Development of automotive industry has driven mold industry. Currently, more and more automotive interior parts use plastic parts, and their structures are becoming increasingly complex, making their molding processes more complicated. Under this trend, higher requirements are placed on injection mold design. A certain type of car requires a batch of central control screen frame shells, as shown in Figure 1. Material is ABS. This plastic part has a large external dimension, reaching 495 mm * 146 mm * 30 mm, with many molding features and uneven wall thickness. Inner wall thickness is 1 mm, and outer wall thickness is 2.5 mm. No modifications to plastic part structure are allowed, and surface roughness requirement is high, with no deformation, scratches, or flash allowed.

 

Figure 1 Central Control Screen Frame Shell

The overall accuracy of central control screen frame shell is ±0.1 mm. Although its accuracy is not high, its large external dimensions and complex features present certain difficulties in both injection molding and demolding. Plastic part has a frame structure with upper and lower sections. Undercut portion of feature I in Figure 1 is one of difficulties in molding. Feature II requires an external core-pulling structure. Feature III consists of two small undercuts forming a snap-fit mechanism, with many reinforcing ribs nearby, and is not on outer side, requiring an internal core-pulling structure. Feature IV is on outermost side of plastic part and also requires an external core-pulling structure.
In summary, features II and IV are molded using a standard external core-pulling structure, feature III is molded using an internal core-pulling structure, while feature I, due to its undercuts and limited space, requires a specially designed external core-pulling structure.

Molding scheme for plastic part is shown in Figure 2. Thickened lines around edges are parting lines, and the entire part is demolded along ±Z direction. Feature I is core-pulled along +Y direction; Feature II is core-pulled along ±X directions respectively; Feature III is restricted by wall edge of plastic part to be molded on both sides of X direction, so core-pulling is chosen along ±Y direction; two Features IV are 168 mm apart, and using a one-to-two side core-pulling structure would result in a large slider size and reduced accuracy, so each side core-pulling slider is used to achieve individual core-pulling, both along -Y direction.

 

Figure 2 Molding Scheme

Molding parts of mold core are shown in Figure 3. Due to numerous and small features on lower surface of plastic part, molding parts are complex. An interlocking structure is adopted, and the overall structure of plastic part is symmetrical. Interlocking core reduces processing difficulty of molding parts, and molding parts are easily replaced if damaged due to long-term contact with high-temperature and high-pressure melt. Upper surface structure of plastic part is simple, without small features, and does not require ejection. Cavity plate adopts an integral structure, which also reduces assembly difficulty.

 

Figure 3. Core Molding Parts
1. Wedge Block 2. Core Wedge Block 3. Ejector Block 4. Tapered Jaw Block 5. Ejector Pin 6. Ejector Pin 7. Square Ejector Pin 8. Throttling Block 9. Edge Ejector Pin 10. Square Ejector Pin
Due to large mold size, in actual production, mold is installed on a horizontal injection molding machine. Under its own weight, core and cavity plate are prone to vertical misalignment during mold closing. Therefore, a tapered jaw block 4 is designed on core. During mold closing, tapered surfaces of core and cavity plate fit tightly together, preventing misalignment. To further ensure accuracy of molded plastic part, five wedge blocks 1 are designed on core fixing plate. These blocks have high installation accuracy and are positioned at the bottom using pins. During mold closing, tapered surfaces of core fixing plate and cavity plate fixing plate fit tightly together, preventing misalignment between moving mold and fixed mold, and serving as a secondary positioning function. To prevent core displacement due to high pressure during filling stage, which could cause deviations in wall thickness of molded plastic part, four inclined core wedge blocks 2 were designed. Adjusting fixing screws on wedge blocks ensures core reference surface is tightly fitted to core fixing plate, guaranteeing a secure hold. Because plastic part has a thin wall thickness (1 mm on inner side and 2.5 mm on outer side), shrinkage force after cooling is significant, making it difficult to eject part effectively using only push rods. Therefore, four ejection blocks 3 are symmetrically placed on outer side of molded part, with two push rods connected to push plate at the bottom to eject part. To avoid interference with edge push rods 9, appropriate clearance was provided. Both core wedge blocks 2 and ejection blocks 3 have threaded process holes for easy disassembly. Push rods 6, square push rods 7 and 10, and edge push rods 9 are evenly distributed throughout the entire annular shape of molded part, serving both molding and ejection functions. Since push rod 6 is machined from a round shape, but molding part is curved, its bottom requires flattening for positioning.

As shown in Figure 1, several features of plastic part require side core pulling. Feature IV requires a core pulling distance of 9.5 mm. Slanted guide post side core pulling structure shown in Figure 4 meets this requirement. Core and slider are connected by simple pins. Positioning steel balls limit slider's stroke. A circular arc notch corresponding to stroke is made at the bottom of slider for secondary positioning. There is some interference between reset rod 2 and slider pressure bar; therefore, pressure bar is designed to create a clearance. To prevent interference between slanted guide post and core fixing plate, clearance grooves are machined on core fixing plate. This slanted guide post is φ16 mm with an inclination angle of 18°, achieving a core pulling distance of 12.5 mm during mold opening, with a 3 mm safety distance that will not affect subsequent demolding.

 

Figure 4. Feature IV Side Core Pulling Mechanism
1. Connecting Pin 2. Reset Rod 3. Positioning Steel Ball 4. Limiting Block
In Figure 1, Feature II has a shorter core pulling distance and area than Feature IV, with a distance of 4.5 mm. It employs a different inclined guide post side core pulling mechanism than Figure 4, as shown in Figure 5. This side core pulling mechanism has a smaller slider and a simpler structure. Its wear-resistant clamping block is fixed to fixed mold side. Inclined guide post is shorter, suitable for short-distance core pulling. Inclined guide post dimensions are φ16 mm * 77 mm, with an inclination angle of 18°. It can achieve an 8 mm core pulling distance during mold opening, with a 3.5 mm safety distance that will not affect subsequent demolding.

 

Figure 5. Side core-pulling mechanism for features II and III
1. Angled push rod 2. Slider base 3. Angled push rod slider
In Figure 1, space for snap-in of feature III is limited, with a side core-pulling distance of only 1 mm. A half-structure angled push rod side core-pulling mechanism as shown in Figure 5(c) is used. Angled push rod 1 serves both forming and ejection functions; slider base 2 is relatively tall, effectively reducing length of angled push rod and increasing rigidity; angled push rod slider 3 allows angled push rod to move upwards, left and right simultaneously, completing side core-pulling of snap-in. Angled push rod has an inclination angle of 3°, and slider base moves upwards by 30 mm during mold opening, allowing angled push rod to complete a unidirectional core-pulling distance of 1.57 mm. A safety distance of 0.57 mm is maintained, ensuring normal subsequent demolding.
In Figure 1, feature I is an undercut structure. Side core-pulling distance is a challenge in mold design, with a vertical core-pulling distance of 1.8 mm and a side core-pulling distance of 7.2 mm. Figure 6(a) shows the overall structure of this feature molding. It uses a slanted guide post side core-pulling method. Complex undercut structure results in a long slider movement distance; therefore, a 20 mm slanted guide post with a length of 216 mm and an inclination angle of 18° is used, and a clearance groove is designed for slanted guide post. Figure 6(b) shows internal structure and connection structure of slider and side core. Core is divided into two parts: slider core 5 and core 8 can move relative to each other. Two T-slots are used to ensure movement accuracy, with T-slots inclined at 12°. Core 8 and slider 10 are connected and fastened by screws 9. Bottom of slider core push plate 6 has two positioning slots that mate with positioning block shown in Figure 6(c). Two springs are installed at the bottom between slider core push plate 6 and slider 10, allowing them to be separated during mold opening. Top is a wear-resistant block 7, positioned by pins 3 and fastened with screws. Screws 1 pass through slider core push plate 6 and fix it to slider 10. Guide posts 2 are fixed to slider core push plate 6, serving a guiding function. Specific movement is shown in Figure 6(d). During mold opening, under action of inclined guide pillar, slider 10 drives core 8 to move to the right. At this time, slider core push plate 6 is fixed in place under action of positioning groove. When movement reaches a stroke of 15 mm, slider core 5 has moved downwards by 3.2 mm, which is greater than undercut of plastic part by 1.8 mm. Continuing to open mold, slider core push plate 6 disengages from its limiting position, pushing slider core 5 to the right until it stops at a core-pulling distance of 24 mm. This structure achieves secondary delayed core-pulling of undercut feature I in Figure 1, which first moves downwards by 3.2 mm and then to the right by 9 mm.

 

Figure 6. Delayed Side Core Pulling Structure
1. Screw 2. Guide Post 3. Pin 4. Screw 5. Slider Core 6. Slider Core Push Plate 7. Wear-resistant Block 8. Core 9. Screw 10. Slider

Due to large size of plastic part, to improve molding efficiency and based on appearance requirements of plastic part, a hot runner + multi-point gate gating system is adopted, using a branch runner method. Inner edge of plastic part to be molded is fed through 8 side gates. Using side gates can reduce post-processing steps of molded plastic part. Runner and side gate dimensions are shown in Figure 7. Advantages of this gating system: ① Because hot runner maintains a constant temperature, no condensed material is generated, saving material; ② Gate mark on the surface of plastic part is small, which can ensure appearance quality; ③ Hot runner has a higher filling efficiency than ordinary runner.

 

Figure 7. Gating System.
Size of runner can be adjusted using throttling block shown in Figure 3 to control filling speed and time. Moldflow simulation shows that injection pressure of hot runner + multi-point gating scheme is 71 MPa, and filling time is 2.4 s. Predicted warpage of main body of plastic part is within 1.0 mm, with small weld lines and good quality, as shown in Figure 8. Venting design is needed at locations prone to air entrapment; this mold mainly uses inserts for venting.

 

Figure 8. Plastic Part Quality.

Plastic part has a small height difference and a long length. Cooling water channels should be arranged as evenly as possible. Fixed mold adopts a cooling strategy with an additional water well in the middle. Due to large number of ejector pins in moving mold, it is not possible to effectively install water wells. Analysis shows that 12 sets of cooling water channels were designed, all with a size of φ12 mm, as shown in Figure 9. Due to small size of side core-pulling mechanism and small height of plastic part, no water channels were arranged on slider.

 

Figure 9. Cooling Water Channels
Channels 2-7 are located on fixed mold cavity plate, primarily cooling interior of cavity. This area is near hot runner gate, where heat is concentrated and difficult to dissipate. A φ18.5 mm water well and baffle plate are used in the middle to improve cooling efficiency. For channels 3 and 6, to avoid interference with hot runner, they are designed as a ring, with sealing rings installed at both top and bottom. Channel 12 serves to cool fixed hot runner plate. Channels 9-11 are arranged on moving mold core, following shape of plastic part as much as possible to ensure uniform cooling. Mold temperature is set to 60 ℃, and inlet water temperature is set to 25 ℃. Some channels are simplified and imported into Moldflow. Coolant temperatures are shown in Table 1.
Table 1. Coolant Temperatures

 

Analysis results show that temperature difference between mold inlet and outlet water is controlled within 3 ℃, which will not cause molded plastic part to shrink too quickly. Cooling water channel design is relatively reasonable.

Bottom of plastic part is flat, and area available for ejector pins is limited. Flat, round ejector pins 3 are symmetrically arranged along inner side of long side of plastic part. Most of remaining ejector pins serve both molding and ejecting functions. These ejector pins need to be able to be positioned correctly. To ensure even force distribution on plastic part during ejection and avoid damage, ejector pins are best arranged symmetrically. Ejector pins 2 at edge are symmetrically arranged along middle of long side of plastic part; square ejector pins 5 are located on short side of plastic part; top-rounded ejector pins 6 are present on both sides; ejection block 1 connects to flat ejector pins 14 at the bottom. Ejection mechanism is shown in Figure 10.

 

Figure 10 Ejection Mechanism
1. Ejection Block 2. Edge Ejector Rod 3. Flat Round Ejector Rod 4. Ejector Rod 5. Square Ejector Rod 6. Upper and Lower Round Ejector Rod 7. Limiting Post 8. Ejector Plate 9. Limiting Pin 10. Lifter Rod 11. Dust Cover 12. Support Post 13. Reset Rod 14. Flat Ejector Rod
During mold opening, driven by injection molding machine slider, moving mold and fixed mold open by 80 mm. Subsequently, ejector plate drives various types of ejector rods to move, traveling 30 mm under action of limiting post. At the same time, half-type lifter rod completes a bidirectional 3.14 mm side core pulling, which is greater than minimum core pulling distance of 2 mm for plastic part clips, enabling subsequent demolding.