As an engineer specializing in injection mold design for EV cars, I recently developed a large, precision mold for producing inner brackets of rearview mirrors in electric vehicles. The EV car industry demands high-performance components with strict dimensional accuracy, and this project addressed unique challenges such as complex geometry, large lateral undercuts, and the need for efficient production cycles. The inner bracket, a critical structural part in EV cars, requires MT2 precision according to GB/T14486-2008, and it is molded from polypropylene with 45% glass fiber reinforcement (PP/45% GF), which offers excellent dimensional stability but introduces issues like high shrinkage and abrasiveness. This article details my design approach, incorporating innovations like a fixed-mold spring block mechanism, conformal cooling channels, and enhanced side core-pulling safety distances to overcome common failures in similar molds for EV cars.
In the initial phase of designing the injection mold for EV car rearview mirror inner brackets, I analyzed the part’s structure to identify key requirements. The bracket features multiple assembly posts, reinforcing ribs, and two inclined undercuts, which complicate demolding and increase the risk of defects. For EV cars, weight reduction and durability are paramount, so the material PP/45% GF was selected, with a shrinkage rate of 0.5%. This shrinkage must be accounted for in mold dimensions to achieve the required precision. The mold is designed for a two-cavity layout to produce left and right brackets simultaneously, optimizing production efficiency for high-volume EV car manufacturing. To quantify the material behavior, I used the following shrinkage formula: $$ S = S_0 \times (1 + k \cdot GF) $$ where \( S \) is the effective shrinkage, \( S_0 \) is the base shrinkage of PP (approximately 1.5%), \( k \) is a correction factor (0.2 for GF), and \( GF \) is the glass fiber content (0.45). This calculation helped ensure dimensional accuracy for EV car components.
The gating system was designed using a conventional runner with a tapered side gate to facilitate smooth melt flow and minimize pressure loss. For EV car parts, which often have thin walls and complex features, this approach reduces shear stress and prevents material degradation. The runner cross-section is U-shaped to enhance flow efficiency, and the gate dimensions were optimized based on mold flow simulations. Below is a table summarizing key parameters of the gating system for the EV car bracket mold:
| Parameter | Value | Description |
|---|---|---|
| Runner Type | U-shaped | Minimizes flow resistance for PP/45% GF |
| Gate Type | Tapered Side Gate | Reduces jetting and ensures uniform filling |
| Gate Width | 6 mm | Balances fill time and gate vestige |
| Runner Diameter | 8 mm | Optimized for two-cavity layout in EV car molds |
Moving to the mold’s forming components, I employed a split insert structure for both the moving and fixed halves to simplify manufacturing and maintenance. The fixed mold insert is made of NAK80 steel (hardness 37-43 HRC) for wear resistance against the abrasive GF-filled material, while the moving mold insert uses P20 steel for cost-effectiveness. To address the issue of the part sticking to the fixed cavity—a common problem in EV car mold designs—I integrated a fixed-mold spring block mechanism. This innovation ensures that during mold opening, the spring block ejects the part slightly, preventing adhesion and distortion. The mechanism’s effectiveness is quantified by the ejection force formula: $$ F_e = \mu \cdot P \cdot A $$ where \( F_e \) is the ejection force, \( \mu \) is the coefficient of friction (0.3 for PP/GF), \( P \) is the residual pressure, and \( A \) is the contact area. By minimizing \( F_e \), this design enhances reliability for high-volume EV car production.
The guidance and positioning system of the mold incorporates conical interlocking surfaces with a 5° taper on both moving and fixed inserts to counteract lateral forces during injection. This is crucial for EV car molds, which experience high clamping pressures. Four guide pins (ϕ50 mm × 330 mm) and bushings ensure accurate alignment, while additional pins on the ejector plate maintain stability during part ejection. The positioning system’s stiffness can be modeled using: $$ K = \frac{E \cdot A}{L} $$ where \( K \) is the stiffness, \( E \) is the modulus of elasticity of steel (210 GPa), \( A \) is the cross-sectional area, and \( L \) is the length. This design guarantees the mold’s longevity, supporting over 2 million cycles for EV car component manufacturing.
For the side core-pulling mechanisms, I designed two inclined sliders to handle the undercuts S1 and S2, which are angled at 95° to the mold opening direction. This non-standard angle required custom slider paths to avoid interference. In previous EV car molds, insufficient safety distances led to part-slider collisions during demolding. To resolve this, I increased the safety distance for slider S2 to 35 mm, resulting in a total core-pulling distance of 45 mm. The core-pulling force can be estimated with: $$ F_c = \sigma \cdot A_c \cdot \tan(\theta) $$ where \( F_c \) is the core-pulling force, \( \sigma \) is the material stress, \( A_c \) is the undercut area, and \( \theta \) is the angle. External blocks and springs were used for slider positioning, reducing mold size and weight—a benefit for EV cars where efficiency is key. The table below compares the slider parameters:
| Slider | Core-Pulling Distance (mm) | Safety Distance (mm) | Angle (°) |
|---|---|---|---|
| S1 | 62 | 27 | 95 |
| S2 | 45 | 35 | 95 |
The temperature control system was designed with conformal cooling channels to achieve uniform cooling and reduce cycle times—a critical factor for EV car mass production. I implemented 9 cooling circuits: 2 for the fixed mold inserts and 7 for the moving side, including 4 baffle-cooled water wells for intense cooling around core features. The cooling time can be approximated by: $$ t_c = \frac{h^2}{\pi \cdot \alpha} \ln\left(\frac{4}{\pi} \cdot \frac{T_m – T_w}{T_e – T_w}\right) $$ where \( t_c \) is the cooling time, \( h \) is the part thickness, \( \alpha \) is the thermal diffusivity, \( T_m \) is the melt temperature, \( T_w \) is the coolant temperature, and \( T_e \) is the ejection temperature. By maintaining a mold temperature differential below 10°C, this system eliminates weld lines and warpage, ensuring MT2 precision for EV car brackets. The conformal channels follow the part geometry closely, enhancing heat exchange efficiency.
Venting is another critical aspect, as trapped air can cause burns and defects in EV car components. I placed 8 venting slots per cavity on the parting surface, with a primary slot depth of 0.05 mm, optimized for PP/GF’s flow characteristics. The venting capacity is given by: $$ Q_v = A_v \cdot v \cdot P $$ where \( Q_v \) is the venting flow rate, \( A_v \) is the vent area, \( v \) is the air velocity, and \( P \) is the pressure. Additional vents on ejector pins and sleeves further improve air escape, contributing to a defect-free surface finish essential for EV car interiors.
The mold’s operation sequence begins with melt injection through the sprue into the runners and gates, filling the cavities. After cooling and solidification, the mold opens, and the fixed-mold spring block activates to prevent part adhesion. The sliders S1 and S2 retract via angled guide pins, with distances controlled by external stops. The ejection system then pushes the parts off the cores using pins and sleeves, facilitated by hydraulic cylinders. This process achieves a cycle time of 30 seconds, a 10% reduction compared to conventional molds for EV cars, boosting productivity. The overall mold dimensions are 750 mm × 500 mm × 540.5 mm, classifying it as a large-scale tool suitable for EV car assembly lines.
In testing, the mold achieved first-trial success, with produced brackets meeting all specifications for EV cars. The innovations in spring blocks, cooling, and slider safety eliminated common failures, resulting in a robust design. For future EV car projects, this approach can be adapted to other complex parts, emphasizing precision and efficiency. The integration of these elements underscores the importance of tailored solutions in the rapidly evolving EV car industry, where component reliability directly impacts vehicle safety and performance.