Figure 1 shows a two-tone headlight lens. It is composed of a white and black sheet. White sheet is made of transparent PC (Sabic Lexan LS1-111H) and weighs 309g. Black sheet is made of opaque black PC (Sabic LS1-701) and weighs 376g. White sheet is located on the outermost edge of two-tone headlight lens. Black sheet partially covers inner edge of white sheet and is joined to maximum contour of white sheet. Primary function of white sheet is to transmit light source within product through white sheet with minimal loss to meet lighting, light distribution, and regulatory requirements. Mounting structures such as screw posts and clips are designed into black sheet to secure headlight lens to lamp housing and to bumper bracket, ultimately assembling the entire lamp to the vehicle body.

 

Figure 1 Two-color headlight lens
Product has a shrinkage rate of 0.6%, an average wall thickness of 2.8mm, and dimensions of 606mm * 197mm * 347mm. Because majority of white sheet is joined to black sheet, reinforcing ribs cannot be designed on melt surface of white sheet. To prevent warping after the first-color white sheet is injected and mold is opened, a clamp is designed on movable mold side. During mold opening, clamp remains stationary, ensuring that the first-color white sheet remains on movable mold side, prevents shrinkage and warping. When mold is rotated to close second-color mold, clamp is removed by second-color injection wedge, without affecting molding of second-color black sheet. Black sheet has multiple latches on the outside, requiring design of three sliders. Furthermore, black sheet also has undercuts on the inside, facing mold opening direction, requiring design of an inclined push mechanism.

To improve production efficiency, reduce material waste, enhance product quality, and reduce production costs, mold utilizes a hot runner system for injection molding. Furthermore, to address issue of gate stringing and resulting product appearance defects associated with open hot runners, an HRS needle-valve hot runner was selected.
As shown in Figure 2, hot runner feed system for the first color, white, is used to mold transparent semi-finished products. Each semi-finished product is designed with an independent feed point, and gate is designed directly on internal mold surface without a conventional runner. Hot runner feed system for the second color, black, is used to mold black products. Each part is designed with two independent gates, and gates are designed directly on the side ribs of part without a conventional runner. Designing gates directly on part avoids material waste associated with designing conventional runners later and reduces need for manual gate solidification removal. Hot runner system design was simulated and analyzed using Moldflow software. White sheet achieved a filling time of 4.1 seconds, an injection pressure of 104 MPa, and a clamping force of 4670 kN, as shown in Figure 3(a). Black sheet achieved a filling time of 4.1 seconds, an injection pressure of 143 MPa, and a clamping force of 5600 kN, as shown in Figure 3(b). A 17,000 kN injection molding machine was sufficient to meet production requirements.

 

Figure 2 Gating system

 

Figure 3. Gating system simulation analysis
Injection molding process parameters for mass production were generally consistent with mold flow analysis. Actual production verification showed that molded products exhibited no obvious defects such as weld lines or gate marks.

Because white sheet lacked reinforcing ribs on movable mold side, it remained there after injection. This resulted in warping and sticking to fixed mold after first white sheet melt was injected and mold opened. To address this issue, this two-color headlight lens features a pressure block structure on movable mold side. As shown in Figure 4, pressure block structure consists of a wedge block, a pressure block, a limit screw, a base, and a set screw. Base is used to position pressure block and guide its movement. It is secured to movable mold cavity plate with four screws. Wedge block is secured to fixed mold core with a set screw, driving pressure block's movement during mold opening and closing. A limit screw is secured to bottom of pressure block to limit its travel distance. Pressure block's operating process is shown in Figure 5. When first-color white film melt is injected, pressure block, under action of first-color fixed mold wedge block, moves to its initial position to form white film. During mold opening, pressure block remains stationary, compressing white film, preventing shrinkage, warping and remaining in movable mold cavity. When first-color white sheet is transferred to second-color injection molding station for black sheet, pressure block, under action of second-color fixed mold wedge, moves to stop screw position, clearing white sheet. This prevents pressure block from interfering with black sheet's molding process, thereby addressing shrinkage, warping, and sticking to fixed mold.

 

1. Wedge block 2. Wedge block fixing screw 3. Pressure block 4. Limit screw 5. Base fixing screw 6. Base
Figure 4. Pressure block structure

 

1. First-color movable mold cavity plate 2. Movable mold cavity fixed plate 3. Second-color movable mold cavity plate 4. Fixed mold core fixed plate 5. First-color fixed mold core 6. Second-color fixed mold core 7. Slider 8. Base 9. Pressure block 10. First-color fixed mold wedge 11. First-color white sheet 12. Second-color black sheet 13. Second-color fixed mold wedge
Figure 5: Position of the first and second color pressure blocks

To address undercut structure of black sheet's outer surface, three slider structures are designed on fixed mold for second-color black sheet. Slider structure, shown in Figure 6, primarily comprises a slider, inclined guide pins, pressure strips, wear plates, limit blocks, and travel switches. All sliders are mechanically driven by inclined guide pins, with separate water channels designed for each slider. During mold opening, inclined guide pins drive slider; pressure strips secure slider to fixed mold core and guide its movement. Wear plates are located on the bottom and inclined surfaces of sliders, and oil grooves are provided to lubricate slider during movement, preventing burning and jamming. Limit blocks limit slider's travel distance. Travel switch monitors slider's movement and transmits a signal to injection molding machine to initiate next mold operation. During mold opening, fixed and movable molds separate along parting line. Each slide, guided by an inclined guide post, moves away from product to achieve lateral core pulling. When slide reaches stop, it stops and simultaneously strikes travel switch, signaling injection molding machine that slide has reached its designated position, completing lateral core pulling. During mold closing, inclined guide post drives slide to simultaneously close and reset mold.

 

1. Slider 2. Slider 3. Slider 4. Inclined guide post 5. Extended water pipe 6. Stop block 7. Hold-down strip 8. Wear plate 9. Travel switch
Figure 6 Slider structure

To address issue of undercuts on inner side of blank in mold opening direction, an inclined push solution was adopted to simplify mold structure, reduce mold size, and lower mold manufacturing costs. Inclined push structure, shown in Figure 7, primarily consists of an inclined push block, an inclined push rod, a T-shaped guide rail, and a slide. Inclined push block and push rod are connected by a fixed block for easy assembly and disassembly for maintenance. The other end of inclined push rod is screwed into slide, which is fixed to the back of push plate. Push plate drives inclined push block, enabling undercut to be properly released from mold. To ensure stability during pusher's movement, a guide groove is designed on the back of pusher. A T-shaped guide rail is screwed to core. Guide groove and core are connected via T-shaped guide rail, and pusher moves under guidance of T-shaped guide rail. Due to large size of pusher structure, a conformal water channel is designed within pusher to achieve optimal cooling. This channel is manufactured using 3D printing technology. Extended water pipes on pusher plate connect water channel to mold temperature controller, enabling independent control of mold temperature and preventing product deformation.

 

1. Product 2. Sliding block 3. Fixed block 4. T-shaped guide rail 5. Sliding rod 6. Extension pipe 7. Push plate 8. Sliding seat
Figure 7 Sliding block structure

Mold cooling system is crucial in injection mold design. A uniform and well-designed cooling water channel significantly impacts product quality and cycle time. Dual-color headlight lens cooling system features following features: The entire mold utilizes a "conformal water channel + water well" cooling system. Cooling water channel is designed to conform to product's contours. Water wells are added to provide supplemental cooling in areas where cooling is insufficient, ensuring uniform cooling. Water channel design for pusher structure was performed using 3D printing. To facilitate adjustments to product deformation during later production, core waterway, slider waterway, oblique pusher waterway, and hot runner gate insert waterway all utilize separate mold temperature controllers for temperature control. Mold flow analysis and actual waterway design verification show that this conformal mesh waterway design achieves a cooling area of 90%, inlet and outlet water temperature differential can be controlled within 5℃, as shown in Figure 8(a).

 

Figure 8 Cooling channel simulation analysis and product deformation analysis
Product deformation can be controlled within 2mm. During actual production, local temperature differentials are adjusted using separate mold temperature controllers, resulting in actual product deformation of approximately 1.5mm. Mold flow analysis and actual product deformation are shown in Figure 8(b). This meets product assembly requirements and avoids costly changes that would result from excessive deformation later in production process.

Since white piece is a cosmetic component, it requires a smooth surface and no ejection marks. Ejection system is located on the black piece, primarily consists of oblique pushers and push blocks. Five push blocks are designed around black piece to prevent imbalanced ejection. Oblique pushers remove undercut and assist in ejection. Pusher and pusher block are fixed to push plate using a push rod and a push rod. Push plate is driven by hydraulic cylinder piston rod to push out push plate, as shown in Figure 9.

 

Figure 9 Push-out system

During production, two-color headlight lens mold is installed on a 17,000 kN two-color injection molding machine. Two-color injection molding machine is a diagonally rotating type. Mold's operating principle and process on injection molding machine are shown in Figure 10. On moving side of injection molding machine, movable platen, turntable, and movable mold magnetic plate are arranged in order. On fixed side, fixed platen, fixed mold magnetic plate, main nozzle injects black melt, and side nozzle injects white melt. Movable mold is fixed to moving side of injection molding machine by movable mold magnetic plate, while fixed mold is fixed to fixed side of injection molding machine by fixed mold magnetic plate. During the first shot, side nozzle only injects the first color white melt; main nozzle does not inject second color black melt. After the first-color white film melt injection is complete, movable mold platen moves movable mold back laterally to open mold. The first-color white film remains in movable mold cavity. Movable mold then rotates 180° under injection machine's turntable, shifting cavity for the first-color white film to second-color injection position and cavity for second-color black film to the first-color injection position, switching black and white film cavities in movable mold. After switchover is complete, side plates of movable mold platen move forward to complete mold closing. During second-color injection, side nozzles inject the first-color white film melt while main nozzle injects second-color black film melt. Second-color injection completes two-color molding of black and white films, resulting in finished two-color lens. Simultaneously, the first-color injection molds transparent part in white film mold, preparing for next two-color injection molding. After two-color injection is complete, injection machine platen moves movable mold back laterally to complete mold opening. Piston rod of hydraulic cylinder on fixed mold, controlled by injection machine's hydraulic system, pushes push plate out of mold, ejecting finished two-color lens.

 

1. Moving platen 2. Turntable 3. Moving mold magnetic plate 4. First color white film 5. Side nozzle 6. Fixed mold magnetic plate 7. Fixed platen 8. Main nozzle 9. Second color black film 10. Two-color lens
Figure 10 Two-color lens injection process
Next, machine's robotic arm removes finished product, and hydraulic cylinder piston rod, controlled by injection molding machine's hydraulic system, retracts mold push plate, completing two-color injection and demolding of first mold. Movable mold then rotates 180° under injection molding machine's turntable, rotating cavity for first-color white film to second-color injection position, and cavity for second-color black film to the first-color injection position, switching movable mold's black and white film cavities. After switch is complete, injection molding machine's movable mold side plate moves forward, completing mold closing action and entering two-color injection cycle.