Automobile glove box is a challenging component in automotive interior design. Its main components include glove box compartment and box body. Designing molds for these two plastic parts is a complex process involving multiple technical aspects and challenges. Key issues include: firstly, material selection and improvement. Glove box compartment and box body are generally manufactured using injection molding, typically using polypropylene (PP) and polycarbonate acrylonitrile-butadiene-styrene copolymer (PC/ABS). These two materials each have their advantages and disadvantages. Most OEMs prioritize PP for cost considerations. PP has excellent performance, low density, is easy to mold, and is inexpensive, but it has slightly poor wear resistance, a large molding shrinkage rate, poor low-temperature performance. Therefore, when designing glove box molds, improvements need to be made to address shortcomings of PP, such as adding materials like EPDM rubber, talc (TD), and glass fiber (GF). Second, there are many potential design pitfalls in mold structure design that are easy to overlook, which can easily lead to failure of mold structure design. Among them, more typical design difficulties are as follows: (1) Preventing hydraulic cylinder from retracting and jamming. When designing mold structure of outer panel of glove box, special attention should be paid to preventing hydraulic cylinder from retracting and jamming. (2) Inverted mold structure. In view of structural characteristics of glove box compartment, mold mostly adopts an inverted mold structure. Gating system and demolding system are both on moving mold side of mold. Since injection molding machine ejector is not on fixed mold side, fixed mold pusher is pushed out and reset by hydraulic cylinder. (3) Application of leather texture process. Due to requirements of users, periphery of box body often adopts leather texture surface. (4) Application of flocking process. In high-end cars, flocking process is usually used for glove box storage space to increase comfort. Flocking process is a process of attaching fine fluff to surface of parts. Generally, glue is first applied to corresponding parts of parts, and then fluff is sprayed onto corresponding area. (5) Complex internal structure of plastic part, many features that make demolding difficult require comprehensive application and innovative structural design to solve problem of automated injection molding production. These difficulties need to be fully considered and addressed when designing glove box mold to ensure that final mold has a reasonable structure, stable performance, can meet production needs. Especially in mold structure design stage, using mold structure design methods to address molding process requirements of plastic part can effectively save mold manufacturing costs. This paper, combined with molding of a new car glove box body, designs a hot runner two-plate mold. Design methods adopted in mold to solve problems that easily occur in molding of plastic part have good reference value for production practice.

Shape of automotive glove box body plastic part is shown in Figure 1(a). Features affecting mold structure design of plastic part are as follows: From top-view axonometric drawing of plastic part, it can be seen that right side front P1 of plastic part has 4 textured surface features, and left side front P2 has 1 textured surface feature. There are many holes in four edge areas B1, B2, B3, and B4. Central part is box-and-hopper mounting area Xd, with hole H6 in the middle. From bottom-view isometric view of plastic part, it can be seen that there are several features on the back of part that are difficult to demold, including nine retaining plates K1~K9, two side grooves G1 and G2, waist-shaped holes H2, H4 and one round hole H3 on two sides of box-and-hopper, beveled holes H1, H5, H7, and H8 at both ends of side B4, and side hole H9 of retaining plate K9. Seven side groove features, c1~c4 and d1~c3, are provided on outer wall of side B1.

 

Figure 1: Plastic part of automotive glove box body
Structural dimensions of plastic part are shown in Figure 1(b), with external packaging dimensions of 658.3 mm * 365.8 mm * 223.2 mm. Demolding depth dimensions affecting demolding of plastic parts for each feature are as follows: K1 lateral core-pulling depth is 14.7 mm, K2 is 16 mm, K3 is 8.6 mm, K4 is 13.7 mm, K5 is 13.6 mm, K6 lateral core-pulling depth is 12.4 mm, K7 is 12.9 mm, K8 is 27.9 mm, K9 has no lateral core-pulling depth in body, but its wall hole H9 has a lateral core-pulling depth of 2 mm. G1 and G2 have a lateral edge depth of 5 mm. Holes H1 and H5 have a wall thickness depth of 3.1 mm, H2 and H4 have a lateral depth of 4.5 mm, H7 and H8 have a lateral depth of 3.4 mm. Molding precision of plastic parts is MT4-MT6, with a quantity of 200,000 pieces.
Plastic part material uses PP modified with 20% talc (TD20) and toughening agents, with a shrinkage rate of 0.54%~0.79%. Use of activated talc to fill and modify PP significantly improves material's rigidity and surface finish, effectively reducing warpage. Small spherulites in PP result in good dimensional stability, significantly improved elastic modulus, impact resistance, increased strength, reduced shrinkage, high flexural strength, notched impact strength, heat resistance.

Design challenges of plastic part molding molds include following: First, inconsistent demolding directions of nine retaining platforms K1~K9, holes H1~H9, main body make main mold opening direction difficult. Second, concentrated features in certain areas limit design space for demolding mechanism, making design of molded parts in these difficult-to-demold areas particularly challenging. Concentration of multiple demolding mechanisms in these areas further complicates design of these mechanisms. Third, textured surfaces are required in certain areas of plastic parts. Careless design, processing of these textures can easily lead to scrapping of molded parts, resulting in production waste. Fourth, large size and irregular shape of plastic parts make them prone to injection, ejection imbalances, leading to numerous potential defects or ejection deformation, resulting in unstable molded dimensions that are difficult to control precisely.

To address molding difficulties of plastic parts, following approach is proposed:
(1) First, main parting surface of mold cavity is determined using demolding direction integration design method to simplify mold structure design. For large automotive plastic parts with multi-directional demolding features, main molded parts of mold cavity and other secondary demolding direction molded parts are designed using a demolding direction integration method followed by setting of separate demolding directions. This method, which addresses demolding of most features on plastic part before setting separate demolding mechanisms, simplifies the overall mold structure design, reduces difficulty of mold design and manufacturing. First, identify maximum outer contour line and internal hole line of plastic part, as shown in Figure 2(a). Then, based on integration direction of demolding direction of most features of plastic part, determine demolding directions of other branches, as shown in Figure 2(b). Combining shape characteristics of plastic part, after a concentrated analysis of demolding directions of all features on plastic part, determine placement of plastic part in mold cavity as shown in Figure 2(b). Main demolding direction of this placement is FZ-. Based on this main demolding direction, use maximum outer contour line of plastic part under this placement [LP line in Figure 2(a)] to set parting surface of plastic part as PS surface. Under this main parting setting, after further local area concentration of each feature in different demolding directions, set local molded parts and corresponding demolding mechanisms as shown in Figure 2(c). Difficult demolding characteristics and corresponding demolding mechanism settings are as follows: For retaining platforms K1, K2, K3, K4, K5, K6, and K8, respectively, lifters are used for forming and side core pulling demolding. Corresponding lifters are XK1, XK2, XK3, XK4, XK5, XK6, and XK8. If an lifter is set for K7, it will inevitably interfere with lifter XK8. Therefore, using a slider SK7 towards FY+ for core pulling can avoid this situation. For K6, K5, and K4, it is actually possible to use an FX+ slider for side core pulling and integrate it on one slider for core pulling demolding. However, considering processing cost, it is more economical to use three lifters XK6, XK5, and XK4 respectively. XK4 also integrates core pulling demolding of hole H3. Demolding direction of K9 is FZ-, and no separate demolding mechanism is required. However, side wall hole H9 on it integrates lifter XH2 through hole H2 for side core pulling demolding. Like H2, hole H4 is also demolded using lifter XH4. On side B1, features c1~c4 and d1~d4 on its outer wall can only be demolded using FY direction. Therefore, slider SB1 is used to demold side wall of B1 and its features using FY direction. Due to limited space between lifter XH2 and slider SB1, it is more suitable to use lifters XK1, XK2, and XK3 for demolding K1, K2, and K3 within this space. Demolding of side grooves G1 and G2 faces similar issues, so lifters XG1 and XG2 are used for demolding. In main mold opening direction FZ-down, demolding direction of holes H1 and H5 is angled; therefore, sliders SH1 and SH5 are more suitable for demolding them using side core-pulling mechanism. After this optimization, arrangement of molded parts on moving mold side is shown in Figure 2(d). Four slider mechanisms (SK7, SH5, SH1, SB1) and eleven lifter mechanisms (XK8, XK6, XH4, XK5, XK4, XK3, XK2, XK1, XH2, XG1, XG2) are needed on moving mold side to achieve molding and demolding of difficult-to-demold features on inner surface of plastic core.

 

B1, H1~H9, G1, G2, K1~K9 (see Figure 1); S_K7, S_H5, S_H1, S_B1 - slider numbers; X_K8, X_K6, X_H4, X_K5, X_K4, X_K3, X_K2, X_K1, X_H2, X_G1, X_G2 - lifters; P - parting surface; F_Y-, F_Y+, F_X-, F_X+, F_Z-, F_Z+ - mold coordinate axis directions; LK - hole line.
Figure 2 Mold Cavity Parting and Design of Difficult-to-Demold Molded Parts
(2) Hole parting is set on moving mold core side to avoid flash affecting appearance of plastic part. Based on automatic demolding of plastic part, mold cavity design also needs to consider convenience and economy of molding processing, and also needs to avoid molding defects such as flash as much as possible. As shown in Figure 3(a), among all parting lines LK of parting surfaces inside plastic part, except for holes H2 and H4, all other parting lines LK are set on one side of inner core surface, so that possible flash defects appear on inner wall side of plastic part as much as possible without affecting front appearance of plastic part.

 

L_K, P_a, H₂, P_s (see Figure 2); P₁, P₂ - leather-textured surface.
Figure 3 Hole filling design and texture surface setting
(3) A texture scheme with a texture depth of 30 μm and a draft angle of 2.4° is selected to form texture surfaces of P1 and P2. Mold structure design for texture of P1 and P2 surfaces. As shown in Figure 3(b), in order to meet requirement of setting texture appearance on P1 and P2 surfaces, texture shape required for surface of corresponding fixed mold cavity forming surface P1 and P2 is texture sample shape shown in Figure 3(b), texture depth of P1 and P2 surfaces is 30 μm, draft angle is 2.4°. Texture design is a technically intensive step in plastic part mold making, involving control of multiple steps and parameters. During process of creating textures on P1 and P2 surfaces of mold platen, multiple immersions in chemical agents for etching and sandblasting are required to remove residual chemicals, adjust gloss. To ensure texture effect, mold platen is made of S136 alloy steel. Mold design measures are taken to address molding shrinkage error (setting different shrinkage rates in X, Y, and Z directions) and mold temperature control (using contoured water channels) to avoid molding defects such as surface blooming on plastic part. In handling relationship between parting line and texture, parting line is positioned below texture surface with a 0.3 mm margin to prevent texturing. Before texturing on P1 and P2 surfaces, mold needs to be polished. Polishing requirements correspond to texture depth as follows: polishing with 1000# sandpaper, texture depth 30~40 μm.
(4) A hot runner + cold runner gating system is used to balance gating of mold cavity. As shown in Figure 4, gating system uses a hot runner + cold runner composite gating system. Hot runner uses two hot nozzles 10 and 11 for material supply. Hot nozzles 10 and 11 are controlled by pneumatic needle valves 4 and 3, respectively. In Figure 4(a), hot nozzle 10 supplies material to cold runner R1 in Figure 4(b). R1 then supplies material to two side gates g1 and g2. Similarly, hot nozzle 11 supplies material to cold runner R2, R2 supplies material to side gate g3 alone. Positions and dimensions of g1, g2, g3 are obtained from production experience. Cross-sectional dimensions of rectangular side gates g1 and g2 are 12 mm * 2 mm, and cross-sectional dimensions of g3 gate are 15 mm * 2 mm. Material supply path of hot runner system is as follows: injection molding machine nozzle supplies material to hot runner in hot runner plate 7 through insulated sprue sleeve 12. Molten material in hot runner plate 7 is supplied to gates g1 and g2 through hot nozzle 10 and cold runner R1, respectively. At the same time, molten material in hot runner plate 7 is also supplied to gate g3 through hot nozzle 11 and cold runner R2, respectively. Valve needle opening sequence in hot nozzle 10 and hot nozzle 11 is: hot nozzle 10 from 0 to 14.8 s, hot nozzle 11 from 4 to 14.8 s. This setting time was obtained through CAE simulation analysis.

 

1 - Temperature control wire socket; 2 - Junction box; 3 & 4 - Needle valve cylinder; 5 - Solenoid valve; 6 - Pressure gauge; 7 - Hot runner plate; 8 - Moving mold plate; 9 - Wire; 10 & 11 - Hot nozzle; 12 - Insulated gate sleeve; P - Plastic part.
Figure 4 Gating system settings
(5) Cooling system design adopts a contoured water channel to implement uniform cooling of mold cavity. Mold cooling system is also a key aspect of mold design. As shown in Figure 5, 18 water channels (WL1-WL18) are constructed for cooling mold cavity. WL1-WL4 are used for cooling one side of cavity, and WL5-WL18 are used for cooling core side.

 

WL1-WL18 - Waterway Number; P_a - Plastic Part
Figure 5: Mold Cavity Cooling Water Channel Layout
Based on shape characteristics of plastic part and size of mold structure, four orifice diameters (ϕ6 mm, ϕ8 mm, ϕ10 mm, and ϕ12 mm) are selected for mold cavity cooling. WL1-WL4 use ϕ12 mm pipes, WL5 and WL6 use ϕ10 mm pipes, WL7-WL12 use ϕ10 mm pipes, WL13 and WL17 use ϕ6 mm pipes, WL14, WL15, WL16, and WL18 use ϕ8 mm pipes. Cooling water circuit is designed with a balanced approach to effectively control deformation of plastic part; In cooling pipe layout, distance from cooling water channel to cavity surface is 15 mm. To ensure a balanced and evenly spaced distribution of cooling water channels, spacing between channels is 4d (pipe diameter). During pipe arrangement, due to higher mold temperature near gate, cooling water channels are arranged in a cooling sequence from inside (closest to gate) to outside (farthest from gate). In cooling water channel spacing design, spacing between intersecting cooling water holes that are not on same plane is 15 mm to prevent hole wall damage and cooling water leakage caused by drill misalignment. Top of water holes should be designed as a 120° conical surface. In pipe fitting design, PT1/8” fittings are used for fittings with water hole diameters d=6 mm/8 mm; PT1/4” fittings are used for fittings with water hole diameters d=10 mm; PT3/8” fittings are used for fittings with water hole diameters d=12 mm. Distance between cooling channels and ejector pins, sleeves, inserts, inclined guide pillars is controlled to be more than 5 mm. CAE analysis verified that, as shown in Figure 5(c), cooling system is effective, mold cavity surface temperature is uniform, temperature difference between inlet and outlet of cooling channels is less than 5 ℃, and cooling effect is good, meeting needs of plastic part molding process. Insufficient cooling of moving and fixed molds will not cause uneven shrinkage, warping, or prolonged molding cycles, affecting dimensional stability of product. Plastic part production cycle can be controlled within 95 days.

As shown in Figure 6, mold structure is a hot runner two-plate mold with one opening surface, which is a PS surface. Mold cavity layout is a one-cavity layout. Cavity forming surface is directly milled on fixed platen 15, which is made of alloy steel S136. Core forming surface is directly milled on moving platen 16, which is made of alloy steel H13. Direct machining of main forming surfaces of cavity on mold plates reduces amount of machining required and enhances overall structural strength of cavity. Gating system uses hot runner + cold gate composite gating system shown in Figure 4. Cooling system has 18 cooling channels as shown in Figure 5. To achieve balanced ejection of 11 lifters and several ejector pins shown in Figure 2, ejector plate 18 of mold uses four hydraulic cylinders 20 for ejection. Ejection of ejector plate 18 is a single ejection.

 

5 - Solenoid valve; 6 - Pressure gauge; 13 - Fixed mold base plate; 14 - Hot runner baffle; 15 - Fixed mold plate; 16 - Moving mold plate; 17 - Cover plate; 18 - Push plate; 19 - Moving mold base plate; 20 - Hydraulic cylinder; 21 - Limit switch; 22 - Mold foot; 23 - Guide pillar; 24 - SK7 slider; 25 - SB1 slider; 26 - Reset rod; P - Parting surface.
Figure 6 Mold Structure

Figure 7 shows installation of main mechanical components in mold. Of four slider mechanisms shown in Figure 2, sliders SK7, SH5, and SH1 are horizontal core-pulling type inclined guide pillar slider mechanisms, slider SB1 is an inclined slider mechanism at a certain angle to horizontal direction. All four slider mechanisms use a common type of inclined guide pillar driven slider mechanism. Inclined guide pillars and locking blocks of mechanisms are installed on fixed mold plate 15, while sliders, their guiding and positioning mechanisms are installed on moving mold plate 16. Mold push plate 18 is pushed out in a balanced manner using four hydraulic cylinders 22, which are evenly arranged on the outside of moving mold plate 16.

 

10-26 are shown in Figure 6; 27, 28 are ejector pins; 29 is a round lifter rod; 30 is a sliding lifter seat; 31 is a rotating block; 33 is an lifter guide post; F_Z-, X_K4,P_s, X_K8, X_G1, S_H1, S_K7, S_B1 are shown in Figure 2.
Figure 7 Installation of Mechanism in Mold
Referring to Figure 7, arrangement of lifter mechanism and slider mechanism in mold is shown in Figure 8(a). Ejector rod components of 11 lifter mechanisms shown in Figure 2 are divided into three types, as shown in Figure 8(b): one is round lifter rod + ejector block structure shown in lifter XH4 and XK4 [Type I in Figure 8(b)]; another is square ejector rod 37 structure shown in lifter XK3 [Type II in Figure 8(b)]; and third is composite lifter mechanism with inclined guide post shown in lifter XK8 [Type III in Figure 8(b)].

 

18~33 Same as Figure 7; S_K7, S_H5, S_H1, S_B1, X_K8, X_K6, X_H4, X_K5, X_K4, X_K3, X_K2, X_K1, X_H2, X_G1, X_G2, F_Y+, F_X+, F_{Z -}- Same as Figure 2; a, b - Reference points on rotating block; F_A1 - Oblique ejection direction; T_a, T_b - Motion vector triangles
Figure 8 Ejection Mechanism Installation in Mold
Composite lifter mechanism is a type of lifter mechanism reinforced with a cylindrical lifter rod and an angled guide post. Its working principle is as follows: As shown in Figure 8(c), push plate 18 pushes sliding lifter seat 30 upwards in FZ+ direction, thereby pushing rotating block 31 and round ejector rod 29 outwards in FA1 direction. During this process, guide angled guide post 33 drives its corresponding rotating block 31' to move in horizontal groove of sliding lifter seat 30 in FX+ direction. Meanwhile, rotating block 31' is simultaneously pulled by two intermediate blocks 32 on both sides in horizontal groove of sliding lifter seat 30 in FX direction. Moving in + direction, vector motion triangle Tb of point b on rotating block 31 is same as vector motion triangle Ta of point a on rotating block 31'. This mechanism effectively reduces sliding resistance in FX+ direction at lower end of ejector pin 29, reduces torque generated by resistance, lowers risk of breakage of ejector pin 29, ensures flexible movement of ejector pin 29 during ejection, and improves working life of mold.
As shown in Figure 8(d), to ensure balanced ejection of plastic part and prevent ejection deformation due to imbalance, ejector pins of various diameters are evenly arranged on push plate 18. Ejector pin diameters are 8, 9, 10, 12, and 14 mm. Selection of type (mm, etc.) and diameter depends on narrowness of the space where ejector head is located.

After mold is installed on injection molding machine, its injection working principle is as follows:
(1) Mold Closure Injection. As shown in Figure 7, after mold closes at Ps surface, injection molding machine nozzle completes injection into mold cavity through hot runner gating system. After pressure holding and cooling processes, mold cavity awaits opening.
(2) Ps Surface Opening. Moving platen of injection molding machine drives moving mold below Ps surface to press FZ-down. Mold moves downwards, causing plastic part to be ejected from cavity forming surface of fixed mold plate 15. During downward movement, sliders of four slider mechanisms, including sliders 24 and 25, are driven by their respective inclined guide pillars to complete side core-pulling action.
(3) Ejection. After moving mold has descended a certain distance, it stops. Four hydraulic cylinders, including hydraulic cylinder 22, move synchronously, driving 11 lifters and ejector pins 27 on push plate 18 to eject plastic part from core forming surface of moving mold plate 16, achieving complete demolding of plastic part. Resulting plastic part is shown in Figure 9.

 

Figure 9 Molded Plastic Part
(4) Reset. Reset process is reverse of mold opening process. After reset, mold starts next injection cycle.

(1) To address molding difficulties of automotive glove box body plastic parts, demolding direction integration method was first used to set main demolding direction of plastic part, then other demolding directions were obtained. Based on this, main parting line and main parting surface of plastic part were selected, resulting in 14 secondary demolding directions for difficult demolding features. Four sliders and 11 lifters were set to implement side core pulling demolding for these difficult demolding features.
(2) In molding surface, scheme used for molding of textured surface is a texture depth of 30 μm and a draft angle of 2.4. Molding material is S136 alloy steel, and texture is polished with 1000# sandpaper. Three types of lifter mechanisms are used for side core-pulling demolding to address difficulty in demolding. Composite lifter mechanism with angled guide pillars allows for a relatively large side core-pulling distance within a small ejection space, effectively reducing mold height and manufacturing cost. Mold cavity cooling system incorporates 18 conformal water channels, enabling injection molding cycle to be controlled within 95 seconds, effectively ensuring high production efficiency.