End cover of washing machine is shown in Figure 1. The overall size is 553 mm*415 mm, wall thickness is 3 mm, material of plastic part is ABS, diameter of middle hole is φ384 mm, there are several buckles on outer surface of four sides, and no buckle on inner surface.

 

Figure 1 Plastic structure

Since there is a large round hole in the middle of plastic part, parting surface is divided into two parts, one parting surface is designed for middle hole, and one parting surface is designed around plastic part. Parting surface of middle hole is flat, at maximum contour of hole; surrounding parting surface is a curved surface created by maximum contour line of plastic part radiating to surroundings. Molded plastic part is relatively neat, so created parting surface is also relatively smooth, without sharp corners, corners and other features. Size of molded plastic part is large, mold adopts a structure of one cavity, fixed mold and movable mold adopt an integral design. In order to protect parting surface, wear-resistant sheets are installed on movable mold plate, and wear-resistant sheets bear pressure of injection molding machine; In order to strengthen positioning accuracy of fixed mold and movable mold, set positioning pits and bosses at four corners of fixed mold and movable mold, and matching slope of side is 5. Structure of movable and fixed mold cavity plates is shown in Figure 2.

 

Figure 2 cavity plate structure

There are two tall ribs next to round hole in the middle of plastic part, with a height of about 38 mm and a thickness of 2.28 mm. In order to prevent cavity from being insufficiently filled, an insert is used on fixed mold corresponding to place where reinforcing rib is formed, and an exhaust groove is set on insert. Insert is located inside round hole of plastic part to be formed, as shown in Figure 3. Due to large volume of two inserts, in order to keep mold temperature uniform, a straight-through cooling water circuit was designed on two inserts.

 

Figure 3 Insert design

There are 10 buckles on 4 outer sides of plastic part. Size of buckles is small and demoulding distance is short. Inclined guide post is used to drive slider mechanism to demould. Layout of slider is shown in Figure 4(a). T-shaped slot of slider is formed by fixing bead on chute with screws. Width of five sliders at both ends of molded part is 40 mm. Due to limited space of mold, in order not to reduce strength of mold base, sliders are set It is a unilateral T-shaped groove, as shown in Figure 4(b). In order to prevent slider from being stuck, a wear-resistant sheet is installed at the bottom of slider, and an oil groove or graphite is added to bead to increase lubrication of slider.

 

Figure 4 Slider Design

Size of molded plastic part is large, height difference between the two ends is 206 mm, drop is large, and there is a large round hole in the middle of plastic part, which affects flow of melt in cavity. Two ribs with higher height and thinner wall thickness on contour of round hole of molded plastic part require sufficient pressure so that melt can fill cavity at rib. Based on above characteristics, molded plastic parts are injected with multi-point hot runners, and three hot runner pipes are controlled by sequence valves. Gating system is shown in Figure 5.

 

Figure 5 gating system
In order to judge rationality of gating system shown in Figure 5, it is verified by mold flow analysis software.
(1) Distribution of weld lines is shown in Figure 6(a), which are mainly distributed at confluence of melts of different hot runner tubes. Reason for formation of these weld lines is mainly temperature of melt front when melt flows in cavity, temperature of melt front is low, and when two melts meet, an irregular curve is formed, which is inevitable. However, color of weld line can be lightened to meet appearance requirements of plastic part by adjusting mold temperature or setting a push rod at position where weld line may appear.
(2) Position distribution of trapped air is shown in Figure 6(b), mainly located at corners of ribs and buckles on the side of plastic part, and exhaust at these positions is more difficult. Solution is to set push rods where air trapping may occur or set exhaust grooves on parting surface so that air in cavity can be discharged smoothly.

 

Figure 6 Mold flow analysis

There is no buckle on inner surface of plastic part, and it is pushed out by a push rod. Since there is a long groove on inner surface and reinforcement ribs are unevenly distributed, tightening force of molded plastic part is different at different positions, and layout of push rod is also uneven. In position where ribs are dense and grooves are dense, wrapping force is greater, and density of push rods is also higher. In flat position of molded plastic part, wrapping force is smaller, and density of push rods is also smaller. Diameter of push rod is φ16 mm, and there are 21 push rods in total, and pushing system is shown in Figure 7.

 

Figure 7 Launch system

In order to quickly cool, shape and solidify high-temperature melt in cavity, mold temperature must be controlled within a certain range, and mold temperature must be uniform to prevent molded plastic parts from deforming during cooling process after demoulding. Cooling system of movable mold is shown in Fig. 8(a). It adopts combination of straight-through waterway and water well waterway. Five water wells are set at position of plastic part pit. Diameter of water well is φ20 mm. In order to make temperature of mold uniform, 5 water wells are connected in parallel with a straight-through waterway, and straight-through waterways are used in other positions of movable mold, and distance from cavity wall is kept consistent, about 15 mm. Cooling system of fixed mold is shown in Fig. 8(b), which mainly adopts a straight-through water channel with a diameter of about φ10 mm and a distance of about 15 mm from cavity wall. All waterways are set in parallel to ensure that cooling water entering pipeline is at room temperature, which is conducive to maintaining uniform temperature of mold. Connecting all direct waterways with water collecting blocks facilitates connection of waterways when mold is disassembled.

 

Figure 8 cooling system

Appearance structure of mold is shown in Figure 9(a). A heat shield is installed on fixed mold seat to cut off heat transfer between injection molding machine and mold, and a cooling water circuit is arranged on fixed mold seat to control fixed mold seat plate to control temperature of fixed mold seat plate; set prying groove on mold to prevent fitter from beating mold when disassembling and assembling mold; set a chip groove between movable mold seat plate and push plate to prevent dust from entering between movable mold seat plate and push plate, which will affect reset of ejection system; set mold foot on mold, when mold is placed on the ground, quality of mold is supported by mold foot, and electrical components of hot runner are protected; clamping strip is set between fixed platen and movable plate, to prevent mold from opening during transfer process, and a lock module is also provided to enhance positioning accuracy of movable and fixed mold plates; in order to keep push plate movement balanced, 6 push plate guide columns and return springs are set; in order to strengthen dynamic For the strength of movable mold base, 8 support columns are set on movable mold seat plate, as shown in Figure 9(b).

 

Figure 9 Die appearance and support column and return spring
Mold adopts a two-plate mold structure and a hot runner gating system, and two-dimensional structure is shown in Figure 10. Working process of mold: molten plastic enters hot runner tube for further heating, then sequence valve 14 controls valve needle of hot runner tube to open, and high-temperature melt enters cavity. After filling, pressure maintaining, solidification, cooling and shaping, movable mold and fixed mold are opened at parting surface, inclined guide post 18 drives slider 19 to perform demoulding movement. After movable mold is completely separated from fixed mold, injection molding machine pushes pushing plate 3, push rod 8 and other pushing mechanisms of mold to push out molded plastic parts. After molded plastic part is taken out, push-out mechanism resets under push of return spring 6, then movable mold and fixed mold are closed. After mold is completely closed, next injection can be started.

 

Figure 10 Two-dimensional structure of mold
1. Moving mold seat plate 2. Pad block 3. Push plate 4. Push rod fixed plate 5. Push plate guide column 6. Return spring 7. Moving template 8. Push rod 9. Fixed template 10. Hot runner tube 11. Heat flow Channel plate 12. Hot runner electric device 13. Heat insulation plate 14. Sequence valve 15. Positioning ring 16. Fixed mold seat plate 17. Insert 18. Inclined guide column 19. Slider 20. Slider spring 21. Support column 22 .Limiting column 23. Chip flute limiting column