Figure 1 shows refrigerator zero-degree chamber card strip, which is made of high-impact polystyrene (HIPS), with a shrinkage rate of 1.005, an average wall thickness of 2.0 mm, and a smooth surface. The overall dimensions are approximately 788 mm*124.4 mm*40 mm. It is a large plastic part with following characteristics: ① Plastic part has three undercuts in different directions, one of which has a longer span and uses a slider core pulling mechanism to pull core, and middle undercut uses an inclined push rod core pulling mechanism to pull core; ② There are many long and deep reinforcing ribs distributed on card strip. In order to facilitate processing and easy exhaust, 5 moving mold inserts are set; ③ Plastic part is an appearance part with high dimensional accuracy and surface roughness requirements. Defects such as warping, welding marks, and spots are not allowed, and mold should be cooled evenly.

 

Figure 1 Refrigerator zero-degree room card strip

According to structural characteristics of plastic part and actual production requirements, a three-point integral pouring system is adopted, as shown in Figure 2. Three undercuts of plastic part use a "slider + inclined push rod" to achieve lateral core pulling. According to process analysis of plastic part, fixed mold adopts an integral structure and movable mold is an inlaid structure. Injection mold uses a non-standard mold base with a size of 1 030 mm * 470 mm * 611 mm.

 

Figure 2 Mold structure
1. Fixed mold base plate 2. Fixed mold plate 3. Fixed mold insert 4. Guide column 5. Small slider 6. Moving mold plate 7. Oblique push rod 8. Moving mold pad 9. Oblique push guide block 10. Support column 11 Oblique push seat 12. Push plate fixing plate 13. Push plate 14. Moving mold base plate 15. Positioning ring 16. Gate sleeve 17. Diverter plate 18. Hot nozzle 19. Slider insert 20. Large slider 21. Push rod 22. Pad

Based on comprehensive analysis of size and molding quality of plastic parts, "hot runner + ordinary runner" casting system can effectively reduce filling pressure and improve molding quality of plastic parts. As shown in Figure 3, a 3-point integral needle valve hot nozzle is used for feeding. Needle valve hot nozzle is 115.5 mm long and φ39 mm in outer diameter. Spacing is 250 mm. Gate is set on the back of plastic part to be molded, with a width and thickness of 6.7 mm and 1.2 mm respectively. Runner pull rod is designed to form a cold material hole to receive condensate.

 

Figure 3 Casting system design
1. Plastic part 2. Ordinary runner 3. Positioning ring 4. Gate sleeve 5. Diverter plate 6. Needle valve hot nozzle 7. Temperature control box

According to structural characteristics of plastic part, fixed mold molding part is an integral structure, and movable mold molding part design is more complicated, as shown in Figure 4. Since left and right sides of plastic part are symmetrical, movable mold inserts 2 and 8, movable mold inserts 3 and 7, movable mold inserts 4 and 6 are symmetrical parts. In assembly, movable mold insert is fixed to movable mold plate by locking screws. All molding parts of mold adopt an inlay combination structure, which is easy to process and maintain. In addition, it should be noted that different codes should be set for movable mold inserts with same appearance to avoid reverse installation.

 

Figure 4 Structure of movable mold
1. Movable mold core 2. Movable mold insert 3. Movable mold insert 4. Movable mold insert 5. Movable mold insert 6. Movable mold insert 7. Movable mold insert 8. Movable mold insert

(1) Design of slider core pulling mechanism. Two undercuts on outer groove of plastic part are formed by a large slider, a left slider and a right slider, as shown in Figure 5. Since left and right structures of plastic part are symmetrical, two grooves on outer surface are formed by four slider inserts, and four slider inserts are connected to large slider with screws. Pressure strip 10 is fastened to movable mold plate with screws, and inclined guide column 8 provides power for slider to move. Inclined surface designed on fixed mold plate has a locking effect on slider, and limit block is designed to limit slider to ensure accurate mold closing. Since demolding stroke of large slider is large, slider seat is designed to extend slider stroke considering cost economy, and slider seat is fastened to movable mold plate with screws.

 

Figure 5 Structure of large slider
1. Large slider insert 2. Small slider insert 3. Slider seat 4. Limit block 5. Wear plate 6. Large slider 7. Center guide rail 8. Inclined guide column 9. Nozzle 10. Pressure strip
Left and right sliders are composed of a pressure plate 1, a wear plate 2, an inclined guide column 3, a small slider 4 and a limit block 5 respectively. Compared with left slider, right slider has an additional positioning spring. In addition, other parts of right slider are symmetrical with corresponding parts of left slider. Structure of left slider is shown in Figure 6.

 

Figure 6 Structure of left slider
1. Pressure plate 2. Wear plate 3. Inclined guide column 4. Small slider 5. Limit block
(2) Design of inclined push rod core pulling mechanism. For undercut in the middle of plastic part, inner side core pulling mechanism of inclined push rod is designed for molding, as shown in Figure 7. Width of undercut in the middle of plastic part is 10.88 mm. Demoulding stroke of inclined push rod should be greater than 10.88 mm, and there should be enough safety distance. Mold ejection stroke is 60 mm, angle of inclined push rod is 13°, and designed inclined push rod core pulling distance is 13.85 mm. Under premise of ensuring undercut demoulding stroke of 10.88 mm, there is about 3 mm of safety distance, which can ensure that undercut of plastic part is completely demoulded.

 

Figure 7 Oblique push structure
1. Oblique push rod 2. Oblique push rod guide block 3. Oblique push seat

Due to large size of plastic part, in order to ensure cooling efficiency, fixed mold cooling system is designed with 2 sets of straight-through circulating water circuits, movable mold cooling system is designed with 4 sets of straight-through water pipe inlet and outlet water circuits. Cooling system and water pipe aperture is φ8.5 mm, and spacing is about 50 mm. Large slider, left and right small sliders are large in size. In order to ensure cooling efficiency, a straight-through cooling water circuit is also designed. There is an insert screw blocking middle position of large slider, which is divided into 2 groups of water transport. Size of inclined push rod is small, and there is no space to set up a cooling water circuit.

In order to ensure smooth demolding of plastic parts and good surface quality, "push rod + oblique push rod" combined demolding structure is adopted after analysis. Flat push rods are designed for long and deep ribs of plastic parts to push out, increase push force, and ensure smooth demolding of ribs. A total of 20 flat push rods are set in mold. In parts where plastic parts need to have a large push force, there are no oblique push rods and flat push rods, round push rods are designed according to size of space. A total of 16 φ8 mm and 4 φ6 mm round push rods are designed for mold, and 3 φ8 mm round push rods are evenly designed at diversion channel. Due to large number of push rods and some push rods are under slider, a forced reset mechanism needs to be set to make push rods fully retracted to avoid damaging push rods under slider when slider is reset. Its structure is shown in Figure 9.

 

Figure 9 Ejection system
1. Runner condensate pull rod 2. Oblique push rod 3. Flat push rod 4. Round push rod

In order to ensure complete filling of cavity, mold adopts form of exhaust groove to achieve exhaust, as shown in Figure 10. Exhaust groove is designed at the edge of fixed mold about 4 mm away from plastic part, with a width of about 5 mm, a depth of 0.3 mm, and a matching width of parting surface of about 30 mm. In addition, matching gap between mold forming parts also plays a role in exhaust.

 

Figure 10 Exhaust system

After mold is closed, molten material flows through hot runner main nozzle through channel inside hot runner manifold, then enters cavity through ordinary manifold and side gate to complete filling. When cooled to demolding temperature, movable mold side begins to move. After mold is opened, fixed mold plate drives large slider, left and right sliders to pull core in different directions. Large slider pulls the core 55 mm, left and right sliders pull core 27.5 mm until core is pulled. Push plate drives push rod and oblique push rod to start ejecting plastic part. Slider mechanism completes undercut core pulling at two outer grooves, and oblique push mechanism completes undercut core pulling at one middle position. When ejection stroke reaches set 60 mm, it stops, plastic part is taken out, and ejection system is reset. Finally, slider of injection molding machine drives dynamic mold side mechanism to close mold, slider is reset under drive of fixed mold plate, and mold is accurately reset under action of reset rod before starting next injection process.