With development of automotive industry, headlights, as important exterior components of automobiles, are becoming increasingly complex in structure, placing higher and higher demands on their appearance quality and molding performance. Headlight mask products are gradually shifting from single-color to bi-color and multi-color. Bi-color grille masks, as atmospheric exterior components of automobiles, are becoming increasingly common.
Bi-color injection molds generally consist of two sub-molds combined together. By rotating and changing cavities, two-color injection molding is performed in two overlapping injection stages to obtain bi-color molded plastic parts. Compared with single-color injection molding, bi-color injection molding has advantages such as high production efficiency and good product quality, making it increasingly suitable for modern production needs. Conventional bi-color headlight molds are single-layer injection molds, which suffer from problems such as large mold cross-sectional area, high product unit energy consumption, and low production efficiency. This paper designs a high-efficiency, energy-saving through-beam stacked injection mold, composed of two sub-molds combined back-to-back, effectively improving equipment utilization and production efficiency while reducing costs.

Two-color grille mask product is made of transparent polycarbonate (PC) resin and black PC resin. Transparent part is located on the outside of two-color grille mask, covering the entire black part, requiring high light transmittance. Its weight is 550 g, its function is to meet light transmission and decorative lighting requirements of grille light. Black part is located inside two-color grille mask, with a weight of 453 g. Its function is to block light, and design of black part's mounting surface, including snap-fit and welded ribs, facilitates installation and fixation of grille mask in corresponding areas. Structure of two-color grille mask product is shown in Figure 1.

 

Figure 1. Two-color grille cover
Figure 2 shows a schematic cross-section of overlap between transparent and black parts of grille cover. Transparent part is located outside black part. A suitable angle and a wide contact area are designed between them to enhance bonding strength and prevent grille cover from cracking during mold ejection or use in extremely cold or hot environments.

 

Figure 2. Cross-section of overlap between transparent and black plastic parts

2.1.1 Injection sequence
Based on analysis of structural characteristics of two-color product in this case, black product has a sloping core-pulling structure, as shown in Figure 3. If black part is injected first, then transparent part is injected, A-side of transparent part will have a sloping core-pulling mark line, affecting product's appearance quality. To avoid this situation, two-color grille mask mold is chosen to first complete transparent part molding, then inject black part. Lifter is 0.5 mm away from A-side of transparent part, as shown at point E in Figure 3. Lifter and transparent part make partial contact and sealing, as shown at point F in Figure 3.

 

Figure 3: Cross-section of grille mask core-pulling location
2.1.2 Mold Scheme
Inverted Mold: According to product molding scheme, transparent part needs to be injected first, then black part is injected, with transparent part covering outside of black part. Two-color grille mask is ejected through mounting surface of black part. Since injection system and ejection mechanism are on same mold side of finished product, two-color mold structure adopts an inverted mold, as shown in Figure 4.

 

Figure 4 Schematic diagram of inverted mold
Two-color injection mold for through-shot type: Two-color plastic parts are mass-produced using a through-shot type two-color injection molding machine. Therefore, mold is designed with a front and rear two-color injection system based on through-shot machine. Two-color grille cover mold includes two independent sub-molds (1ST/2ND), whose moving mold cavity side structures are consistent and respectively match two cavity core sides. Two moving mold cavity sides are fixed back-to-back on central rotating table of injection molding machine, as shown in Figure 5.

 

Figure 5 Two-color injection mold for through-shot type

Grille cover is flat and elongated in shape, with a cavity design of 1 mold and 2 cavities. PC plastic molding adopts HRS brand hot runner system. Injection process for first color transparent plastic part is relatively long. A single product is designed with 3-point sequential valve injection. Sequential valve facilitates adjustment and control of weld line position. Gate type adopts a side gate, as shown in Figure 6(a). Second-color black plastic part has a hollow center. A single product is designed with a 4-point sequential valve injection system, using 2 submarine gates and 2 sliding tunnel side gates, as shown in Figure 6(b). After initially determining number and location of gates, mold flow analysis was performed using Moldflow software to verify feasibility.

 

Figure 6 Hot runner system

2.3.1 First-color transparent part mold structure
2.3.1.1 Mold core and cavity design
Figure 7 shows core and cavity structure of transparent part sub-mold. Mold core steel uses German 1.2343ESR material. After rough machining, further tempering heat treatment is required to achieve a hardness requirement of HRC46-48. Core and cavity molding surfaces need to be mirror polished to ensure transparency of molded plastic part.

 

Figure 7. Core and Cavity of First Color Transparent Part Mold
Core and cavity mold cores are relatively large. Sides that mate with mold plate are designed with two straight surfaces (reference) and two inclined surfaces (3°). Adjustment plates are attached to inclined surfaces for easy mold adjustment. Transparent plastic part has a large side projection area and bears significant lateral pressure during injection. Therefore, bosses and grooves are designed on the front of core and cavity to provide positioning and interlocking during mold closing and injection.
2.3.1.2 Rotating Fixing Mechanism on Moving Mold Cavity Side
After first color transparent part is injection molded, based on inward shrinkage characteristic of plastic part, under normal circumstances, transparent part will remain on fixed mold core side after mold opening. To ensure that transparent part remains on moving mold cavity side during mold opening, a "rotating fixing mechanism" is designed on moving mold cavity side of mold, as shown in Figure 8. When first color injection molding is completed and mold opens, transparent part is held in place by "rotation and fixing mechanism" on moving mold cavity side. After cavity rotates 180°, mold closes again for second color injection molding. After injection molding is completed, with mold closed, "rotation and fixing mechanism" is ejected a distance S (S=7 mm for this mold) by hydraulic cylinder on moving mold cavity side, as shown in Figure 9. After fixing of two-color grille cover is released, mold opens again under drive of hydraulic cylinder. At this time, two-color grille cover remains on fixed mold core side, is then ejected by hydraulic cylinder on fixed mold core side.

 

1. Lifter block 2. M4 screw 3. Locating pin 4. Guide block 5. Connecting block 6. M5 screw 7. Ejector rod 8. Guide sleeve 9. I-beam block
Figure 8 Rotation and Fixing Mechanism

 

Figure 9 Rotation and Fixing Mechanism Precautions
2.3.1.3 Design Precautions for Rotation and Fixing Mechanism
To ensure smooth ejection movement of fixing mechanism when mold closes, angle of mating surface is 3° larger than angle of guide block, as shown in Figure 9(a). Fixed mold core side of mold for second color black part has sufficient space for clearance (=mold closing ejection distance S+5), as shown in Figure 9(b).
2.3.2 Second Color Black Part Mold Structure
2.3.2.1 Mold Core and Cavity Design
Figure 10 shows core and cavity structure of black part mold. Cavity steel of black part mold also uses German 1.2343ESR material. After rough machining, it needs further tempering and heat treatment to achieve a hardness requirement of HRC 46~48; cavity forming surface also needs to be mirror polished. Core steel of black part mold uses pre-hardened steel XPM, with a hardness requirement of HRC 38~42; core surface does not need polishing after finishing.

 

Figure 10 Core and Cavity of Second Color Black Part Mold
Core and cavity mold core are relatively large. Sides that mate with mold plate are designed as two straight surfaces (reference) and two inclined surfaces (3°). Adjustment plates are attached to inclined surfaces for easy mold adjustment. Side projection area of black plastic part is large, it bears greater pressure during injection. Therefore, front of core and cavity is designed with bosses and grooves, which play a positioning, interlocking role when mold is closed for injection.
2.3.2.2 Rotation and Fixing Mechanism for Moving Mold Cavity
Moving mold cavities of two sub-molds need to be interchanged after first-color transparent part is injection molded. Therefore, both must have same mold structure. An identical rotation and fixing mechanism is also designed for moving mold cavity of black part sub-mold.
2.3.2.3 Runner and Sprue Slider Mechanism for Fixed Mold Core
Inner edge of black plastic part is flush with outer edge of transparent plastic part. An embedded runner and sprue slider structure is used, as shown in Figure 11. Runner and sprue are located at the bottom of slider, thus bypassing already molded first-color transparent part to achieve injection molding of black part. Slider uses a slanted guide post drive to achieve gate ejection. Correspondingly, corresponding position on first-color fixed mold core side should be cleared to avoid interference from slanted guide post during mold closing after moving mold cavity side rotates.

 

Figure 11. Slider for runner gate of second-color black part
2.3.2.4 Tunnel slider core-pulling mechanism on the side of fixed mold core
Because demolding direction of mounting clip position of black part is different from main mold ejection direction, i.e., undercut position, a slide core-pulling mechanism was designed on the side of fixed mold core to complete demolding of undercut structure of black part, as shown in Figure 12. This core-pulling mechanism mainly consists of a slide linkage block and a slide, is driven by a slanted guide post and a spring. Correspondingly, corresponding position on the side of first-color fixed mold core is cleared to avoid interference from slanted guide post when moving mold cavity rotates.

 

Figure 12. Tunnel slider core-pulling mechanism
2.3.2.5 Angled core-pulling mechanism on the side of fixed mold core
Figure 13 shows use of an lifter core-pulling mechanism to solve problem of demolding direction of local undercut position of black part being different from main mold ejection direction. This lifter has a large contact area and is designed with an internal water cooling system.

 

Figure 13 Lifter Mechanism

2.4.1 Ejection Mechanism for First Color Transparent Part
After transparent part is formed, it is fixed in cavity by a rotating fixing mechanism on moving mold cavity side. Then, it is rotated 180° and mold is closed again to complete injection molding of second color black part. Therefore, transparent part sub-mold does not require an ejection mechanism.
2.4.2 Ejection Mechanism for Second Color Black Part
2.4.2.1 Ejection Design of Moving Mold Cavity "Rotating Fixing Mechanism"
After second color black part is injected (i.e., after two-color grille cover is formed), it needs to remain on fixed mold core side. If mold closure is required, rotating fixing mechanism on moving mold cavity side is ejected 7 mm, detaching mechanism from product. Rotating fixing mechanism is fixed to ejector plate, and ejector plate is driven by a hydraulic cylinder for ejection, as shown in Figure 14.

 

Figure 14. Ejection Design of "Rotational Fixing Mechanism" for Moving Model Cavity
2.4.2.2 Design of Ejection Mechanism for Fixed Model Core Side
After second-color black part is molded, two-color grille cover shrinks inward due to cooling, generating a clamping force and remaining on core side of black part sub-mold. An ejection mechanism consisting of ejector pins and ejector blocks is designed on core side, with driving cylinders on both sides. Ejector pins and ejector blocks are fixed to ejector plate, and ejection movement is achieved by driving ejector plate through cylinders, as shown in Figure 15.

 

Figure 15. Ejection Mechanism for Second Color Black Core
2.4.3 Auxiliary Mold Opening Cylinder Ejection
In mold opening sequence of injection molding machine, second color fixed mold platen moves, opening second color fixed mold and moving mold; then intermediate turntable moves horizontally, opening first color fixed mold and moving mold. Therefore, intermediate turntable will bear a large torque. Thus, auxiliary mold opening cylinders are installed at four corners of moving and fixed molds, as shown in Figure 16. When moving and fixed molds open, auxiliary cylinders eject synchronously, separating moving and fixed molds, reducing torque force on turntable. Cylinders at four corners of auxiliary mold opening mechanism must have similar hydraulic pressure, be synchronized with mold opening action of injection molding machine.

 

Figure 16. Auxiliary Mold Opening Cylinders for Moving and Fixed Molds

Different plastic materials correspond to different mold temperatures. Mold temperature requirement for PC material of this two-color grille cover is between 85~95℃. Water channels of moving and fixed molds have following characteristics: Mold as a whole adopts a cooling design method combining conformal water channels and water wells. Conformal water channels are designed according to shape of two-color grille cover. Water wells are designed to assist cooling in areas with insufficient cooling, ensuring uniform product cooling, as shown in Figures 17 and 18. To ensure cooling effect, mold hot nozzles and lifter mechanism are equipped with independent cooling water channels. Mold length is extended, divided into 3 independent cooling zones, facilitating adjustment of product deformation. A mold temperature controller is used to separately control temperature of fixed mold core side and moving mold cavity side of two sub-molds.

 

Figure 17 Cooling system of first-color transparent part mold

 

Figure 18 Cooling system of second-color black part mold
Cooling system of this mold has a water channel diameter of Φ11.5 mm, a water well diameter of Φ18 mm, a water channel spacing of 45~65 mm, and uses imperial PT threads. Minimum safe distance between water channels and mold core requiring quenching treatment is required to be at least 12 mm, distance between water well and product is required to be at least 20 mm to avoid risk of quenching cracking.

Working principle of through-shot two-color stacked injection mold is shown in Figures 19 and 20. Following steps describe mold operation under continuous cyclic production, i.e., during simultaneous injection in step ② below, 2ND side already contains first-color product: ① Mold closing. Mold temperature controller heats mold to specified temperature, barrel heats and melts plastic, and other relevant process parameters are set. ② Injection. Simultaneously open two gating systems on first-color fixed mold side (FS side) and second-color fixed mold side (MS side) to complete injection + holding pressure + cooling molding process. The first color is injected with transparent PC on fixed mold side, and second color is injected with black PC on fixed mold side. The first color sub-mold completes injection molding of transparent part, and second color sub-mold completes molding of two-color face mask. ③ Mold opening and part removal. Fixed mold core side and moving mold cavity side of second color black part sub-mold are respectively designed with a lifter mechanism and a slider mechanism. Before mold opening, moving mold ejection cylinder ejects 7 mm, causing "rotation fixing mechanism" to detach from product, so that two-color grille face mask remains on second color fixed mold due to cooling and shrinkage. Parting surface cylinder assists in mold opening. Second color fixed mold and central rotating platform move to the left at the same time. Moving distance of second color fixed mold is greater than moving distance of central rotating platform, thus completing mold opening of first color sub-mold and second color mold. Then, hydraulic cylinder on core side of second-color mold pushes ejection mechanism to eject two-color grille cover plastic part 80 mm, which is then removed by a robotic arm. ④ Moving mold interchange. Two moving molds rotate 180° with central rotating platform. The first-color transparent part is fixed to first-color moving mold by a "rotation fixing mechanism" to prevent it from falling off during rotation. ⑤ Mold closing again. Slider resets, mold closes again, and next injection molding cycle begins.

 

Figure 19 Two-dimensional diagram of a through-shot stacked injection mold

 

Figure 20 Two-color plastic part molding process

(1) Two-color grille mask mold adopts an inverted mold structure. The first color is used for injection molding of transparent part, and second color is used for injection molding of black part. This solves problem of trace lines caused by lifter mechanism on appearance of grille headlight mask, and improves appearance quality;
(2) A product fixing mechanism was designed so that transparent part is fixed in moving mold cavity after mold is opened; after second color is injected, it is pushed out by hydraulic cylinder of moving mold side cavity of black part. Rotating fixing mechanism pushes out a distance in closed mold state, loosening fixing of product. After mold is opened, product stays on fixed mold side of black part, is then pushed out by fixed mold of black part.
(3) For undercut parts of black parts with different main ejection directions of mold, a slider mechanism and a lifter core-pulling mechanism were designed to achieve core-pulling and demolding of undercut area;
(4) Cooling system adopts a combination of conformal water channels and water wells, with uniform spacing between water channels. Independent cooling water channels are provided for hot nozzles and lifters, ensuring molding quality of two-color grille mask;
(5) During production, two-color grille mask mold runs smoothly and stably, injection molding cycle meets requirements, appearance quality and dimensional accuracy of injection molded samples meet customer needs, achieving expected goals.