Abstract: To integrate dual requirements of functionality and aesthetics, taillight cover of a certain car model is designed in three colors. A three-color, three-station injection mold was designed for this cover, consisting of three sub-molds arranged at 120° intervals. To address issues of steep slopes, large height differences, and potential tearing due to undercut structure in mold opening direction of face mask products, an internal pulling mechanism and parting hook were designed. Placing plastic parts at a specific angle ensures that internal pulling motion aligns with mold opening direction. During mold opening, parting hook opens moving platen, separating internal pulling mechanism and completing core-pulling motion. Square guide pillars were also designed to increase mold's positioning accuracy and support strength. Each sub-mold is equipped with a needle valve hot runner system, cooling water jackets were designed at valve needle gates of second and third colors to enhance cooling. Ejection system is hydraulically driven, with ejection direction at an angle to mold opening direction, effectively reducing height difference of mold, saving steel and increasing mold strength. Cooling system employs a combination of water channels and wells that conform to mold's shape. Actual testing has shown that this three-color mold operates stably and reliably, products meet production requirements.
Car lights are often referred to as "eyes" of a car, people have increasingly higher requirements for appearance and quality of cars. Therefore, structural design of car lights is becoming more and more complex. Car taillight covers are usually composed of various colored plastics, commonly two-color and three-color. Multi-color plastic molds are usually complex in structure, requiring consideration of their appearance, material, molding process characteristics to obtain qualified products. Design difficulties lie in: First, precise positioning and repeatability during rotation must be met, and components must not interfere with each other during opening, closing, and rotation; second, multi-color plastics are prone to color bleeding, requiring a reasonable overlapping method to avoid color bleeding defects while ensuring bonding strength; third, plastic parts are formed through multiple molding processes, each sub-module requires an independent runner system and cooling system.
Based on bonding characteristics of plastic parts, three-color mold structures can be divided into three-color two-station, three-color three-station types. This case uses a three-color three-station mold structure. Three-color three-station molds are usually composed of three sub-molds. Fixed mold is changed twice by rotating injection molding machine turntable, and three-color molded plastic parts are obtained by overlapping injection twice. Compared with single-color injection molding, multi-color injection molding has advantages of good product quality and high production efficiency, which is very much in line with current production needs of industry. This article introduces design scheme and working principle of a taillight cover mold from five aspects: gating system, molding system, core pulling system, demolding system, and cooling system.

Taillight cover product is composed of three colors: black, transparent, and red. Material is polymethyl methacrylate (PMMA) resin. Black part weighs about 61 g and is located around taillight cover as first color. It plays a role in blocking light. Design of black part mounting surface welding rib and other structures facilitates installation and fixation of taillight cover in corresponding area. Transparent part and red part are second and third colors respectively, covering black part. Weights are 39 g and 122 g respectively. Their function is to meet light transmission and decorative lighting requirements of taillight. Structure of three-color taillight mask is shown in Figure 1.

 

Fig. 1 Tricolor taillight mask product structure
Fig. 2 shows overlapping cross-section of three-color taillight mask. As can be seen from Figure 2, a 0.2 mm forward gap is designed at parting line to avoid reverse gaps caused by machining accuracy and mold positioning accuracy. To enhance bonding strength of different colored plastics, prevent cracking during ejection and demolding or use under extremely cold or hot conditions, three colored plastics are designed with a wider contact area and an appropriate overlapping angle.

 

Fig. 2 Cross-section of three-color mask

Conventional product placement method in mask mold design is shown in Figure 3a. In this case, ejection direction is consistent with mold opening direction, with a drop of 367 mm. This presents problems such as excessive mold thickness, difficult machining, high pressure in mold's lateral cavity, and difficulty in mold locking. If product is rotated 30° by internal pulling angle, so that ejection direction is 30° with mold opening direction, as shown in Figure 3b, this design can reduce mold height difference by about 181 mm, while eliminating slider connected to internal pulling, saving steel and increasing strength of mold. However, how to reasonably arrange internal pulling and mold opening ejection mechanisms, avoid interference is a major challenge.

 

Fig. 3 Schematic diagram of mold ejection direction and mold opening direction

Three-color face mask is mass-produced using a horizontal multi-color injection molding machine. Mold is designed with three stations. Moving mold side turntable of injection molding machine rotates 120° clockwise each time, re-closes mold for injection, and completes part removal in third sub-mold. Due to inherent errors in mold processing, dimensional accuracy of moving molds of three sub-molds is not completely consistent; and mold is relatively heavy, so positional displacement is prone to occur during rotation, thus failing to guarantee installation accuracy. However, three-color, three-station mold requires moving mold to mesh with each of three stationary molds after each rotation. Therefore, ensuring positioning accuracy during dynamic, repeated rotation of mold is a challenge.

This taillight cover is made by sequential injection molding of three colors of plastic. Based on product's structural characteristics, first color is black, second color is transparent, and third color is red. All three gating systems utilize hot runner valve pin gates.
Three sub-mold cavities all adopt a mirrored, two-cavity design. PMMA plastic is molded using HRSflow brand hot runner system. First color (black) uses a two-point valve pin gate on product surface, as shown in Figure 4a; second color (transparent) uses a one-point valve pin gate on product surface, as shown in Figure 4b; and third color (red) uses a one-point valve pin gate on product surface, as shown in Figure 4c. Cooling water jackets are designed at valve pin gates of second and third colors to enhance cooling, prevent heat transfer defects on product surface. After initially determining number and location of gates based on product structure, feasibility was verified through mold flow analysis. Figure 5 shows optimal gate location cloud map obtained from Moldflow software, which is consistent with actual gate location.

 

Figure 4 Mold runner system

 

Figure 5 Optimal gate location analysis

3.2.1 Core Design: Core structures of three sub-molds are identical. Core steel uses German 1.2343ESR material. After rough machining, it needs to be heat-treated to ensure its hardness meets requirement of 46~48 HRC. Black product surface of core side uses 1200# sandpaper for polishing, while product surfaces of transparent and red parts need to be polished to a mirror finish. Due to large size of core, for easy assembly and disassembly, sides that mate with mold frame are designed as two straight surfaces as references and two inclined surfaces with a draft angle of 3°, as shown in Figure 6. Taillight cover has a large side projection area, and product must withstand significant lateral pressure during injection molding. Therefore, bosses and grooves are designed on the front of core and cavity to provide positioning and interlocking.

 

Fig. 6 First color, second color and third color sub-mould core
3.2.2 Cavity Design: Cavity structures of three sub-molds are different, as shown in Figure 7. Width of middle sealing surface is designed to be 2 mm to avoid obvious indentation on plastic part. All steel used is German 1.2343 ESR material. After rough machining, it needs to be heat-treated to ensure a hardness of 46~48 HRC. Molded surface of cover needs to be mirror polished. Due to large cavity size, for easy assembly and disassembly, sides that mate with mold frame are designed as two straight surfaces as references and two beveled surfaces with a draft angle of 3°.

 

Figure 7. First color, second color, and third color sub-mold cavities

Internal pulling angle is designed to be consistent with mold opening direction, eliminating need for a tunnel slider structure, reducing mold height, saving steel, and increasing mold's own strength. At this point, ejection direction is 30° to mold opening direction. Mold uses a parting hook to separate moving mold plate from internal pulling, completing core-pulling motion. Ejection system is driven by a hydraulic cylinder to eject product.
Internal pulling structure in mold-closed and mold-opening states is shown in Figure 8. Internal pulling block is fixed to base plate with screws, while moving mold plate is not fixed to base plate. When mold opens using parting hook, moving mold plates and cores of first and second color sub-molds do not separate from base plate; moving mold plates and cores of third color sub-mold separate from base plate by 25 mm, internal pulling block detaches from plastic part. Square guide pillars are designed between moving mold plate and base plate to provide support and guidance during opening and separation of moving mold plate. Four spring supports are also designed between moving mold plate and base plate to assist in opening process. Additionally, four tie rods are designed between moving mold plate and base plate; after inner core is pulled out during opening, tie rods pull moving mold plate and moving mold together to open mold.

 

Figure 8. Internal pumping in the state of closing, opening and ejection of ejector plate
1—Core; 2—Moving Plate; 3—Core-pulling mechanism; 4—Base plate; 5—Ejector plate; 6—Ejector pin; 7—Ejector sleeve

3.4.1 Parting Hook: Parting hook of American DME standard is adopted as internal pumping opening drive structure. Hook 1 and lever 1 of the first and second sub-molds are fixed to base plate on moving mold side. Positions of mold closing hook and lever are shown in Figure 9a, positions of mold opening hook and lever are shown in Figure 9b. Hook 1 pulls moving mold plate, moving mold plate and base plate do not separate when mold opens. Pull hook 1 and lever 1 of third die mold are fixed to base plate on moving mold side, while pull hook 2 and lever 2 are fixed to fixed mold plate. When mold closes, lever 2 lifts pull hook 1, separating it from moving mold plate. Pull hook 2 then engages with moving mold plate, with pin of pull hook 2 25 mm from trapezoidal block of lever 1, as shown in Figure 9c. When mold opens, pull hook 2 pulls moving mold plate, separating it from base plate. Inner core is then pulled out. When separation reaches 25 mm, lever 1 lifts pull hook 2, disengaging it from moving mold frame. Moving mold plate and base plate then open together, as shown in Figure 9d.

 

Figure 9. Schematic diagram of hook and lever operation
1—Hook 1; 2—Lever 1; 3—Base plate; 4—Moving Plate; 5—Fixed plate; 6—Hook 2; 7—Lever 2
3.4.2 Square Guide Posts: To achieve inward pulling motion by opening moving mold plate and base plate by 25 mm, circular guide posts alone are insufficient for positioning and support. Therefore, square guide posts are designed on outer sides of base plate and moving mold frame to increase positioning accuracy and support strength of mold. Square guide posts are fixed to base plate, and adjusting blocks are fixed to moving mold plate, as shown in Figure 10.

 

Figure 10. Square guide pillar structure diagram
1—Base Plate; 2—Moving plate; 3—Fixed plate; 4—Square guide post; 5—Adjusting block; 6—Wear plate; 7—Screw
3.4.3 Ejection Mechanism Design: Ejection systems are designed on moving mold side for colors 1, 2, and 3. Ejection direction is at a 30° angle to mold opening direction. Two hydraulic cylinders drive ejection and resetting movements. Straight ejector blocks are arranged around plastic part, and two round ejector pins are arranged in the middle of plastic part, as shown in Figure 11. Traces of ejector blocks and ejector pins are all on the black plastic part, which can cover them up and make them difficult to see visually. Ejection distance is set at 30 mm.

 

Figure 11 Ejector Mechanism
1—Ejector sleeve; 2—Return pin; 3—Cylinder; 4—Limit switch; 5—Ejection stop block; 6—Guide pin; 7—Ejector pin; 8—Ejector plate

Tri-color taillight cover is made of PMMA material, and its mold temperature is generally around 60 ℃. A mold temperature controller is used to separately control temperature of stationary mold and moving mold. A water channel was designed to fit taillight cover's structure, and water wells were also designed to assist in cooling insufficiently cooled areas, thus ensuring uniform cooling of product. To ensure cooling effect, independent cooling water channels were designed for mold's hot runner and internal extraction structure. 3D model and mold flow analysis results are shown in Figure 12, showing that surface temperature distribution of plastic part is uniform.

 

Figure 12. Mold Cooling System
Mold cooling system has a water channel diameter of 11.5 mm, a water well diameter of 18 mm, and a water channel spacing of 45-65 mm. Threads are imperial pipe threads. For mold cores requiring quenching, minimum safe distance between water channels and mold core must be at least 12 mm, distance between water well and product must be at least 20 mm to prevent cracking during quenching.

Assembly diagram of automotive taillight cover injection mold is shown in Figure 13. The overall dimensions of mold are 1980 mm * 1795 mm * 1040 mm. Mold is a "three-mold, six-cavity" three-color injection mold.

 

Fig. 13 Mold assembly drawing
1—First locating ring; 2—Second locating ring; 3—Third locating ring; 4—Insulation plate; 5—Clamping plate; 6—Hot runner plate; 7—Fixed plate; 8—Moving plate; 9—Locator; 10—Base plate; 11—Hook and lever; 12—Hydraulic cylinder; 13—Ejector plate; 14—Fixed mold core; 15—Guide bushing; 16—Guide post; 17—Moving mold core
Working principle of three-color three-station injection mold is shown in Fig. 14. Mold operation steps under condition of continuous cyclic production [that is, when step (3) is performed below, second color sub-mold already contains first color plastic part, third color sub-mold already contains first and second color plastic parts] are as follows: (1) Mold closing. (2) Injection molding. Simultaneously, three gating systems for colors 1, 2, and 3 are opened to complete injection + holding pressure + cooling molding process. (3) Mold opening. Moving mold side of color 3 opens through parting hook, and inner core is pulled out by 25 mm. Hydraulic cylinder of sub-mold of color 3 drives ejection mechanism to eject tricolor taillight mask product by 30 mm, and robot arm completes part removal. (4) Moving mold rotation. Hydraulic cylinder of color 3 drives ejection mechanism to reset, and injection molding machine turntable rotates counterclockwise by 120°. (5) Mold closing again. Inner core and slide are reset, mold is closed again for injection molding, and cycle repeats.

 

Fig. 14 Tricolor plastic part molding process
Through trial molding verification of assembled mold, tricolor taillight mask product is obtained as shown in Fig. 15. It can be seen that product surface is smooth and without molding defects.

 

Fig. 15 Actual product of tricolor taillight mask

(1) A "three-mold, six-cavity" tricolor injection mold was designed to address structural characteristics of tricolor taillight mask. By rotating product 30° in inward pulling direction and adjusting ejection direction, problem of large height difference in mold structure was solved. This saved steel while enhancing mold strength, achieving goal of cost reduction and efficiency improvement.
(2) To facilitate smooth mold demolding, an inward pulling mechanism and a parting hook were designed. Parting hook enables moving mold plate to open and separate inward, completing core pulling motion. Square guide pillars were also designed to increase mold's positioning accuracy and support strength.
(3) Mold cooling system was designed with a combination of water channels and wells that conform to mold shape. Independent cooling water channels were provided for both hot nozzle and inward pulling structure, with uniform channel spacing, ensuring uniform cooling of mask product.
(4) Mold trial run was stable, product molding cycle met requirements, appearance quality and dimensional accuracy of tricolor taillight mask met requirements, achieving expected goals.