Working environment of plastic parts of engineering machinery engines is harsh, requirements for use of plastic parts are relatively high, especially for aging and higher strength requirements in thermal environment. Wall thickness of plastic parts is generally more than 2mm. In view of functional requirements of plastic parts, structure of plastic parts is relatively complex, which is composed of a variety of complex features, which brings greater difficulties to design of molding die structure, especially demolding of plastic parts, which requires design of multiple demolding mechanisms, or design of multi-directional demolding functions on demolding mechanism mechanism. An injection mold with multiple core-pulling mechanisms is designed for excavator engine air intake cover. Mold is equipped with a fixed mold Hough slider mechanism, a movable mold hydraulic cylinder secondary core-pulling mechanism, and a push plate ejection mechanism.

 

Structure of air intake cover of excavator engine is shown in Figure 1. There are multiple holes of different sizes on cylindrical tube and slot cover, and logo characters are set on the side of outer wall of lower end. Inner wall of card slot cover is provided with a reinforcing rib feature, upper and lower sides of opening of card slot cover are provided with inverted buckle grooves, which makes it difficult to demold inner wall of card slot cover. Plastic parts are made of nylon, with a shrinkage rate of 0.62% and an output of 120,000 pieces. Combining characteristics of materials and processes, precision level of plastic parts is MT5.

 

Parting of mold adopts following scheme: plastic part to be formed is placed in mold according to orientation shown in Figure 2, and single-point hot nozzle is used for direct casting. It is necessary to set 5 parting faces for parting of plastic part, namely P1, P2, P3, P4, P5. P1 is used to split cavity plate insert on the top of plastic part, P2 is used to split molded parts of inner wall and outer wall of cylindrical tube; P3 divides cylindrical tube and outer wall forming parts of card slot cover into two halves, and side core of Huff slider; P4 is used for parting of outer wall molding parts of slot cover and inner wall core; P5 is used for parting of inner wall core and inner wall core of cylindrical tube. In this parting method, demolding of plastic parts includes following parts: firstly, cavity plate is separated from plastic part, secondly, inner wall of card slot cover is separated from plastic part, then left and right slide blocks are separated from plastic part. Finally core is separated from plastic part.
To integrate demolding process of plastic parts, mold needs to design two more complex demolding mechanisms, one is fixed mold Hough slider mechanism for driving core pulling on left and right slider sides; the other is secondary core pulling mechanism for hydraulic cylinder used to drive core pulling on inner wall core side.
Left and right sliders are mainly combined with demolding of outer wall of cylindrical tube and slot cover. Target features include outer wall hole, cylindrical tube, side undercut formed by joint part of slot cover, and logo text on outer wall of cylindrical tube. Inner wall core is used to form inner wall, ribs and wide ribs of slot cover. Separation of core and plastic part also needs to consider inverted grooves at upper and lower ends of slot. Core-pulling direction of molded parts is inconsistent with core-pulling direction of inner wall core. It is necessary to set core pulling mechanism of inner wall core with a secondary core pulling mechanism, that is, inner wall core is a combination of 3 molded parts. First, upper and lower inverted grooves must be core-pulled, then inner wall core core-pulling is completed .

 

For core-pulling and demolding of a single cavity, fixed-mold Hough slider mechanism for core-pulling on the left and right slider sides is shown in Figure 3. Mechanism is installed in fixed mold plate, side core is pulled out by oblique sliding out method. When mold is opened, it is firstly activated by pull hook and then driven by drive spring. Movement of slider is guided by inclined T-shaped guide rail.

 

1. Hydraulic cylinder 2. Hydraulic cylinder seat 3. Outer slider 4. Slot plate 5. Plastic parts 6. Outer slider 7. Slot plate 8. Upper inner slider 9. Center rod 10. Center block 11. Lower and inner slide Block 12. Inner wall core
Secondary core pulling mechanism of hydraulic cylinder. Structure of mechanism is shown in Figure 4. Demolding of inner wall of plastic part slot cover is carried out in two steps: ①Central rod of hydraulic cylinder pulls central block to left by a distance t, right end of central block drives upper and lower inverted grooves forming parts (upper inner slide and lower inner slide) to move to center synchronously to complete core pulling; ②Center rod continues to be pulled out to the left, center block will push outer slide, groove plate, upper inner slide, lower inner slide , inner wall core to move to the left together, and continue to complete side core pulling of inner wall core.

 

1. Hot nozzle 2. Fixed mold base plate 3. Hot runner plate 4. Fixed mold plate 5. Push rod 6. Push rod fixed plate 7. Unloading plate 8. Movable mold plate 9. Right slider
Mold structure is shown in Figure 5, with a 2-cavity layout, a four-plate mold base, and a hot runner pouring system. Mold is opened in 2 times to drive mechanism moves. The first time is used for drive of fixed mold Hough slider mechanism and opening of cavity, the second time is used for discharge plate 7 to push plastic part from core insert to complete demolding of plastic part. Fixed mold Hough slider mechanism (represented by right slider 9) is installed in fixed mold plate 4, secondary core pulling mechanism of hydraulic cylinder is installed on discharge plate 7. Four push rods 5 are installed at four corners of discharge plate 7 to push discharge plate 7 out, and upper end of push rod 5 is tightly connected with discharge plate 7.

 

10. Spring 11. Spring pin 12. Left slider 13. Insert plate 14. T-shaped guide block 15. Cavity plate insert 16. Core insert 17. Inner wall insert
Working process of mold is shown in Figure 6, and mold is opened twice.
(1) Mold is closed. After mold is installed on injection molding machine, it is closed, and injection molding machine is ready to open mold after injection is completed.
(2) Secondary core-pulling mechanism of hydraulic cylinder is core-pulling. Before mold is opened, hydraulic cylinder of secondary core pulling mechanism of hydraulic cylinder moves first to complete core pulling of inner wall core 17 of plastic part.
(3) Open at H1. Driven by slider of injection molding machine, movable mold of mold moves downwards, first opens at H1, cavity plate insert 15 is separated from plastic part.
(4) Core is pulled from side of Hough slider. When H1 is opened, hook on unloading plate 7 pulls right slider 9 and left slider 12 down, two sliders implement side core pulling simultaneously. After opening H1, plastic part is completely separated from cavity plate insert 15, right slider 9, and left slider 12.
(5) Open at H2. Movable mold continues to descend. Ejector rod of injection molding machine drives push rod fixing plate 6, thereby pushing push rod 5 to push discharge plate 7 out, discharge plate 7 pushes insert plate 13 to push out plastic part from core insert 16 to achieve complete demoulding of plastic part.