Abstract: Parametrically illuminating automotive headlights is an effective way to enhance technological feel and brand recognition of lighting fixtures. Addressing problems of low part qualification rate, low production efficiency, high cost caused by post-processing techniques such as laser engraving and hot stamping for parametric illumination of inner light distribution ring in automotive headlight systems, this paper proposes a solution based on a two-color injection molding process for inner light distribution ring of automotive headlights. This process involves injection molding a non-transparent resin base ring to block light path, and injection molding a translucent resin base ring to form light-emitting area of inner light-distributing ring. Parametrically designed translucent area is created by filling translucent resin with a parametrically hollowed-out parametric shape in base ring. Furthermore, an engineering constraint model was developed for parametrically designed two-color overlapping structure based on two-color mold processing technology and mold structure requirements, effectively solving defect of discontinuous perimeter boundaries of parametric features. This paper designs a two-color injection mold based on part structure. Addressing problems of undercut structures on two-color connecting surfaces, surfaces without draft angles, difficulties in molding gate and runner for single-color molded parts, mold design incorporates two sets of fixed mold spring mechanisms to mold single-color molded part, two-color molded part, gate and runner, respectively. Mold has been designed, processed, applied in actual production. Practical experience has proven that mold structure is stable and reliable, part quality meets design requirements, design scheme can provide a reference for development of similar products and has high engineering practice guidance significance.
As "eyes" of a car, headlights provide illumination and safe driving for drivers in dark, and are an inseparable part of car's driving function; In addition, headlights, like eyes, express emotions of car and convey its spirit, playing a major role in expressing car's emotions and are an important part of car's styling. With development of automotive lighting systems, car headlights have gradually evolved from their initial function of illumination to become an important carrier for decoration, brand characteristics implantation, showcasing technical strength and design capabilities of OEMs. Parametric lighting function is one of trends in headlight styling design. According to light distribution structure, regulatory requirements and material properties of car lights, in existing technical means, hot stamping and laser engraving processes are usually used in light-transmitting area of inner light distribution ring to achieve effect of parametric pattern lighting. Huayu Vision Technology (Shanghai) Co., Ltd. heats opaque parametric pattern patch to a preset temperature, then uses a preset pressing force and preset pressing time to press it onto light-transmitting inner light distribution ring body. After lamp is lit, opaque pattern shape can be highlighted. Hubei Huazhong Marelli Automotive Lighting Co., Ltd. sets multiple layers of paint on light-transmitting area of inner light distribution ring, then uses laser engraving to remove paint layer of parametric shape to achieve effect of parametric lighting of lamp. Both of above post-processing processes require investment in expensive special equipment, tooling fixtures and production lines. Parts produced are high-tech products with high added value and are usually used in high-end models. With increasing application of parametric pattern lighting design in automotive lights, laser engraving and hot stamping processes are no longer suitable for increasingly competitive market. Therefore, a low-cost dual-color molding process that can achieve parametric lighting has emerged.
This paper takes inner light trim ring of a taillight of a Wuling model as research object, introduces dual-color injection molding process in detail, explains parametric structure of product and key points of mold design according to process requirements.

Dual-color injection molding refers to molding process of plasticizing two different materials or different colors separately, then injecting them into mold cavity in sequence or simultaneously to obtain plastic molded products of two different colors or different materials. As shown in Figure 1, dual-color injection molding uses two injection systems and one mold closing system. Its mold structure is that two cores of moving mold are same, and two cavities of fixed mold are different, which are used to mold one-color molded parts and two-color molded parts respectively. Injection molding machine adds a moving mold turntable to enable moving mold to rotate.

 

Figure 1. Schematic diagram of double injection molding process

Figure 2 shows shape of inner light-distributing ring. Position lights and parametric ambient lights are non-connected areas. Parametric ambient light unit is a rectangle with a width of 2.0 mm to 3.8 mm and a length of 13.5 mm to 46.3 mm. Function definition is that when position lights are lit, parametric rectangular ambient lights also illuminate. Material definition is that position lights and ambient lights are milky white light-diffusing panels, molded from optical-grade (polycarbonate) PC material. Inner light-distributing ring base is high-gloss piano black with a matte, finely frosted black leather texture, injection molded from PC material. Because part is an optical structural functional component, product surface must achieve a mirror-polished A0 level on light-inlet surface, light-outlet surface, and visible surface, defects such as splicing lines, gate residue, and shrinkage marks are not allowed.

 

Figure 2. Simplified diagram of inner lens design surface and sectional diagram.

As shown in Figure 3, taillight inner lens trim is divided into an inner lens (single-color molded part) and a trim base (two-color molded part) according to a two-color structure. Trim base has a hollowed-out light-transmitting area for inner lens. Non-connected inner lenses are connected by a guide channel with a thickness of 2 mm to 2.5 mm, which is covered and invisible by trim base.

 

Figure 3. Two-color structure diagram for Inner lens

2.3.1 Complex gating system design: Inner light-distributing ring has visible surfaces, and gate marks are not allowed. Following conventional two-color mold gating system design, single-color molded part uses a hot runner needle valve point gate located at runner. Two-color molded ring base covers runner, making gate invisible and improving perceived quality of appearance. As shown in Figure 4, in this product structure, a gate needs to be designed in ambient light area to ensure the overall filling and pressure holding of plastic part. Distance between two adjacent parametric rectangles of ambient light is 5.8 mm, after placing hot runner gate in this area, core forms a thin steel, which is prone to breakage during production; designing a bottom-edge gate increases connection area between gate and product, making it difficult to trim gate and leaving residue, resulting in light leakage at that point when part is lit. To meet product appearance requirements and improve mold stability, single-color molded part is designed with a gate guide channel for gating. Demolding undercut formed by this structure is achieved through a fixed mold spring block.

 

Fig. 4 Schematic diagram of parameterized area setting for each gate of the first shot
2.3.2 Appearance defects of parameterized area: Inner light distribution ring designs parameterized rectangular ambient light and ring base as same curved surface. Due to limitations of mold processing technology and characteristics of two-color mold structure, plastic part molded under this design forms short shots around parameterized feature, resulting in discontinuous boundaries (Fig. 5), which seriously affects dynamic and static perception quality of lamp. As shown in Figure 6, single-color cavity of molding parametric feature is a groove. Groove is rounded to a radius of 0.1 mm to 0.3 mm at its root through CNC machining or mirror EDM. This results in a rounded perimeter of molded parametric feature. During injection molding of two-color parts, sharp corners around parametric feature make it difficult to fill completely. In two-color molds, A-side of single-color molded part serves as sealing surface. To prevent overflow during injection, two-color cavity is pre-compressed with an additional 0.01 mm to 0.02 mm based on mold's designed shrinkage rate to compact A-side of single-color molded part. Due to varying shrinkage rates in different areas of single-color molded part and errors in machining accuracy, different angles of sharp corners around parametric feature after compaction cause variations in filling of black adhesive at these sharp corners, resulting in unevenness at parametric feature boundary.

 

Figure 5. Schematic diagram of parameterized area defects

 

Figure 6. Profile diagram of double injection molding in parameterized area of inner lens
To address issue of discontinuous boundaries of parameterized features in two-color molding process, several car models with illuminated parameterized features on the market were benchmarked (Table 1). Based on mold processing technology and structural requirements of two-color mold, a cross-section of a two-color product containing parameterized features was designed (Figure 7). Engineering constraint models are shown in Table 2.

 

Table 1 Comparison of parameterized lighting of tail lamps for different vehicle models under different molding processes

 

Fig. 7 A parameter modeling engineering restriction model for double injection molding process

 

Table 2 Engineering restriction model parameters

In the early stage of double-color injection mold design, it is necessary to determine single-color molding part and two-color molding part according to product appearance requirements, injection materials and double-color overlapping structure of product. An analysis of two-color structure of inner light-distributing ring reveals that part only has a snap-fit with an undercut in mold opening direction (Figure 8). Both parts are molded from same material. If black ring base is used as a single-color molded part, milky-white molten plastic from second-color injection will melt surface of black ring and transfer black plastic to surface of inner light-distributing lens, resulting in an appearance that does not meet usage requirements. Furthermore, product's two-color overlapping structure involves inner light-distributing lens's runner design being located below B-side of ring base. Product's structure and appearance requirements dictate that inner light-distributing lens is a single-color molded part, while ring base is a two-color molded part. Two-color injection mold adopts a front-mounted mold design. After injection molding, single-color molded part remains on moving mold core side. Then, core rotates 180° to participate in molding of second injection ring base.

 

Figure 8. Analysis of slope of forming surface in direction of inner lens mold opening

Light-in and light-out surfaces of single-color molded parts and visible surfaces of two-color molded parts require a high-gloss mirror finish. Core and cavity of mold need to be polished to a mirror A0 grade, which requires mold steel to have a hardness of HRC48 or higher. Mold needs to use expensive pre-hardened steel or quenched steel. To reduce cost of mold steel, facilitate machining and heat treatment of core and cavity, shorten mold processing cycle, core and cavity of mold adopt an insert design. Two-color injection mold with inner lens ring adopts a 4-cavity layout, left and right inner lens rings are formed in one molding cycle. Figure 9a shows that fixed mold adopts an insert design. Insert size of single-color cavity and two-color cavity is 778 mm * 378 mm * 476 mm, which is fixed to mold frame by internal hexagonal head screws. Fixed mold frame is made of S50C steel with a hardness of 28~32HRC. Inserts for single-color and two-color cavities are made of imported 1.2343ESR hardened steel, which has a higher surface hardness (48~52HRC) and better polishing performance compared to S50C steel. There are three fixed mold springs around product perimeter, mainly used for molding gates, snap-fitting, and sealing of single-color molded parts. Each reciprocating motion of fixed mold springs involves continuous contact and friction with product and core surfaces, thus requiring high strength and wear resistance. Fixed mold springs are made of Cr12 mold steel with a hardness of 58~62HRC, which has high strength and wear resistance, improving service life of fixed mold springs. Figure 9b shows design of moving mold forming system. Based on structural characteristics of two-color mold, single-color and two-color cores have same design, their cooling water channels and ejection mechanisms are also identical. Similar to fixed mold, moving mold frame is made of S50C steel, and core is made of 1.2343ESR imported hardened steel. Moving mold is designed with an 8-slider core-pulling structure, using Cr12 mold steel, for snap-fit structure of trim ring base.

 

Fig. 9 Design of double-color injection molding system for inner lens

As shown in Figure 10, two independent hot runner injection systems are designed for single-color and two-color molded parts respectively. Gating systems for single-color and two-color molded parts are designed with 3-point and 5-point needle valve hot runner to cold runner systems respectively. Single-color molded part is formed with two gates per piece, and two-color molded part is formed with three gates per piece. Based on above analysis of difficulties in molding inner lens ring, to avoid light leakage caused by exposed white lens gates, a gating channel design is adopted for single-color molded parts. Gating channel is located inside parting line of single-color molded part to avoid exposure, and end connects to cold runner. This design creates an undercut in mold opening direction, and mold needs to add a fixed mold spring block to realize gating system design for single-color molded parts; two-color molded parts are designed with a bottom-side gating.

 

Fig. 10 Gating system of double color injection mold for inner lens

Based on structural characteristics of two-color injection molds, moving mold cores and structures of one-color and two-color molded parts are identical. When undercut area of two-color molded part overlaps with one-color molded part, a fixed mold spring block is typically used for molding. As shown in Figure 11, snaps of inner decorative rings are all undercut structures, molded from two-color black material, avoiding light leakage defects caused by molding milky white material. Snaps at point A and point B of product are different. Snap at point A is designed below one-color molded part, while one at point B is designed on two-color molded part. Snap at point B can be formed by moving mold lifter or slider. If same structure is designed at point A, milky white material will form snap when one-color molded part is injected. To realize snap structure at point A of product, a one-color fixed mold spring block and a two-color fixed mold spring block are designed for one-color and two-color cavities of fixed mold, respectively. One-color fixed mold spring block acts as a sealant when molding one-color molded part, while two-color fixed mold spring block completes molding of snap.

 

Figure 11. Cavity plate spring block structure of parameterized area clips.
As shown in Figure 12, as automakers increasingly emphasize continuity of illumination between inner trim ring position lights and tailgate through position lights, C-surface of inner trim ring is typically designed with no demolding angle in demolding direction. This leads to tearing defects in molding of this surface. To solve this problem, mold is designed with a fixed mold spring block structure so that relative movement of C-surface with mold forming surface during mold opening process is normal to C-surface. Fixed mold spring blocks are divided into single-color fixed mold spring blocks and two-color fixed mold spring blocks. Single-color fixed mold spring blocks act as a sealant when molding single-color molded parts, while two-color fixed mold spring blocks mold X and Y surfaces of trim ring base that overlap with C-surface.

 

Fig. 12 Structure of fixed mold spring block on 0° demolding surface of inner lens

Form of ejection mechanism is related to shape, structure and plastic properties of plastic part. Number and position of ejection mechanism are designed according to appearance and functional requirements of plastic part and demolding resistance. Ejection force balance ensures that plastic part is not deformed or damaged. Mold ejection should be smooth, stable and reliable. Since moving mold of this two-color mold will be interchanged during injection molding process, ejection mechanism of one-color molded part and two-color molded part are exactly same. As shown in Fig. 13, mold ejection mechanism is mainly composed of round ejector pin, flat ejector pin, push tube, lifter block, straight ejector block, ejector plate and ejector base plate. Screw column is designed for push tube ejection, snap-fit mounting seat is designed for lifter block molding and ejection. All ejection mechanisms are fixed between ejector plate and ejector base plate. Ejector plate is driven by push rod of injection molding machine to perform ejection movement.

 

Fig. 13 Ejection structure of double color mold for inner lens

One of design principles of system is that distance from each part of water channel to molding surface of mold is approximately equal, so as to achieve a roughly uniform temperature across molding surface. As shown in Figure 14, in order to avoid uneven cooling of plastic part leading to a longer molding cycle or warping deformation, cooling system of mold adopts a combination of "straight-through water pipe + inclined water pipe + water well". Cooling system is designed with conformal water channels along shape of plastic part, and water wells are designed for areas with uneven cooling. Diameter of cooling water channel is designed to be 11.5 mm, diameter of water well is 18 mm, distance between water channels is 45-50 mm, distance between water channel and surface of plastic part is 20-25 mm to ensure uniform cooling of plastic part.

 

Fig. 14 Cooling system of double color mold for inner lens
Since the overall appearance of plastic part is a high-gloss mirror surface and it is made of PC, mold temperature needs to be set above 80℃ to ensure appearance quality. For this reason, mold temperature controller is connected to each of eight core cavities of mold, and a separate cooling water channel is set at hot runner nozzle. Temperature of hot runner, moving and fixed molds is controlled independently.

Double color mold is produced by a rotary double color injection molding machine. Working principle of two-color mold with an inner polished ring is as follows:
(1) As shown in Figure 15a, mold is placed from top to bottom between four columns of injection molding machine via a trolley. Fixed mold is accurately aligned with injection molding machine nozzle via a positioning ring. Moving mold is precisely positioned on moving platen of injection molding machine using two positioning pins. Injection molding machine drives moving and fixed molds to close. Under guidance of round guide pillars, square guide pillars, precise positioning of mold, moving and fixed molds are fully engaged. Then, moving and fixed molds are fixed on moving and fixed platen of injection molding machine using mold locking brackets. After fixing mold, mold inlet and outlet water channels are connected to mold temperature controller, and corresponding water channel temperature is set. Hot runner is a needle valve type hot runner, which requires connection to a solenoid valve to control opening of valve needle. Once mold temperature reaches approximately 85℃, plasticized material is ready for injection molding.

 

Fig. 15 Schematic diagram of double-color mold for inner lens
(2) As shown in Fig. 15b, during first injection, nozzle needle valve of first-color hot runner is open, and nozzle needle valve of second-color hot runner is closed. Mold only injects first-color molded part, and second-color molded part is not injected. When first-color molded part is completed and mold opening motion is initiated, plastic part is fixed to first-color core side under clamping force of fixed mold spring block and screw pillar. Ejection system on moving mold does not perform ejection action. Subsequently, moving mold rotating disk of injection molding machine drives moving mold of double-color mold to rotate 180° clockwise, and mold closes, realizing cavity changing action of double-color mold.
(3) During second injection molding, needle valves of both single-color and double-color hot runner nozzles open simultaneously, allowing for simultaneous injection molding of single-color and double-color molded parts. Double-color cavity completes two-color molding of both parts, while single-color cavity simultaneously injection molds single-color part, preparing for next molded double-color part. After double-color part has undergone pressure holding, cooling, and solidification, mold opens. Driven by a spring and guided by a T-shaped wedge, fixed mold spring block, in conjunction with limiting block, moves 9 mm towards undercut direction, molding decorative ring base so that sprue can be ejected in forward direction. Driven by inclined guide post, slider, in conjunction with limiting block, moves 6 mm towards undercut direction, completing side core-pulling action. Snap-fit undercut is completely disengaged, and plastic part remains on moving mold (Figure 15c).
(4) As shown in Figure 15d, after product temperature reaches ejection temperature, mold opens. Moving mold of single-color mold does not eject. Under action of ejector pin of injection molding machine, moving mold of two-color mold moves along guide post by 35 mm with ejector plate and ejector base plate, driving round ejector pin, flat ejector pin, push tube, lifter block and straight ejector block to slowly eject plastic part. After plastic part is ejected, robot arm picks up part. After part is picked up, moving mold of two-color mold rotates 180° counterclockwise, mold closes, and two-color injection molding cycle begins.

In context of intelligent and parameterized automotive headlights, this paper proposes to use two-color injection molding process to replace costly laser engraving and hot stamping process to achieve effect of parameterized lighting of headlights. Paper focuses on process flow of two-color injection molding, difficulties in product and mold design. After product and mold structure scheme is verified by mass production, mold structure is stable and product quality meets requirements of car manufacturing. Based on development process of two-color molding process for inner decorative ring, following conclusions were drawn:
(1) Compared with post-processing hot stamping and laser engraving, two-color molding method for inner decorative ring reduces development costs and increases product qualification rate. However, parametric modeling is constrained by mold manufacturing feasibility and injection molding process, requiring engineering constraint models during development stage.
(2) Two-color mold for inner decorative ring features two sets of mold schemes with different spring block structures on fixed mold side. This effectively achieves molding of 0° demolding surface, gate guide channel, and snap-fit at two-color overlap, avoiding surface scratches and light leakage defects. Furthermore, spring block structure design greatly simplifies mold structure of two-color mold.
(3) Inner light-distributing ring molding surface adopts a core-cavity inlay design. Each cavity uses an independent flowing water channel + water well for individual temperature control, improving heat transfer and cooling efficiency of mold molding surface while reducing development cost of mold steel. Dual-color mold core cavity is temperature-controlled by a mold temperature controller. By adjusting mold temperature, molding cycles of first-color and second-color molded parts are made closer, improving cooling efficiency of inner light-distributing ring mold.
Currently, dual-color injection molding process cannot meet requirement of arbitrary parametric design and lighting of automotive lights. However, with continuous development of materials and light sources, continuous innovation of injection molding machines and mold structures, new molding methods and corresponding processes for achieving parametric lighting of lamps still need further research and exploration.