As shown in Figure 1, automotive glove box is a combination of interior and functional components. Its appearance requires a leather-like texture, thus demanding high precision and surface roughness. Its function is to provide a storage compartment and function as an air conditioning filter cover, accessible from passenger side.

 

Figure 1: Automotive Glove Box

Glove box has external dimensions of 590 mm * 362 mm * 263 mm. Material is PP+TD20, a polypropylene-based composite material with 20% talc as a reinforcing filler. Melt flow rate is 1.8 g/min, and shrinkage rate is 1.1%. This material possesses high rigidity, dimensional stability, and heat resistance. After molding, glove box must not have defects such as shrinkage marks, pits, flash, sun spots, or weld lines. Plastic part has a bucket-shaped deep cavity structure with many undercuts on inner and outer sides for assembly, as shown in Figure 2. Mold design requires a dense multi-directional slider core-pulling structure. Core-pulling distribution is as follows: ① Four lateral undercuts (8-12 mm deep) in fixed mold direction, as shown in Figure 3; ② Nine undercuts (including five deep cavity irregular undercuts, 15 mm deep) in moving mold direction, as shown in Figure 4.

 

Figure 3 Fixed Mold Slider Structure

 

Figure 4 Moving Mold Slider Structure
1. Hydraulic Cylinder Core-Pulling Slider 2. Mechanical Turning Slider 3. Conventional Slider 4. Mechanical Turning Slider

Plastic part has a bucket-shaped deep cavity structure with multiple undercuts, presenting following challenges in mold design:
(1) Gating System Design. Plastic part has a textured appearance and requires high surface quality. Gate cannot be placed on the surface, and plastic part to be molded is surrounded by slider structures, limiting space for gate location selection. Setting appropriate gate and weld line locations through mold flow analysis is crucial. Analysis shows injection location of plastic part to be molded is shown in Figure 5, employing a hot runner system and a 4-point needle valve gating.

 

Figure 5: 4-point needle valve gating analysis
(2) Core-pulling system design. Due to structure of plastic part, 4 sets of undercuts are formed on fixed mold side, and 9 sets of undercuts are formed on moving mold side. Mold needs to be designed with multiple sets of inclined slider core-pulling structures. Due to limited space, traditional hydraulic cylinder core-pulling cannot be arranged on inner undercuts. Considering all factors, a new mold structure is needed, namely a three-plate mold driven slider core-pulling and a turning slider core-pulling.
(3) Mold component strength design. There are a total of 13 slider core-pulling mechanisms in moving and fixed molds, and inclined sliders need to be machined with turning structures, resulting in a large amount of hollowing out of moving and fixed mold plates. Effective bearing surface decreases by 35%, reducing strength. Therefore, the overall mold plate support design and strength analysis verification are key to ensuring subsequent mold mass production.

Due to large size of plastic part, injection mold adopted a one-mold-one-cavity structure. During injection molding, a four-point needle valve feeding system is used, with sequential valves G1~G4 opening sequentially. Molten plastic enters hot runner through nozzle and fills cavity through needle valve gate. Plastic part has 13 undercuts. Mold employs a three-plate mold with a slider core-pulling mechanism, a mechanical turning slider, a hydraulic cylinder slider core-pulling mechanism, and a slanted push structure for core-pulling. The overall dimensions of mold are 1400 mm * 1400 mm * 1436 mm, with a total weight of approximately 12,100 kg, classifying it as a large and complex injection mold.

Fixed mold forms four sets of undercuts. Since cores for forming undercuts need to be demolded from inside of mold, a traditional mechanical spring structure to drive slider demolding cannot be designed. Furthermore, limited internal space of mold also precludes use of a hydraulic cylinder for core-pulling. Therefore, an innovative three-plate mold structure with sequential opening is adopted to achieve slider core-pulling. Wedge block is fixed to runner plate and engages with inclined slider via a T-block, as shown in Figure 6. Driving principle is as follows:

 

Figure 6 Fixed mold slider core-pulling structure
1. Runner plate 2. T-block 3. Fixed mold plate 4. Moving mold plate 5. Inclined slider 6. Inclined slider 7. Wedge block 8. Pull-out clip 9. Spring 10. Wedge block
(1) First step: Mold opening. During mold opening, runner plate and fixed mold plate are first opened at PL1 by spring drive. Moving and fixed mold plates are locked by a pull-out clip. Runner plate opening simultaneously drives wedge block to move, and wedge block drives inclined slider to pull core via T-block.
(2) Second step: Mold opening. After runner plate's mold opening stroke reaches 66 mm, core pulling on fixed mold side is completed, and pull-out clip is released. Moving and fixed mold plates open at PL2, and molded plastic part remains on moving mold side.

Moving mold side of molded plastic part has three undercuts, designed for conventional mechanical sliding block molding, driven by angled guide pillars with a 15° angle and a maximum stroke of 25mm. Deep cavity of plastic part has five undercuts. Due to angle of inclination greater than 30°, conventional sliding block molding is not feasible; only a mechanical turning slide and angled slide core pulling can be used, as shown in Figure 7. Driving principle is: initial drive by wedge block → secondary stroke extension by turning slide (T-slot guide engagement). Turning slide stroke distribution: wedge block and T-block drive turning slide stroke to 46.6 mm, with a mold opening angle of 30°; turning slide drive angled slide core pulling stroke to 19.14 mm.

 

Figure 7. Moving Mold Slider Mechanical Turning Drive
1. Wedge Block 2. T-Block 3. Turning Slider 4. Angled Slider 5. Turning Slider 6. Wedge Block
Molded plastic part has one undercut on the side, with an undercut amount of 70 mm. A hydraulic cylinder piston rod drives slider to pull core, as shown in Figure 8. To save mold space, an internal hydraulic cylinder is designed. Plastic part has two conventional undercuts inside. A slanted push structure is designed for demolding. Slanted push rod has an inclination angle of 5° and an ejection stroke of 180 mm. Undercut amount of molded plastic part is 12 mm, and demolding stroke of slanted push structure is 15.75 mm.

 

Figure 8. Hydraulic Cylinder Drive for Moving Mold Slider
1. Angled Push Block 2. Angled Push Rod 3. Hydraulic Cylinder 4. Large Side Slider

Due to numerous core-pulling structures in mold, fixed and moving mold plates are extensively hollowed out. To ensure structural strength of mold parts, strength of mold plates needs to be optimized: fixed mold plate is thickened from 300 mm to 400 mm, and moving mold plate is thickened from 400 mm to 500 mm. Inlay structures are used in some areas to avoid thin-walled areas that would result in insufficient strength. H13 steel inserts are used in core-pulling intersection area, with a heat treatment hardness of 48~52 HRC. Wear-resistant plates and guide rails of slider are made of self-lubricating aluminum bronze + graphite material.

Due to large inclination angle of glove box's inner hopper and crisscrossing reinforcing ribs, moving mold side experiences significant clamping force after plastic part is molded. Furthermore, core-pulling structure occupies a large amount of space, affecting arrangement of ejection mechanism. Therefore, mold ejection system ultimately incorporates a flat ejector rod (6 mm * 9 mm) within slide core-pulling area. In main body area of molded plastic part, ejector rods, ejector blocks, and a lifter structure are used for auxiliary ejection, as shown in Figure 9. To prevent molded plastic part from getting stuck on lifter structure and ejector block after ejection, a secondary ejection scheme is designed. Lifter structure, ejector block, and flat ejector rod are mounted on primary ejector plate, with an ejection stroke of 180 mm. Ejector rod is mounted on secondary ejector plate, with an ejection stroke of 30 mm. Secondary ejection drive structure is shown in Figure 10.

 

Figure 9 Ejection Structure Layout
1. Ejector Block 2. Lifter Structure 3. Ejector Rod 4. Lifter Structure 5. Flat Ejector Rod

 

Figure 10 Ejection Drive Structure
1. Pulling Mold Buckle 2. Primary Ejector Plate 3. Secondary Ejector Plate 4. Limiting Post 5. Hydraulic Cylinder
Both fixed mold and moving mold adopt conformal water cooling, as shown in Figures 11 and 12. Water channel diameter is φ12 mm. Fixed mold has 8 sets of cooling water channels, and moving mold has 7 sets of cooling water channels. Cooling water channels are 20~30 mm away from molded plastic parts, rationality and economy of water channel design are verified by Moldflow cooling analysis module.

 

Figure 11 Fixed Mold Cooling Water Channel

 

Figure 12 Moving Mold Cooling Water Channel

(1) Injection. Molten plastic enters ordinary runner through injection molding machine nozzle via hot runner 4-point needle valve, then enters cavity through side gate of ordinary runner. Cavity is filled, pressure held, and cooled before solidification.
(2) Core Pulling from Fixed Mold. Driven by injection molding machine slider, mold opens. Due to spring between runner plate and fixed mold plate, runner plate opens first under spring force, while inclined slider pulls core out of mold.
(3) Mold Opening. Driven by injection molding machine slider, moving and fixed mold plates open, leaving molded part on moving mold side. During mold opening, slider fully opens under action of inclined guide post and wedge block, while hydraulic cylinder piston rod completes core pulling.
(4) Ejection. After mold opening and core pulling, four hydraulic cylinder piston rods and a pull mold buckle drive two ejections. A push rod, flat push rod, push block, and inclined push structure are used to eject molded part mold, then a robot arm removes part.
(5) Mold Closure. After molded part is removed, hydraulic cylinder piston rod resets secondary and primary push plates. Then, mold closes under drive of injection molding machine slider, awaiting next cycle.