Abstract: Considering characteristics of automotive window frame trim parts—large size, thin wall thickness, complex shape and structure, high requirements for appearance quality and dimensional accuracy—an injection mold with an accelerated ejection mechanism was designed. Gating system adopts a combination of hot runner and cold runner. Molten plastic first enters primary hot runner main nozzle through injection molding machine nozzle orifice, then transfers to secondary needle valve hot runner, and finally enters cold runner. Cold runner section adopts a U-shape and is designed on moving mold parting surface. Gate is a side gate located on fixed mold side. A total of 15 needle valve-type two-stage hot runners and 29 side gates are designed, achieving uniform material feeding and facilitating removal of solidified material in later gating system. Addressing issue of a large undercut area and numerous gates in U-shaped region around plastic part, resulting in a wide distribution of solidified material, demolding system adopts a combination of "direct ejector + ejector pins," and uses an accelerated ejection mechanism for secondary ejection. After ejection, part is removed by forced demolding, with demolding power provided by a hydraulic cylinder. To improve production efficiency, cooling system is designed with densely arranged layered water channels, achieving efficient and uniform cooling, increasing production efficiency by approximately 20% compared to previous similar products. Venting system is located in fixed mold core. After reserving a 20mm sealing surface on parting surface, remaining areas are treated with a 0.2mm clearance to ensure rapid gas discharge from mold cavity and facilitate mold fitting. Production practice has shown that mold structure is reasonable and reliable.
Today, automobiles have become an indispensable part of life, and users have increasingly higher demands for car quality, moving beyond simple transportation functions to focus on comfort, personalization, and aesthetics. Interior and exterior trim parts play a decorative and beautifying role in automobiles, and their quality directly affects the overall image of car. Therefore, manufacturers have placed higher demands on quality and precision of molds used to produce automotive trim parts. Automotive window frame trim strips are important decorative accessories. These products are typically U-shaped, large in size, thin in wall thickness, have many curved surfaces, large undercut areas, and complex structures. Designing a structurally sound and reliable mold while ensuring appearance quality and dimensional accuracy of finished product has become a challenge for mold design engineers. This paper takes a window frame trim strip of a certain car model as an example, focusing on design points and difficulties of its gating system, molding components, ejection system, and cooling system, providing a reference for design of similar molds.

Figure 1 shows the 3D model and engineering drawing of a window frame trim plastic part for a certain car model. It is U-shaped, with dimensions of approximately 959.4 mm * 515.1 mm * 64.3 mm. Large external dimensions are due to curved outer surface and the entire U-shaped area of inner surface being flanged and undercut except for two ends, as shown in blue area of Figure 1b. Specific shape and structural dimensions of plastic part are shown in Figure 1c. Wall thickness of plastic part model was checked and analyzed using Moldflow software, and results are shown in Figure 2. Wall thickness of most areas of plastic part is approximately 1.5 mm. Plastic part is made of glass fiber (15% by mass) reinforced polyamide 6 (PA6+GF15), with a shrinkage rate of 0.5%, moderate fluidity, good mechanical properties, temperature resistance, creep resistance. Window frame trim is an exterior component, requiring high surface quality; defects are not permitted after molding. Part also demands high dimensional accuracy and assembly precision, and has a complex structure. Therefore, a single-cavity mold design is adopted, classifying it as a large, thin-walled precision mold. Due to complex structure and thin-walled characteristics of part, mold ejection system design is quite challenging.

 

Fig. 1 Three-dimensional model and engineering drawing of automobile window frame trim (units: mm, unless otherwise noted)

 

Fig. 2 Runner system design

Following points should be considered when designing a large hot runner gating system:
(1) Edges of hot runner plate should be rounded or chamfered. Wiring channels should ideally be straight and all rounded to facilitate wiring and processing, and to avoid cutting or damaging wires.
(2) Hot runner nozzle design must consider thermal deformation, and sufficient space for thermal deformation must be reserved at parting line.
(3) Junction box and solenoid valve should be installed on the top or operating side of mold, higher than cooling water pipe joint, to prevent leakage and short circuit, for convenient operation.
(4) At the injection point of needle valve hot runner nozzle, a straight section of approximately 0.3 mm should be left to avoid repeated wear of mold steel at this point by gate, which could lead to damage.
Design of automotive window frame trim gating system is shown in Figure 2. Due to large size of plastic part and moderate fluidity of PA6+GF15 material, a multi-gate injection system is required. Analysis using Moldflow software shows that gating system adopts a hot runner-to-cold runner combination, with side gates and 15 needle valve hot runners. Mold flow parting results show that there is a joint line between every two gates, but junction temperature is high, and two-stage needle valve hot runner system does not require a sequence valve control. Each hot runner needle valve transitions to a U-shaped cold runner at parting line, and cold runner then transitions to a side gate, resulting in a total of 29 side gates that guide molten plastic into mold cavity. Dimensions of U-shaped cold runner and side gates are shown in Figure 2b. This combined hot runner-to-cold runner gating system avoids heat loss from molten plastic, ensuring a higher melt temperature at weld line, allows for flexible design of runners and gates, meeting product's requirements for multiple gates and gate types, and facilitating uniform melt filling.

Mold adopts a one-cavity layout. Plastic part is an appearance part. To ensure appearance quality of plastic part and facilitate smooth demolding, maximum contour line of plastic part is extracted as parting line, as shown by pink line in Figure 1b. Parting surface is created from parting line, and parting surface is mostly curved. Fixed mold core molds outer surface of plastic part, which requires a high-quality appearance and cannot have any splicing marks. Plastic part is also relatively large; therefore, fixed mold core is designed as a single, inlaid structure, measuring 1135 mm * 665 mm * 183 mm, and is directly inlaid onto fixed mold plate. Advantage of this inlaid structure is that mold core and mold plate can be made of different steels; mold core is made of alloy mold steel, and mold plate is made of ordinary carbon 45 steel, saving material costs, facilitating processing and maintenance. Because both moving and fixed mold cores are relatively large, a conical positioning structure is designed between moving and fixed molds to ensure mold closing accuracy. Four sides of fixed mold core, which are higher than fixed template, are designed with a 7° slope to mate with moving mold plate; large U-shaped parting surface of fixed mold core features a U-shaped groove with a 7° slope on the sides, also mates with moving mold core. A blind groove shaped like mold core is cut into fixed mold plate. Mold core is installed in blind groove of fixed mold plate. Due to large size of mold core, it is positioned within fixed mold plate using wedge blocks. Wedge blocks and fixed mold plate have an H7/m6 transition fit, and wedge blocks are fastened to fixed mold plate with hexagonal socket head cap screws. Then, mold core is fastened to fixed mold plate with hexagonal socket head cap screws. Mold core is made of 1.2344 hot work die steel, heat-treated to a hardness of HRC48-50. Design of molded components of fixed mold is shown in Figures 3a and 3b.

 

Fig. 3 Design of molded components
Design of molded components of moving mold is shown in Figures 3c and 3d, and is used to mold inner surface of plastic part. U-shaped undercut area around inner surface of plastic part is relatively large, requiring secondary disassembly of moving mold forming components. As shown in Figure 3c, moving mold core, ejector pin 1, ejector pin 2, ejector pin 3, and ejector pin 4 are obtained. Considering shape and structure of ejector pins, shape and strength of ejector pin groove on moving mold plate, moving mold core needs to be designed as an irregular shape. Dimensions of moving mold core are 1032 mm * 539 mm * 162 mm. Blind grooves of corresponding shapes are made on moving mold plate according to shape of moving mold core. Moving mold core and moving mold plate are positioned by transition fit, then moving mold core and moving mold plate are fastened with hexagonal screws. Large U-shaped parting surface of moving mold core is designed with a U-shaped boss, and side of boss is designed with a 7° slope to fit with fixed mold core. To facilitate mold assembly, top surface of moving mold core boss does not need to fully mate with fixed mold core; a 1mm clearance is provided on this surface. Eight balancing blocks are designed on the top surface of moving mold core boss; only top surfaces of these balancing blocks need to mate with fixed mold core. Moving mold core is also made of 1.2344 hot work mold steel, with a heat treatment hardness of HRC45-48.

Demolding system design is shown in Figure 4. To ensure smooth demolding, a combination of "straight ejector + ejector pin" ejection method is adopted. U-shaped undercut area around plastic part is ejected by four straight ejector blocks, followed by a secondary ejection by ejector pins to push out cold runner, then plastic part is forcibly ejected. A seesaw-type accelerated ejection mechanism is installed to achieve secondary ejection, and demolding power is provided by a hydraulic cylinder. Because the entire U-shaped inner ring area of plastic part, except for both ends, is undercut, and undercut area is large with high clamping force, four straight ejector blocks need to be designed in undercut area to eject plastic part. Each straight ejector block is equipped with two straight ejector rods with a diameter of 35 mm. One end of each straight ejector rod is connected to straight ejector block using a "pressure block + screw" method, and the other end is fixed to ejector base plate using a nut for locking. Straight ejector rods are not flush with ejector plate. A schematic diagram of straight ejector 1 structure is shown in Figure 4b. Similar to straight ejector 1, its three straight ejector structures consist of four straight ejector blocks, eight straight ejector rods, an ejector base plate, eight locking nuts, and sixteen straight ejector rod guide sleeves, forming the first set of demolding components. At cold runner of gating system, 30 ejector pins with a diameter of 6 mm are designed to eject solidified slurry. These 30 ejector pins are fixed to ejector plate with 15 ejector plate clamps. Together, these 30 ejector pins, 15 ejector plate clamps, and ejector plate constitute second set of demolding components. To achieve primary and secondary ejection movements of two sets of demolding components, four seesaw-type accelerated ejection mechanisms are installed on ejector base plate. Accelerated ejection clamps are installed on the bottom surface of moving mold plate. A schematic diagram of accelerated ejection structure is shown in Figure 4c. Ejection system has eight straight ejector pins. Sufficient space needs to be provided on the base plate and ejector pin base plate to install these pins. However, this prevents injection molding machine's ejector rollers from providing sufficient ejection power to mold. Therefore, hydraulic cylinders are required to provide ejection power. Four ejection cylinders are fixed to base plate, and their piston pins are engaged with U-shaped blocks, which are mounted on ejector pin base plate.

 

Figure 4 Ejection system design
Accelerated ejection mechanism is the key and challenging aspect of ejection system design. Its working principle is shown in Figure 5. Accelerated ejection mechanism includes an accelerated ejection 31, an accelerated ejection pressure block 32, a stop block 32 mounted on ejector plate 16, and an accelerated ejection fixing plate 33. When ejection cylinder is working, piston pin drives ejector base plate 18 and ejector plate 16 through U-shaped block, causing four straight ejectors and 30 ejector pins to eject together at the beginning. When seesaw-type accelerated ejection 31 fixed on ejector base plate 18 contacts accelerated ejection pressure block 34 fixed on the bottom surface of moving mold plate 13, one ejection cycle ends. During subsequent ejection cycles, under action of accelerated ejection 31, movement speed of ejector plate 16 is higher than that of ejector base plate 18, making ejection speed of 30 ejector pins higher than ejection speed of straight ejector blocks. Solidified material in gating system is ejected from straight ejectors and separates from plastic part, equivalent to a secondary ejection, until ejection stroke ends, and then plastic part is forcibly removed.

 

Figure 5 Working principle of accelerated ejection mechanism
13—B plate; 16—Ejector retainer plate; 18—Ejector plate; 19—Bottom clamp plate; 27—Straight top pin; 28—Lock nut; 29—Straight top guide bush; 30—Straight top 3; 31—Accelerated ejection; 32—Accelerated ejection pressure block; 35—Ejector pin; 40—Setting block

Following are design considerations for cooling system of large precision injection molds:
(1) Temperature difference between inlet and outlet of cooling water should generally not exceed 3 ℃.
(2) Cooling water for moving and stationary molds should preferably be arranged in a cross-grid pattern. Where a cross-grid pattern is not possible, a cross-grid pattern should be used where space allows.
(3) Cooling water design should aim for parallel water and material flow; cooling water should be designed wherever there is plastic.
(4) Spacing between cooling water outlets should be 3 to 6 times diameter of cooling water holes, distance between cooling water outlets and material position should be controlled at 12 to 20 mm. Distance between cooling water holes and edges of other holes should be greater than 5 mm, distance between sealing rings and edges of other holes should be greater than 3 mm to prevent leakage due to machining errors in cooling water holes.
Cooling water channel design requires that distance from cooling water to mold cavity surface be as equal as possible, amount of cooling water be as large as possible, diameter of cooling water holes be as large as possible, temperature distribution be as uniform as possible, and cooling balance be achieved as much as possible.
Fixed mold is assembled from fixed mold core and A plate, and its cooling system design is shown in Figure 6a. Cooling system for fixed mold core employs a combined "straight-through + inclined + baffle" water cooling system. Baffle cooling pipes have a diameter of 19 mm, while the other cooling pipes have a diameter of 12 mm. Cooling system for plate A uses a combined "straight-through + inclined" water cooling system. Main loop pipe connecting cooling water to external water source has a diameter of 30 mm. In Figure 7a, cooling pipes in gray loops 7 and 8 have a diameter of 15 mm, while the other cooling pipes have a diameter of 12 mm. Sealing rings are installed at connection points between cooling pipes of fixed mold core and cooling pipes of fixed mold plate to prevent leakage. All loops on fixed mold core and fixed mold plate combine to form cooling system for fixed mold, consisting of 8 branch cooling loops and 2 main loops. These are connected to external water source via inlet and outlet holes on two main water pipe connectors, facilitating operation during production.

 

Figure 6. Temperature control system

 

Figure 7. Venting system design
Moving mold mainly consists of a moving mold core and four ejector blocks. Moving mold core is fixed to moving mold plate, while four ejector blocks are moving parts and are relatively large, requiring cooling water. Therefore, cooling system design of moving mold is quite complex, as shown in Figure 6b. Cooling systems for moving mold core, moving mold plate, and four ejector blocks all adopt a combination of "straight-through + inclined" cooling methods, with all cooling pipes having a diameter of 12 mm. Sealing rings are installed at connection points between cooling pipes of moving mold core and moving mold plate to prevent leakage. Cooling water for moving mold core and moving mold plate is connected into six cooling loops: loops 1 to 4, loop 8, and loop 9. Cooling water on each ejector block is led out through two ejector rods fixed to it, then introduced into =ejector base plate and ejector face plate via water pipes. From there, water is concentrated onto moving mold plate via water pipes. Cooling water on four ejector blocks is connected into three loops: loop 5, loop 6, and loop 7. Cooling water in moving mold core, moving platen, and ejector block is connected into nine loops, all concentrated on moving platen. These loops are connected to an external water source via a main inlet/outlet water pipe connector, facilitating operation during production.
Mold's densely packed, layered water channels provide excellent cooling and a short molding cycle. Injection cycle is successfully controlled to approximately 50 seconds, reducing molding cycle by about 20% compared to similar plastic parts for other car models, thus improving production efficiency.

This mold produces large, thin-walled products with high requirements for surface quality and dimensional accuracy. Poor venting can easily lead to molding defects such as trapped air, burning, and insufficient material. In addition to auxiliary venting at parting surface, ejector mating surface, and ejector pin mating surface, a dedicated venting system is required to ensure smooth discharge of gas from mold cavity during molten plastic filling. Typically, venting grooves or venting clearance surfaces are located on fixed mold. Venting system is located on fixed mold core. After leaving a 30-40 mm sealing surface on parting surface of fixed mold core, all other parting surfaces are left with a 0.2 mm clearance to facilitate gas escape from mold cavity and also to facilitate mold fitting by fitter. Figure 7 shows venting clearance surface on fixed mold core. Yellow area indicated by arrow is sealing surface area of parting surface, and purple area indicated by arrow is 0.2 mm clearance area on parting surface, which is venting clearance surface.

Maximum external dimensions of mold are 1150 mm * 1895 mm * 1090 mm, weighing approximately 12 t, classifying it as a large injection mold. A Haitian 900T horizontal injection molding machine is selected for molding. Mold structure diagram is shown in Figure 8.

 

Fig. 8 Structure drawing of mold (units:mm)
1—Locating ring; 2—Nozzle pressure ring; 3—Hot runner main nozzle; 4—Top clamp plate; 5—Needle valve nozzle; 6—Hot runner plate; 7, 8—Support plate; 9—A plate; 10—Taper block set; 11—Cavity insert; 12—Core insert; 13—B plate; 14—Ejector pin; 15—Spacer block; 16—Ejector block retainer Plate; 17—Ejector pin pressing plate; 18—Ejector plate; 19—Bottom clamp plate; 20—Craft screws; 21—Stop Bolt; 22—Ejector guide bush; 23—Ejector guide pin; 24—Return pin contact block; 25—Return pin; 26—Support pillar; 27—Straight top pin; 28—lock nut; 29—Straight Top guide bush; 30—Straight top 3; 31—Accelerated ejection; 32—Accelerated ejection block; 33—Accelerated ejection retainer plate; 34—Accelerated ejection pressure block; 35—Hydraulic cylinder; 36—Hot runner junction box; 37—Ejector pin; 38—Guide bush; 39—Guide bush pressure plate; 40—Guide pin; 41—Support pillar

(1) Injection Molding. Molten material enters the primary hot runner main nozzle 3 under action of injection molding machine, enters sub-runner within hot runner plate 6, then enters secondary needle valve hot runner, flows into cold sub-runner through secondary needle valve hot runner, finally enters mold cavity through side gate for filling, holding pressure, and cooling.
(2) Mold Opening. After cooling is complete, mold opens, and product remains on one side of moving mold, moving with moving mold.
(3) Ejection. After mold opening stroke is completed, ejector cylinder starts working. Ejector base plate 18 and ejector panel 16 cause four straight ejectors and 30 ejector pins to eject together at the beginning of ejection process. When seesaw-type accelerated ejector 31 fixed on ejector base plate 18 contacts accelerated ejector block 34 fixed on the bottom surface of moving platen 13, this is the first ejection, straight ejectors and ejector pins are ejected together. Ejection continues. Under action of accelerated ejector 31, movement speed of ejector panel 16 is higher than that of ejector base plate 18, so that ejection speed of 30 ejector pins is higher than ejection speed of straight ejector block. Solidified material of gating system is ejected from straight ejectors and separated from plastic part, which is equivalent to a second ejection, until ejection stroke ends, and then plastic part is forcibly removed.
(4) Reset. After plastic part is removed, mold begins to reset. Ejector cylinder drives ejector base plate 18 and ejector panel 16 to reset. At this time, due to action of accelerating ejector block 34, ejector panel 16 resets first. Ejector panel 16 drives ejector pin to perform the first stage of reset. When accelerating ejector block 32 separates from accelerating ejector 31, ejector base plate 18 and ejector panel 16 together drive straight ejector block and ejector pin to begin second stage of reset, until mold is completely reset.
(5) Mold closing. Under action of injection molding machine, moving mold moves towards fixed mold. It is guided and positioned by guide pillars, guide sleeves, conical positioning of moving mold core and fixed mold core to achieve accurate mold closing and start next injection molding.

(1) Due to large size and heavy weight of mold, moving and fixed mold insertion surfaces and surrounding mold closing surfaces are designed with a 7° slope for conical positioning to ensure mold closing positioning accuracy of large molds.
(2) Mold adopts a "hot runner + cold runner" gating system with a side gate, facilitating melt filling and removal of solidified material from gating system, ensuring molding quality of finished product.
(3) Mold's demolding system employs a two-stage ejection structure combining "straight ejector blocks + ejector pins," with ejection power provided by an ejection cylinder. Ejection speed of ejector pins is higher than that of straight ejector blocks. Upon ejection, plastic part is forcibly removed.
(4) Multiple cooling water channels are arranged in a dense, layered configuration, enabling rapid cooling, stabilizing mold temperature, and reducing molding cycle to approximately 50 seconds, saving about 20% of molding time and improving production efficiency. Practice has shown that mold structure design is reasonable and reliable.