For cylindrical plastic parts with circumferentially distributed side holes, if side holes have undercut structures that cannot be resolved by forced demolding, a half-parting structure is usually required to achieve circumferential molding. When axial dimension of plastic part is large, if a traditional axial demolding method is used (i.e., axial direction of plastic part is consistent with opening and closing direction of injection molding machine), mold closing height will increase significantly, and mold opening stroke will also increase, thus requiring a larger tonnage injection molding machine. This not only increases equipment costs but also reduces production economy. If a layout scheme in which axial direction of plastic part is perpendicular to opening and closing direction of injection molding machine is adopted, this scheme can effectively reduce mold thickness. However, if a conventional side-pulling core mechanism is used, side-pulling distance will increase with length of plastic part. A more reasonable solution is to use a side-ejection mechanism instead of traditional core-pulling structure, so that plastic part is demolded along side. This improvement not only significantly shortens mold opening stroke but also reduces complexity of mold structure, making it more economical and feasible in practical engineering applications.

Elongated basket-shaped plastic part is shown in Figure 1. It is produced in large batches, made of polypropylene (PP), with a shrinkage rate of 2% and good toughness. Part is a cylindrical basket-like component with circumferentially distributed side holes. Outer diameter of large end is 30 mm, diameter of small end is 22 mm, there is a 2 mm groove at the bottom of small end, rib thickness is 2 mm, and total length of part is 205 mm, with a relatively large axial dimension. Part structure ensures structural rigidity while also considering demolding processability.

 

Figure 1 Part Diagram

Through a comprehensive analysis of structural characteristics of plastic part and molding process requirements, elongated basket injection mold structure shown in Figure 2 was designed. This plastic part has a circumferentially distributed undercut structure with side holes, and measured undercut depth is 2 mm. According to injection molding theory, when undercut depth does not exceed 5% of undercut dimension (diameter), a forced demolding process can be used. Undercut dimension of this plastic part is 30 mm, and calculated allowable critical demolding depth is 1.5 mm (30 * 5%). Since actual undercut depth (2 mm) exceeds this critical value, it does not meet process conditions for forced demolding.

 

Figure 2 3D model of mold
Given this structural characteristic, mold cavity needs to adopt a half-parting structure. Two possible half-parting structure design schemes are compared and analyzed below:
Scheme 1: Half-parting direction is perpendicular to mold opening direction. In this scheme, axial direction of plastic part is consistent with mold opening direction. However, due to large height of plastic part, mold closing height will increase significantly, requiring a larger tonnage injection molding machine. Considering equipment investment and production costs, this scheme is less economical.
Scheme 2: Half-parting direction is parallel to mold opening direction. This scheme requires a side core-pulling mechanism to achieve core demolding. However, analysis shows that core length is large, and required core-pulling stroke increases accordingly. Even with a hydraulic cylinder-driven side core-pulling mechanism, excessive core-pulling stroke still complicates mold structure and significantly prolongs molding cycle, impacting production efficiency.
After a comprehensive evaluation of above solutions' process feasibility and economics, a design scheme using a movable core and a side ejection mechanism was ultimately selected: plastic part is demolded via side ejection mechanism, then a robotic arm removes part. This scheme effectively optimizes mold structure complexity, production costs while ensuring molding quality.
Considering relatively large size of plastic part (Φ30 mm * 205 mm), hot runner gating was adopted to improve melt flow properties and enhance molding quality. This effectively reduces surface defects such as flow marks or shrinkage marks and shortens molding cycle. As shown in Figure 3, a balanced injection method with 5 hot nozzles is used, resulting in uniform pressure distribution at each gate and reducing injection pressure. To reduce impact of hot melt plastic on cavity and core, gates are located at rib positions of plastic part, and rib structure enhances melt guidance.

 

Figure 3 Hot runner
Structural design of molding part is shown in Figure 4. Die adopts a half-type parting structure, as shown in Figures 4(a) and 4(b), consisting of a fixed die assembly and a moving die assembly. Considering large size of die structure (length L>205 mm), a split design is adopted to reduce processing difficulty, ensure processing accuracy, and facilitate heat treatment. Split die assembly is positioned by a high-precision connecting block and fastened with hexagonal head screws. Finally, moving and fixed die cavities are fixed to moving and fixed die base plates by countersunk screws. Core assembly adopts a modular design, as shown in Figure 4(c). Core and fixed block are precisely positioned by a riding block, and circumferential positioning is achieved by a cylindrical pin. Riding block is fixed to fixed block with screws. For groove structure at the bottom of plastic part basket, a side core-pulling mechanism is designed. Slider driven by inclined guide post forms this feature to ensure smooth demolding, as shown in Figure 4(d). This structural design fully considers factors such as processing capability, assembly accuracy requirements, mold reliability.

 

Figure 4 Molded Part
Demolding process begins with small-end push plate and large-end push plate simultaneously lifting movable core assembly and plastic part. Then, demolding hydraulic cylinder pushes plastic part out.
Small-end push plate mechanism is shown in Figure 5. It is located in a groove between moving mold base plate and moving mold cavity. Space between push plate 1 and moving mold cavity is ejection distance. This mechanism is powered by two symmetrically arranged hydraulic cylinders, which are rigidly connected to push plate 1 by bolts. Guiding system consists of four guide pillars fixed to moving mold cavity, guide sleeves pressed onto push plate 1, forming a high-precision sliding pair. Support pillars are fastened to moving mold cavity by hexagonal socket head cap screws, and push rod is fixed to push plate 1 by screws using a pressure block.

 

Figure 5 Small-End Push Plate
Large-end push plate mechanism is shown in Figure 6. Mechanism for lifting movable core assembly is powered by two symmetrically arranged hydraulic cylinders, which are rigidly connected to push plate 2 by bolts. Ejector plate 2 is connected to fixed block by screws, as shown in Figure 6(b). Guiding system consists of four guide pillars fixed to moving mold base plate and guide sleeves pressed onto fixed block.

 

Figure 6 Large-end ejector plate and demolding mechanism.
Demolding hydraulic cylinder is fixed to fixed block via a bracket, as shown in Figure 6. Piston rod of demolding hydraulic cylinder is connected to ejector plate via a T-slot. Ejector plate is connected to plastic part ejector plate via four connecting rods. Upper two connecting rods pass through riding-shaped pressure block, and lower two connecting rods pass through fixed block for guidance.
As shown in Figure 1, mold closing guide between moving mold and fixed mold adopts a guide pillar guiding mechanism arranged symmetrically at four corners. To further improve mold closing positioning accuracy and prevent lateral displacement of mold during injection molding, a tiger-mouth positioning structure is added at four corners of cavity mold, specific structure of which is shown in Figures 4(a) and (b). For guiding system of small-end ejector plate 1, a precision guiding method using guide pillars and guide sleeves is adopted (Figure 5). Considering ejection space between small-end ejector plate and moving mold cavity, which keeps moving mold cavity suspended at this position, two high-strength support pillars are added to effectively suppress elastic deformation of mold plate caused by injection pressure. Ejection movement of large-end movable core assembly is precisely guided by four large-diameter guide pillars and guide sleeves (Figure 6). To ensure accurate positioning of movable core assembly during mold closing, two pressure pillars are installed on fixed mold base plate for radial positioning, while a wedge block mechanism is used for reliable axial locking, as shown in Figure 1.
To address problem of poor venting caused by large length of molded part, and to effectively eliminate defects such as bubbles, scorching, and incomplete filling caused by gas retention during injection molding, this study designed 16 sets of 6 mm diameter vent pillars at the bottom of moving mold cavity, along with a venting groove structure with a 0.04 mm gap. This venting system uses a straight-through design to directly discharge gas outside mold body (Figure 1). Experimental results show that this design significantly improves venting efficiency, ensuring dimensional stability and surface quality of product.
Cooling time accounts for 50% to 70% of the entire injection molding cycle. Efficient cooling can significantly shorten molding cycle. Maintaining uniform mold temperature reduces problems such as warpage and shrinkage marks, improves dimensional accuracy and surface quality. Cooling system design in this study features eight independent cooling channels evenly distributed throughout each component cavity, following shape of plastic part. Two independent loop-shaped water channels are also installed inside movable core, as shown in Figure 7. This symmetrical and uniform water channel layout avoids localized overheating or overcooling.

 

Figure 7 Cooling Channels
After injection molding, following pressure holding and cooling, moving and fixed molds open. During mold opening process, inclined guide pillars drive slider to complete core-pulling action of bottom concave side of plastic part. As shown in Figure 8, demolding process is divided into two stages: In the first stage, large-end hydraulic cylinder and small-end hydraulic cylinder are started simultaneously. Piston rods of hydraulic cylinders push support block 1 and support block 2 respectively, lifting movable core assembly and plastic part. During this process, small-end hydraulic cylinder plays an auxiliary ejection role. Piston rod stroke of small-end hydraulic cylinder is 8 mm (Figure 9), completing demolding of side hole core of plastic part on moving mold cavity side (core height 2 mm); large-end hydraulic cylinder continues to lift movable core assembly to a safe height (lower edge of plastic part push plate exceeds height of moving mold cavity and slider). In second stage, demolding hydraulic cylinder drives piston rod to push push plate, which in turn drives plastic part push plate to push plastic part, causing plastic part to pre-separate from core. Finally, robot arm completes part removal. Mold closing process: First, piston rod of demolding hydraulic cylinder is driven to reset, which drives push plate and plastic part push plate to reset. Then, piston rods of large-end hydraulic cylinder and small-end hydraulic cylinder are driven to reset. Large-end push plate drives movable core assembly to reset, and at the same time, small-end push plate drives push rod to reset. Moving and fixed molds close together. Two pressure pillars fixed on fixed mold platen radially position movable core, while wedge blocks fixed on fixed mold platen axially lock movable core (as shown in Figure 1). This multi-stage demolding system effectively solves demolding problem of long cores through timing control of hydraulic actuators.

 

Figure 8 Ejection Mechanism Diagram

 

Figure 9 Small End Ejector Plate 1 Ejection Distance

(1) Unconventional layout of movable core combined with side ejection mechanism (plastic part axis is perpendicular to mold opening direction) significantly reduces mold closing height compared to axial demolding scheme;
(2) Hydraulically driven multi-stage ejection (small end ejector plate + large end core component ejector plate + plastic part ejector plate) achieves precise demolding of plastic part through timing control, greatly shortening stroke of core-pulling mechanism;
(3) Symmetrical 5-point gate setting of hot runner at rib position effectively reduces occurrence of spray marks and shortens molding cycle; layout of cooling and venting devices helps to reduce molding time and improve surface quality;
Through innovative layout of demolding direction and synergistic design of composite ejection mechanism, limitations of traditional injection molding processes on aspect ratio plastic parts can be broken, providing a cost-effective solution for similar structural products.