Glove compartment cover of a new energy vehicle is an important component of storage compartment. Located on the outside of passenger side, it has large length and width dimensions but small thickness, requiring high precision in assembly dimensions and high appearance quality. To meet four major requirements of quality, efficiency, cost, and environmental protection for new energy vehicles, customers demanded further reduction in weight, improvement in precision, shortening of molding cycle, and full automation of production compared to ordinary automotive glove compartment covers. This significantly increased molding difficulty and placed higher demands on mold structure. Therefore, author further optimized plastic part structure, innovated and optimized mold structure. All indicators of molded plastic part met design requirements. This successful experience provides valuable reference for design of injection mold structures for large, thin-walled plastic parts.

Figure 1 shows a part drawing of a storage box cover for a new energy vehicle. It features an aesthetically pleasing spatial design with a relatively complex parting line. Material is PP/EPDM-T20 with a shrinkage rate of 1.1%. Plastic part has assembly requirements with box body, and assembly dimensional accuracy must meet MT3 (GB/T 14486—2008). Outer surface of plastic part requires texturing, and inner surface roughness is Ra=0.04~1.25 μm. Maximum length of plastic part is 555.5 mm, maximum width is 200.4 mm, and maximum height is 300.5 mm. Based on customer requirements, we optimized plastic part structure with conformal reinforcing ribs, reducing average wall thickness from 2.5 mm to 2 mm and weight from 420 g to 360 g, saving approximately 15% of material and meeting lightweight requirements of new energy vehicles. Vibration and stress tests (20~2000 Hz) showed no structural failure. Plastic part is a large, thin-walled component with two undercuts, S1 and S2, making melt filling and demolding difficult. Molding defects such as weld lines, shrinkage dents, spots, flash, and gate marks are not permitted on the surface of part.

 

Figure 1: Part of a New Energy Vehicle Storage Box Cover

Due to large size, complex spatial shape, significant parting line difference of automotive storage box cover, and with customer's consent, both fixed and moving mold parts are designed as a single piece. Cavity and core are directly machined on fixed mold plate A and moving mold plate B. Mold plate is molding part. When ordering mold base, it must be specifically specified that both moving mold plate B and fixed mold plate A are made of P20 mold steel. Compared to a pieced structure, single-piece structure is more compact, has better rigidity, avoids cumbersome processes such as frame opening, frame fitting, wedge manufacturing, and can reduce size of mold base, reducing cost per mold set by approximately 18%.
Because car storage compartment cover has an irregular spatial curved surface and a complex spatial curve parting line, an asymmetric variable curvature parting surface was adopted to reduce impact of parting line on aesthetics and lower manufacturing costs. Due to large height difference of molded part, lateral expansion force of cavity during injection molding is significant. To prevent lateral displacement between mold plates, mold uses four conical surfaces for positioning, with a taper of 5° to ensure precise positioning.
Mold cavity utilizes nano-coating technology, spraying a diamond-like carbon (DLC) coating onto cavity surface. This significantly reduces melt flow resistance and demolding resistance, thereby greatly reducing injection pressure and clamping force, extending mold life by an estimated 30%.
To ensure appearance quality of storage compartment cover and improve melt filling, mold uses a combination of hot runner and conventional runner systems: molten plastic enters conventional runner through hot runner, and then enters cavity through two submarine gates, as shown in Figure 2(a). Conventional runner uses a U-shaped cross-section, while submarine gate uses an arc-shaped structure. Specific dimensions are shown in Figure 2(b).

 

Figure 2: Mold Gating System
During injection molding, arc-shaped submarine gate can automatically cut off. This in-mold gate system ensures appearance quality of molded plastic part and enables fully automated production, reducing scrap rate from 5% to 1%, thus improving green manufacturing technology of new energy vehicles.
To verify rationality of above gating system design, author used MoldFlow software to simulate and analyze mold injection molding process, optimized shape and position of gate based on analysis results. Figures 3-7 show results of analysis of car storage compartment cover filling, filling contour analysis, injection pressure curve analysis, clamping force curve analysis, weld line and air trapping analysis, respectively.

 

Figure 3 Melt Filling Analysis

 

Figure 4 Melt Filling Contour Analysis

 

Figure 5 Injection Pressure Curve Analysis

 

Figure 6 Clamping Force Curve Analysis

 

Figure 7 Weld Line and Air Trapping Analysis
Based on mold flow analysis results, it can be concluded that: plastic part uses a two-point sequential valve hot runner system, with injection point on the side of plastic part. Filling time is 4.6 s, resulting in good filling effect, balanced flow, no poor filling or air trapping, uniform filling pressure, and a scientifically sound injection scheme.
Molded plastic part has two inner undercuts S1 and S2, with opposite core-pulling directions. Due to limited space, only a lateral core-pulling mechanism with angled push rods can be used. This mechanism consists of side core pullers 27 and 28, angled push rods 31 and 37, angled push rod slides 32 and 34, four guide sleeves 26, 29, 33, and 36, as detailed in Figure 8. Both undercuts have a depth of 4.71 mm. Adding a safety distance, side core-pulling distance is 8.40 mm. Based on part height, demolding distance is 80 mm. Using graphical methods or trigonometric calculations, tilt angle of lifter pin is 6°.

 

26 - Angled push rod guide sleeve 1; 27 - S1 side core pull; 28 - S2 side core pull; 29 - S2 Angled push rod guide sleeve 1; 30 - Push rod; 31 - S2 Angled push rod; 32 - S2 Angled push rod slide; 33 - S2 Angled push rod guide sleeve 2; 34 - S1 Angled push rod slide; 35 - S1 Angled push rod; 36 - S2 Angled push rod guide sleeve 2; 37 - S1 Angled push rod
Figure 8: Design of lifter side core-pulling mechanism.
Since two lifters are close together, it is essential to ensure they do not collide when pulling core inwards. As shown in Figure 8, distance between two lifters when mold is closed is 22.53 mm, which is greater than lateral movement distance of 8.40 × 2 = 16.80 mm, ensuring safety.
Storage box lid is a large, thin-walled plastic part with a small inner surface roughness and an outer surface requiring texturing. Its structure includes snap-fit components and reinforcing ribs, making it susceptible to shrinkage marks and warping due to uneven cooling. Therefore, mold temperature control system adopts conformal cooling, with cooling water channels conforming to curved shape of cover plate (20-25 mm from cavity surface). Main water channel has a diameter of ϕ12 mm, and branch water channels have a diameter of ϕ8 mm.
Secondly, we implemented independent zone control: mold is divided into a gate area, a center area, and an edge area, with each area having its own independent water circulation system. Cooling water channel parameters are designed as follows: during injection molding, cooling water flow rate is controlled between 5 and 10 L/min to ensure cooling water is always in a turbulent state (Reynolds number > 4000) to enhance heat transfer. Spacing between water channels is 3-5 times pipe diameter, with a staggered arrangement to avoid cooling blind spots (see Figure 9). To improve cooling efficiency and prevent rust on inner walls of water channels, all cooling water channel inner walls are chrome-plated to reduce scale buildup. To provide real-time data feedback to control system and achieve closed-loop control, intelligent IoT integrated monitoring technology is employed during injection molding process. Temperature sensors are embedded within inserts to monitor mold cavity filling status in real time. Combined with AI algorithms, injection parameters are dynamically adjusted, keeping mold temperature difference within ±2℃ and improving yield rate to over 99.5%.

 

Figure 9: Mold Temperature Control System.
Precise mold temperature control ensures rapid and even cooling, preventing deformation caused by localized temperature differences. Molding cycle is shortened from 35 seconds to 26 seconds, cooling efficiency is improved by 25%, and dimensional accuracy reaches MT3 (GB/T 14486—2008).
Injection mold for automotive storage box covers has a design life of 500,000 cycles. Due to its large size, long lifespan, and high precision, mold needs to withstand prolonged, high-frequency opening and closing actions. Therefore, durability and accuracy of guiding and positioning system are crucial.
Guide pillars of injection mold for car storage box cover are made of high-carbon chromium bearing steel SUJ2, quenched, with a surface hardness of 60-62 HRC; guide sleeves are made of copper alloy to reduce friction and wear. Diameter of guide pillar is usually 1/10 to 1/8 of mold parting surface width; in this mold, it is ϕ50mm. Length of guide pillar needs to exceed closing height of moving mold by 10-15mm; in this mold, it is 370mm. There are four guide pillars, arranged at four corners of mold, fixed with flanges for easy disassembly and maintenance (see Figure 10). Keyways or flat surfaces are added to ends of guide pillars to prevent rotational offset.

 

Figure 10: Mold Guiding and Positioning System.
Injection mold for car storage box cover uses a conical precision positioning structure to assist guide pillars and guide sleeves for high-precision alignment during mold production. Taper of positioning surface is 10°, and contact area needs to be 85%. To improve mold life and facilitate maintenance, 12 wear-resistant blocks are designed on conical surface. Guide pillars and guide sleeves are fitted with an H7/f7 clearance to ensure lubrication and thermal expansion space.
To prevent whitening and deformation during demolding of car storage box cover, mold adopts a combined ejection scheme of "push rod + angled push rod + hydraulic ejection cylinder". There are 12 ejector pins, each with a diameter of ϕ8 mm, evenly distributed, with 2 sets of lifters to handle side-clamping during demolding. Ejection stroke is 80 mm, ejection speed is 0.5 m/s, and ejection force is approximately 8 t. Demolding system is driven by two hydraulic cylinders (25), ensuring smooth and reliable ejection, resetting without whitening or deformation. Injection mold for car storage box cover is a large mold; to ensure safe operation of ejector plate, six guide pillars and six reset rods are designed. A hard block with a diameter of ϕ40 mm is also designed at contact point between fixed mold plate A and reset rod. Hard block is made of S50C mold steel with a nitrided surface.
There is a large amount of air in the cavity of injection mold for car storage box cover. During injection molding process, this gas must be expelled promptly. Simultaneously, during mold opening, outside air must enter cavity in a timely manner to prevent a vacuum from forming and causing demolding difficulties. Storage box cover injection mold primarily uses venting grooves on parting surface for venting, as shown in Figure 11. Additionally, author also added venting by slotting ejector pin sidewalls (depth 0.02~0.03 mm), which proved very effective. PP material has good fluidity; depth of primary venting groove should not exceed 0.04 mm, as excessive depth can easily cause flash. Depth of secondary venting groove can be 0.5~0.8 mm. Venting groove width is 10 mm, venting groove spacing is 30 mm to ensure rapid gas expulsion. This efficient and reliable venting system effectively eliminates molding defects such as weld lines and poor filling.

 

Figure 11 Mold Venting System Design
In summary, mold adopts a hot runner gating system, and two undercuts on inner side of plastic part use a lateral core-pulling mechanism with lifters (also known as angled pushers). Mold's external dimensions are: length 1100 mm, width 730 mm, height 840 mm, and total mass approximately 3 t, classifying it as a large injection mold. See Figure 12 for detailed structure.

 

1-Heat Insulation Plate; 2-Fixed Mold Fixing Plate; 3-Frame Plate; 4-Positioning Ring; 5-Primary Hot Injector Nozzle; 6-Positioning Pillar; 7-Hot Runner Plate; 8-Hot Runner Plate Wiring Socket; 9-Fixed Mold A Plate; 10-Cooling Water Connector; 11-Moving Mold Small Insert; 12-Guide Bushing; 13-Guide Pillar; 14-Moving Mold Insert; 15-Square Iron; 16-Ejector Plate Guide Pillar; 17-Ejector Plate Guide Bushing; 18-Moving Mold Fixing Plate; 19-Ejector Base Plate; 20-Ejector Fixing Plate; 21-Support Column; 22-Wear-Resistant Block; 23-Parting Surface Hard Block; 24-Secondary Hot Injector Nozzle; 25-Hydraulic Cylinder; 26-Lifter Rod Guide Bushing 1; 27-S1 Side Core Pulling; 28-S2 Side Core Pulling; 29-S2 Lifter Rod Guide Bushing 1; 30-Ejector Rod; 31-S2 Lifter Rod; 32-S2 Lifter Rod Slide; 33-S2 Angled push rod guide sleeve 2; 34-S1 Angled push rod slide; 35-S1 Angled push rod; 36-S2 Angled push rod guide sleeve 2; 37-S1 Angled push rod 38-Limit rod
Figure 12: Structural diagram of injection mold for automotive storage box cover
(1) Mold closing: Moving and fixed molds of mold close under drive of injection molding machine, forming a sealed cavity.
(2) Injection filling: Molten polypropylene plastic enters ordinary runner between mold parting surfaces through hot runner gating system, and finally enters mold cavity through arc-shaped submarine gate.
(3) Holding pressure and cooling: During holding pressure stage, pressure is continuously applied to compensate for material shrinkage; cooling system (water system) rapidly cools plastic through circulating cooling water, allowing it to solidify and set.
(4) Mold opening, ejection, and core pulling: Under drive of injection molding machine, moving mold moves backward, mold opens, and after opening distance reaches 400 mm, hydraulic cylinder 25 pushes ejector plate 19, simultaneously pushing all ejection parts to push molded plastic part away from moving mold. During ejection process, angled push rods 31 and 35 push lateral core pullers 28 and 27 inwards respectively.
(5) Part removal and resetting: Ejection stroke is 80 mm, controlled by limit rod 38. Then robot arm removes product, hydraulic cylinder 25 pushes demolding system and lifter to reset, and mold enters next injection molding.

(1) Through optimized design of conformal reinforcing rib structure, average wall thickness of plastic part was reduced from 2.5 mm to 2 mm, saving about 15% of material, which meets lightweight requirements of new energy vehicles.
(2) Through mold flow analysis optimization, mold determined optimal combination of "hot runner + ordinary runner + arc-shaped submarine type" gating system, which not only ensured appearance quality of molded plastic part, but also realized fully automated production, reducing scrap rate from 5% to 1%, improving green manufacturing technology of new energy vehicles.
(3) By adopting cavity nano-coating technology, partitioned conformal water channels, and a closed-loop temperature control system, molding efficiency and quality of mold have been significantly improved. Dimensional accuracy has reached MT3 (GB/T 14486—2008), production cycle has been shortened from 35 s to 26 s, and cooling efficiency has been increased by 25%, thus enhancing rapid and precision molding technology for new energy vehicles.
(4) This mold is innovatively designed around four dimensions: quality, efficiency, cost, and environmental protection. Its advanced and reasonable structure reduces cost of a single mold set by 18% and increases overall production capacity by 2 times. Trial molding was successful on the first attempt, molded plastic parts passed vibration and stress tests, with all indicators meeting design requirements.