Abstract: Mold flow process of car drawer inner panel was analyzed and optimized in depth using Moldflow software. Based on detailed simulation analysis results and unique structural characteristics of plastic part, a one-point open hot runner gating system was designed. Potential problems encountered during injection molding process were predicted, and finally, a one-cavity hot runner injection mold was designed. To overcome demolding problem caused by excessive clamping force on slider due to ribs in injection-molded plastic part, a composite core-pulling mechanism with an integrated ejector pin in slider was designed. This mechanism integrates multiple components of "slider + ejector pin + spring + sliding block," and effectively prevents plastic part from sticking to slider by implementing a two-step core-pulling strategy. Guiding and positioning system, through strategy of "guide pillar and guide sleeve + fixed platen positioning," achieves precise and reliable reset function during mold opening and closing. To efficiently drive and reset ejection mechanism, a combination of "ejector roller + nitrogen spring" is adopted, reducing ejection reset time by approximately 25% compared to using a hydraulic cylinder. A cooling system with a near-conformal water channel design was designed, forming a three-dimensional and dense three-dimensional mesh cooling network, enhancing cooling effect and shortening molding cycle by approximately 8%. After mold was put into production, its operation was stable and reliable, and quality of produced plastic parts fully met predetermined standards, providing a reference for mold design of similar plastic parts.
Drawer panel is a key part of vehicle interior. It not only provides a convenient storage solution for drivers and passengers, but also greatly enhances decoration effect and practicality of car interior with its unique design and high-quality material selection. In the early stage of mold design and manufacturing, Moldflow software can effectively assist in planning gating system, identify and evaluate defects that may occur in molding process of plastic parts in advance, implement fine adjustment and optimization of process parameters in processing flow. Application of this technology has significantly shortened molding cycle of mold, and at the same time greatly reduced defect rate in production of plastic parts, improved the overall production efficiency and plastic part quality.
In the field of mold design, many scholars have conducted extensive and in-depth research and practice on application of Moldflow. Zhou Junjie et al. used Moldflow software to accurately locate injection injection position in drain pump filter and carried out cooling water channel layout. They then explored influence of different process parameters on degree of warpage deformation of final product. Based on these analysis results, they successfully designed an injection mold that met given requirements. Fan Yu et al. used mold flow simulation software to conduct a detailed simulation analysis of cooling effect and warping deformation of battery positive electrode bracket; through this process, they successfully determined optimal injection position of melt on plastic part, thus achieving a significant optimization effect. Wang Jinrong et al. used advanced CAE software tools to conduct an in-depth simulation analysis of snap-fit parts containing metal inserts; through this process, they accurately identified optimal gate position, effectively predicted possible plastic part defects, and optimized production process accordingly, significantly improving production efficiency and quality of plastic parts. Wang Wei et al. used powerful functions of CAE software to accurately identify and determine optimal gate position, carefully designed molding ejector system to ensure accurate positioning and stability of inserts inside plastic part. Zhang Weihe et al., Xu Yonglin et al., Hong Wei et al., Wu Junchao et al., Fu Yinglong et al., and Fu Lihua et al. have all successfully designed injection molds using professional software. During design phase, they used mold flow simulation software to predict potential quality problems during molding process and took corresponding preventative measures, producing plastic products that met quality standards. Concepts and research results of these scholars in the field of mold design undoubtedly provide inspiration and reference for us. However, these theories and methods cannot completely solve mold design problems discussed in this paper. Therefore, taking inner panel of a car drawer as a case study, we systematically explored how to solve problem of plastic part following slider during side core pulling, finally designed a hot runner injection mold with one cavity. After mold was put into use, it showed good operating performance, produced products met quality standards. This mold design method and concept provide reference for design of injection molds for plastic parts with same structure.

Structure of car drawer inner panel is shown in Figure 1. Raw material used is a composite material of polypropylene (PP) + ethylene propylene diene monomer (EPDM) rubber - 20% talc (T20) by mass, with a shrinkage rate of 1%. Addition of EPDM significantly enhances toughness and impact resistance of PP material, allowing material to remain intact, less prone to breakage when facing large impact forces and pressures. Maximum external dimensions of plastic part are 336 mm * 303 mm * 235 mm, with a main wall thickness of 2.5 mm and a local wall thickness of 2.2 mm. Wall thickness at connection between undercut reinforcing rib (circled in Figure 1c) and plastic part is 1.0 mm. The overall shape of plastic part is irregular. For drawer inner panel plastic part, inner surface is outer surface. Traditionally, outer surface is formed using a fixed mold. However, this design would leave plastic part in fixed mold, requiring a separate ejection mechanism, which would make mold structure more complex. Therefore, this mold proposes to use a moving mold slider to form the entire inner surface of plastic part. Due to complex shape of inner surface, how to design slider at this position is one of difficulties in mold design. Plastic part has 12 undercut features similar to those circled in Figure 1c. This requires a reasonable arrangement of angled core-pulling mechanism to prevent interference between mechanisms. Product surfaces indicated by directions B, C, and D have a large number of hexagonal shapes composed of reinforcing ribs. Surface B has 85 reinforcing ribs with a height of 4 mm, surface C has 46 reinforcing ribs with a height of 4 mm, and surface D has 93 reinforcing ribs with a height of 3 mm. Undercut features on surfaces B and C are planned to be formed using a hydraulic cylinder side-pulling mechanism. After mold opening, due to large clamping force, plastic part will move along with large slider. Therefore, how to prevent plastic part from moving with slider is another design challenge.

 

Fig. 1 Structural diagram of automotive drawer inner panel

Automotive drawer inner panel is an interior part, its appearance has strict requirements, injection defects such as shrinkage marks and inclusions are not allowed. Based on characteristics of plastic part structure and extensive experience in engineering practice, a composite gating system combining a single-point open hot runner and a cold runner was designed, as shown in Figure 2.

 

Figure 2. One-point open hot runner

3D design of plastic part was loaded into Moldflow software, then meshing was performed. According to statistics of meshing results, number of triangular mesh elements was 119,804, the total number of connected nodes was 59,870, there were no invisible triangles, surface area was 5,427.97, volume according to element type was 616.64, maximum aspect ratio of mesh elements was 14.95, minimum was 1.15, average aspect ratio was 1.88, there were 179,706 shared edges, and number of incorrectly aligned elements, intersecting elements, and completely overlapping elements was 0. There were no free edges or multiple edges, mutual percentage was 91.1%, and matching rate was 92.8%, ensuring high quality and high efficiency of mesh. Based on characteristics of PP+EPDM-T20 composite material, injection molding process parameters were set in mold flow analysis software, as shown in Table 1. Figure 3 visually presents results of mold flow analysis, which helps to understand material's flow behavior and filling situation in mold. In Figure 3a, blue line marks the area where melt begins to enter mold, i.e., starting point of filling, while red line marks end position of filling process, i.e., end of filling. Analysis results show that melt flows smoothly throughout filling process without any stagnation or blockage. In addition, Figure 3a also shows that the entire filling process takes 4.813 s, demonstrating material's efficient filling ability under given conditions. Figure 3b shows pressure distribution during speed/pressure control transition. Observing Figure 3b, it can be seen that pressure distribution shows a relatively uniform gradient without significant abrupt changes. During volume/pressure control transition, recorded pressure value is 61.05 MPa, which is within a suitable range and conducive to achieving effective filling. Figures 3c and 3d show distribution of air pockets and weld lines in plastic part, respectively. Analysis results show that there are no trapped air pockets or weld lines on main surface of part, and the overall appearance meets requirements. A small number of trapped air pockets and weld lines are mainly concentrated at ribs. Venting inserts can be installed at reinforcing ribs to enhance venting, reduce impact of air pockets and weld lines. Figures 3e and 3f show distribution of freezing layer factor and volume shrinkage rate during ejection. Gradual cooling and solidification of plastic part from end to gate is clearly observed. Holding pressure process is sufficient, ensuring density of plastic part. After holding pressure and cooling processes are completed, when plastic part is ejected from mold, volume shrinkage in most areas shows a relatively uniform distribution, indicating that the entire injection molding process is well controlled and product quality is stable.

Table 1 Setting of injection molding process parameters

 

Fig. 3 Moldflow analysis results

Conventional mold design method places outer surface of plastic part on fixed mold side, while outer surface of car drawer inner panel is located on its inner side. If designed conventionally, plastic part would remain in fixed mold after mold opening, requiring a separate fixed mold ejection mechanism, making mold structure more complex and increasing costs. To ensure plastic part remains on moving mold side after mold opening, a side core pulling mechanism is needed to form inner surface of plastic part. However, due to complex shape of inner surface of plastic part, conventional core pulling cannot be completed, which is one of difficulties in core pulling structure design. Therefore, after repeated demonstrations and based on past engineering experience, this mold uses a "tilted slider + hydraulic cylinder" core pulling mechanism on moving mold side to form outer surface of plastic part, as shown in Figure 4. The overall dimensions of side core pulling mechanism are 764 mm * 570 mm * 480 mm, belonging to a large slider core pulling mechanism. Based on shape of inner surface of plastic part, calculations show that to ensure smooth core-pulling, large slider needs to be designed at a 10° angle to Y-axis (as shown in Figure 4a). Slider is fixed to plate B, limit switch is fixed to pressure plate, pressure plate, limit post, guide block, and hydraulic cylinder are fixed to slider with screws. Cylinder mounting base is connected to large slider with screws. When mold opens from parting surface, two locking blocks fixed to plate A gradually disengage from large slider. After mold opening, limit switch receives a command, and T-shaped pull rod connected to hydraulic cylinder drives large slider to move along guide block. When slider travels 220 mm, side core-pulling action is completed. Since core-pulling mechanism is located on the side of mold at this position, a limit post with a diameter of 48 mm and a height of 30 mm is designed to prevent large slider from falling.

 

Figure 4 Side core pulling mechanism on D-side of plastic part.
Based on previous analysis of plastic part structure, it is known that sides B and C have numerous reinforcing ribs and undercut features. It is proposed to use an integral slider to mold sides B and C. Due to large clamping force exerted by hexagonal reinforcing ribs and undercut features on slider, plastic part will follow slider when side core pulling mechanism moves. How to solve this problem is a design challenge. After repeated demonstrations, a built-in ejector pin was finally adopted to solve problem of plastic part sticking to slider. That is, a composite mechanism of "slider + ejector pin + spring + sliding block" was designed, structure of which is shown in Figure 5. Shapes of sides B and C of plastic part are different, but composite core pulling mechanism used is similar. To more clearly observe internal structure of mechanism, three-dimensional view of C-side core pulling mechanism hides slider and slide block, as shown in Figure 5a. Two push tubes with mounting platforms and guide rods are fixed to slider seat and covered by a pressure plate; two push tubes and a circular ejector pin with a diameter of 8 mm are fixed to sliding block. At the start of operation, hydraulic cylinder drives slider and slide block to move. Sliding block remains stationary under spring force, thus fixing two circular ejector pins and push tube, which continue to hold plastic part in place. When core-pulling mechanism moves 10 mm, slider has slid out from ribs and some undercut features on B and C sides of plastic part. At this point, sliding block, push tube, and ejector pins continue to move backward with core-pulling mechanism under guidance of guide rod. When core-pulling stroke reaches 60 mm, side core-pulling action is completed, solving problem of plastic part sticking to slider during core pulling.

 

Fig. 5 Side core pulling mechanism on B-side and C-side of plastic part

To ensure stability and precise guidance of mold structure, circular guide pillars with a diameter of 60 mm and a length of 400 mm are installed at each of four corners of mold. These guide pillars are firmly fixed to A plate with M16 bolts, as shown in Figure 6. This design not only simplifies machining complexity of guide post through holes but also facilitates mold assembly. Furthermore, to enable robotic arm to accurately grasp plastic products from moving mold side during automated production, guide posts are specifically positioned on fixed mold side. This arrangement fully considers convenience and efficiency of production operations. Four corners of mold employ a 6° gripper-mouth positioning method to ensure mold's strength and stability.

 

Fig. 6 Mold guidance positioning system

As a deep-cavity plastic product, interior panel of a car drawer requires a relatively long ejection stroke after injection molding due to its depth. Therefore, designing and applying a proper ejection mechanism is crucial to effectively prevent defects such as ejection marks and cracks, ensuring perfect product quality and appearance. This mold integrates a comprehensive demolding mechanism consisting of an ejector pin system, push tube device, lifter block, flow guide hook, and ejector pins built into slider, as shown in Figure 7. This design aims to achieve an efficient and precise demolding process, ensuring integrity and surface quality of molded product. At the start of ejection, injection molding machine's top roller drives ejection mechanism components. When stroke reaches 105 mm, ejection ends, and robotic arm picks up plastic part. During reset, four sets of nitrogen springs, precisely guided by reset rod, drive push plate and its fixing plate back to their initial position. This reset mechanism significantly improves reset efficiency, effectively reduces the overall molding cycle, and further optimizes production process. Compared to using a hydraulic cylinder, ejection reset time is reduced by approximately 25%.

 

Figure 7. Mold Ejection System

A well-designed cooling system is crucial to ensure molding quality of plastic products and reduce molding time. By optimizing layout of cooling channels and flow path of cooling medium, mold temperature can be precisely controlled, thereby significantly improving production efficiency while ensuring excellent dimensional consistency and a smooth surface finish of plastic parts. Figure 8 shows cooling system design used in this mold. This system adopts a layout close to a "conformal water channel," with cooling channels intricately interwoven inside mold to construct a three-dimensional grid-like cooling network, shortening molding cycle by approximately 8%. This design ensures that direction of water flow is highly consistent with flow direction of plastic melt, while distance between adjacent cooling channels is controlled within range of 45–65 mm to optimize cooling effect.

 

Figure 8. Mold cooling system

Figure 9 shows assembly diagram of car drawer inner panel mold. This mold adopts a two-plate structure design, uses a one-point open hot runner gating system, has an ejection stroke of 105 mm, and the overall mold dimensions are 1300 mm * 1200 mm * 1314 mm. Fixed mold mass is approximately 4200 kg, and moving mold mass is approximately 4350 kg, which belongs to large injection molds. The overall mold operation process is summarized as follows: Hot runner controller 28 of mold is connected to power supply and heated to preset suitable temperature. Then, using pressure generated by injection system, fully plasticized molten plastic is injected into hot runner plate 1. Subsequently, under precise control of solenoid valve, molten plastic is precisely injected into mold cavity through primary hot nozzle 3 and secondary hot nozzle 27. After injection, in order to ensure that melt can completely fill cavity and maintain its shape, a certain holding pressure needs to be applied to molten plastic. After holding pressure, shrinkage compensation, sufficient cooling and solidification process are completed, moving mold plate B 22 and fixed mold plate A 8 are successfully separated by applying mold opening force. After moving mold stops moving, combined core-pulling mechanism for surfaces B, C, and D of plastic part is activated, pulling corresponding sliders outward. This process continues until limit rods of each slider touch corresponding limit switches, marking completion of core-pulling action. Next, ejection mechanism begins operation. When ejection stroke reaches 105 mm, ejection is complete, and robotic arm picks up plastic part. Subsequently, guided by reset rod, four sets of nitrogen springs 20 drive ejection mechanism components back to their original positions. Then, injection molding machine closes moving mold, and all mechanisms successively return to their pre-opening state, initiating next injection cycle.

 

Fig. 9 Mold assembly drawing
1—Hot runner plate; 2—Positioning ring; 3—First level hot nozzle; 4—Base plate; 5—Fixed clamp plate; 6—Hexagon socket head cap screw; 7—Frame plate; 8—Fixed mold A plate; 9—Water collection block; 10—Guide pillar; 11—Guide sleeve; 12—Hanging rod; 13—Large slider; 14—Plug; 15—Travel switch; 16—C plate; 17—Moved clamp plate; 18—Push plate fixing plate; 19—Pushing board; 20—Gas spring; 21—Limit column; 22—Moving mold B plate; 23—Sprue puller; 24—Moving mold core; 25, 32—Moving mold inserts; 26—Wear plate; 27—Secondary hot nozzle; 28—Hot runner controller; 29—Slider; 30—Guide 31—Spring; 33—Oil cylinder fixed seat; 34—Slide base

Mold adopts a one-cavity layout, utilizes a gating system combining a single-point open hot runner and a cold runner. Detailed injection molding simulations were performed using Moldflow, potential injection defects were thoroughly analyzed and evaluated. A slider-embedded ejector mechanism, i.e., a composite core-pulling mechanism of "slider + ejector + spring + sliding block," was designed. Through two-step core-pulling, plastic part is prevented from being carried away by slider during movement. A cross-layout three-dimensional mesh cooling system was adopted, which significantly improved cooling efficiency. A comprehensive demolding mechanism consisting of "ejector system, push tube device, lifter block, flow guide hook, and ejector pins built into slider" solved problem of difficult demolding. Since its commissioning, mold has operated smoothly without faults, ensuring that quality of produced products fully meets established standards and requirements.