Integrated design of flow channel frame and gasket of flow battery under two-color injection molding process

Redox flow battery is a long-term energy storage technology suitable for large-scale application, safe, stable, green and environmentally friendly. The flow channel frame is a key component in the flow battery. It has the important role of providing electrolyte flow channel, supporting other components and sealing, and is usually molded by injection molding. Based on the two-color injection molding process, VET Energy designs an integrated structure of flow channel frame and gasket. The structure can use modified polypropylene as the flow channel frame body material and dynamically vulcanized thermoplastic elastomer as the gasket material, and is molded by two-color injection molding process.

VET Energy also determines the feasibility of material selection and injection molding process through simulation analysis, and conducts injection molding test to obtain an ideal flow channel frame-gasket integrated product with a warpage of less than 1 mm. Finally, the product is assembled into a single battery. After 40 cycles of charge and discharge performance test, the results show that the electrochemical performance is not attenuated, and there is no leakage or deformation.

The flow channel frame plays an extremely important role in the battery stack. It is a carrier for the circulation of electrolyte in the battery. It not only provides support and assembly positions for various components in the battery stack, but also provides a uniform electrolyte flow channel, while also meeting the sealing requirements. With the expansion of the scale of flow battery energy storage applications, the quality stability and production efficiency of the flow channel frame are particularly important.

Conventional flow channel frames are produced by injection molding. Most of the materials are common plastics such as polypropylene and polyethylene. After injection molding, rubber seals or gaskets are installed manually or automatically. This production and assembly method is not suitable for large-scale flow channel frame production.

VET Energy proposes a new integrated design of flow channel frame and sealing gasket for liquid flow battery based on two-color injection molding process. This design uses modified polypropylene material as the main body of the one-shot flow channel frame and EPDM/PP dynamically vulcanized thermoplastic elastomer material as the second-shot sealing gasket. VET Energy first designs the integrated structure of the flow channel frame and the sealing gasket based on the requirements of the two-color injection molding process, and then determines the material, mold gate position and quantity through simulation analysis. Subsequently, through experimental verification, the injection molding product of the flow channel frame of the liquid flow battery that meets the use requirements is obtained. Finally, the battery flow channel frame is installed into a battery, and after a long-term charge and discharge test, the results show that the battery performance is good, and there is no leakage and deformation.

01 Two-color injection molding process

Two-color injection molding is a plastic injection molding process that is both common and has a high technical content. It injects two plastic materials of different plastic materials or the same plastic material but different colors together to form an injection molded product. The advantages of two-color injection molding include high product precision, stable quality, good structural strength and good durability.

Two-color injection molding can be achieved by two injections using an ordinary injection molding machine, or by using a two-color injection molding machine to complete the injection molding of two different plastics on the same machine. The former does not require high equipment, but has low production efficiency and poor precision. The latter has a wide range of applications and good product quality, high production efficiency, which is the current trend and the method to be adopted in this article: The specific working steps are shown in Figure 1. The material pipe 1 of the injection molding machine injects raw material A into the lower mold cavity to form the first shot product. After the mold is opened, the machine rotates 180° in the plane and rotates the lower mold to the top. Then, raw material B is injected into the upper mold cavity through material pipe 2 to form the second shot product. At the same time, material pipe 1 continues to inject raw material A into the lower mold cavity.

02 Selection of injection molding materials

The selection of injection molding materials for the flow battery flow channel frame needs to meet the following requirements:

(1) The material can adapt to the operating temperature range of the flow battery (50~70 ℃)

(2) The material needs to have strong resistance to aging, strong acid and other chemical media;

(3) Both the flow channel frame body material and the sealing ring material need to meet the requirements of the injection molding process, have good fluidity, and the two materials must be chemically compatible to reduce the warping deformation of the injection molded product;

(4) The sealing ring material has good heat resistance, mechanical properties and sealing properties;

(5) The flow channel frame body material needs to have excellent mechanical properties and high temperature resistance;

(6) The cost of the material should be low and the supply should be sufficient

2.1 Body material

Polypropylene (PP) is a general thermoplastic resin with the advantages of regular body structure, high crystallinity, easy processing and molding, high bending strength, good electrical insulation and good mechanical properties at high temperatures. However, general polypropylene products have problems such as dimensional instability and large shrinkage. Therefore, the industry often uses the method of adding fillers (such as inorganic fillers and reinforcing fibers) to modify polypropylene.

Filling and modifying polypropylene with inorganic fillers such as talcum powder, wollastonite, and calcium carbonate can increase the rigidity of the product and reduce shrinkage and deformation. The use of glass fiber reinforced polypropylene can significantly improve the overall mechanical properties and heat resistance of the product, reduce shrinkage and deformation, and has good acid resistance, which is a better modification method.

2.2 Sealing materials

TPV is a special type of thermoplastic elastomer. It was proposed by American Gessler in the 1960s. It is a thermoplastic elastomer made by dynamically vulcanizing a mixture of thermoplastic resin and elastomer. In 1981, the American ONSANTO company successfully achieved industrial mass production of EPDM/PPTPV and registered its product as Santoprene[u-2]. Compared with ordinary thermoplastic elastomers, the rubber component of TPV is completely vulcanized and evenly dispersed in the thermoplastic matrix, so its physical and mechanical properties and processing stability are significantly improved, and it has broad application prospects in the fields of automobiles, electronics, etc. L3]

TPV materials concentrate the characteristics of both rubber and plastic materials. The specific characteristics are as follows:

(1) It has the plasticity of plastic and can be processed in various ways like plastic, such as extrusion, injection molding, blow molding, etc., and can be bonded with PPEPDM, etc.;

(2) It has the elasticity of rubber and can be used for some elastic products, such as shock absorption, sealing, etc.;

(3) It has good aging resistance;

(4) It has good acid and alkali resistance and oil resistance;

(5) It is pollution-free, environmentally friendly and reliable;

(6) It is recyclable and does not lose mechanical properties after repeated processing. TPV has the above excellent properties, so it can replace rubber as a sealing gasket, and has achieved good results in the automotive industry. However, due to the small scale of my country's liquid flow battery field and the incomplete construction of the upstream and downstream industrial chain system, the production of battery stack liquid flow frames and the installation of sealing rings are still relatively extensive, mostly relying on manual assembly, and it is difficult to obtain products with stable quality and high production efficiency.

VET Energy introduces TPV materials into the production of liquid flow battery flow frames to replace traditional rubber sealing rings, which will be more conducive to the development and large-scale production of key components in the field of liquid flow batteries.

03 Flow frame model design

The prototype design of the flow frame is shown in Figure 2. The length and width of the flow frame are 354 mmx97mm, and the size of the middle electrode frame is 250mmX40mm. The flow channel consists of two parts. One part is a serpentine flow channel connected to the liquid inlet hole, with a flow channel groove of 1.7mm deep and 4mm wide, for the electrolyte to flow through. The other part is a uniform flow channel with a flow channel depth of 0.85mm and a uniformly distributed resistance boss, which allows the electrolyte to enter the electrode evenly.

The wall thickness of the thinnest part of the flow channel frame is 0.8mm, while the wall thickness of the thickest part is 3.2mm, and the thickness varies greatly with the structure, which is designed for uneven wall thickness. Uneven wall thickness will cause uneven cooling and shrinkage of the product during the injection molding process. This unevenness will cause uneven stress distribution and cause product warping and deformation. Therefore, the wall thickness optimization was carried out in the design of the injection molding model. The buckle design was carried out at the thickest wall thickness around the flow channel frame, so that the overall wall thickness distribution of the flow channel frame is more uniform.

Figures 3 and 4 are the front and back of the injection molding flow channel frame model after optimization, respectively. There are four sealing structures on the flow channel frame, namely the sealing structure of liquid inlet 1, the sealing structure of liquid inlet 2, the sealing structure of the membrane and the sealing structure of the bipolar plate. These four sealing structures are distributed on the front and back sides of the flow channel frame.

In order to meet the process requirements of two-color injection molding, the inlet of the two-shot material will be arranged on the same side, which requires the four sealing rings on the front and back sides to be connected. As shown in Figure 5, the original flow channel frame design has been modified to connect the sealing rings. By opening a through groove and a connecting groove on the flow channel frame sealing groove body, all the sealing rings are connected together to form the final flow channel frame-seal ring integrated model.

04 Bi-color injection molding analysis and experimental verification

4.1 First gating simulation analysis

There are two design schemes for the mold casting system of the flow channel frame body with first gating. Scheme a is to arrange the inlet on the surface of the flow channel frame. The advantages are simple mold design and low injection pressure. The disadvantage is that shrinkage marks will be formed on the surface of the product, affecting the appearance. Scheme b is to arrange the glue inlet on the side of the runner frame. The advantage is that it avoids the surface of the product and does not affect the appearance, but the disadvantage is that the mold design is slightly complicated and needs to be manually processed later. The two design schemes are shown in Figure 6.

Both schemes use a 4-point needle valve hot runner glue inlet system, the hot runner diameter is 10mm, and the hot nozzle gate diameter is set to the molding conditions: material temperature 250℃, mold temperature 2.5mm. 45℃, maximum shear stress 0.25MPa, maximum holding pressure 60MPa. The results obtained by simulation analysis of the two design schemes are shown in Table 3.

The glue filling of both schemes is relatively smooth, evenly distributed, the molding pressure is small, there is no shrinkage mark, and the product deformation analysis results are shown in Figure 7. The maximum warping deformation in the Z direction of scheme a is small, and the yield rate of this scheme is high, avoiding the manual post-processing of scheme b. Therefore, the first gating glue inlet adopts scheme a design.

4.2 Second gating simulation analysis

Since the thickness of the second gating seal ring is only 1mm and the width is only 2.5mm, the injection molding process has high requirements for the fluidity of the TPV material: to avoid the problem of "glue breaking". In the calculation, it is assumed that the mold surface temperature is 40℃, the material thickness is 205℃, the maximum shear stress is 0.3 MPa, and the maximum holding pressure is 53MPa. The glue feeding system is shown in Figure 8.

After simulation analysis, the results are summarized in Table 4. The second gating glue can fill the mold evenly, without problems such as trapped air and overflow, the molding pressure is small, there are small shrinkage marks, and the volume shrinkage is relatively uniform.

4.3 Experimental verification

According to the above simulation analysis, the mold design was completed, and the injection molding production verification was carried out. The actual runner frame is shown in Figure 9. The warping deformation in the Z direction does not exceed 1mm, which is consistent with the simulation analysis results.

It was assembled into an iron-chromium liquid flow single battery and charged and discharged. The single cell is composed of an upper end plate, an upper insulating plate, a positive current collector, a plate frame, a positive electrode, a membrane, a negative electrode, a plate frame, a negative current collector, a lower insulating plate and a lower end plate, as shown in Figure 10.

The test conditions are: constant current charging and discharging at a current density of 130mA/cm', a flow rate of 1mL/min for a cross-sectional area of 1cm', and an operating temperature of 50℃; the electrode area is 100 cm', the thickness is 3.6mm, and the membrane thickness is 60m. After 40 cycles, the average energy efficiency reached about 76%, and there was basically no attenuation. The test data is shown in Figure 11.

During the test, the flow channel frame did not have problems such as leakage or deformation, which further verified the feasibility of the flow channel frame in terms of materials, mechanical design, injection molding, etc.

Conclusion Two-color injection molding is a mature plastic product molding process, which is widely used in automobiles, home appliances, medical devices and other fields. As one of the key components in the liquid flow battery, the manifold frame has a harsh working environment, usually in an acidic, high-temperature and charged environment, which places high demands on the material, mechanical design and molding process of the manifold frame. In order to improve the production efficiency and product consistency of the manifold frame, this study completed the design of the manifold frame structure based on the requirements of the two-color injection molding process and the actual application needs of the product, and determined the mold inlet layout and the first and second gating materials through simulation analysis. Finally, through experimental verification, the two-color injection molded manifold frame was successfully prepared, and the performance test was good after being assembled into a battery.