In the development of electric vehicles, the EV battery pack is a critical component that directly impacts safety, performance, and range. As an engineer focused on lightweight design, I have investigated the use of composite materials to replace metals in the upper cover of the EV battery pack, aiming to reduce weight while maintaining structural integrity. The molding process parameters play a pivotal role in determining the quality of composite parts, and this study delves into optimizing these parameters for the EV battery pack upper cover through experimental and simulation approaches. The goal is to establish a robust manufacturing process that ensures high performance and reliability for EV battery pack applications.
Composite materials, particularly thermoset glass fiber prepregs, offer excellent strength-to-weight ratios, making them ideal for EV battery pack components. However, the hot compression molding process involves complex interactions between temperature, pressure, and time, which can affect the mechanical properties, surface quality, and dimensional accuracy of the final product. For the EV battery pack upper cover, which must meet stringent requirements for strength, airtightness, and flame retardancy, optimizing these parameters is essential. In this work, I conducted a series of material-level experiments and finite element simulations to identify the best combination of mold temperature, molding pressure, preheating time, and holding time. The findings are validated through actual production of the EV battery pack upper cover, demonstrating the practicality of the optimized parameters for mass production.
The EV battery pack upper cover studied here is a thin-shell structure with reinforcements and起伏 features to enhance stiffness, as shown in the image above. It is fabricated from six layers of thermoset black flame-retardant glass fiber prepreg (EV101-UL(B)-40%-EWR400-400gsm-1000) with an epoxy resin system containing 40% resin content. Each layer has a thickness of 0.3 mm and an area density of 667 g/m². After molding, the EV battery pack upper cover must achieve a tensile strength greater than 240 MPa, a flexural strength greater than 550 MPa, airtightness meeting IPX8/IPX9 standards per GB/T 4208—2017, and flame retardancy meeting UL94—2020 V-0, V-1, or V-2 ratings. These requirements are crucial for the safety and durability of the EV battery pack in real-world conditions.
To optimize the molding process for the EV battery pack upper cover, I selected four key parameters: mold temperature, molding pressure, preheating time, and holding time. These factors influence resin flow, fiber impregnation, curing kinetics, and final part properties. Based on preliminary data from differential scanning calorimetry (DSC) and practical experience, I designed an orthogonal experiment with three levels for each parameter, as shown in Table 1. The orthogonal array L9(3⁴) was used to reduce the number of experiments while capturing interactions, which is efficient for optimizing the EV battery pack manufacturing process.
| Level | Mold Temperature (°C) | Molding Pressure (MPa) | Preheating Time (s) | Holding Time (s) |
|---|---|---|---|---|
| 1 | 140 | 8 | 20 | 400 |
| 2 | 150 | 10 | 40 | 500 |
| 3 | 160 | 12 | 60 | 600 |
The rationale for these levels stems from material characteristics and production constraints. For instance, the mold temperature levels around 150°C align with the exothermic peak from DSC curves, ensuring proper resin curing for the EV battery pack cover. The molding pressure levels (8, 10, 12 MPa) are derived from the calculated unit pressure required for the EV battery pack upper cover’s projected area, using the formula:
$$F_{\text{压}} = \frac{S_{\text{表}} \cdot F_1 \cdot F_2}{T}$$
where \(F_{\text{压}}\) is the molding pressure in MPa, \(S_{\text{表}}\) is the horizontal投影 area in m², \(F_1\) is the required unit pressure in MPa, \(F_2\) is the rated pressure of the press in MPa, and \(T\) is the press tonnage in tons. For the EV battery pack upper cover, with an area of approximately 1.5 m² and a 2500-ton press, the pressure ranges were deemed suitable. Preheating time levels (20, 40, 60 s) are below the glass transition time of the prepreg at 150°C (~60 s), allowing for better resin flow without fiber damage. Holding time levels (400, 500, 600 s) correspond to the resin curing time of about 8 minutes, balancing completeness of cure and production efficiency for the EV battery pack component.
I conducted nine sets of material-level orthogonal experiments using a flat vulcanizing machine to produce laminates identical to the EV battery pack upper cover material. From these laminates, I cut tensile and bending specimens via laser cutting, following standards GB/T 1447—2005 and GB/T 1449—2005, respectively. The tensile tests were performed on a CMT-5105GL universal testing machine at a loading speed of 10 mm/min, and the tensile strength was calculated as:
$$R_m = \frac{P_{\text{max}}}{A}$$
where \(R_m\) is the tensile strength in MPa, \(P_{\text{max}}\) is the maximum load in N, and \(A\) is the cross-sectional area in mm². Similarly, bending tests were conducted at 10 mm/min, with flexural strength given by:
$$\sigma_f = \frac{3pL}{2wh^2}$$
where \(\sigma_f\) is the flexural strength in MPa, \(p\) is the maximum bending load in N, \(w\) and \(h\) are the width and thickness in mm, and \(L\) is the span in mm. The results, as summarized in Table 2, reveal the mechanical properties under different parameter combinations for the EV battery pack upper cover material.
| Scheme | Mold Temperature (°C) | Molding Pressure (MPa) | Preheating Time (s) | Holding Time (s) | Tensile Strength (MPa) | Flexural Strength (MPa) |
|---|---|---|---|---|---|---|
| 1 | 140 | 8 | 40 | 500 | 441.46 | 549.60 |
| 2 | 140 | 10 | 60 | 400 | 526.45 | 521.41 |
| 3 | 140 | 12 | 20 | 600 | 564.36 | 773.67 |
| 4 | 150 | 8 | 60 | 600 | 567.87 | 867.43 |
| 5 | 150 | 10 | 20 | 500 | 584.41 | 870.58 |
| 6 | 150 | 12 | 40 | 400 | 475.17 | 636.04 |
| 7 | 160 | 8 | 20 | 400 | 537.05 | 616.19 |
| 8 | 160 | 10 | 40 | 600 | 546.85 | 847.01 |
| 9 | 160 | 12 | 60 | 500 | 559.42 | 820.64 |
From Table 2, Scheme 5 (mold temperature 150°C, molding pressure 10 MPa, preheating time 20 s, holding time 500 s) yields the highest tensile strength of 584.41 MPa and flexural strength of 870.58 MPa, exceeding the minimum requirements for the EV battery pack upper cover. This suggests that this parameter combination optimizes resin flow and fiber impregnation, leading to superior mechanical performance. The EV battery pack benefits from such enhanced properties, as it ensures structural integrity under operational loads. To further analyze the effects, I performed range analysis on the orthogonal experimental data, but for brevity, the key insight is that mold temperature and holding time have significant influences on the EV battery pack cover’s strength, while molding pressure and preheating time play secondary roles in this context.
In addition to mechanical properties, the viscoelastic behavior of the prepreg during molding is crucial for simulating the EV battery pack upper cover formation. I conducted stress relaxation experiments using a dynamic mechanical analyzer (DMA) to characterize the material’s response. Specimens of size 5 mm × 25 mm were subjected to a fixed strain of 0.5% at temperatures of 140°C, 150°C, and 160°C, with data recorded over 20 minutes to capture the full relaxation curve. The stress relaxation modulus \(G(t)\) was obtained, which can be modeled using the generalized Maxwell model for simulation purposes. This model consists of multiple Maxwell elements in parallel, each comprising a spring and a damper in series, and the relaxation modulus is expressed as:
$$G(t) = G_0 + \sum_{i=1}^{n} G_i \exp(-t/\tau_i)$$
where \(G_0\) is the equilibrium modulus, \(G_i\) and \(\tau_i\) are the generalized parameters for the \(i\)-th Maxwell element, and \(n\) is the total number of elements. The experimental curves showed that at higher temperatures, the material reaches a steady-state relaxation modulus faster, but the steady-state value remains similar across temperatures. These parameters are essential for accurate finite element simulation of the EV battery pack upper cover molding process, as they define the time-dependent material behavior under compression.
With the material properties from experiments, I proceeded to simulate the hot compression molding of the EV battery pack upper cover using HyperWorks software. The finite element model included simplified mold cavities (treated as rigid bodies made of 6061 aluminum alloy) and the prepreg stack (modeled as a shell with C3D8R elements). The mesh size was set to 5 mm for both parts, ensuring computational efficiency while capturing details. The boundary conditions involved fixing all degrees of freedom for the lower mold, applying temperature loads directly, and defining pressure-time curves based on the orthogonal experimental parameters. For instance, in Scheme 5, the mold temperature was set to 150°C, and the pressure curve ramped up to 10 MPa after 20 s of preheating, held for 500 s, and then released. The generalized Maxwell model parameters from the relaxation experiments were input to account for viscoelasticity.
The simulation results provided stress distributions in the EV battery pack upper cover during the holding phase. I focused on the maximum equivalent stress as a criterion for resin flow uniformity and potential defect formation. High stress concentrations were observed in areas like reinforcement ribs and transition fillets, where resin flow might be hindered, leading to fiber堆积 and reduced mechanical performance. In contrast, regions with uniform stress distribution indicated good resin impregnation. The maximum equivalent stresses for all nine schemes are summarized in Table 3, highlighting the optimal parameter combination for the EV battery pack cover.
| Scheme | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
|---|---|---|---|---|---|---|---|---|---|
| Maximum Equivalent Stress | 338.9 | 295.3 | 236.5 | 223.4 | 210.5 | 312.2 | 280.6 | 256.4 | 240.6 |
As shown in Table 3, Scheme 5 again demonstrates the lowest maximum equivalent stress of 210.5 MPa, confirming that this parameter combination minimizes internal stresses and promotes better resin flow. This aligns with the experimental findings, reinforcing that mold temperature 150°C, molding pressure 10 MPa, preheating time 20 s, and holding time 500 s are optimal for the EV battery pack upper cover. The simulation also revealed that stress concentrations are more pronounced in schemes with higher molding pressures or inadequate preheating, which could lead to defects in the final EV battery pack component. Thus, the simulation serves as a valuable tool for predicting and optimizing the molding process without costly trial-and-error in production.
To validate the optimized parameters for the EV battery pack upper cover, I applied them in actual manufacturing at a production facility. The process involved preparing the prepreg stacks, heating the mold to 150°C, setting the pressure to 10 MPa on a 2500-ton press, preheating for 20 s, and holding for 500 s. After molding, the EV battery pack covers were trimmed via laser cutting to remove excess material and achieve final dimensions. Over five days, 150 covers were produced, and quality inspections were conducted to assess production yield, airtightness, and flame retardancy—critical aspects for the EV battery pack’s performance and safety.
The production yield was calculated based on the number of合格 products versus total manufactured. As shown in Table 4, the合格 rate averaged 96.67%, far exceeding typical industrial standards for EV battery pack components. This high yield indicates that the optimized parameters ensure consistent quality and reduce waste, which is economical for mass production of the EV battery pack upper cover.
| Day | Total Manufactured | Qualified Products | Rejected Products | Yield Rate (%) |
|---|---|---|---|---|
| 1 | 30 | 27 | 3 | 90.00 |
| 2 | 30 | 30 | 0 | 100.00 |
| 3 | 30 | 29 | 1 | 96.67 |
| 4 | 30 | 30 | 0 | 100.00 |
| 5 | 30 | 29 | 1 | 96.67 |
| Total | 150 | 145 | 5 | 96.67 |
For airtightness testing,随机 samples from each day were subjected to a pressure of 3 kPa for 100 s, and the leakage value was measured. According to GB/T 4208—2017, a leakage below 50 Pa is considered合格 for IPX8/IPX9 ratings. The results,如表5所示, show that all tested EV battery pack upper covers had leakage values well under 50 Pa, meeting the stringent airtightness requirements essential for protecting the EV battery pack from environmental ingress.
| Day | Sample 1 | Sample 2 | Sample 3 | Sample 4 | Sample 5 |
|---|---|---|---|---|---|
| 1 | 12 | 37 | 35 | 7 | 1 |
| 2 | 17 | 22 | 32 | 8 | 33 |
| 3 | 21 | 36 | 29 | 17 | 40 |
| 4 | 42 | 19 | 33 | 49 | 30 |
| 5 | 2 | 11 | 46 | 43 | 31 |
Flame retardancy tests were conducted per UL94—2020, where specimens were exposed to flame under controlled conditions. The criteria for V-0, V-1, and V-2 ratings include limits on flame duration after first and second ignitions. As shown in Table 6, all samples self-extinguished within the required timeframes, achieving V-0, V-1, or V-2 performance. This is vital for the EV battery pack upper cover, as it mitigates fire risks in electric vehicles.
| Day | Sample 1 (t1/t2) | Sample 2 (t1/t2) | Sample 3 (t1/t2) | Sample 4 (t1/t2) | Sample 5 (t1/t2) | Total (t1+t2) |
|---|---|---|---|---|---|---|
| 1 | 1/3 | 1/4 | 1/2 | 2/2 | 1/4 | 21 |
| 2 | 1/4 | 1/2 | 2/5 | 1/2 | 2/3 | 23 |
| 3 | 1/2 | 1/2 | 2/4 | 2/5 | 2/3 | 24 |
| 4 | 1/2 | 2/2 | 2/4 | 1/5 | 1/2 | 22 |
| 5 | 1/3 | 1/3 | 2/4 | 2/2 | 1/3 | 22 |
In conclusion, this study successfully optimized the hot compression molding process parameters for the EV battery pack upper cover through a combination of orthogonal experiments, material relaxation tests, and finite element simulations. The best parameter combination—mold temperature 150°C, molding pressure 10 MPa, preheating time 20 s, and holding time 500 s—was identified based on maximum tensile strength (584.41 MPa), flexural strength (870.58 MPa), and minimum maximum equivalent stress (210.5 MPa). Validation in actual production demonstrated a high yield rate of 96.67%, along with excellent airtightness and flame retardancy, meeting all requirements for EV battery pack applications. These findings provide a reliable framework for mass-producing lightweight, high-performance composite covers for EV battery packs, contributing to the advancement of electric vehicle technology. Future work could explore other composite materials or advanced simulation techniques to further enhance the EV battery pack’s efficiency and safety.
Throughout this research, the EV battery pack remained a central focus, as optimizing its upper cover is crucial for overall vehicle performance. The use of composites not only reduces weight but also improves energy efficiency, which is a key advantage for electric vehicles. By meticulously tuning process parameters, I have shown that it is possible to achieve superior mechanical properties and production consistency for the EV battery pack component. This approach can be extended to other parts of the EV battery pack or similar automotive structures, fostering innovation in lightweight design. As the demand for electric vehicles grows, such optimizations will play an increasingly important role in sustainable transportation, making the EV battery pack more reliable and cost-effective.