Figure 1 shows two similar middle cover grilles of a fan disk, maximum external dimensions are 200.3 mm * 164.2 mm * 58.7 mm, material is ABS without any fiber filler, using 4 cavity injection molding. There are differences in number of grids and material thickness of grid plastic parts, and cavity layout is actually 2+2.

 

Figure 1 Middle cover grille

Features and technical requirements of plastic parts: ①There are many inner clips (inverted) buckles, and there are 6 single plastic parts, which belong to assembly structure and require high precision; ②Plastic grille and rib structure are densely covered, thickness of grille is thin, forming and demoulding are difficult, there is a risk of material shortage and mold sticking during injection;③ Height of plastic grille ribs is high, and it is easy to stick to mold parts during injection ;④Quality of appearance surface of plastic parts is high, and some areas need to be treated with dermatoglyphics. Appearance surface must not have gate marks, defects such as protrusions, depressions, shrinkage printing, trapped air, scorching, and flashing are not allowed; ⑤ In order to control costs and avoid material waste, it is necessary to control volume of condensate.
Mold adopts a 4-cavity structure, combined with factors such as size of plastic parts, quality requirements, and production batches. If common runner is used for feeding, material will be wasted and runner will be long, molded plastic parts will have risks such as material shortage and shrinkage, and it is difficult to ensure molding quality of plastic parts. No gate marks are allowed on the surface of plastic part, and gate cannot be directly set on the surface of plastic part to be formed. Finally, feeding method of hot runner to ordinary runner is adopted. Size of hot runner is φ18 mm and diameter of hot nozzle is φ6 mm, size of common runner is set according to different schemes.

After feeding method is determined as hot runner to ordinary runner, considering feasibility of designing gate surface and mold structure of plastic part to be formed, common runner gate type can only choose side gate or latent gate. For this reason, five pouring schemes are proposed, as shown in Figure 2. Through CAE simulation analysis, advantages and disadvantages of five schemes are compared, and final gating system is determined. Pouring plan: ① One side gate feeding on side wall, gate size φ6 mm*1.2 mm, runner size φ9 mm*7 mm*7 mm; ② 2 side gate feeding on side wall, gate size φ6 mm*1 mm, runner size φ9 mm*7 mm*7 mm; ③4 side gates for feeding, gate size φ5 mm*1 mm, runner size φ9 mm*7 mm*7 mm; ④ 6 side gates for side wall feeding, gate size φ4 mm*1 mm, size of runner is φ9 mm*7 mm*7 mm; ⑤Middle rib is lurking feed, size of gate is φ4 mm*1 mm, and size of runner is φ8 mm*6 mm*6 mm.

 

Figure 2 Pouring scheme
CAE simulation analysis of above five pouring schemes is carried out through Moldflow software, Final pouring system is determined by comparing filling balance, rationality of injection pressure, feasibility of weld line, rationality of temperature at flow front, uniformity of volume shrinkage rate, and volume of condensate.

1. Filling balance. As shown in Figure 3, Schemes 1 and 2 have problem of unbalanced flow, which cannot ensure that each cavity is evenly filled with melt. Schemes 3 to 5 can reach end of flow at the same time, which is more reasonable.

 

Figure 3 Filling balance

2. Maximum injection pressure is shown in Figure 4. Maximum injection pressure of Schemes 1 and 2 is large, exceeding 70 MPa, and maximum injection pressure of Schemes 3 to 5 is about 60 MPa. A larger injection pressure will cause a larger injection pressure. Residual stress of , so schemes 3 to 5 are more reasonable.

 

Figure 4 Maximum injection pressure

3. Welding line is shown in Figure 5. Plastic part is a grid structure, and there are certain differences in thickness of each area, so no matter what kind of pouring scheme is used, there will be multiple welding lines. However, color of plastic part is white, traces of weld line on appearance surface are small, and its influence on the appearance of plastic part can be ignored, but it should be noted that influence of weld line on structural strength of molded plastic part should be avoided. During assembly and disassembly of grille in daily use, two side walls are stressed areas, and its strength needs to be ensured. As shown in Figure 5, weld lines of Schemes 1, 2, and 5 all appear on sidewalls on both sides, which have a greater impact on structural strength. Schemes 3 and 4 weld lines appear at grid position, which has little impact on the overall structural strength, schemes 3 and 4 are more reasonable.

 

Figure 5 Weld line

4. Temperature at flow front is shown in Figure 6. On the surface of plastic part to be formed, temperature of solutions 1, 2, and 5 all decrease greatly at the end of flow, and if it exceeds 5℃, stagnation will occur. In schemes 3 and 4, temperature reduction is small, and maximum difference is less than 3℃, which is within reasonable range of material flow.

 

Figure 6 Flow front temperature

5. Volume shrinkage rate is shown in Figure 7. Difference between maximum surface shrinkage rates of Schemes 1 and 2 is large, exceeding 3%, and there will be shrinkage marks on appearance surface. Schemes 3 to 5 have a smaller difference in maximum surface shrinkage.

 

Figure 7 Volume shrinkage
(6) Comparing condensate volume of 5 pouring schemes, scheme 4 has the largest volume of condensate and wastes more materials. Volume of condensate produced by schemes 1, 2, 3, and 5 is relatively small. Considering cost and waste, Option 4 is poor.
According to summary and comparison of above parameters, scheme 3 is better in ensuring cavity filling, ensuring molding quality of plastic parts, and controlling material costs. Finally, casting method of scheme 3 is determined.

When plastic parts are formed, mold parts are easy to stick, especially positions of buckles, screw columns and grille ribs. Therefore, a balanced push-out mechanism should be set up in these positions to ensure that molded plastic parts can be demolded smoothly. A push-tube mechanism is set at position of screw column of molded plastic part, and second push is used to prevent position from sticking to mold part. At rib position in the middle of molded plastic part, set a flat push rod, thickness of which is smaller than thickness of rib by 0.1 mm on one side, to avoid risk of rib sticking to mold parts. In addition, a direct push mechanism is arranged on the two side walls to realize the overall push of plastic parts and ensure push-out balance of molded plastic parts. Design of push-out system is shown in Figure 8.

 

Figure 8 Push out system

According to design principles of cooling system and structural characteristics of plastic parts, cooling system shown in Figure 9 is designed. There are 8 cooling water circuits, 4 for core and 4 for cavity plate, diameter of water path is φ10 mm to ensure a sufficient flow rate of cooling liquid. In addition, in combination with demoulding direction of plastic part, in order to make cooling water channel have same size from surface of molded plastic part and ensure uniform cooling of molded plastic part, water channel is arranged in a curve along edge of plastic part to be formed, so as to realize a network water channel structure that adapts to characteristics of plastic part and its shape.

 

Figure 9 Cooling system

Mold structure is shown in Figure 10. In actual injection production process, molten plastic enters mold hot runner through injection molding machine nozzle, enters cavity after continuous heating by hot runner 10 and hot nozzle 11. After filling, pressure maintaining, and cooling, plastic part 13 is formed. When molded plastic part reaches push-out temperature, fixed die plate 12 and movable die plate 5 are separated by slider of injection molding machine, then plastic part 13 is pushed out by ejector rod, push plate 9, push rod fixing plate 8, push-out mechanism components, etc. , that is, to complete its injection process, then close mold and enter next injection cycle.

 

Figure 10 Mold structure
1. Fixed die seat plate 2. Hot runner plate 3. Fixed die insert 4. Water pipe joint 5. Moving template 6. Support column 7. Push rod 8. Push rod fixed plate 9. Push plate 10. Hot runner 11. Heat Nozzle 12. Fixed mold plate 13. Plastic part 14. Locking block 15. Moving mold insert 16. Oblique push guide block 17. Pad block 18. Inclined push block 19. Positioning block 20. Moving mold seat plate