Design of Two-stage Overmolding for Automobile Headlight Cover

Abstract

This paper presents the design of an overmolding system for the headlight cover of a specific vehicle model, utilizing two-stage overmolding. The process employs a two-stage injection molding approach: first, the white plastic component is molded, followed by the molding of the black plastic component in the second stage. To ensure process efficiency and stability, a tilting ejector mechanism is integrated into the dynamic mold cavity of the white plastic sub-mold, allowing the molded white plastic part to remain securely in position during mold opening. Ejector mechanisms are installed on both the fixed and dynamic mold sides of the black plastic sub-mold. The tilting ejector mechanism of the black plastic sub-mold, powered by a hydraulic cylinder, is synchronized with the mold opening action, facilitating the smooth demolding of the two-stage overmolding component. The part stays in the fixed mold side of the black plastic sub-mold, where the ejector mechanism on the fixed mold core ejects the component, which is then removed by a robotic arm. Additionally, a tunnel core-pulling mechanism is integrated into the fixed mold core of the black plastic sub-mold to assist in demolding undercut features that do not align with the primary ejection direction. Testing has demonstrated that the inverted two-stage overmolding mold design is structurally stable, operates reliably, and produces components that meet the required quality standards.

1. Analysis of Two-stage Overmolding Plastic Parts for Headlight Cover

Polycarbonate (PC) resin is a high-performance thermoplastic characterized by excellent impact resistance, creep resistance, and dimensional stability. It is also one of the few engineering plastics that maintain high transparency. The two-stage overmolding plastic parts of the headlight cover consist of a transparent white PC component and an opaque black PC component (as shown in Figure 1). The white plastic part is positioned on the inner side of the headlight cover, primarily responsible for light transmission and meeting lighting and light distribution requirements. The weight of the white plastic part is 0.59 kg. The black plastic part is positioned on the outer rim of the cover, providing structural support and facilitating installation, with a weight of 0.4 kg. The rear of the part is equipped with mounting features, such as screw columns, ensuring a secure connection between the headlight cover and the entire lamp assembly. The interface between the two plastic parts is shown in Figure 2. The black plastic part envelops the outer surface of the white plastic part, and the bond strength between the two is improved by increasing the contact area, which helps reduce the risk of cracking under extreme temperature variations.

Figure 1 Headlight cover

Figure 2 The overlapping section of the white plastic part and the black plastic part of the headlight cover

2. Working Principle of Overmolding

The two-stage overmolding mold for the headlight cover is mounted on an overmolding machine for production, as shown in Figure 3. The process is described as follows:

First injection stage: The white plastic part sub-mold is injected first, while the black plastic part sub-mold remains inactive during this stage. Once the injection is complete, wall panel 2 of the machine drives the fixed mold core of the black plastic part sub-mold to retract to the open position. Simultaneously, wall panel 3 moves the movable mold cavities of both the white and black plastic part sub-molds to the right, opening the white plastic part sub-mold. At this point, the white plastic part remains in the movable mold cavity. The turntable then rotates 180°, enabling the mold cavity switch for the second injection stage.

Second injection stage: The movable mold cavities of both the white and black plastic part sub-molds align to form the mold for the second injection. Wall panel 2 then drives the fixed mold core of the black plastic part sub-mold to the left, closing the mold. During the second injection, both sub-molds are filled simultaneously: the black plastic part sub-mold completes the formation of the black plastic part, while the white plastic part sub-mold continues molding the white part for the next cycle.

Demolding and ejection: As the mold opens, wall panel 2 drives the fixed mold core of the black plastic part sub-mold to the right, while the hydraulic system of the injection molding machine activates the ejection mechanism on the dynamic mold cavity side, ensuring that the two-stage overmolding plastic part remains securely in the fixed mold core of the black plastic part sub-mold. Wall panel 3 then retracts both dynamic mold cavities, completing the mold opening for the white plastic part sub-mold. The ejection mechanism on the fixed mold core side of the black plastic part sub-mold is triggered to eject the two-stage overmolding plastic part, which is then removed by the robotic arm.

Cycle production: After the part is ejected, the turntable rotates 180° counterclockwise to switch the mold cavities and initiate the next overmolding cycle.

Figure 3 Two-stage overmolding

3. Working Principle of Overmolding (New Cooling System)

The cooling system design plays a crucial role in the molding quality and production efficiency of two-stage overmolding plastic parts during the overmolding process. This mold incorporates a "conformal water channel plus water well" cooling solution to optimize thermal management and improve production efficiency.

(1) Conformal Water Channel

Conformal water channels are strategically positioned along the contours of the plastic part to ensure the cooling system aligns with the part’s structure, particularly in the white transparent PC area. By optimizing the layout of these channels, cooling efficiency is enhanced, reducing defects such as shrinkage and air marks.

(2) Water Well

Water wells are integrated into critical areas of the black plastic part sub-mold to improve partial cooling. This design reduces cooling time, minimizes temperature variations across the mold, and enhances the dimensional stability and surface quality of the two-stage overmolding plastic parts. Production tests confirm that this cooling system mitigates the risk of deformation caused by uneven mold temperatures, reduces cycle time, and improves product quality and consistency.

4. Production Practice and Mass Production Stability Analysis (New Mass Production Data)

To evaluate the stability of the overmolding process, mass production trials were conducted, and the molding cycle times and product qualification rates were analyzed and assessed.

Production Efficiency

In actual production, the molding cycle for each two-stage overmolding mold is maintained at 45 seconds, representing a 25% improvement over the traditional overmolding process, which typically takes around 60 seconds per mold.

Product Qualification Rate

During the continuous production of 500,000 two-stage overmolding plastic parts, dimensional accuracy was maintained within ±0.1 mm, with excellent surface quality. The qualification rate reached 98%, meeting the required quality standards for automotive headlight covers.

Mold Operation Stability

After 5,000 hours of continuous production, test data confirm that the mold operated smoothly, with no abnormal wear or failures. The ejection, core-pulling, and inclined ejector mechanisms maintained optimal synchronization, ensuring smooth demolding of the two-stage overmolding plastic parts.

In summary, production trials confirm that the overmolding process is robustly designed, ensures efficient and stable production performance, and meets the technical requirements for mass production.

5. Conclusion

(1) The inverted mold design is used, with the white plastic part injected first, followed by the black plastic part. This approach effectively prevents color mixing between the black and white components during the overmolding process, ensuring high-quality two-stage overmolding molded parts.

(2) An inclined ejection mechanism is integrated into the dynamic mold cavity to ensure the white plastic part remains securely in place after the first injection, even during mold opening. Additionally, an ejection cylinder on the dynamic cavity side of the black plastic sub-mold ensures synchronized operation between the inclined ejection mechanism and the mold opening action, facilitating smooth demolding of the black plastic part.

(3) A tunnel core-pulling mechanism is incorporated into the fixed mold core side of the black plastic sub-mold to facilitate the demolding of the black plastic part’s undercut features, which are not aligned with the mold’s primary ejection direction. This mechanism ensures the structural integrity of the part while enhancing demolding efficiency.

(4) The mold features a cooling system with conformal water channels and water wells, optimizing cooling efficiency, improving molding quality, and reducing cycle times to support mass production demands.

(5) During mass production, the two-stage overmolding mold for the headlight cover operates reliably, with the mold structure ensuring stability throughout the injection cycle. Production results demonstrate that the mold consistently achieves a high qualification rate and meets the expected design objectives.