Water dispenser cover is shown in Figure 1. Material is ABS, the overall dimensions are 254 mm * 165 mm * 42 mm, wall thickness is 3 mm, and there are two φ4 mm blind holes on the left side (see Figure 2 (a) ). After turning plastic part over, there are two hollow structures on inner surface corresponding to blind hole, as shown in Figure 2(b). There is a U-shaped bracket on the right side of plastic part. There is a round hole on each side wall of bracket. There are 2 cylinders inside the bracket. Height of cylinder is 5 mm. Axis direction of round hole is perpendicular to axis direction of cylinder inside U-shaped bracket.

 

Figure 1 Two-dimensional structure of water dispenser cover

 

Figure 2 Three-dimensional structure of water dispenser cover
(a) Outer surface (b) Inner surface

Since outer surface of plastic part is not allowed to have gate marks or welding marks to be formed, it is suitable to pour from inner surface and set gate on inner surface of plastic part to be formed, as shown in Figure 3. Gate type is point gate, and gate diameter is φ3.5 mm.

 

Figure 3 Gate location
During Moldflow Insight mold flow analysis, according to injection molding machine, process parameters, plastic part quality and other requirements, material is set to ABS, maximum injection pressure is set to 125 MPa, melt filling time is set to 4s, plastic temperature is set to 240℃, mold temperature is set to 60℃, grid size is set to 5 mm, and number of tasks is set to 8. Obtained mold flow analysis results are shown in Figure 4.

 

Figure 4 Mold flow analysis results
As can be seen from Figure 4, filling time at both ends of plastic part is the longest, about 4.126 s, as shown in Figure 4(a); there are scattered pores at both ends of plastic part, located at the end surface of bracket, cylinder and round hole, as shown in Figure 4(b); plastic part has good filling and formability, as shown in Figure 4(c); injection pressure at pouring point is the largest, 27.449 MPa, and injection pressure at both ends of plastic part is the smallest, 2.745 MPa, as shown in Figure 4(d); maximum shear stress is located near two circular holes, which is about 1 MPa, and minimum shear stress is located at four corners of plastic part, which is 0~0.16 MPa, as shown in Figure 4 ( As shown in e); there are two welding marks on plastic part, and length is short, as shown in Figure 4(f).
It can be seen from mold flow analysis results that injection time required to mold plastic part is short, and maximum injection pressure is only 27 MPa. Generally, injection molding machine can meet production requirements. For air trapped on end cylinder of plastic part, an inlay can be set at corresponding position part, so that gas can be discharged from gaps around inserts, which is helpful to increase exhaust effect and eliminate trapped air in plastic parts.

Since demoulding directions of cylinder inside U-shaped bracket on the side of plastic part and round hole on side wall of bracket are perpendicular to each other, and distance between the two is relatively close, if separate demoulding mechanisms are designed separately, structure will be crowded, affecting strength and production efficiency of mold base. Demoulding mechanism with above characteristics is combined to form a combined slider demoulding structure, which is composed of an inclined wedge, an inclined guide pillar, a large slider seat, a large slider, a small slider seat, a small slider, etc., as shown in Figure 5(a) ) shown. After hiding molded slider and large slider seat of U-shaped bracket, small slider and small slider seat are displayed as shown in Figure 5(b). Joint surface between small slider and small slider seat is an inclined plane. A T-shaped block is provided on small slider seat, as shown in Figure 5(c). A T-shaped slot is also provided on small slider. In order to make small slider slide along side of large slider, guide bars are provided on small slider, as shown in Figure 5(d). Cross-sectional structure of combined slider is shown in Figure 5(e), cross-sectional structure of small slider and small slider seat is shown in Figure 5(f).

 

Figure 5 Combined slide demoulding mechanism
1. Large slider 2. Small slider 3. Small slider seat 4. Inclined guide post pressure block 5. Inclined guide post 6. Inclined wedge 7. Large slider seat
As can be seen from Figure 5 (e) and (f), effective height of inclined guide pillar that guides movement of small slider seat is 33 mm, and inclination angle is 25°. When mold is closed, there is a distance between wedge and large slider seat, and its vertical height is 50 mm. Demoulding distance of middle hole of round hole on the side wall of bracket is 4 mm. Mating surface of small slider and small slider seat is an inclined plane, and angle with horizontal plane is 70°. The two are connected with an inclined T-shaped slot. Inclined surface of small slider seat is aligned with inclined surface of large slider seat, and both are pressed by inclined wedge of fixed mold. Demoulding distance of small slider guided by inclined guide pillar: 33*tan25°*tan20°=5.6 mm, which is larger than demoulding distance of middle hole of round hole on side wall of bracket (4 mm), which can ensure demoulding of round hole on the side wall of bracket and hole in the middle of round hole. Movement process of combined slider is as follows.
(1) When mold opening distance between moving and fixed molds is 0~33 mm, inclined guide pillar 5 guides small slider seat 3 to move in F1 direction. At this time, large slider seat 7 remains stationary. Under guidance of side of large slider seat 7, small slider 2 slides in F2 direction to demould round hole on the side of U-shaped bracket.
(2) When mold opening distance between moving and fixed molds is 33~50 mm, inclined guide pillar 5 has separated from the small slider seat 3, small slider seat 3 and small slider 2 stop moving.
(3) When mold opening distance is greater than 50 mm, inclined surface of wedge 6 contacts inclined surface of large slider seat 7, guiding large slider seat 7 and large slider 1 to move in F1 direction, causing U-shaped bracket and cylinder to disengage mold.
(4) When mold is closed, motion processes of moving and fixed molds are opposite.

There are two blind holes on the side of plastic part, and corresponding positions on inner surface are hollow structures. Hollow structures are demoulded using an inclined push rod. Since blind hole cannot be demoulded by an inclined push rod, a slider demoulding mechanism needs to be used. Two demoulding mechanisms need to be designed, outer one is slider demoulding mechanism, and inner one is oblique push rod demoulding mechanism, as shown in Figure 6. Demoulding distance of small hole is 11 mm, effective height of inclined guide post is 41 mm, and tilt angle is 18°. Therefore, distance inclined guide post guides movement of slider: 41*tan18°=13.32 mm, which is 11 mm larger than demoulding distance of small hole.

 

Figure 6 Demoulding structure of slider and inclined push rod
1. Slider 2. Inclined guide column 3. Slider seat

Mold adopts a flip-chip structure. Ejection system and pouring system are both located on movable mold. Positioning ring is fixed on movable mold base plate. Hot runner system is selected for pouring, and hot runner trunking is opened on movable mold plate, as shown in Figure 7. In order to realize movement of ejection system, a hydraulic cylinder is installed on mold, and piston rod of hydraulic cylinder drives movement of ejection system to eject molded plastic parts. Install hydraulic cylinder on upper surface of push rod fixed plate, fix a screw on movable mold base plate, and connect piston rod of hydraulic cylinder to screw, as shown in Figure 8. When piston rod extends, screw remains stationary, hydraulic cylinder piston rod drives push plate and push rod fixed plate to move, pushing out molded plastic part.

 

Figure 7 Feeding system

 

Figure 8 Launching system

Set up a reasonable cooling water path to cool surface of molded part to keep temperature uniform throughout mold, which can effectively avoid deformation of plastic part. Mold cavity plate is relatively flat and adopts a straight-through cooling water path. In order to prevent cooling water path from being too long and causing uneven mold temperature, cooling water path at the bottom of cavity plate is designed into two independent parts. There is a slope on the bottom of cavity plate. In order to achieve a good cooling effect, an independent waterway is opened at the position of slope. Therefore, fixed mold is equipped with three cooling water channels, of which water channel 3 is a slope cooling water channel, as shown in Figure 9(a). Since there are many push rods in movable mold, they must be avoided when designing water path. Movable mold adopts a water well cooling system and connects water wells with a straight water path, as shown in Figure 9(b). Slider is large in size and adopts a direct waterway, as shown in Figure 9(c). Slider and inclined push rod at the other end of plastic part are small and not suitable for opening a cooling water path.

 

Figure 9 Cooling water path design
(a) Fixed mold cooling water path (b) Moving mold cooling water path (c) Slider cooling water path

Due to high quality requirements for outer surface of plastic parts, gate marks are not allowed on outer surface. Mold adopts an inverted structure, pouring from inner surface of plastic part to be formed, and uses a hot runner pouring system. Mold structure is shown in Figure 10.

 

Figure 10 Mold structure
1. Heat insulation board 2. Electrical device connector 3. Hot runner electrical device 4. Screws 5. Push plate 6. Push rod fixing plate 7. Hydraulic cylinder 8. Hot runner tube 8. Core 9. Inclined push rod 10. Moving Template 11. Copper sleeve 12. Inclined wedge 13. Side slide mechanism 14. Hot runner sleeve 15. Fixed mold insert 16. Fixed mold base plate 17. Fixed template 18. Mold feet 19. Movable mold insert 20. Combination slide Block mechanism 21. Push rod 22. Push plate guide column 23. Limiting column 24. Reset rod 25. Moving mold base plate 26. Positioning ring
During injection production, movement of mold is as follows.
(1) When injection starts, injection material enters mold cavity through hot runner tube 8. After injection is completed, it is cooled under pressure to set shape of plastic part, then fixed platen 17 is separated from movable platen 10.
(2) When movable mold and fixed mold are separated, side slider mechanism 13 demolds small hole on the side of plastic part, combined slider mechanism 20 demolds bracket on the side of plastic part, round hole on the side of bracket, and cylinder inside bracket.
(3) After fixed platen plate and movable platen plate are completely separated, piston rod of hydraulic cylinder 7 extends, driving push rod fixed plate 6 and push plate 5 to move, thereby driving inclined push rod 9 and push rod 21 to push out molded plastic part (see figure 11).

 

Figure 11 Actual plastic parts
(4) After taking out plastic part, piston rod of hydraulic cylinder 7 contracts, driving push rod fixed plate 6, push plate 5, inclined push rod 9 and push rod 21 to reset.
(5) When movable mold and fixed mold are closed, reset process of each part on mold is opposite to mold opening process. After mold is completely closed, production of next plastic part can begin.