In injection molding, plastic parts with complex geometric features such as side holes, bosses, bends often require a side core-pulling mechanism for demolding. Side core-pulling mechanism typically uses inclined guide pillars driven by cylinders or hydraulic cylinders to complete lateral movement of slider. Side core-pulling mechanism can be located on either moving mold side or fixed mold side. Since moving mold side does not need to support gating system and has sufficient structural space, it is more suitable for arranging side core-pulling mechanism. In contrast, fixed mold side, constrained by spatial limitations of gating system layout, usually cannot accommodate complex lateral movement mechanisms. Based on structural features of a curved spout funnel plastic part, this paper innovatively designs a side-gate cold runner mold with fixed mold core-pulling and secondary parting. This mold has a compact structure, reduces manufacturing costs, has been verified in production to have certain practical application value.

Molded plastic part is a funnel with a curved spout, as shown in Figure 1. Curved end of funnel forms a 30° angle with centerline. Three 0.5 mm high annular bosses are evenly distributed on straight wall of large end entrance to prevent shrinkage deformation. A 5 mm high annular T-shaped reinforcing rib is added to bottom edge. Maximum diameter of large end of plastic part is 60 mm, and funnel wall thickness is 1 mm.

 

Figure 1 Funnel Plastic Part Diagram
Material is polypropylene (PP), with a shrinkage rate of 1.8%. It has good heat resistance, is lightweight, has good toughness, and is low in cost.
In mold design, parting surface is selected at location with the largest projected area in mold opening direction.

Based on structural analysis of plastic part, mold adopts a half-block structure to form main shape of plastic part. Inclined slider mechanism, along with concave mold 2 and fixed mold plate, constitutes concave mold. Inner hole uses an inclined core-pulling design, and bottom flange of plastic part is formed by an ejector plate, as shown in Figures 2 and 3.

 

Figure 2: Angled core-pulling structure for first parting

 

Figure 3: Half-type structure for second parting
If an angled guide pillar structure is used for molding of curved nozzle, mold structure will be more complex and mold size will increase. Mechanism optimization is achieved by using an angled slider side core-pulling mechanism. Side wall of angled slider has a straight guide rail that cooperates with a straight groove on fixed mold plate, controlling core-pulling stroke within mold's internal space, improving stability of mechanism's movement. This simplifies side-pulling mechanism. Curved nozzle core is fixed to slider with a pin. A T-slot is provided on the top of slider, and a T-block cooperates with T-slot. T-block is fixed to fixed mold base plate with screws. As mold opens, slider moves relative to fixed mold plate through T-block guide. Slider and curved nozzle core move obliquely upwards to pull out plastic part, completing fixed mold core-pulling movement, as shown in Figure 2. Cavity mold 2 is fixed to fixed mold platen by screws. Cavity mold 2 and fixed mold platen together form another part of outer contour of plastic part. As injection molding machine opens, spacer screws drive fixed mold platen, causing fixed mold platen, cavity mold 2, and plastic part to separate, completing demolding of plastic part from cavity mold, as shown in Figure 3.
Gating system uses a cold runner system, and runner layout is shown in Figure 4. Theoretically, main runner should be symmetrically arranged along central axis of mold body to reduce pressure imbalance and ensure uniform filling of cavity by molten plastic. Because cavity mold uses a half-gating structure, center of mold body coincides with half-gating block joint, which is not conducive to fit between main runner and fixed mold platen. Therefore, main runner is positioned radially offset on cavity mold 2, as shown in Figure 4, using a side gate for feeding. Pull rods are provided at the bottom of main runner and sub-runners to keep solidified material in moving mold during mold opening. There is relative movement between main runner and mold platen. To reduce friction and improve fit accuracy of movement, a tapered structure optimization design was implemented at the bottom of main runner.

 

Figure 4 Gating System
To achieve molding of inner hole and irregular outer contour of plastic part, this mold adopts a structural scheme combining oblique core pulling and two-stage parting, as shown in Figure 5. Considering that gating system is a cold runner side gate, a three-plate mold (a three-plate mold with an ejector plate) is selected for mold base. Sequence control of parting process is achieved through combined action of a fixed-distance spring, damper, and fixed-distance screw. In this fixed-mold core pulling structure, parting spring first provides initial driving force, and damper controls parting sequence, causing mold to part along PL1 parting surface, completing oblique core pulling of inner hole of funnel-shaped nozzle and one side of half-structure. After being fixed by fixed-distance screw, as mold continues to open, mold parts from PL2 parting surface, completing demolding of other side of half-structure cavity and plastic part.

 

Figure 5 Mold Structure Diagram
During injection molding process, cooling time usually accounts for 50% to 80% of the entire production cycle. By optimizing cooling circuit, cooling time can be significantly shortened, thereby improving production efficiency. Uniform cooling reduces stress concentration within product, preventing warping, ensuring dimensional stability and surface quality. Forming die employs a split structure, creating a separate cooling circuit, as shown in Figure 6. Separate cooling circuits are installed on slider and die 2. A water well-type cooling device is installed inside core.

 

Figure 6 Cooling Circuit Layout
Plastic part is ejected by an ejector plate. This ejection leaves no marks on plastic part, resulting in good surface quality. Ejector plate also functions as a cavity forming device; its inner edge contour mates with mold core to form forming area of plastic part's flange structure. Ejector plate is connected to ejector rod by screws, ejector rod is fixed by an ejector rod fixing plate and ejector plate, as shown in Figure 7.

 

Figure 7 Demolding Mechanism
The overall mold design considers multiple objectives, including mold function, cost control, and manufacturability. To improve molding quality and system stability, following details were carefully considered during design process:
(1) When PL1 parting surface separates, main runner needs to pass through fixed mold plate. To reduce friction, lower end of main runner is designed as a cone.
(2) During injection molding, to prevent overflow and flash from occurring on PL2 main parting surface due to uneven injection pressure, six balancing blocks are symmetrically arranged on opposite sides of fixed mold plate at PL2 parting surface, as shown in Figure 8.

 

Figure 8: Balancing block layout
(3) To prevent deformation of mold plate due to injection pressure, which would affect molding quality, two support pillars are set at corresponding positions on moving mold base plate and cavity projection area.
(4) For mold venting, in addition to venting through core, a 0.02 mm deep venting groove around core is set on working surface of ejector plate, as shown in Figure 9, to improve molding quality of plastic part.

 

Figure 9. Layout of exhaust groove on push plate
(5) Upper end of main core has a certain curvature. Since core is a rotating body, its position is easily rotated during assembly. Therefore, an asymmetric boss positioning method is adopted. One side of boss position is milled off to improve positioning accuracy of core, as shown in Figure 10.

 

Figure 10. Fixing form of core and push rod
(6) In addition to pushing out plastic part, push plate also serves as a reinforcing rib for bottom flange of molded plastic part. To improve positioning accuracy of push plate, an anti-rotation structure is set at step position of push rod, and circumferential constraint is achieved through D-shaped cross-section fitting, as shown in Figure 10.
After pressure holding and cooling, injection molding machine is opened. Under action of parting spring and damper, mold opens from PL1, as shown in Figure 11. Slider and main runner separate from fixed mold plate along with fixed mold base plate. Due to action of pull rod 1 and pull rod 2, solidified material of main runner and sub-runner separates from main runner, remains in moving mold part. After mold opens to distance screw and fixed mold plate contact limit, it opens from parting surface PL2. Fixed mold plate stops moving and drives cavity mold 2 to separate from plastic part. Plastic part remains on moving mold core. Ejection mechanism of injection molding machine pushes ejector plate through push rod assembly to demold plastic part. Mold closes, and under action of return spring, it drives return rod to return to original position. One injection cycle is completed.

 

Figure 11 Mold cross-section

(1) A two-stage parting spacing mechanism is adopted, and cavity mold half-structure achieves graded demolding. During the first parting, a portion of cavity mold half-structure demolds along with fixed mold base plate and main runner. When limiting device triggers second parting, the other portion of half-structure demolds along with fixed mold plate, completing extraction of the entire plastic part from cavity mold.
(2) In fixed mold core pulling process, inclined slider is connected to fixed mold base plate via a T-shaped slider. With mold opening motion, inclined slider's side core pulling is achieved. This side core pulling feature within mold body simplifies mold structure.
(3) Inclusion of a balance block and support column structure improves injection molding accuracy. Anti-rotation structure of core and ejector pins prevents rotational offset, improving molding accuracy.
(4) Production verification shows that mold operates reliably, and this design provides valuable reference for similar injection mold structures.