Electric vehicle front hood is shown in Figure 1. Average wall thickness is 2.2mm. Outer wall of plastic part has a smooth shape and no complex curved surfaces. However, inner wall has many features, which increases difficulty of mold design and manufacturing. These features include undercut grooves k1~k8, k1’~k8’, screw columns b1, b2, oblique holes h1, h2, and semi-encircled local wrapped grooves z1, z2.

 

Plastic parts are injection molded using fiber-modified reinforced polypropylene PP plastic (PP+20%GF) to enhance heat resistance and rigidity of material, which can reduce dimensional shrinkage and deformation of material. Material shrinkage rate is 1.13%~1.22%. Glass fiber modified PP is a non-toxic, odorless and tasteless milky white highly crystalline polymer with a density of only 0.90~0.91g/cm3. It is particularly stable to water, with a water absorption rate of only 0.01%. It has good formability, but has a large shrinkage rate. Surface of molded plastic parts has good gloss, and is suitable for molding exterior parts of electric vehicles. However, PP plastic is easy to age. It usually ages and becomes brittle in atmosphere for about 12 days. It will deteriorate after 4 months when left indoors. Specific antioxidants need to be added to improve its weather resistance. In view of structural characteristics of plastic parts and good fluidity of PP materials, fluidity of material becomes worse after adding modified glass fiber. Therefore, when designing mold, hot runner pouring system is preferred for pouring, which can avoid insufficient filling of cavity and problems such as material shortage and trapped air caused by poor material fluidity.

(1) The overall dimensions of plastic part are about 615mm*450mm*277mm, and size of molded parts required for a single plastic part is >800mm*650mm. It is more suitable for mold to adopt a 1-cavity layout. After plastic part is placed as shown in Figure 2, use maximum outer contour of plastic part as parting line to obtain main parting surface PL of cavity.

 

(2) Undercut grooves k1~k8, k1’~k8’, screw columns b1 and b2 all use side core pulling of lifter mechanism to achieve automatic demoulding, while partial wrapping grooves z1 and z2, inclined holes h1 and h2 requires design of two special secondary core pulling and demoulding mechanisms to achieve automatic demoulding.
Main molded parts adopt an integral structure, as shown in Figure 3. Cavity is processed from fixed mold plate and core is processed from movable mold plate. Therefore, movable and fixed mold plates need to use mold steel with better performance. Since plastic part material is glass fiber reinforced PP, cavity wall is subject to greater friction during injection. Cavity molded parts need to have good wear resistance. Considering requirements of injection pressure, polishing, mold service life, etc., cavity plate material is S136, and core material is 2738. S136 with anti-corrosion and anti-rust functions can meet requirements of high wear resistance of molded parts.

 

Figure 3 Design of molded parts
1. Cavity plate 2. Core 3. First insert 4. Second insert
Considering structural strength factors, use of local small inserts should be avoided as much as possible in molded parts. For required through-hole designs, such as inclined ejector holes and push rod holes, small apertures should be used; for necessary functional inserts, such as the first insert, second insert, etc.,it should be kept away from parting line to ensure strength of side of cavity. Effective strength width of side of cavity is set to more than 70mm. Since electric vehicle front cover is a large plastic part, heat dissipation is essential. Both cavity plate and core adopt multi-pipeline parallel balanced water cooling. Inclined head and side sliders that are large enough must also be cooled by water channels to ensure uniform cooling of the entire mold cavity, prevent warpage and deformation of plastic parts due to uneven cooling. Cooling pipe diameter is φ12mm. Cavity is poured using two side gates, but material supply to gates uses a hot runner for close-range supply to ensure adequacy of material flow.

(3) In design of side core-pulling molded parts, a total of 14 lifter mechanisms (L1~L7 and L1'~L7') and 2 secondary slider core-pulling mechanisms (S1, S1') are set up for core-pulling and demoulding of plastic parts that are difficult to demold, as shown in Figure 4. Lifter mechanism L1~L5 corresponds to forming and side core pulling and demoulding of undercut grooves k1~k5. Lifter mechanism L6 corresponds to forming and side core pulling and demoulding of undercut grooves k6~k8. Lifter mechanism L7 corresponds to forming and side core pulling and demoulding of screw column b1. Lifter mechanism L1’~L5' corresponds to forming and side core pulling and demoulding of undercut grooves k1'~k5', lifter mechanism L6' corresponds to forming and side core pulling, demoulding of undercut grooves k6'~k8'. Lifter mechanism L7' corresponds to molding and side core pulling, demoulding of screw column b2. Slider mechanism S1 corresponds to forming and demoulding of oblique hole h1 and local wrapping groove z1, slider mechanism S1' corresponds to forming and demoulding of oblique hole h2 and local wrapping groove z2. Since lifter mechanism has a large weight, it requires a large driving force, and friction with core is also large. Lubrication measures need to be taken at lifter position, relatively dense annular grooves are set at corresponding positions to store lubricating oil to reduce its motion friction.

 

Pouring system uses a hot runner + ordinary runner, with a total of 3 needle valve hot nozzles for feeding, 1 side gate (G1) and 2 latent gates (G2, G3) for feeding, which facilitates pressure maintenance adjustment and control, as shown in Figure 5(a). CAE simulation analysis of gating system is shown in Figure 5(b). Gates G1, G2, and G3 can effectively fill cavity and ensure fullness of cavity filling. Cavity filling time is 4.278s.

 

Mold structure is shown in Figure 6, which is a single-cavity two-plate mold. A heat insulation plate 2 must be installed on the top of fixed mold base plate to prevent excessive heat loss from mold, and hot runner plate 11 must be installed using a heat insulation pad.

 

1. Positioning ring 2. Heat insulation plate 3. Hot nozzle 4. Fixed platen 5. Moving platen 6. Pulling rod 7.15 Inclined top mechanism 8. Push plate 9. Moving mold base plate 10. Hot nozzle 11. Hot runner plate 12. Wear-resistant plate 13.16 inclined push rod 14, guide wear-resistant block 15. Inclined top seat 16. Pad block 17. Push rod 18. Top block 19. Push plate guide post 20. Return spring 21. Secondary core 22. Support pad 23. Guide groove wear-resistant plate 24, bent rod 25. Hydraulic cylinder 26. Wear-resistant slide plate 27. Primary core 28. Transverse locking block
(1) Cavity is processed from fixed mold plate 4, and core is processed from movable mold plate 5. Cavity is cooled by a φ12mm water-cooled pipe. Inlet water temperature is 25℃, and mold temperature is controlled at 45~47℃. An oil-type mold temperature controller cannot be used because temperature of oil-type mold temperature controller needs to rise to about 60℃ to be stable, while a water-type mold temperature controller can keep mold temperature within set value. Using a water-type mold temperature controller can keep mold temperature constant within the set value. Since there are assembly gaps between 14 lifter mechanisms and 2 slide blocks used in mold, fixed platen plate 4 and movable platen plate 5, assembly contact area is large enough to discharge gas in cavity, there is no longer an exhaust slot around cavity.
(2) 14 lifter mechanisms all adopt same structure. Lifter base 15 and L6 tilting push rod 13 are assembled in the form of T-shaped slots and T-shaped guide rails. Guide wear-resistant blocks 14 and L6 must be provided under moving mold plate 5. Inclined push rod 13 has a sliding fit to prevent L6 inclined push rod 13 from wearing.
(3) Plastic parts are demoulded using multiple push rods 17. Push rods 17 push ejector block 18 by ejector rod of injection molding machine to drive push plate 8 to push out.
(4) Slider mechanisms S1 and S1' have similar structures. They are both secondary core-pulling slider mechanisms. Taking slider mechanism S1 as an example, its structural components include a secondary core 21, a support pad 22, a guide groove wear-resistant plate 23, a bent rod 24, a hydraulic cylinder 25, a wear-resistant sliding plate 26, and a primary core 27. Hydraulic cylinder 25 pulls primary core 27 through bent rod 24 to complete core pulling first. During this process, primary core 27 lifts secondary core 21 through inclined guide rail, completes core pulling at a certain distance in lifting direction, then drives secondary core 21 to be extracted from local wrapping groove of plastic part.
Principle of secondary slider core-pulling is shown in Figure 7. Compared with ordinary secondary core-pulling slider mechanism, this mechanism has following characteristics: ① Hydraulic cylinder 25 pulls primary core 27 to move in F1 direction through bent rod 24, and primary core 27 must be guided by guide groove wear-resistant plate 23; ② Primary core 27 and secondary core 21 move through T-shaped guide rail. When primary core 27 moves in F1 direction, secondary core 21 is limited in F1 direction by guide fins in guide groove wear-resistant plate 23, and can move in F2 direction, so that after primary core 27 moves d1 in F1 direction, secondary core 21 moves d2 in F2 direction, ensuring that lower end of secondary core 21 head will not be blocked when moving out in F1 direction, both primary core 27 and secondary core 21 can be moved out of plastic part; ③ After core pulling of secondary core 21 is completed along F2 direction, its guide fin must move in lower groove of guide groove wear-resistant plate 23 to ensure that mechanism can be accurately reset and closed; ④ When closing mold, use a side slider to lock primary core 27 by inserting its slider head into locking groove on primary core 27.

 

21. Secondary core 23. Guide groove wear plate 24. Bent rod 25. Hydraulic cylinder 27. Primary core

After mold is injected, maintained and cooled, demoulding of plastic part is completed according to following steps.
(1) PL side is open. Slider of injection molding machine pulls movable mold back, PL surface of mold is opened, and plastic part remains on movable mold plate 5.
(2) Hydraulic cylinder core pulling. After PL surface is opened, hydraulic cylinders 25 of slider mechanisms S1 and S1' drive primary core 27 and secondary core 21 to complete core pulling.
(3) Demold plastic parts. Ejector pin of injection molding machine pushes push plate, forcing pull rod 6 and push rod 17 to push flow channel aggregate and plastic part from driven mold plate 5 to achieve complete demolding of plastic part.
(4) Reset. Push plate is reset first, then PL surface is closed, finally slider mechanisms S1 and S1' are reset. Next injection cycle starts after mold is completely closed.

For processing of movable mold plate 5 and fixed mold plate 4, due to their large size, rough machining and finishing must be separated when using CNC machining. Generally, following principles are followed: ① Processing of previous process cannot affect positioning and clamping of next process, and machining processes interspersed with general machine tools should also be considered comprehensively; ② Process inner cavity first and then process outer shape; ③ Processes processed with same positioning and clamping method or using same tool should be processed continuously to reduce number of tool changes due to repeated positioning; ④ Principles of CNC programming should be adhered to principles of coarse first and then fine, first main and then second, surface first and then hole, and benchmark first.
Processing process of moving mold plate 5: back drilling, front drilling, and four side drillings; front CNC processing to complete CNC milling of core insert installation slot, slider mechanism installation slot, etc. Processing process of fixed mold plate 4 is as follows.
(1) Rough machining: ① Level with reference angle, center four sides, and bottom is facing tool; ② Fine mill back periphery, C corner, chamfer, C corner of gate and exhaust groove to drawing size; ③ Rough milling of front parting surface steps, plastic part molding positions, reference angle marks, fonts and other features, leaving a 1mm margin during rough machining; ④ Drill holes on the front and back sides (leaving clamping platforms at four corners); ⑤ Process all screw holes, cooling water channel holes, drill eye screw holes, feed inlet holes, exhaust holes and other features; ⑥ Weigh and use a marker to make a reference mark and send it to next process for processing; ⑦ Heat treat hardness to 46~48HRC.
(2) Finishing: ① Precision grinding shape, grinding bottom surface to a bright level (Ra0.8μm) and ensuring six right angles, leveling with datum plane, and setting tool on 2 sides of datum angle;  ② Precision cutting feed port hole; ③ Level with datum plane, calibrate tool on two sides of datum angle; ④ Fine mill front parting surface step, plastic part forming position, exhaust groove, reference angle mark, all fonts and other features. When fine milling plastic part forming position, leave a 0.1mm margin for polishing.