Air outlet blade trim is shown in Figure 1. The overall structure is a long strip with a U-shaped cross-section. Material is electroplating grade PC+ABS, and mold design shrinkage rate is 1.005. Each car has 4 air outlet blade trims. Outer dimensions of left and right air outlet blade trims are 150 mm * 5 mm * 4.5 mm, and structure is symmetrical; outer dimensions of middle left and middle right air outlet blade trims are 130 mm * 5 mm * 4.5 mm, and structure is symmetrical. Main wall thickness of 4 air outlet blade trims is 1.5 mm, and each has 4~5 inverted buckles in main demolding direction. Wall thickness of 4 air outlet blade trims has sudden changes in many places, with the thinnest being 0.7 mm and the thickest being 2 mm. Parting line around tuyere blade trim is in appearance area and cannot have appearance defects.

 

Figure 1 Automobile tuyere blade trim
Design of tuyere blade trim mold has three difficulties: ① Wall thickness of plastic part changes suddenly, which is easy to cause shrinkage defects; ② Thin-walled structure of plastic part to be molded is difficult to fill; ③ Parting line position is appearance surface, defects such as step difference and flash cannot be hidden.

Taking into account production cycle, injection molding machine and mold manufacturing costs, mold adopts a 4-cavity layout to produce 4 tuyere blade trims at a time. Since slider molding undercuts need to be designed at both ends of tuyere blade trim, 4 tuyere blade trims to be molded are arranged in parallel and share 2 sliders, as shown in Figure 2.

 

Figure 2 Mold cavity layout

Length of 4 tuyere blade trims is 130~150 mm, which is relatively short, and 1 gate can be arranged for each cavity. Wall thickness of tuyere blade trim is uneven, which is prone to shrinkage defects and requires a larger injection pressure. However, too much injection pressure will cause flash, so it is necessary to consider reducing pressure loss in casting system. Length of ordinary runner should be as short as possible and gate size should be as large as possible. Since parting lines of tuyere blade trim are all in appearance area, a latent gate is used. According to experience, deformation of slender plastic part is small when gate is arranged at 1/3~1/2 of the total length.
Taking above into consideration, casting system is designed as an open hot runner 2-point latent gate, as shown in Figure 3. Cross-section of ordinary runner is U-shaped, with a size of 4.7 mm×3.8 mm and a bottom fillet radius of R2 mm. Gate is designed at wall thickness of plastic part to be molded 2 mm, as shown in Figure 4, and gate size is 4.3 mm×1.3 mm. The total length of ordinary runner plus gate is only 40 mm.

 

Figure 3 Casting system

 

Figure 4 Gate

Moldflow is used to perform mold flow analysis on casting system, focusing on shrinkage and deformation of molded plastic parts. Material used for analysis is defined as electroplating grade PC+ABS, which is same as material used in actual production.

Due to small characteristic size of tuyere blade trim, double-face mesh division is performed according to global mesh side length of 1 mm, and mesh defects are repaired to obtain a perfect mesh. Mesh is statistically analyzed, average aspect ratio is 1.88, maximum aspect ratio is 14.97, and matching percentage is 82.4%, which meets requirements of mold flow analysis. Modeling is performed according to designed casting system size, and casting system model is shown in Figure 5.

 

Figure 5 Casting system model

Process settings are based on actual production process of similar plastic parts of same material: mold temperature is 80 ℃, melt temperature is 250 ℃, injection time is 0.7 s, speed/pressure switching is performed from 99% filling volume, holding time is 4 s, holding pressure is 80% filling pressure, and cooling time is 25 s.

When speed/pressure is switched, pressure at main nozzle is 66.44 MPa, as shown in Figure 6. Pressure is small, indicating smooth filling. Pressure at gate is 46.48 MPa, and pressure drop of casting system is small, only 19.96 MPa.

 

Figure 6 Pressure-time relationship diagram
Before gate waste solidifies, plastic part has been completely solidified, indicating that pressure holding channel is unobstructed, plastic part can be continuously held under pressure and compensated for shrinkage, as shown in Figure 7.

 

Figure 7 Solidification layer factor
Maximum shrinkage mark of plastic part is about 0.02 mm, as shown in Figure 8, which meets shrinkage requirements of plastic part electroplating. Maximum deformation after shrinkage compensation is small, only 1.28 mm, as shown in Figure 9. There is an installation point at maximum deformation position, which can correct deformation of plastic part, and deformation meets requirements. After Moldflow mold flow analysis verification, gating system design can theoretically meet requirements.

 

Figure 8 Shrinkage mark estimation

 

Figure 9 Deformation

Parting surface design is shown in Figure 10. Parting line of long side of tuyere blade trim is relatively straight, stretching horizontally to surrounding and bridging parting surface of other surrounding cavities. Parting surface is a plane, which is convenient for mold part processing and assembly. Parting line of narrow side of plastic part is U-shaped, and parting surface is designed to diffuse outward at 15°~45° from parting line, which can ensure that slider and fixed mold are well sealed and not easy to produce flash.

 

Figure 10 Parting surface design

When designing core pulling mechanism, try to avoid arranging mechanism on parting surface to avoid appearance defects due to insufficient processing accuracy of mold parts.

There are undercuts at both ends of air outlet blade trim strip, and distance between undercuts is 2.6~9 mm. Shape of undercut is shown in B-B in Figure 1 (b). Here, undercut is designed to pull core of movable mold slider, as shown in Figure 11. Core pulling directions of four air outlet blade trim strips are consistent, and two sliders are shared to save mold space. Four inserts are designed on each slider to save materials, facilitate processing and mold repair. Slider is driven by an inclined guide column with an inclination of 18° and a diameter of φ16 mm. Each slider is equipped with two inclined guide columns to ensure force balance of slider. Slider movement stroke is 15.5 mm.

 

Figure 11 Slider mechanism

Each tuyere blade trim strip needs a core-pulling mechanism to complete undercut demoulding, and core-pulling mechanism needs to be arranged on parting surface, which requires a large space, will cause mold size to become larger, increase mold manufacturing cost and production cost. Therefore, oblique push mechanism is selected to complete undercut demoulding at gate, as shown in Figure 12. There is an undercut at gate in demoulding direction, and a core-pulling mechanism is required to complete demoulding. Undercut distance is 4.3 mm, inclination of oblique push rod is 12°, actual push height is 25 mm, and core-pulling distance is 25tan12°=5.3 mm.

 

Figure 12 Oblique push structure

Width of tuyere blade trim strip is only 5 mm. In order to avoid flash and step defects caused by setting core-pulling mechanism on parting surface, each tuyere blade trim strip is designed with 2 core-pulling slider mechanisms driven by dovetail slot inserts to complete inner undercut demoulding, as shown in Figures 13 and 14. Dovetail slot insert and slider are designed within parting line, no defects such as flash and steps will be generated on parting line.

 

Figure 13 Slider mechanism layout

 

Figure 14 Partial cross-sectional view of slider mechanism
Enlarged structure of dovetail slot insert and slider is shown in Figure 15. Dovetail slot insert and slider are connected by a dovetail slot. Except for thin position (only 3.5 mm) of dovetail slot insert and slider in contact with plastic part, strength of other positions is good.

 

Figure 15 Enlarged structure of dovetail slot insert and slider
Movement principle of slider is shown in Figure 16. Dovetail slot insert is fixed on fixed plate, pressure plate is fixed on movable mold insert to form slide groove of slider, and movable mold insert is fixed on movable mold. When mold is opened and pushed out, under action of injection molding machine's push force, movable mold and fixed plate are separated by 35 mm, limited by limit screw. Dovetail slot insert and movable mold insert produce relative movement, driving slider to move 4.9 mm in horizontal direction relative to movable mold insert to achieve undercut demolding.

 

Figure 16 Slider movement principle
(a) Before mold opening (b) After mold opening

Tuyere blade trim has no deep ribs or other difficult-to-eject structures, and clamping force on movable mold part is not large. As shown in Figure 17, double-section push rods are evenly arranged at position without core pulling mechanism. Two longer tuyere blade trims are designed with 5 push rods, and the other two trims are designed with 4 push rods. Small end diameter of double-section push rod is φ1.2 mm and height is 95 mm; large end diameter is φ3 mm and height is 145 mm. In addition, 5 runner push rods with a diameter of φ4 mm are designed. In order to prevent plastic part from sticking to oblique push rod at gate during ejection, push rod is designed to sink into plastic part 0.2 mm to play a positioning role, as shown in Figure 18.

 

Figure 17 Push rod arrangement

 

Figure 18 Local cross-sectional structure at the push rod

Fixed mold cooling system is shown in Figure 19. Due to short length of air outlet blade trim and two hot nozzles blocking middle of mold, cooling water channel is arranged in length direction of plastic part. There are 3 groups of cooling water channels in total, and diameter of cooling water channel is φ8 mm. Due to core pulling mechanism and ejection mechanism blocking movable mold, cooling water channel can only be arranged in length direction of plastic part, as shown in Figure 20. There are 3 groups of cooling water channels in total, and diameter of cooling water channel is φ8 mm.

 

Figure 19 Fixed mold cooling system

 

Figure 20 Moving mold cooling system
Cooling water channel model is imported into Moldflow for cooling analysis. Temperature difference between inlet and outlet water is only 1.5 ℃, as shown in Figure 21, indicating that cooling is uniform and cooling effect is good.

 

Figure 21 Cooling water temperature

Mold structure is shown in Figure 22. Since plastic parts need to be electroplated, mold steel material is required to be high. In order to reduce manufacturing cost, movable and fixed molds are designed as insert structures, and steel grade is 1.2344ESR. Mold adopts a two-plate mold non-standard mold frame, a fixed plate 32 is added between movable mold plate 33 and pad 17.

 

Figure 22 Mold structure
1. Manifold plate 2. Fixed mold base plate 3. Hot runner plate 4. Fixed mold plate 5. Oblique guide column pressure plate 6. Slider 7. Moving mold insert 8. Slider 9. Dovetail groove insert 10. Counter 11. Limit screw 12. Spring 13. Moving mold base plate 14. Push plate 15. Push rod fixing plate 16. Reset rod 17. Pad 18. Oblique push seat 19. Pressure plate 20. Oblique push rod 21. Oblique wedge 22. Cavity plate 23. Positioning ring 24. Hot nozzle 25. Press block 26. Spring block 27. Push block 28. Support column 29. Connecting block 30. Limit block 31. Push rod 32. Fixed plate 33. Moving mold plate 34. Inclined guide pin
When injection is completed and mold is opened, fixed mold plate 4 and movable mold plate 33 are separated, and slider 6 moves horizontally relative to molded plastic part under drive of inclined guide pin 34 to complete demolding of two ends. After mold is opened, injection molding machine pull rod pushes connecting block 29 to drive push plate 14 and push rod fixing plate 15. Push plate 14 and push rod fixing plate 15 transmit force to movable mold plate 33 through push block 27 and spring block 26, so that it is separated from fixed plate 32. Dovetail slot insert 9 is fixed on fixed plate 32, and generates relative movement with movable mold plate 33, driving core-pulling slider 8 to move horizontally relative to molded plastic part to escape undercut. When ejection stroke reaches 30 mm, limit screw 11 fixed on movable mold plate 33 is blocked by fixed plate 32, movable mold plate 33 and fixed plate 32 no longer move relative to each other. At this time, spring block 26 is pressed back by inclined surface on pressing block 25, there is no channel for transmitting force between push plate 14 and movable mold plate 33. Ejection continues, push plate 14 pushes push rod 31 and inclined push seat 18 to move. Inclined push seat 18 applies force to inclined push rod 20, inclined push rod 20 performs a composite movement in ejection direction and lateral direction under joint action of inclined push seat 18 and movable mold plate 33. At this time, molded plastic part is ejected from movable mold plate 33 under action of push rod 31, and inclined push rod 20 moves lateral to plastic part to complete undercut demolding at gate. When ejection stroke reaches 55 mm, limit block 30 is blocked by fixed plate 32, and ejection process ends. After plastic part is removed, injection molding machine pull rod pulls connecting block 29 back. Connecting block 29 transmits force to movable mold plate 33 through push plate 14, push rod fixed plate 15 and reset rod 16 (with steps at both ends), driving the entire movable mold to return to its original position, which is opposite to ejection action [8].

Mold production machine uses a 2000 kN injection molding machine, and plastic part material is electroplating grade PC+ABS. Injection molding machine barrel temperature is set to 250 ℃, hot runner temperature is set to 240 ℃, and mold temperature is set to 80 ℃. Filling is divided into 3 stages: injection pressure of the first stage is 3 MPa, and injection speed is 35%T (T is maximum injection speed of injection molding machine); injection pressure of second stage is 2.5 MPa, and injection speed is 25%T; injection pressure of third stage is 1.2 MPa, and injection speed is 15%T. Injection pressure of holding stage is 1 MPa, injection speed is 2%T, and time is 4 s. Molded plastic parts are shown in Figure 23. They have good appearance and no defects such as shrinkage and flash. After electroplating verification, finished products have few defects and high yield, as shown in Figure 24, which meets requirements of mass production.

 

Figure 23 Actual plastic parts

 

Figure 24 Electroplated finished products