This paper aims at design difficulties of molds with complex core head structures, innovatively proposes a mold core component structure scheme with a vertical and horizontal locking inlay structure, which cleverly solves design problem of complex core head structure. Actual operation proves that its molding effect is good and meets quality requirements of plastic parts.
With rapid development of modern industry, CAD/CAE/CAM technology has been widely used in modern mold design and production. Application of this technology can greatly improve product quality, shorten R&D and production cycles, reduce production costs, and enhance core competitiveness of enterprises.
Level of mold design and manufacturing technology is an important indicator of a country's manufacturing level. Success or failure of mold design also directly affects mold manufacturing and product production. While applying CAD/CAE technology, mold design should also pay attention to importance of innovative design to solve various technical problems faced in production. Author designed an injection mold for a cage-type box seat plastic part.

Structure of plastic part is shown in Figure 1. Material of plastic part is acrylonitrile-butadiene-styrene copolymer (ABS), and its brand is HF380. Plastic part is generally a fence-type structure around perimeter, a hole-grid structure on the top, with deep hole characteristics, snap-on structure and side holes, and product size accuracy level is MT3. Structure of plastic part is complicated, especially top structure, which brings difficulties to mold design, manufacturing and product molding.

 

Figure 1 Schematic diagram of plastic part structure

Molding process parameters of ABS HF380 are shown in Table 1.

Table 1 ABS HF380 molding process parameters

According to shape and technical requirements of plastic part, a single gate solution and a four-gate solution are proposed, and Moldflow MPI filling analysis is used to compare solutions. Main analysis results are shown in Table 2.

Table 2 Comparison of single gate solution and four-gate solution
It can be seen from Table 2 that four-gate solution can effectively reduce cavity pressure, shear rate and shear stress, maintain a balanced flow state, and mold structure is feasible compared with single gate solution.

Due to deep cavity structure characteristics of product, product needs to be fully cooled, especially core part. Therefore, this design sets cooling pipes on upper, lower, surrounding and deep cavity of cavity. Moldfiow MPI is used for flow, cooling and warpage analysis. Cooling system design solution and analysis results are shown in Figure 2, and warpage analysis results are shown in Figure 3. As shown in Figure 2, temperature difference between cooling water inlet and outlet is 0.3℃, which meets basic requirements of molding process. As shown in Figure 3, global deformation of product is 0.64mm, of which deformation caused by shrinkage is 0.638mm, deformation caused by uneven cooling is 0.09mm, and deformation caused by orientation is 0.1mm. Deformation is mainly caused by shrinkage, which can be eliminated by adding shrinkage to mold size to meet product size accuracy requirements.

 

Figure 2 Cooling analysis results

 

Figure 3 Warpage analysis results

The overall structure scheme of mold is shown in Figures 4 and 5.

 

Figure 4 Mold assembly view

 

1-fixed mold plate; 2-nylon anti-hook; 3-inclined slider core pulling mechanism; 4-moving mold plate; 5-moving mold plate fixed distance screw; 6-fixed mold plate fixed distance screw; A-moving and fixed mold plate guide device; B-moving mold plate guide device; C-pushing guide device;
Figure 5 Mold assembly three-dimensional exploded view
Action process of mold is: mold first opens from point I. Fixed mold plate 1 moves with movable mold due to action of nylon hook 2, and fixed mold plate stops moving when fixed mold plate fixed distance screw 6 travels. Due to action of nylon hook, movable mold plate 4 and push plate assembly stick to fixed mold plate, stop moving, rest of movable mold (movable mold seat) continues to open mold, and mold is separated at point III, driving core assembly to open mold, then realizing ejection action of push plate movable mold plate assembly relative to core; when movable mold seat moves to stroke of movable mold plate fixed distance screw 5, it drives movable mold plate and push plate assembly to overcome tension of nylon hook and disengage from it, separates at point II, continues mold opening stroke. Inclined slider core pulling mechanism 3 starts to move to realize lateral core pulling. When mold is opened to a sufficient stroke, hydraulic push rod of injection molding machine pushes mold push rod ejection device to realize large stroke ejection of product. Product and pouring system are removed manually or by a robot.

(1) Ejection mechanism design. Product has characteristics of deep cavity and thin wall. In order to realize reliable ejection, this scheme adopts push plate ejection. Since push plate ejection stroke is limited, it is combined with push rod ejection to implement push plate and push rod secondary ejection.
(2) Core pulling mechanism design. Since there are large grids and side holes around workpiece and core pulling distance is not large, it is suitable to use a lateral core pulling mechanism with an inclined slider.
(3) Design of guiding mechanism. This mold has many parting surfaces, mold plate components need to ensure good guidance and good rigidity. Fixed mold part is guided by movable fixed mold guiding device A; movable mold part is guided by movable mold guiding device B and ejection guiding device C because movable mold push plate and other components need to move to realize ejection of workpiece.
(4) Design of the overall rigidity of mold.
Since lateral molding area of product is large, mold cavity needs to withstand very large lateral pressure. Therefore, fixed mold of mold adopts form of an integral cavity and ensures sufficient side wall thickness. Support columns are added under support plate to improve rigidity of each template of movable mold part. Each mold plate and molding part adopts a suitable structural size to ensure overall and local rigidity. Movable mold guide device B also bears role of supporting quality of movable mold push plate and other components during ejection to ensure rigidity of movable mold part during mold opening movement.

Observing plastic part, the entire top of core is a molding surface covered with holes, structure of top and back is very complex, as shown in Figure 6. If an integral core is used, it will bring great difficulties to processing and polishing of molded parts. How to solve this problem? Author refers to structure of "beamless hall" style building in ancient China and innovatively designs a vertical and horizontal lock-type inlay structure, which solves this problem well.

 

Figure 6 Plastic part top and back structure core component structure design
As shown in Figure 7. From bottom of core seat, bottom of core seat is very simple and structural position is sufficient, as shown in Figure 8.

 

Figure 7 Schematic diagram of core components

 

Figure 8 Bottom view of core components
How are longitudinal core inserts and transverse core inserts assembled and fastened? Implementation method is to open longitudinal and transverse grooves on the top of core seat part, as shown in Figure 9, for assembling longitudinal and transverse core inserts.

 

Figure 9 Core seat top groove structure
During assembly, longitudinal core insert is inserted into longitudinal groove at the top of core seat part. At this time, longitudinal core insert also has longitudinal and vertical freedom. As shown in Figure 10.
Figure 11 shows a schematic diagram of external structure of a longitudinal core insert. After longitudinal core insert is assembled, two sides of bottom surface of core seat groove are exposed, forming a buckle boss.

 

Figure 10 Assembling longitudinal insert on the top of core seat

 

Figure 11 Schematic diagram of external structure of longitudinal core insert
After longitudinal core insert is assembled, transverse core insert is buckled in, as shown in Figure 12, and transverse core insert structure is shown in Figure 13. Horizontal core insert, due to alignment limit between side of insert and groove wall of core seat, has its own longitudinal position limited, and longitudinal freedom of longitudinal insert is also limited; after horizontal core insert is buckled in, its lower buckle groove and buckle boss of longitudinal insert form a buckle fit, and limiting effect of buckle fit realizes limitation of horizontal freedom of horizontal core insert assembly.

 

Figure 12 Assembly of horizontal core insert

 

Figure 13 Schematic diagram of the appearance of horizontal core insert
Horizontal core insert is tightened with screws at the bottom of core seat, so that assembly of core insert is completed, as shown in Figure 14. Horizontal insert is tightened with screws to limit vertical freedom of longitudinal and horizontal core inserts in core seat at the same time. So far, assembly freedom of components in core seat is completely limited.
Actual operation proves that its molding effect is good and meets quality requirements of plastic parts.

 

Figure 14 Core insert screw fastening

Molding process of cage-type box seat plastic parts was analyzed by mold flow CAE technology, optimized process scheme was proposed for pouring system and cooling system design; detailed design of injection mold was carried out, combination of sequential parting, lateral parting core pulling, and secondary ejection was reasonably adopted to realize molding of complex products; structure of mold core component was innovatively designed, problem of complex core head structure design was cleverly solved by adopting vertical and horizontal locking inlay structure, which can be used in design of injection molds for plastic parts with similar structures.