With continuous development of automotive industry towards lightweighting and functional integration, plastic storage boxes have been widely used in automotive manufacturing field due to their significant advantages such as light weight, corrosion resistance, and ease of molding. However, design of automotive storage boxes usually includes multi-directional ear-like structures, deep cavities, and complex rib features, which poses high technical requirements for mold design. This paper takes a certain model storage box plastic part [size 354 mm * 84 mm * 266 mm, material is polyoxymethylene (POM)] as an example, explores key technical issues in mold design through in-depth analysis of its structure and processability. Research focus of this paper is to solve problems of parting surface optimization, multi-directional core pulling, mold nozzle eccentricity, and molding defect control.

Figure 1 shows 3D contour shape and structural features of plastic part. As clearly seen in image, deep internal cavity in the middle of plastic part serves as a storage area and is considered an exterior surface (commonly known as a Class B surface). There are five non-exterior surfaces on the outside, with reinforcing ribs and holes. On upper left and right sides and bottom of plastic part, there are five outward-extending ear-like structures (A, B, C, D, E), which also have reinforcing ribs and holes. Three of these are on upper left and right sides (A, B, C), two are on the bottom (D, E), and their ejection directions are different, making selection of mold parting surface a significant challenge. Plastic part has dimensions of 354 mm * 84 mm * 266 mm, a main wall thickness of 2.2 mm, and a weight of approximately 400g, classifying it as a small to medium-sized plastic part. It is made of POM material with a shrinkage rate of 1.6%. Plastic part has a uniform wall thickness, and rib thickness is reasonably designed, meeting requirements of injection molding. As a functional structural component, this plastic part has high strength requirements and must meet product functional testing standards. In terms of appearance, product's surface must meet requirements for Grade B. On non-appearance surfaces, gate marks, slider clamping lines, and parting lines are permissible, but maximum clamping depth must be less than 0.1 mm, defects such as flash, shrinkage, deformation are not allowed. Setting of other core-pulling mechanisms will be determined based on selection of parting surface; details are provided below regarding mold parting surface and core-pulling mechanism.

 

Figure 1: 3D Schematic Diagram of Plastic Part Structure
As shown in Figures 1 and 2, 3D structural schematic diagram and parting scheme diagram of plastic part in this case are presented. Figure 1 clearly shows basic shape and structure of plastic part. Analysis of dimensions of plastic part in Figure 2 reveals that its width is only 84 mm, which is disproportionate to its length of 354 mm and height of 266 mm. If mold is made according to conventional parting scheme, several disadvantages will be exposed. Taking Scheme 1 in Figure 2(a) as an example, following analysis will be conducted:

 

Figure 2: Schematic diagram of parting scheme
Large side core pulling volume: Taking Z-axis as ejection direction, based on structural characteristics of part, reinforcing ribs and ear-like structures on both sides require design of side core pulling S1 and S2. These two side core pullings cover plastic part's length of 354 mm, height of 266 mm, resulting in a large volume, increasing difficulty of mold design and manufacturing.
Eccentricity of moving mold core: Moving mold core contains plastic part's height of 266 mm and width of 84 mm, but due to its small size and thinness, eccentricity is prone to occur during injection molding, affecting quality of plastic part.
Difficult ejection and demolding: Deep internal cavity of plastic part belongs to Class B appearance surface, which does not allow ejector pin marks, so ejector pins cannot be used for ejection. If a push-plate structure is designed, parting surface of push-plate will be uneven with a large drop, and it will also contain reinforcing ribs, making ejection impossible and posing a significant challenge to ejection design.
Mold Dimension Inconsistency: Ejection at 266 mm in height direction of plastic part will result in an excessively thick mold, while width direction of 84 mm will be too thin, causing a significant imbalance in the overall length, width, and height dimensions of mold, which is detrimental to mold manufacturing and use.
Consider Scheme Two, as shown in Figure 2(b), which also uses Z-axis as ejection direction. Its advantages are:
Reduced Mold Thickness and Harmonized Dimensional Proportions: In original Scheme One, S1 and S2 become fixed and moving mold cavities, with cavity filling located at 84 mm in width direction of plastic part. This reduces mold thickness, dimensional proportions of length (354 mm) and width (266 mm) are more harmonious, which is more conducive to mold manufacturing and maintenance.
Large Side Core Pulling Stroke: Moving mold core becomes a side core puller S1, with a large core pulling stroke along 266 mm height of plastic part. Simultaneously, a new side core puller S2 is added on the other side. Although side core pulling stroke is increased, this problem can be solved through reasonable structural design.
Ejection Problem Solving: Since outer side of plastic part is not an appearance surface, ejector pins can be placed as needed, effectively solving ejection problem and ensuring smooth demolding of plastic part.
Hydraulic Cylinder Driven Core Pulling: Due to large stroke of side core puller S1, a hydraulic cylinder driven core pulling method is adopted, as shown in Figure 4. This ensures smooth completion of core pulling action and guarantees normal operation of mold.

 

Image 3 3D Schematic Diagram of Gating System

 

Image 4 3D Schematic Diagram of Core Pulling Mechanism Layout
Based on above analysis, Scheme 2 is more suitable for this case. From actual results, mold made according to Scheme 2 proceeded very smoothly from trial molding to mass production. For more detailed considerations regarding mold structure, please refer to mold structure design below.

Figure 3 presents a 3D schematic diagram of plastic part's gating system. As clearly seen in Figure 3, plastic part uses a large-gate side gate for injection. Gate position was carefully set using mold flow analysis software (MOLDFLOW), specifically located on the side of plastic part, at the top of slider core, at the center of stationary and moving mold cavity. This ingenious design effectively avoids vertical eccentricity of slider core during injection, thus preventing misalignment of plastic part. Compared to large-gate direct gate injection method, although filling efficiency of large-gate side gate injection is relatively low, considering characteristics of this plastic part, appearance quality of plastic part can be significantly improved by rationally designing secondary runner and setting up a cold slug well. This design effectively prevents injection molding defects such as runner defects and cold slug defects. At the same time, injection pressure loss of this injection method is relatively limited, which is sufficient to meet molding requirements of plastic part.
Figure 4 shows a 3D schematic diagram of core-pulling mechanism layout. In this case, mold has two core-pulling mechanisms, located on the top and bottom sides respectively. Side core-pulling mechanism A (S1), located on the top side, is a secondary core-pulling mechanism with an ultra-large stroke. First core pulling is driven by a slanted pin, using a motion-priority fixed-distance mold opening core-pulling method; second core pulling is driven by a hydraulic cylinder, achieving ultra-large stroke core pulling. The entire core-pulling process is completed in two steps. The other side core-pulling mechanism, B (S2), is located on the bottom side of mold, using a conventional core-pulling structure and driven by a slanted pin. Specific core-pulling process will be described in detail in subsequent core-pulling steps.
Figures 5(a) to (d) show 3D schematic diagrams of side core-pulling mechanism B (S2) in closed and open mold states. As can be seen from figures, side core-pulling mechanism B (S2) is not related to PL1 fixed-distance parting mold opening. Its structure is fixed to fixed mold plates A and B, consists of a core-pulling block, core-pulling base, shovel base, slanted pin, spring, wear-resistant block, pressure plate, limit screw, belonging to a common side-sliding core-pulling mechanism. Its working principle is as follows: When mold opens, inclined pin drives slider base, causing core-pulling block to move laterally, thereby disengaging undercut. Limit screw controls core-pulling stroke, spring maintains core-pulling state to ensure smooth mold closing. When mold closes, shovel base locks slider base to ensure core-pulling mechanism resets. Wear-resistant blocks are set at moving parts of core-pulling mechanism to facilitate subsequent maintenance and replacement. Figures 5(b), (c), and (e) show 3D schematic diagrams of side core-pulling A (S1) in closed mold state, PL1 open mold state, and PL2 open mold state. It can be seen that side core-pulling A (S1) core-pulling mechanism completes core-pulling in two stages. First core-pulling is completed by inclined pin when fixed-distance parting surface PL1 is opened; second core-pulling is completed by hydraulic cylinder after parting surface PL2 is opened. Its structure includes a core-pulling block, a core-pulling base, a shovel base, an inclined pin, an inclined pin fixing plate, a spring, a wear-resistant block, a pressure plate, a hydraulic cylinder fixing plate, a T-block, and a hydraulic cylinder. Working principle of fixed-distance parting sequence mold opening and side core pulling A (S1) is as follows: Shovel base is fixed on panel, and a spring is set on parting surface PL1 to assist PL1 in opening mold first; a resin opening and closing device is set on parting surface PL2, which uses friction between device and A plate to achieve mold opening delay of PL2. During mold opening, parting surface PL1 opens first under action of the spring, and PL1 limit screw controls mold opening stroke to 30 mm. During this period, PL2 has not yet opened, and the entire cavity is in a closed state. Shovel base is released, and inclined pin drives core pulling base to move core pulling block to side. Spring assists core pulling block to disengage from core. Front end of T-block has a 10 mm limit stroke. Core pulling block is released from plastic part, and fixed mold side insert is released from glue position with panel, minimizing deformation of plastic part. When mold opening stroke exceeds 30 mm, parting surface PL2 opens, and mold completes full opening. Hydraulic cylinder drives core-pulling base to continue ejecting core block until core is completely disengaged, with a total stroke of 220 mm. During this process, hydraulic cylinder's load only needs to support mass of side core-pulling block A (S1), without bearing clamping force of plastic part on core. Hydraulic cylinder load is small, force is stable and constant, ensuring stability and reliability. During mold closing, hydraulic cylinder first drives side core-pulling block A (S1) to reset its core-pulling stroke, then parting surface PL2 resets, finally parting surface PL1 resets. Shovel base locks core-pulling base, ensuring core block is reset. Because side core-pulling block A (S1) is in direction of force on injection side, a reverse angle is specially designed on shovel base so that it inserts into moving mold and locks shovel base during mold closing, preventing core block from retracting due to excessive injection pressure, thus avoiding quality defects in plastic part.

 

Figure 5. 3D schematic diagram of sequential parting and core pulling states.
Parting surface of plastic part is irregular, with many reinforcing ribs, some of which are quite deep. Parting surface has both an insertion surface and two side core pulling mechanisms, S1 and S2, with sealing positions distributed in various directions. Therefore, design of inserts and venting plays a crucial role in filling efficiency and appearance quality of plastic part. Considering demolding, cooling, ejection, and core strength of plastic part, a total of 6 inserts are designed on fixed mold side. Five of these are movable inserts, fixed to panel for demolding purposes, and ejected when parting surface PL1 opens for the first time. This design maximizes quality of plastic part. The other insert is fixed inside fixed mold core and is a through-face insert, facilitating later maintenance, as shown in Figure 6(a). On moving mold side, a total of 3 inserts are provided. Two are designed on one side of insertion surface of two pillow positions, and one is designed on a deep rib position. This design improves processing efficiency, facilitates venting, helps plastic part fill more smoothly, as shown in Figure 6(c). In addition, a complete venting system is designed across the entire parting surface, encompassing mold core, side core-pulling slider mechanism, as shown in Figure 6(d). Mold's venting system is divided into two stages: a primary venting depth of 0.5 mm and a secondary venting depth of 0.02 mm. Simultaneously, inserts and ejector pins designed in conjunction with the entire mold (ejector pins themselves have auxiliary venting functions), as shown in Figure 6(b), can maximize discharge of gas generated during molten plastic from mold, effectively preventing problems during plastic part filling.

 

Figure 6. 3D schematic diagram of venting system, inserts, and ejector pins
Through analysis of structural characteristics of plastic part and requirements of production process, mold structure has undergone comprehensive and balanced consideration in design of parting surface, gate, side core-pulling mechanism, venting system, ejection mechanism, and cooling system. Mold base adopts a three-plate mold structure with a one-mold-one-cavity layout. A standard I-beam three-plate mold base (700 mm * 750 mm * 576 mm) was modified by adding a mold base balancing device to address excessive eccentricity of nozzle caused by top-side core-pulling mechanism A (S1). Face plate and base plate were each lengthened by 290 mm, and hydraulic cylinder portion of top-side core-pulling mechanism A (S1) was extended by 620 mm. Maximum dimensions of mold are 800 mm * 1660 mm * 576 mm, with a weight of approximately 2.8 t, placing it in medium to large mold category. Mold base balancing device adopts a modular design concept, consisting of multiple components, is adjustable in real-time on-site, enabling dynamic balance control throughout injection molding cycle. Its innovation lies in combining static balance design with a dynamic response mechanism. Mold base balancing device design provides a good initial balance foundation for mold base through careful static design, while relying on a dynamic response mechanism to address potential imbalances under various dynamic operating conditions. Core function is to balance clamping pressure of injection molding machine when mold experiences severe eccentricity, preventing flash defects caused by uneven stress on mold parting surface, and adjusting in real time to resolve flash defects. It also reduces mold costs by eliminating need to enlarge the entire mold set. In addition to special design of mold base balancing device, multiple balancing block structures are set on parting surface PL2, especially at four corners of guide pillars. These blocks are hardened to HRC 50~54° through heat treatment to increase their hardness, disperse clamping pressure on mold core on parting surface PL2, avoid risk of crushing, and ensure smooth venting. Furthermore, to ensure fitting accuracy between fixed and moving molds, a jaw structure is designed on mold core; a parting surface positioning block structure and 0-degree side locking structures are added to mold base. A 3D schematic diagram of mold is shown in Figure 7.

 

Figure 7 3D schematic diagram of mold
Mold working process: (1) Mold closing and machine installation; (2) Injection, pressure holding, and cooling, first molding is completed by injection molding, ensuring that material fully fills cavity and solidifies; (3) Mold opening, parting surface PL1 opens first under action of spring and resin opener, limiting stroke to 30 mm, fixed mold insert comes out, side core pulling mechanism A (S1) moves 10 mm to upper side under action of inclined pin and releases from plastic part; Continue to open mold, resin opener/closer fails, and parting surface PL2 begins to separate until it is fully open, completing mold opening action. Side core-pulling mechanism B (S2) moves 30 mm to ground under action of inclined pin to complete demolding of plastic part undercut. Side core-pulling mechanism A (S1) moves 220 mm to top under action of hydraulic cylinder to separate from plastic part and remove undercut. (4) Ejection: Ejection system completes ejection action, and machine operator or manual can remove part. (5) Mold closing: Before mold closing, hydraulic cylinder of side core-pulling mechanism A (S1) is reset to its initial state. Then injection molding machine completes mold closing and enters next molding cycle. Above actions are repeated for batch production.
In summary, by optimizing mold structure design (such as using a two-parting and two-core-pulling design), mold met mass production requirements in first trial molding. Currently, mold has successfully completed more than 10,000 production tasks without any abnormalities (such as unsmooth core pulling or poor venting). This proves success of design scheme and efficiently meets production needs.

(1) Step-by-step demolding design: For situations where plastic part exerts a large clamping force on mold core or core-pulling mechanism, step-by-step demolding method can effectively reduce deformation of plastic part and improve product quality. Furthermore, this method enhances stability and durability of demolding mechanism, thereby extending service life of mold.
(2) Application of balancing device: In mold structure design process, when encountering problem of excessive eccentricity of nozzle, this paper innovatively designs a modular mold frame balancing device. This device effectively solves problem of mold instability caused by eccentricity, ensuring smooth operation of mold, and improving production efficiency and product quality.
(3) Insert design optimization: For deep reinforcing ribs or irregular ear-shaped structures on plastic part, mold manufacturing process becomes more efficient through optimized insert design. Simultaneously, adding venting structures to inserts, using them in conjunction with all-around venting system on parting surface significantly improves filling efficiency and appearance quality of plastic part. It is worth noting that insert design should comprehensively consider the overall structure of mold, including strength of mold core and ease of later maintenance and repair of cooling water channels.