure 8. Moldflow is still used for injection molding simulation analysis, and an optimized three-dimensional solid model of the retaining frame is established with dimensions of outer diameter 47.9 mm, inner diameter 38.9 mm, and width 19.3 mm. A modified mesh model with 3200 triangles is obtained by dividing the surface mesh type, and the retaining frame material and related parameters remain unchanged.
Figure 8: Structural diagram of optimized cage
3.1 Optimized Position and Quantity of Retainer Gates
The gate is set in the middle of the cage crossbeam, which will form weld marks on other cage crossbeams, reducing the mechanical performance of the cage crossbeam. After optimizing the structure, the position of the retaining frame gate is set on the inner ring of the ring beam. The design scheme for the position and quantity of gates is as follows.
(1) The sprue is set on the ring beam, with one sprue. The distribution of fusion marks after filling analysis is shown in Figure 9. It can be seen from the figure that there are only two fusion marks distributed on the ring beam, and most of the fusion marks are distributed on the crossbeam, which is prone to fracture and reduced tensile strength.
Figure 9: Distribution of weld marks on a ring beam with one sprue
(2) The sprue is set on the ring beam, with 3 sprues evenly distributed. The distribution of weld marks after filling analysis is shown in Figure 10. As can be seen from the figure, there are 3 weld marks distributed on the ring beam at the sprue, 10 weld marks distributed on the ring beam outside the sprue, and no weld marks on the crossbeam.
Figure 10 Distribution of 3 sprue weld marks on the ring beam
3.2 Setting and testing of the optimized cage mold pouring system
Among the two optimized gate position and quantity design schemes, Scheme 1 forms more weld marks on the retainer crossbeam, while Scheme 2 has no weld marks on the retainer crossbeam. During injection molding in Plan 2, the ring beam at the gate is first formed into three weld lines; Continuing with the formation of the retaining frame crossbeam, the distance between the crossbeam and the gate position has different forming times, but no weld marks have been formed at the retaining frame crossbeam; The non sprue ring beam is finally formed, and there are many weld marks distributed. After optimizing the retaining structure, the thickness of the ring beam increases, which can effectively avoid the strength reduction caused by welding marks. Scheme 2 improves the mechanical performance of the retaining frame crossbeam and meets the design requirements.
After optimization, the gate system structure of the retaining frame is shown in Figure 11, and the mold is designed accordingly. The radial tensile strength of the cage after injection molding is 108.3 MPa, and an increase in the cross-sectional area of the cage ring beam can effectively offset the decrease in strength caused by weld lines; The surface of the holder is flat, smooth, and free of cracks and concave convex defects.
Figure 11 Structure diagram of optimized cage pouring system
Due to the unchanged size of the retaining frame window hole, the original tensile fixture is still used for the tensile test of the retaining frame window hole crossbeam. In the tensile test, the tensile curve shows a tensile plateau at around 520 N, indicating that the retaining frame crossbeam has a certain degree of toughness during tensile testing; The minimum tensile force at break is 692 N, which is 65.7% higher than before optimization.
The target bearing of the installation test runs smoothly, and the test parameters such as bearing outer ring temperature, host current, vibration, etc. fluctuate normally with the changes of the test conditions. After the test state stabilizes, the readings of each test parameter remain stable without any abnormal situations.
4. Conclusion
The position and quantity of gates have a direct impact on the quality of injection molded retaining frames. Using CAE software to analyze the optimal gate position can only be used as a reference, and the gate position and quantity of the retaining frame should be adjusted according to the actual situation of the injection molded retaining frame. The use of Moldflow can effectively simulate the distribution of weld lines in injection molded retaining frames based on design schemes, providing scientific basis for the improvement of retaining frame structures and mold design.
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