With development of automotive industry toward lightweight, plastic parts account for an increasing proportion of automotive interior and exterior decoration. In order to meet consumers’ high-quality requirements for intuitively perceptible automotive interior and exterior parts, interior parts of automobiles have also changed from single function to multi-purpose. Structural features of plastic parts become more complex, which brings greater difficulty to injection molding. Lower trim panel on driving side of car belongs to instrument panel assembly series. It is located at driver’s knee, plays role of protecting driver’s knee and covering overlap gap of steering column cover. Now we introduce design of injection mold for forming lower trim of driving side of right rudder car.

 

Figure 1 Driver's side lower trim
Lower trim plastic part of a right rudder car is shown in Figure 1. Material is modified polypropylene PP-TD20. Adding 20% talc to PP can improve impact resistance and surface properties of polypropylene, reduce shrinkage of molded plastic parts. It is widely used in automobiles and electrical products with a material shrinkage of 0.95%. Outer dimension of lower trim is about 504.90mm*357.35mm*190.66mm, average wall thickness is about 2.5mm, and weight of a single piece is 567g. Plastic part has a large external dimension. There are a large number of sheet clamps for installing aluminum plates on the back, multiple installation buckles and screw fixing posts. Injection process of plastic melt in cavity is long, cavity filling is difficult, and gating system is one of design difficulties. Plastic parts require high dimensional accuracy, complex internal structure, and multiple lateral convex-concave structures that affect demolding. Design of lateral parting core-pulling mechanism is also difficult.

 

Figure 2 Gating system
1. Hot runner junction box 2. Hot nozzle control system 3. Hot nozzle heater 4. Main nozzle 5. Pressure gauge 6. Electromagnetic control valve 7. G1 Latent gate insert 8. G2 Latent gate insert 9 .Top block 10. Two-stage needle valve hot nozzle 11. Manifold plate
Due to unique shape of plastic part, complicated internal structure and leather-grained surface of plastic part, it is not possible to set a gate on appearance surface of plastic part to be molded. According to structural characteristics of lower trim, through CAE analysis, hot runner + ordinary runner + latent gate is used for pouring, as shown in Figure 2. Hot runner gating system consists of a main nozzle, multiple hot runner plates and 2 conical needle valve hot nozzles. Parting surface of mold is a complex spatial curved surface. Slope of parting surface is greater than 30°, height deviation on both sides of nozzle is ≥5mm. In order to prevent nozzle from being heated to extend beyond parting surface and collide with molding surface of movable mold part, a 1mm thermal expansion platform is designed on movable mold insert below nozzle. Distances between two needle valve nozzle positions and main nozzle are respectively 154.51 and 77.65mm. Length of nozzle G1 is 143.59mm, length of nozzle G2 is 206.59mm, outer diameter is ϕ35mm, and valve needle diameter is ϕ4.5mm.

 

(A) Slider side core pulling mechanism

 

(B) Inclined push core pulling mechanism
Figure 3 Side parting core pulling mechanism
Movable part of mold is processed with U-shaped cross-section runners, width W=20.3mm, height H=5mm, and taper 15°. Angle between G1 latent gate and parting surface is 50°, angle between G2 latent gate and parting surface is 55°. Latent gate adopts movable mold insert to sneak under parting surface, then opens runner on insert.
Design of lateral core-pulling mechanism is combined with structural characteristics of molded plastic part. Mold is designed with 2 movable mold sliders + oblique guide column lateral core-pulling structure and 2 fixed mold sliders + T-slot lateral core-pulling mechanism. There are 8 undercuts inside part, and 8 moving die oblique push inner core pulling mechanisms are designed, as shown in Figure 3.

 

(A) Slider 1 structure

 

(B) Slider 2 structure
Figure 4 Slider structure
1. Oblique guide post fixed block 2. Slider 1 3. Limit block 4. Oblique guide post 5. Spacer 6. Slider spring 7. Wear-resistant block 8. Screw 9. Slider 2 10. Insert
There are 5 undercuts on left and right sides of plastic part, namely S1~S5. From perspective of mold safety, two integrally connected large sliding blocks + inclined guide pillar core pulling structure are designed, spring baffles are used to limit position. Left side undercut of plastic part is biased to movable mold side, with an inclination angle of 4°, and a downward inclined oblique slider is designed, as shown in Figure 4(a). Maximum undercut distance of plastic part S=15.78mm, distance S1 that slider needs to move in lateral core-pulling direction is 15.78+(2~5)mm,. According to triangle principle, inclination angle of inclined guide post is calculated as α=10°, locking angle of locking block is 12°. Width of slider is 339mm and height is 309mm. Two ϕ30mm inclined guide posts are designed through calculation and correction. There are 2 buckles on right side of plastic part, core pulling distance is 1.05mm. Two inserts are fixed on the slider. Lateral core pulling distance of slider is 4mm. According to triangle principle, inclination angle α of oblique guide post is calculated to be 4°, locking angle of locking block is 6°, two ϕ30mm oblique guide posts are also designed, as shown in Figure 4(b).
Sliding guide part of slider is designed with two T-shaped grooves, and T-shaped block is installed inside slider. Since left and right sides of plastic part to be formed are large sliders, they are divided at R corners of plastic part. In order to make parting line clamp beautiful, each slider needs to be positioned well, two "mushroom heads" are processed on the top of each slider for precise positioning. Fixed mold plate design process screw locks slider to facilitate assembly and polishing of slider and fixed mold plate. Back of slider, two sides, bottom surface, mating surface of slider and core are equipped with wear-resistant blocks with a protruding profile of 1mm.

 

(A) Structure of fixed mold oblique slider 1 (b) Structure of fixed mold oblique slider 2
Figure 5 Fixed die inclined slider
1. Screw 2. Slider 3. Cooling water pipe 4. Pull hook 5. Baffle plate 6. Guide rod 7. Guide block 8. Spring 9. Equal height screw 10. Oblique slider 11. Guide block locking screw
For deep-cavity oblique undercut S6 with outside of plastic part at 13° from horizontal, undercut distance is 4.5mm. If it is designed for movable mold slider molding, only undercut distance is slipped, molded plastic part is not easy to demold. Step difference between block and fixed mold (uneven connection between mold parts causes surface of molded plastic part to have steps) is difficult to accurately control. If fixed mold inclined slider structure is designed to form, gap between it and fixed mold plate can be very small, melt is easy to connect smoothly at parting surface, appearance of molded plastic part is more beautiful, as shown in Figure 5(a). Tilt angle α of inclined surface of guide block is 13°. In order to ensure smooth movement of fixed die inclined slider, reduce workload of processing and mold matching, locking surface of inclined slider is 15° and demolding stroke is 4.55mm. According to triangle principle, when inclined slider moves 30mm along guide chute on both sides, actual stroke of fixed mold inclined slider core is 6.75mm, which is greater than demolding stroke. When mold is opened, under action of hook and spring, spring pushes inclined slider along "7"-shaped guide slider. When limit block reaches end surface of limit groove, movement of fixed mold inclined slider ends, relative position of fixed mold oblique slider and movable mold in height direction remains unchanged.
S7 undercut position on outside of plastic part is far away from movable mold side. Molding area is small and undercut ejection stroke is 0.53mm. It forms an angle of 9° with Z axis direction. Design a small fixed mold inclined slider, angle α of inclined surface of guide block is 5°, when inclined slider moves 25mm along guide chute, actual core-pulling stroke of fixed mold slider is 2.18mm, which is greater than demolding stroke, as shown in Figure 5(b). When mold is opened, under force of hook and spring, inclined slider slides obliquely along guide rail fixed on slider, and is limited by equal height screw on inclined slider. When movement of inclined slider ends, spring resists and does not rebound. When inclined slider returns, it directly presses back on parting surface of movable mold.

 

Figure 6 Oblique push inner core pulling mechanism
Other 8 undercuts (L1~L8) inside plastic part are formed by a movable mold oblique push inner core pulling mechanism. According to size of plastic part's inverted buckle, 5 round-bar split oblique push inner core pulling mechanisms (L1~L5) and 3 integral square oblique push inner core pulling mechanisms (L6~L8) are designed. Mold ejection stroke is 110mm. As shown in Figure 6, to avoid interference between inclined push rods, angles of inclined push rods are 10°, 9°, 7°, 9°, 9°, 10°, 6°, 3 °, core pulling distances of each inclined push rod are 18.11, 17.32, 13.51, 17.42, 17.23, 19.4, 11.56, 5.75mm, core pulling distances required for demolding are 14.88, 14.11, 10.63, 10.24, 14.24, 14.3, 10.58 , 3.11mm, actual core-pulling stroke of each oblique push rod is greater than stroke of inverted stripping.
Undercuts L1, L2, L5, and L8 of plastic parts are inclined downward 21°, 2.5°, 6°, and 3° respectively. Oblique push structure must follow its downward tilt angle, otherwise interference will occur. Inclined pushing angle of formed undercut L1 should exceed 12°. In order to ensure smooth operation of inclined pushing mechanism, it is designed as a parallel guide rod inclined pushing structure. Delay angle of inclined pushing seat is 21° and inclined pushing angle is 10°. Forming undercut L2 and L5 are designed as round rod split oblique push structure, diameter of round oblique push rod is ϕ16mm, delay angle of oblique push seat is 2.5° and 6° respectively. Due to small size of inverted buckle head, L8 is designed as an integral square inclined push structure. Delay angle of inclined push seat is 3°. Sides of each inclined push rod are designed to be ≥3° to ensure oblique push rod will not be strained during long-term use..

 

(A) Fixed mold cooling water circuit

 

(B) Mould cooling water circuit

 

(C) Slide cooling water circuit
Figure 7 Cooling system
Design of mold cooling system should avoid interference with other parts. Cooling system is shown in Figure 7. Moving and fixed mold sides adopt a combination of vertical and spacer-type cooling water channels. There are 3 groups and 8 groups of circulating waterways on moving and fixed mould sides respectively. Diameter of cooling water path is ϕ12mm, diameter of partition type water well is ϕ24mm, number of partition type water wells is 26, and distance between cooling water paths is 50-60mm. Because of large contact area between movable mold slider and fixed mold oblique slider on both sides of melt, a set of circulating water channels combined with vertical and inclined cooling water channels are set up. Diameter of cooling water channel is ϕ12mm. Movable and fixed molds, water passages of slider form a grid, are arranged according to shape of profile, and plastic parts can obtain a good cooling effect.

 

Figure 8 Push out system
Because there are many ribs on the back of molded plastic part, required pushing force is relatively large. Mold adopts a combination of "hydraulic cylinder + push block + oblique push block + push rod + push tube" as shown in Figure 8. Eight single-rod push blocks and one double-rod push block are designed around plastic parts. Large push block with a length of 245mm is designed as a double-rod push block. Each push block is designed with a 3° matching slope around push block. Rod diameter is ϕ20mm. In rectangular deep rib position of molded plastic part, five square push rods with a cross-sectional size of 6mm*4mm are designed, 13 round push rods and one push tube are designed at position where pushing resistance is large. 8 movable mold oblique push blocks are installed in inner undercut of molded plastic part, push rod fixing plate realizes pushing out and resetting of push rod, push block, oblique push block and push tube through two ϕ50mm*130mm hydraulic cylinders fixed on movable mold plate. Hydraulic cylinder realizes oil circuit series arrangement through oil collecting block designed on mold, so that oil circuit design is balanced, and balance of pushing force of mold parts is ensured.

 

Figure 9 Mould structure
1. Movable mold seat plate 2. Push plate 3. Push rod fixing plate 4. Oblique push rod 5. Guide sleeve locking block 6. Guide sleeve 7. Oblique guide column 8. Wear block 9. Movable mold slider 10. Hydraulic cylinder 11. Oblique guide column locking block 12. Water nozzle 13. Oblique push block 14. Fixed template 15. Hot runner junction box 16. Hot runner plate 17. Fixed mold seat plate 18. Hot nozzle control system 19. Manifold plate 20. Two-stage needle valve nozzle 21. Ordinary runner insert 22. Moving mold slider 23. Oblique guide post locking block 24. Inclined guide post 25. Spring 26. Square push rod 27. Square oblique push rod 28. Movable template 29. Limit block 30. Stroke switch 31. Slide block seat locking screw 32. Inclined push rod seat 33. Push rod 34. Limit nail 35. Support column 36. Inclined push rod seat 37. Inclined push rod direction Block 38. Guide post 39. Guide sleeve 40. Spacer 41. Double rod oblique push rod direction block 42. Oblique push rod 43. Lock block screw 44. Guide sleeve lock block 45. Guide sleeve 46. Push block 47. Parting surface pressure block 48. Oblique push block 49. Oblique push block 50. Spring 51. Fixed mold oblique slider 52. Pull hook 53. Limit block 54. Pull hook 55. Guide sleeve 56. Guide sleeve locking block 57. Oblique push rod 58. Oblique push rod direction block 59. Double rod oblique push rod seat 60. Fixed block 61. Secondary guide rod
Mold structure is shown in Figure 9. It adopts a 1-cavity layout with a maximum external size of 1080mm*820mm*922mm. Both movable and fixed molds are integrated. Movable mold plate material is 1.2738 and fixed mold plate material is 1.2738HH. Matching slope of all parting surfaces is 5°, 6 positioning mechanisms are designed between movable and fixed mold plates to ensure precise positioning of mold parts, prevent molded plastic parts from dislocation and appearance defects such as reverse segmentation. Mold working process is as follows.
(1) Mold clamping process. Push plate 2 is driven by piston rod of hydraulic cylinder 10, inclined push rod 4 is reset under drive of push plate 2, movable mold sliders 9, 22 on both sides are reset under drive of inclined guide posts 7, 24 and fixed mold plate locking device. Fixed die inclined sliding block 51 is pressed along guide chute.
(2) Injection process. After mold is closed, melt is injected into cavity through hot runner system, U-shaped runner on movable mold plate, and latent gate.
(3) Mold opening process. Fixed-mold oblique slider 51 slides out along guide chute of fixed-mold slider under action of hooks 52, 54 and spring 50. Large sliding block of movable mold completes lateral core pulling under action of inclined guide post 24, spring 25 and sliding block limiting device. Molded plastic part is left on movable mold core, push plate 2, push rod 33, push block 46, inclined push rod 4 and push tube are driven by piston rod of hydraulic cylinder 10 to push out 110mm together. Plastic part and condensate of gating system are completely pushed out. After molded plastic part is taken out by manipulator, push plate and reset rod are driven to reset by piston rod of hydraulic cylinder 10, mold starts next injection molding cycle.
(1) Appearance quality of lower trim of driving side of car is required to be high. Mold adopts form of hot runner to ordinary runner to flexibly select gate position of plastic part to be molded. Melt enters cavity from inside of plastic part to be molded through latent gate, avoiding influence of gate marks on appearance quality of molded plastic part.
(2) Use of a combination of "hydraulic cylinder + push block + oblique push block + push rod + push tube" can balance forces on molded plastic part when it is pushed out, reduce risk of deformation when plastic part is demolded.
(3) Aiming at complex internal structure of plastic part and multiple lateral concave-convex structures that affect demolding, 2 large movable mold sliders, 2 fixed mold oblique sliders, and 8 movable mold oblique push lateral core-pulling mechanisms are designed to achieve purpose of automatic demoulding of outer wall and internal undercut characteristics of molded plastic part. After debugging and production, mold structure is reasonable, operation is stable, quality of molded plastic parts is good, which meets requirements of customers and has a reference role for mold design of similar plastic parts.