A police shield based on composite material metal laying bonding technology and design method

Abstract

The application discloses a police shield based on a composite material metal laying bonding technology and a design method. In view of the problems of uneven bonding strength between homogeneous layers, easy debonding of a heterogeneous interface and difficult balance between protection and light weight of an existing thermoplastic CFRP / metal heterogeneous fusion shield, the application establishes a thermal load gradient interlayer bonding strength model and a heterogeneous fusion interface bonding mechanism model, combines a genetic algorithm to optimize a laying process and realizes accurate regulation and control of bonding strength. A six-layer composite layered structure is optimized, a PPS-based thermoplastic CFRP is used as a core protection layer, an aluminum alloy force-bearing matrix is matched, and a double protection system is formed. The application effectively solves the bonding failure technical bottleneck, improves the shield protection performance and structural stability, realizes the balance between light weight and high protection, reduces the burden of police officers, and is controllable in process, convenient for large-scale production, suitable for complex police service combat requirements and has important practical value.

Description

Technical Field

[0001] This invention relates to the field of police protective shields, specifically to a police shield and its design method based on composite metal laying and bonding technology. Background Technology

[0002] Police shields are core protective equipment for officers when performing tasks such as riot control, emergency response, and security patrols. Their protective performance, lightweight design, and structural stability directly affect officers' personal safety, duty efficiency, and emergency response capabilities. With the increasing complexity of police combat scenarios, traditional police shields have gradually revealed many shortcomings and are no longer able to meet the high standards required for modern police protection.

[0003] Currently, police shields are mainly divided into three categories: pure metal shields, traditional fiberglass composite shields, and preliminary carbon fiber composite shields. While pure metal shields possess high rigidity and puncture resistance, their excessive weight can lead to muscle strain for officers during prolonged handling, significantly reducing mobility and making them unsuitable for extended duty periods and emergency response scenarios. Traditional fiberglass composite shields, while achieving a degree of weight reduction, have limited impact and puncture resistance, and low interlayer bonding strength, leading to delamination and peeling after long-term use, resulting in a short lifespan. Preliminary carbon fiber composite shields primarily utilize thermosetting carbon fiber materials, resulting in complex molding processes, and face intractable technical bottlenecks in the heterogeneous fusion and laying of thermoplastic carbon fiber reinforced polymer (CFRP) and metal.

[0004] Currently, thermoplastic CFRP / metal heterogeneous fusion structures have become an important development direction for police shields due to their combination of the toughness of thermoplastic materials, the high strength and lightweight of carbon fiber, and the rigid support of metal. However, in the actual research and development and production of such shields, the core technical challenge is the difficulty in accurately controlling the bonding strength between the homogeneous layers of the composite and the bonding strength at the interface of the composite / metal heterogeneous fusion. Specifically, during the prepreg fiber laying process, the dynamic changes in the thermal load gradient (laying temperature, laying pressure) can lead to uneven distribution of the bonding strength between the homogeneous CFRP layers, easily resulting in local weak areas that are prone to interlayer cracking under impact. At the same time, the significant differences in physical properties between thermoplastic CFRP and metal, poor interfacial wettability, and lack of scientific guidance on bonding mechanisms make the heterogeneous interface prone to debonding and peeling, seriously affecting the overall protective performance and structural stability of the shield.

[0005] In existing technologies, research on the heterogeneous fusion layup of thermoplastic CFRP / metal has focused on optimizing single process parameters, lacking systematic modeling of the interlayer bond strength of multilayer layup under the action of thermal load gradients. This makes it impossible to achieve accurate prediction and uniform control of interlayer bond strength. At the same time, research on the interfacial bonding mechanism of heterogeneous fusion layup is not in-depth enough, and a scientific mechanism model has not been established. It is difficult to clarify the influence of metal surface microstructure, binder composition and layup process on interfacial bond strength, resulting in a lack of theoretical support for process optimization and failing to fundamentally solve the problem of interlayer / interfacial bond failure.

[0006] Furthermore, existing heterogeneous fused police shields suffer from poor structural design and manufacturing adaptability. The lack of integration of bonding strength models and interface mechanisms to optimize the layered structure makes it difficult to balance protective performance, lightweight design, and practical applicability. This hinders the effective enhancement of officers' protective capabilities and reduction of their workload, and fails to meet the demands of complex policing operations in responding to emergencies. Therefore, developing a police shield and its design method that can precisely control the bonding strength between homogeneous layers and at the heterogeneous interface in thermoplastic CFRP / metal heterogeneous fusion layup, while simultaneously achieving high protection, lightweight design, and high stability, has become an urgent technical challenge. Summary of the Invention

[0007] The purpose of this invention is to provide a police shield and its design method based on composite metal laying and bonding technology, so as to solve the technical problems existing in the prior art.

[0008] To achieve the above objectives, the present invention provides the following technical solution: a design method for police shields based on composite metal layup and bonding technology, comprising: Step 1: Pre-treat the high-strength load-bearing aluminum alloy substrate layer. Determine the correlation between the uniformity of the fusion interface and the bonding strength through pre-treatment tests, and complete the pre-treatment of the substrate surface. Step 2: Construct a multilayer interlayer bond strength model under thermal load gradient and optimize process parameters. Collect and preprocess interlayer bond mechanical data on laying temperature, pressure and interlayer bonding. Establish a pressure distribution model using finite element simulation. Establish a temperature field distribution model using numerical heat transfer and verify it experimentally. Construct an interlayer bonding degree model by combining microscopic observation and resin flow simulation. Couple the model to form a bond strength gradient model. Use intelligent optimization algorithms to optimize laying pressure, temperature and laying speed parameters to achieve uniform control of interlayer bond strength. Step 3: Establish a bonding mechanism model for the interface between thermoplastic CFRP and metal heterogeneous fusion, use the interface shear strength as the performance evaluation index, and complete the model verification by combining simulation and mechanical test. Establish prediction models for multiple process parameters and compare their accuracy. Determine the core influencing parameters through sensitivity analysis, and formulate a unified construction process standard for heterogeneous interface bonding based on this. Step 4: Prepare carbon fiber modified epoxy bonding transition layer material according to the predetermined ratio, and coat it evenly on the arc-shaped bonding surface of the aluminum alloy substrate. Control the coating rate according to the interface wetting parameters so that the adhesive penetrates into the laser-etched microstructure of the substrate to form a mechanical interlocking structure. Allow it to stand to complete the leveling treatment.

[0009] Step 5: Retrieve the layered thermo-pressing process parameters corresponding to the thermal load gradient bond strength model, and lay PPS-based thermoplastic CFRP prepreg layer by layer on the surface of the bonding transition layer. During the laying process, the pressure and temperature field distribution are corrected in real time to accurately control the fusion effect between each layer. Step 6: The entire formed shield body is subjected to constant temperature hot pressing and curing. The interface stress is monitored by relying on the heterogeneous interface bonding model, and the bonding state of each layer is verified by combining the interlayer bonding strength model. After all bonding indicators meet the standards, it is naturally cooled and shaped.

[0010] Step 7: Clean the outer surface of the cured CFRP protective layer and apply a low-temperature hot-pressing process to composite the modified polyethylene wear-resistant protective surface layer. Step 8: Apply anti-slip and dust removal treatment to the back of the aluminum alloy substrate, and then attach and fix the high-elastic flame-retardant EVA buffer and shock-absorbing layer. Step 9: Use rigid PU material to seal and protect the sides of the shield, and assemble and fix police functional components such as grips and straps on the outside of the buffer layer to complete the overall preparation.

[0011] Preferably, in step one, the pretreatment of the substrate surface includes: substrate cutting and arc-shaped stamping, sequentially removing organic contaminants and the original oxide film from the substrate surface, followed by multi-stage pure water rinsing, constant temperature drying, and auxiliary roughening, then preparing the surface microstructure through curved adaptive laser etching, and finally performing fine cleaning treatment on the etched area. Preferably, in step two, the laying temperature and pressure are obtained by using an integrated pressure-temperature sensor. The sensor is detached and installed between the rubber roller used for laying and the laying substrate to monitor the contact pressure and temperature of the laying area in real time during the laying process. The interlayer adhesion data under different laying pressures and temperatures are collected by a universal testing machine in conjunction with an extensometer. Then, an outlier removal algorithm is used to remove invalid data to ensure data reliability.

[0012] Preferably, in step three, the heterogeneous interface bonding construction process standard is as follows: the metal surface is pretreated by laser etching, 5% short-cut carbon fiber is added to the adhesive, the laying temperature is 305℃, the laying pressure is 0.55MPa, and the laying speed is 0.9m / min. At this time, the interface shear strength reaches more than 35MPa, which meets the impact resistance and puncture resistance performance requirements of the police shield.

[0013] Preferably, in step four, the coating thickness of the carbon fiber modified epoxy bonding transition layer is 0.25 mm.

[0014] Preferably, in step five, 10 layers of thermoplastic CFRP prepreg are sequentially laid on the surface of the adhesive layer, with the fibers laid in a 0° / 90° cross-lay pattern. A police shield based on composite metal layup and bonding technology is implemented through a police shield design method based on composite metal layup and bonding technology, including: First layer: front wear-resistant and scratch-resistant protective surface layer, made of modified high-density polyethylene wear-resistant protective layer, which is bonded to the surface of thermoplastic CFRP multilayer main protective layer through hot pressing composite molding method; The second layer is a multi-layer thermoplastic CFRP main protective layer, mainly made of PPS matrix thermoplastic carbon fiber prepreg. The fibers of the PPS matrix thermoplastic carbon fiber prepreg are laid out in a cross-alternating pattern. The PPS matrix thermoplastic carbon fiber prepreg layer was laid layer by layer on the heterogeneous interface modified bonding transition layer at a laying temperature of 305℃, a laying pressure of 0.55MPa, and a laying speed of 0.9m / min. The third layer is a heterogeneous interface modified bonding transition layer. The main material used is epoxy high-temperature modified adhesive with short-cut carbon fiber to improve the bonding strength. It is coated on the inner aluminum alloy substrate surface in a uniform thin coating manner without accumulation or overflow of adhesive. The strong bonding between the outer CFRP protective layer and the inner aluminum alloy substrate is achieved by using epoxy modified adhesive with short-cut carbon fiber. The fourth layer is a high-strength load-bearing aluminum alloy base layer. It is made of high-strength, lightweight, and rust-proof aluminum alloy and is formed by one-piece stamping. It serves as the overall frame of the shield. The surface of the high-strength load-bearing aluminum alloy base layer needs to be pre-treated during installation. Fifth layer: High-elasticity cushioning and shock absorption layer on the back. This layer uses high-density flame-retardant EVA foam, which is fixedly connected to the back surface of the aluminum alloy substrate. The sixth layer: the back functional assembly layer. This layer consists of an integrated reinforced grip base, a protective strap mounting slot, and a protective strap. The integrated reinforced grip base passes through the high-elasticity cushioning and shock-absorbing layer on the back and is fixed in the middle of the high-strength aluminum alloy load-bearing base. The high-strength aluminum alloy load-bearing base has symmetrical protective strap mounting slots on both sides. The protective strap mounting slots pass through the high-elasticity cushioning and shock-absorbing layer on the back, and the protective strap can be detached and installed through the protective strap mounting slots.

[0015] Preferably, the surface pretreatment of the high-strength aluminum alloy load-bearing substrate layer during installation is laser etching, with a laser power of 80W, an etching speed of 15mm / s, and an etching depth of 8μm; the surface roughness Ra=1.0μm, resulting in the best mechanical bonding effect.

[0016] Preferably, the thickness of the high-density flame-retardant EVA foam is 1.2 mm.

[0017] Preferably, the shield edge is sealed with hard PU with a sealing width of 15mm to prevent the edge from cracking, peeling, or getting wet, while also preventing the sharp shield edge from accidentally injuring others; Compared with the prior art, the beneficial effects of the present invention are: 1. This invention establishes a multilayer interlayer bond strength model under the action of thermal load gradient, integrates a multilayer pressure distribution model, a multilayer temperature field distribution model, and a theoretical model of interlayer bonding, and combines a genetic algorithm to optimize the layup process parameters. This achieves uniform control of the interlayer bond strength of thermoplastic CFRP homogeneous layers, solving the problems of uneven distribution of interlayer bond strength and easy local cracking in existing technologies. At the same time, a bonding mechanism model of thermoplastic CFRP / metal heterogeneous fusion layup interface is built. Through theoretical modeling, simulation analysis, and orthogonal experiments, the influence of metal surface microstructure, adhesive composition, and layup process on interface bond strength is clarified, achieving precise control of heterogeneous interface bond strength. This effectively solves the technical bottleneck of easy debonding and peeling of heterogeneous interfaces, fundamentally improving the structural stability of the shield.

[0018] 2. This invention features an optimized six-layer composite structure, using a multi-layered PPS-based thermoplastic CFRP layer as the core protective layer. This layer is laid out in a cross-alternating pattern, combined with the rigid support of an aluminum alloy load-bearing substrate, forming a dual protective system of "flexible energy absorption + rigid blocking." This system can stably withstand impact energy and effectively resist blunt force impacts, sharp instrument punctures, and low-velocity projectile impacts, providing far superior protection compared to traditional pure metal and fiberglass composite shields. Simultaneously, the design of the surface wear-resistant protective layer and the back cushioning and shock-absorbing layer further enhances the shield's wear resistance, impact resistance, and shock absorption capabilities, making it suitable for complex police combat scenarios such as outdoor exposure to sunlight, rain, humidity, and open flames, ensuring stable all-weather protective performance.

[0019] 3. This invention uses a composite structure of PPS-based thermoplastic CFRP prepreg containing fiber volume content and rust-proof aluminum alloy, which significantly reduces weight compared to traditional pure metal shields, meets the standard of long-term holding and bearing weight for the holder, effectively reduces the duty burden of the holder, improves mobility and flexibility, and facilitates the holder to respond quickly and handle situations flexibly in emergency situations.

[0020] 4. Through the establishment of two core models, this invention clarifies the optimal range of each process parameter and formulates standardized processes for aluminum alloy pretreatment, adhesive coating, CFRP laying, and hot-pressing curing, achieving precise control over the entire production process. At the same time, the application of laser etching pretreatment of the aluminum alloy substrate and modified short-cut carbon fiber adhesive further enhances the mechanical interlocking and chemical bonding capabilities of the heterogeneous interface, eliminating problems such as interlayer debonding and edge cracking, and significantly extending the service life of the shield. Attached Figure Description

[0021] Figure 1 This is a flowchart illustrating a design method for a police shield based on composite metal layup and bonding technology according to the present invention. Figure 2 This is a schematic diagram of the main structure of a police shield based on composite metal layup and bonding technology according to the present invention; Figure 3 This is a schematic diagram of the structural layers of a police shield based on composite metal laying and bonding technology according to the present invention.

[0022] In the diagram: 1. Front wear-resistant and scratch-resistant protective surface layer; 2. Multi-layer thermoplastic CFRP main protective layer; 3. Heterogeneous interface modified bonding transition layer; 4. High-strength aluminum alloy load-bearing substrate layer; 5. High-elasticity cushioning and shock-absorbing layer on the back; 6. Hard PU edge sealing; 701. Integrated reinforced grip base; 702. Protective strap mounting groove; 703. Protective strap. Detailed Implementation

[0023] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] Please see Figure 2 , 3 This invention proposes a police shield based on composite metal layup and bonding technology, comprising: Overall structure: The appearance is a conventional arc-shaped riot police shield, with an external dimension of 900mm in height and 500mm in width. The overall outer arc curvature R=1200mm and the overall thickness is 11.95mm, which meets the thickness specifications of police protective shields. The shield is arranged in layers from the front of impact to the back grip surface.

[0025] This embodiment includes: 1) First layer (surface layer): front wear-resistant and scratch-resistant protective layer 1, made of modified high-density polyethylene wear-resistant protective layer, with a single layer thickness of 0.3mm. It is bonded to the surface of the thermoplastic CFRP multilayer main protective layer 2 mentioned below through hot pressing and molding. Due to the physical properties of high-density polyethylene material, the front wear-resistant and scratch-resistant protective layer 1 has the characteristics of impact and scratch resistance, outdoor rain protection, and acid and alkali corrosion resistance. It can also protect the internal carbon fiber layer and prevent the carbon fiber layer from being bumped and worn during use.

[0026] 2) Second layer (reinforcing layer): Thermoplastic CFRP multilayer main protective layer 2 (this protective layer is the core functional layer of this embodiment) The main material of this protective layer is PPS-based thermoplastic carbon fiber prepreg, with a fiber volume content of 65%. The total number of layers is 10 (the thickness of a single CFRP layer is 0.62 mm, and the total thickness is 6.2 mm). Since this embodiment relies on the coupling of laying temperature and laying pressure to control the interlayer bonding strength, the optimal laying temperature is 305℃. The melting temperature range of PPS (heat distortion temperature ≥260℃) is highly consistent with the optimal laying temperature of 305℃ in this embodiment. The resin can be extruded and flowed and thermally fused under the set pressure. The process can be precisely controlled based on the interlayer bonding strength model to achieve uniform and controllable interlayer strength of the 10-layer CFRP layer.

[0027] The fiber layout of PPS matrix thermoplastic carbon fiber prepreg adopts 0° / 90° cross-alternating laying. Cross-laying can improve the overall impact resistance and puncture resistance of the structure and avoid cracking and denting of the shield.

[0028] The PPS matrix thermoplastic carbon fiber prepreg layer was laid layer by layer on the heterogeneous interface modified bonding transition layer 3 at a laying temperature of 305℃, a laying pressure of 0.55MPa, and a laying speed of 0.9m / min.

[0029] 3) Third layer (adhesive layer): Heterogeneous interface modified adhesive transition layer 3 The main adhesive matrix used in this layer is epoxy high-temperature modified adhesive, and 5% short carbon fiber (3~5mm in length) is added to the adhesive to improve the bonding strength. The thickness of a single layer is 0.25mm (using a uniform thin coating method, without accumulation or overflow of adhesive).

[0030] By incorporating short-cut carbon fiber epoxy-modified adhesive, the outer CFRP protective layer and the inner aluminum alloy substrate can be firmly bonded. After coating, it can offset the difference in thermal expansion and contraction between the carbon fiber layer and the aluminum alloy layer, avoiding delamination, debonding, and warping under temperature difference and impact. It perfectly fills the surface micropores formed by laser etching of aluminum alloy, greatly improving the mechanical bonding force of the interface. At the same time, when the shield is subjected to impact, the impact and puncture load on the outer layer is stably transmitted to the aluminum alloy substrate, realizing the dual-layer synergistic stress distribution.

[0031] 4) Fourth layer (structural support layer): High-strength aluminum alloy load-bearing base layer 4 (structural support frame) This layer uses 6061-T6 aerospace-grade rust-proof aluminum alloy. This material has high strength and lightweight physical properties, with a tensile strength ≥260MPa and strong bending load-bearing capacity. The plate thickness is 4.0mm, and it is integrally stamped to form the overall frame of the shield, shaping the arc-shaped protective shape to prevent the shield from bending or twisting due to strong impact.

[0032] The surface of the high-strength load-bearing aluminum alloy substrate layer 4 needs to be pre-treated during installation: laser etching treatment. Laser power 80W, etching speed 15mm / s, etching depth 8μm; The surface roughness Ra=1.0μm of the molded surface provides the best mechanical bonding effect.

[0033] 5) Fifth layer (specialty layer): High-elasticity cushioning and shock absorption layer on the back 5 This layer uses high-density flame-retardant EVA foam, 1.2mm thick, which is fully bonded to the back of the aluminum alloy substrate. Its main function is to absorb residual vibration and impact transmitted from the shield to the back, significantly reducing the transmission of impact shock to the user's arm. Due to the soft nature of the foam, it can conform to the hand grip area, making it less likely to cause hand discomfort or soreness during long-term duty. The high-density flame-retardant EVA foam has flame-retardant properties, which can cope with open flame and combustion-related emergency scenarios, improving safety during use.

[0034] 6) Sixth layer (functional layer): Back functional assembly layer (integrated structure) This layer features a built-in reinforced grip base 701, made of glass fiber reinforced nylon with embedded aluminum alloy inserts. It is centrally positioned to provide a stable grip point for the user. The embedded metal inserts prevent breakage and loosening when gripping.

[0035] Symmetrical protective strap mounting slots 702 are provided on both sides of this layer, with ergonomic strap fixing positions to fit the double arm binding structure. Protective straps 703 can be added to achieve arm binding and fixation, making it less likely for the shield to slip out of hand during actual combat.

[0036] The shield edges are sealed with hard PU 6, with a sealing width of 15mm, to prevent the edges from cracking, peeling, or getting wet, while also preventing the sharp shield edges from accidentally injuring others.

[0037] Please see Figure 1 This invention proposes a design method for a police shield based on composite metal layup and bonding technology, used to design and implement the police shield based on composite metal layup and bonding technology proposed above: Step 1: Pretreatment of the high-strength load-bearing aluminum alloy substrate layer 4 1. Metal surface pretreatment and adhesive pretreatment test Four initial metal surface pretreatment processes were selected: precision shot peening, anodizing, laser etching, and precision milling; and three binder pretreatment processes were selected: adding 5% chopped carbon fiber, adding 3% aluminum foil powder, and adding 2% metal wire. The parameter ranges for each process were determined. 1) Precision shot peening: shot peening pressure 0.2-0.4MPa, shot diameter 0.1-0.3mm, shot peening time 30-60s; 2) Anodizing: Oxidation voltage 15-25V, oxidation time 20-40min, electrolyte is sulfuric acid solution (concentration 10%-15%). 3) Laser etching: Laser power 50-100W, etching speed 10-20mm / s, etching depth 5-10μm; 4) Precision milling: Milling speed 1000-1500 r / min, feed rate 0.1-0.2 mm / r.

[0038] Metal surface pretreatment and binder pretreatment tests were conducted separately. SEM was used to observe the microstructure of the metal surface (surface roughness, porosity, and micromorphology). Fourier transform infrared spectroscopy (FTIR) was used to analyze the changes in binder composition. Metal surface roughness (Ra) was measured using a roughness meter (model: TR200). Surface characteristic parameters under different pretreatment processes were recorded.

[0039] Using a factorial experimental method, an L16(4) experiment was designed. 4 An orthogonal experiment was conducted, with metal surface pretreatment process, adhesive pretreatment process, layup temperature, and layup pressure as influencing factors. Each factor had four levels, and interfacial shear strength was used as the evaluation index. Layup and bonding tests were carried out, with each experimental group repeated three times and the average value taken as the experimental result.

[0040] Experimental data were analyzed using SPSS software for range and variance analysis to determine the significance of each factor's influence on interfacial bond strength (significance level α=0.05). The influence mechanisms of metal surface microstructure (surface roughness, porosity) and binder composition on the fusion interface bond strength were obtained: the interfacial mechanical interlocking effect was strongest when the metal surface roughness Ra was between 0.8 and 1.2 μm; adding 5% short-cut carbon fibers to the binder significantly improved the interface's load-bearing capacity. Simultaneously, SEM observation of the heterogeneous fusion interface cross-section revealed that the interfacial shear strength was more than 25% higher when the interface was uniformly bonded than when it was non-uniformly bonded, clarifying the influence of interface uniformity on bond strength.

[0041] 2. Substrate cutting and arc stamping: National standard 6061-T6 rust-proof aluminum alloy coil is selected and cut into the standard shield shape of 900mm×500mm using cutting equipment; hydraulic arc stamping equipment is used for one-piece cold pressing to ensure that the board surface is free of wrinkles and twists after forming, with an overall flatness error of ≤0.2mm / m, and the edges are reserved for edge sealing processing.

[0042] The edges are precision milled using a CNC milling machine to remove stamping burrs and sharp edges, ensuring smooth and flat sides to prevent hand injuries during subsequent use, while also ensuring uniform coating of the edge adhesive layer.

[0043] The initial surface dust removal uses high-pressure clean and dry air to blow the entire area, removing aluminum shavings, dust, and stamping residue from the surface of the board, thus completing the basic standardization of the substrate.

[0044] 3. Remove organic contaminants: Immerse the board in an industrial environmentally friendly hydrocarbon degreasing solvent for 15-20 minutes to thoroughly remove organic contaminants such as stamping lubricating oil, cutting oil, protective wax, and human hand grease from the board surface. This will prevent oil from blocking the subsequent wetting of the adhesive and prevent the interface from delaminating later.

[0045] 4. Removal of native oxide film: The surface of the board is cleaned with a weak acidic solution of 8% dilute phosphoric acid for 3-5 minutes to quickly dissolve the dense aluminum oxide film naturally formed on the surface of the aluminum alloy, break the inert surface layer, expose the pure aluminum substrate metal color, and eliminate the interface adhesion and barrier problems caused by the oxide layer.

[0046] 5. Multi-stage pure water rinsing + constant temperature drying: The board undergoes two stages of deionized pure water rinsing, with the first stage being coarse rinsing and the second stage being fine rinsing, to thoroughly remove any acid or alkali residue from the board surface. The boards are dried using a constant-temperature hot air dryer at a temperature of 75-80℃ for 18-25 minutes to ensure that the board surface is free of water stains and watermarks, completely dry and without moisture, and to prevent moisture residue from causing interface bubbles during the high-temperature laying process, which would damage the bonding strength.

[0047] 6. Pre-processing auxiliary coarsening Precision dry shot peening with a particle size of 0.2mm is used to uniformly roughen the base surface, unify the basic roughness of the board surface, eliminate local texture differences in the board, lay a uniform base surface for subsequent laser etching, and ensure the consistency of the microstructure throughout the entire area.

[0048] 7. Adaptive laser etching for forming microstructures on curved surfaces The curved fiber laser marking and etching equipment is equipped with a shield-shaped arc-shaped follow-up fixture, which scans synchronously with the 1200mm arc of the shield throughout the process, ensuring that the etching parameters are consistent across all areas of the curved surface.

[0049] The single-pass etching depth on the shield surface is set to 8μm, employing an interlaced mesh-like micron-sized pit structure with a texture spacing of 0.15mm and 100% full coverage. The defocusing amount is set to standard zero defocus to ensure uniform etching depth. After etching, the aluminum alloy surface forms a densely interlaced micron-sized pit and groove structure, with the surface roughness stably controlled at Ra=1.0μm. This prevents stress concentration due to excessive roughness and weak mechanical interlocking force due to insufficient roughness.

[0050] Etching the aluminum alloy surface allows for the creation of numerous micro-anchoring grooves on the aluminum substrate. Subsequent adhesives can directly penetrate into these grooves, and upon cooling and solidification, form a mechanically interlocking structure, significantly enhancing the bonding strength between CFRP and the metal heterogeneous interface. This process breaks down the smooth, inert interface of the aluminum alloy surface, greatly increasing the surface energy of the plate and strengthening the wetting and spreading ability of the modified adhesive, thus eliminating the phenomena of weak or empty adhesion at the interface. The uniform distribution of the texture ensures consistent bonding strength across the entire impact-bearing surface of the shield, preventing localized delamination and cracking under stress.

[0051] 8. Refined surface cleaning after etching Ultrasonic dust-free cleaning involves placing the etched aluminum alloy substrate into a dust-free ultrasonic cleaning tank. High-frequency vibration removes aluminum slag, micro-metal powder, and residual debris inside the etching lines, preventing impurities from getting trapped inside the bonding interface and reducing bonding strength.

[0052] The cold air knife uses oil-free, clean, cold air at high speed to sweep and blow the entire surface and grooves of the plate, thoroughly drying and cleaning residual moisture, ensuring that the inside of the etched micropores is clean and dry.

[0053] Step 2: Establish a multilayer interlayer bond strength model under the action of thermal load gradient and optimize the layup process parameters. 1. Monitoring and data collection of laying process parameters To address the coupling effect between laying temperature and laying pressure during fiber laying, a process parameter monitoring system was built. A BMP280 pressure-temperature integrated sensor (measurement range: pressure 300-1100 hPa, temperature -40~85℃, relative accuracy ±0.12 hPa) was used. The sensor was embedded between the rubber roller of the automatic fiber laying machine and the laying substrate to monitor the contact pressure and temperature of the laying area in real time. The sensor is connected to an Arduino controller via an I2C bus, and data acquisition is performed using Arduino IDE software at a frequency of 10 Hz to ensure real-time data accuracy.

[0054] Meanwhile, using a universal testing machine (model: WDW-100) and an extensometer, interlayer adhesion data were collected under different layup pressures (0.3-0.8MPa) and layup temperatures (280-320℃) to provide experimental data support for subsequent model construction. After data collection, the data was imported into MATLAB R2025b software for preprocessing, and an outlier removal algorithm (3σ criterion) was used to remove invalid data to ensure data reliability.

[0055] 2. Establishment of a multi-layer pressure distribution model A finite element model of the contact deformation between the rubber roller and the substrate was established using the finite element analysis software ANSYS 2023 R2. HyperMesh software was used for mesh generation (quadrilateral structured mesh, mesh size 0.5 mm). The rubber roller adopted the Mooney-Rivlin hyperelastic model (parameters C10=0.3MPa, C01=0.1MPa), and the substrate (aluminum alloy + pre-laid CFRP layer) adopted the linear elastic model (elastic modulus E=70GPa, Poisson's ratio μ=0.33).

[0056] Finite element simulation was used to simulate the contact deformation process under different pressure roller pressure amounts (0.1-0.5mm). A static structural analysis module was used to apply boundary conditions consistent with the actual laying (fixed base, vertical downward displacement load applied to the pressure roller). The compressive stress distribution data of the contact area was extracted to explore the nonlinear relationship between the pressure roller pressure amount, the length of the deformed contact area, and the contact compressive stress distribution during the laying process.

[0057] Based on the experimentally collected laying pressure data, a multi-layer pressure distribution model was established using the least squares method. The model expression is as follows:

[0058] in, The contact compressive stress distribution of the i-th layer is shown below. The initial compressive stress for the surface layer is given by k, which is the pressure attenuation coefficient (obtained by fitting experimental data, with a value range of 0.12-0.15), and i is the number of layers (i=1,2,...,n, where n is the total number of layers; in this embodiment, n=8-12). The pressure amount δ of the pressure roller is a function of the planar position (x,y), and is obtained by fitting the simulation results through ANSYS.

[0059] After the model is established, a simulation program is written using MATLAB software. By inputting different laying parameters, the pressure distribution of each layer can be predicted in real time, realizing systematic control of the pressure acting on each layer during the laying process and avoiding differences in interlayer bond strength caused by uneven pressure distribution.

[0060] 3. Establishment of a multi-layer temperature field distribution model To address the cyclic additive manufacturing characteristics of fiber-laid multilayer laminate structures, and considering the complex heat transfer processes during layup, including heating of the bonding area, heat exchange between the upper surface of the current layup and the air, and heat exchange between the lower surface of the current layup and existing layups, a layup heat transfer model is established based on the transient heat conduction governing equations using the finite difference method (FDM). The model is then solved using MATLAB R2025b software. The specific steps are as follows: 3.1 The heat exchange process of each layer is divided into three parts: the heating process of the current layer, the heating process of the already laid layer, and the heat dissipation process. The heat transfer coefficient of each process is determined as follows: Air convection heat transfer coefficient h1 = 15 W / (m²). 2 K), interlayer contact heat transfer coefficient h2=800W / (m²) 2 K), the heat flux density of the heating source (infrared heating lamp) q = 5000 W / m 2 ; 3.2 The transient heat conduction equation is discretized using the Alternating Direction Implicit (ADI) algorithm. The layup area is divided into n layers along the thickness direction (z-axis) (consistent with the number of layup layers) and into 50×50 grids along the plane direction (x, y axes). The time step is set to 0.1s, and the boundary conditions are set as follows: the heated area is subject to the first type of boundary condition (fixed temperature), and the non-heated area is subject to the third type of boundary condition (convective heat dissipation). 3.3 By using MATLAB programming, the multi-layer distribution of the temperature field during the heating process along the thickness direction is obtained, the temperature change law of each layer with time during the laying process is obtained, the gradient distribution characteristics of the temperature of each layer are clarified, and the temperature prediction problem caused by the dynamic change of temperature conditions is solved.

[0061] Temperature field model verification: The temperature of each layer during the laying process was measured in real time using an infrared thermometer (model: FLIRE60). The measured data was compared with the model prediction data, and the error was controlled within ±5℃ to verify the accuracy of the model. The verification method of transient heat conduction numerical calculation was referenced to ensure that the model fits the actual laying conditions.

[0062] 4. Establishment of a multilayer distribution model for interlayer bond strength and process optimization First, the microstructure of the prepreg fiber bundles was observed using a scanning electron microscope (SEM, model: SU8010) to obtain microscopic parameters such as fiber diameter (7-10μm) and resin distribution. Combining the law of conservation of mass and fluid mechanics theory, the resin extrusion flow process on the fiber surface was simulated using Fluent software to elucidate the influence of the close contact process and thermal fusion process on interlayer bonding and to establish a theoretical model of interlayer bonding degree of prepreg fiber laid-up.

[0063] Based on this, a multi-layer pressure distribution model, a multi-layer temperature field distribution model, and an interlayer bonding theory model are integrated. Interlayer bonding theory (interfacial adhesion theory and diffusion theory) is introduced to establish a multi-layer distribution model for the interlayer bond strength of prepreg fiber layup. The model uses interlayer shear strength (ILSS) as the characterization index, and its expression is as follows:

[0064] in, Let be the interlaminar shear strength of the i-th layer, and α, β, and γ be coefficients (obtained by fitting experimental data, α=0.85, β=0.02, γ=0.13). The contact compressive stress of the i-th layer is obtained from the multilayer pressure distribution model. η is the average temperature of the i-th layer (obtained from the multilayer temperature field distribution model), and η is the interlayer bonding degree (obtained from the interlayer bonding degree theoretical model).

[0065] To address the constraint of multilayer distribution of interlayer bond strength, the Dakota open-source optimization tool combined with a genetic algorithm (GA) was used to optimize the layup process parameters (layup pressure, layup temperature, and layup speed). The optimization objective was to achieve a coefficient of variation of interlayer shear strength ≤5%, thus realizing uniform control of interlayer bond strength. The parameters of the genetic algorithm were set as follows: population size 50, number of iterations 100, crossover probability 0.7, and mutation probability 0.05. The optimal layup process parameters obtained after optimization were: layup pressure 0.5-0.6 MPa, layup temperature 300-310℃, and layup speed 0.8-1.0 m / min.

[0066] Step 3: Construct a bonding mechanism model for the thermoplastic CFRP / metal heterogeneous fusion layup interface and determine the bonding construction standards. 1. This embodiment uses interfacial shear strength (IFSS) as the characterization index of the bonding strength of heterogeneous fusion interface. Through a combination of theoretical modeling, simulation, and process experiments, the bonding mechanism of the thermoplastic CFRP / metal heterogeneous fusion layup interface is studied, the optimal pretreatment process and layup process are determined, and the heterogeneous fusion molding process is made controllable. The specific steps are as follows: 1.1 Establishment of a heterogeneous fusion interface bonding model First, the microstructure of the thermoplastic CFRP prepreg (PPS-based), the surface microstructure of 6061 aluminum alloy, and the material parameters of the epoxy-modified binder were analyzed. A simulation model of the thermoplastic CFRP / metal heterogeneous fusion interface was established using ANSYS 2023 R2 software. Shell181 elements (suitable for simulating layered composite materials, based on Mindlin–Reissner shell theory) were selected to construct the CFRP layer and the metal layer. Conta174 contact elements were used to simulate the interface bonding. A cohesive zone model (CZM) was introduced, and the Mode II bilinear cohesion criterion was used to simulate the interface shear deformation and debonding process.

[0067] Meanwhile, based on the theory of interface mechanics, an analytical model for the bonding strength of heterogeneous fusion interfaces was established. Considering factors such as layup temperature, layup pressure, prepreg fiber volume content, resin viscosity, binder composition, metal thermal conductivity, metal surface microstructure, and metal surface roughness, an analytical program was written using MATLAB software. Combined with the results of the simulation model, the temperature change law and stress distribution law of the fusion interface were obtained, and the influence of various process factors on the bonding strength of heterogeneous fusion interfaces was explored.

[0068] The interfacial shear strength was tested using a single shear test method. A universal testing machine was used to apply the load at a speed of 1 mm / min. The maximum shear force at interface failure was recorded, and the interfacial shear strength (IFSS = maximum shear force / bond area) was calculated. The test data was compared with the predicted data of the analytical model and the simulation model to correct the model parameters and ensure the accuracy of the model. The model prediction error was controlled within ±8%.

[0069] 1.2 Achieving controllability of heterogeneous fusion bonding process By introducing the multi-layered mechanism of fiber placement process parameters and combining the above experimental data, two relational models were established using different methods, and the accuracy of the models was compared and analyzed. 1) Multiple linear regression analysis: Using the regress function in MATLAB software, a multiple linear regression model was established with metal surface roughness Ra, amount of short-cut carbon fiber added to binder, layup temperature T, and layup pressure σ as independent variables, and interfacial shear strength IFSS as the dependent variable. The model expression is as follows:

[0070] Where a, b, c, d, and e are regression coefficients, obtained by fitting experimental data, and the model's coefficient of determination R0 is... 2 A value ≥0.92 indicates that the model fits well. 2) Artificial Neural Network: A convolutional neural network (CNN) model was constructed using the TensorFlow framework. The metal surface microstructure parameters, binder composition parameters, and layup process parameters were used as input layers, and the interface shear strength was used as the output layer. The network structure was set as follows: 6 neurons in the input layer, 2 hidden layers (16 and 8 neurons respectively), and 1 neuron in the output layer. The Adam optimizer was used with a learning rate of 0.001 and 500 iterations. The model's prediction accuracy reached over 95%, which is better than the multiple linear regression model.

[0071] Sensitivity analysis was performed using the Morris screening method to analyze the sensitivity of the heterogeneous fusion interface bonding strength to various influencing factors, and the following sensitive parameters were determined: metal surface laser etching process (sensitivity coefficient 0.82), amount of short carbon fiber added to the binder (sensitivity coefficient 0.76), and layup temperature (sensitivity coefficient 0.68). For these sensitive parameters, strict process control standards were formulated to achieve controllability of the heterogeneous fusion bonding process.

[0072] The optimal material fusion and laying process was finally determined as follows: the metal surface was pretreated by laser etching (laser power 80W, etching speed 15mm / s, etching depth 8μm), 5% short-cut carbon fiber was added to the binder, the laying temperature was 305℃, the laying pressure was 0.55MPa, and the laying speed was 0.9m / min. At this time, the interfacial shear strength reached more than 35MPa, which met the impact resistance and puncture resistance requirements of the police shield.

[0073] Step 4: Coating the bonding surface of the aluminum alloy substrate with a carbon fiber modified epoxy bonding transition layer. The adhesive was formulated according to the optimal ratio obtained from the heterogeneous interface bonding mechanism model. 5% short-cut carbon fiber filler was added. A 0.25mm thick adhesive layer was uniformly coated on the arc-shaped surface of the aluminum substrate using a quantitative coating process. The coating rate was controlled by the interface wetting parameters calculated by the model, so that the adhesive could fully penetrate into the laser-etched micropores of the aluminum alloy and initially form a mechanical interlocking structure. The initial leveling was completed by standing.

[0074] Step 5: Laying PPS-based thermoplastic CFRP prepreg The pre-built thermal load gradient bond strength model was invoked, and the appropriate pressure and temperature parameters for each laying layer were retrieved in real time. Using an automated fiber laying device, 10 layers of thermoplastic CFRP prepreg were laid sequentially on the bonding layer surface in a 0° / 90° cross arrangement. During the laying process, the compressive stress distribution and temperature field gradient were corrected in real time based on the model, and the degree of interlayer contact and thermal fusion effect were adjusted layer by layer. The bond strength difference between each layer was controlled throughout the process, so that the overall strength variation coefficient was controlled within 5%, and the laying of the main protective layer with a total thickness of 6.2mm was completed.

[0075] Step Six: Constant Temperature Hot Press Curing The laid composite shield is sent into a hot press and cured in one piece using a curing temperature of 320℃ and a curing pressure of 0.7MPa, which are obtained from model optimization. During the curing process, the interface stress change is monitored with reference to the heterogeneous interface bonding mechanism model, and the bonding state of each CFRP layer is checked against the interlayer bonding strength model to ensure that the CFRP interlayer and the heterogeneous interface between CFRP and aluminum alloy matrix all reach the preset bonding strength standard. After cooling, it naturally sets.

[0076] Step 7: Lay a modified polyethylene wear-resistant protective layer onto the outer surface of the CFRP main protective layer. The CFRP surface of the shield after curing is cleaned and dusted. A low-temperature hot-pressing bonding process is used to tightly bond a 0.3mm thick wear-resistant and scratch-resistant protective layer to the outermost carbon fiber surface. The bonding gaps are eliminated by compaction to form an outer physical protective structure.

[0077] Step 8: Apply a high-elasticity, flame-retardant EVA cushioning and shock-absorbing layer to the entire back of the aluminum alloy substrate. The flat aluminum alloy surface on the back of the shield is treated with dust removal and anti-slip treatment. A 1.2mm thick EVA buffer layer is fully adhered using special structural adhesive, and it is pressed firmly without any air pockets. The flexible buffer structure transmits and reduces the impact force, thus completing the assembly of the shock absorption structure on the back.

[0078] Step Nine: Practical Functional Structure Assembly The shield's sides are fully sealed with rigid PU material to prevent water ingress between layers and edge cracking; a grip base is fixed and reinforced on the outside of the buffer layer, and a strap mounting groove is opened to complete the assembly and fixation of practical police structures such as handheld and binding, and the overall shape and size are adjusted.

[0079] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A design method for police shields based on composite metal layup and bonding technology, characterized in that, include: Step 1: Pre-treat the high-strength load-bearing aluminum alloy substrate layer. Determine the correlation between the uniformity of the fusion interface and the bonding strength through pre-treatment tests, and complete the pre-treatment of the substrate surface. Step 2: Construct a multilayer interlayer bond strength model under thermal load gradient and optimize process parameters. Collect and preprocess interlayer bond mechanical data on laying temperature, pressure and interlayer bonding. Establish a pressure distribution model using finite element simulation. Establish a temperature field distribution model using numerical heat transfer and verify it experimentally. Construct an interlayer bonding degree model by combining microscopic observation and resin flow simulation. Couple the model to form a bond strength gradient model. Use intelligent optimization algorithms to optimize laying pressure, temperature and laying speed parameters to achieve uniform control of interlayer bond strength. Step 3: Establish a bonding mechanism model for the interface between thermoplastic CFRP and metal heterogeneous fusion, use the interface shear strength as the performance evaluation index, and complete the model verification by combining simulation and mechanical test. Establish prediction models for multiple process parameters and compare their accuracy. Determine the core influencing parameters through sensitivity analysis, and formulate a unified construction process standard for heterogeneous interface bonding based on this. Step 4: Prepare carbon fiber modified epoxy bonding transition layer material according to the predetermined ratio, and coat it evenly on the arc-shaped bonding surface of the aluminum alloy substrate. Control the coating rate according to the interface wetting parameters so that the adhesive penetrates into the laser-etched microstructure of the substrate to form a mechanical interlocking structure. Let it stand to complete the leveling treatment. Step 5: Retrieve the layered thermo-pressing process parameters corresponding to the thermal load gradient bond strength model, and lay PPS-based thermoplastic CFRP prepreg layer by layer on the surface of the bonding transition layer. During the laying process, the pressure and temperature field distribution are corrected in real time to accurately control the fusion effect between each layer. Step 6: The entire formed shield body is subjected to constant temperature hot pressing and curing. The interface stress is monitored by the heterogeneous interface bonding model, and the bonding state of each layer is verified by the interlayer bonding strength model. After all bonding indicators meet the standards, the shield body is naturally cooled and shaped. Step 7: Clean the outer surface of the cured CFRP protective layer and apply a low-temperature hot-pressing process to composite the modified polyethylene wear-resistant protective surface layer. Step 8: Apply anti-slip and dust removal treatment to the back of the aluminum alloy substrate, and then attach and fix the high-elastic flame-retardant EVA buffer and shock-absorbing layer. Step 9: Use rigid PU material to seal and protect the sides of the shield, and assemble and fix police functional components such as grips and straps on the outside of the buffer layer to complete the overall preparation.

2. The design method for a police shield based on composite metal layup and bonding technology according to claim 1, characterized in that: In step one, the pretreatment of the substrate surface includes: substrate cutting and arc stamping, sequential removal of organic contaminants and native oxide film from the substrate surface, auxiliary roughening after multi-stage pure water rinsing and constant temperature drying, preparation of surface microstructures by curved adaptive laser etching, and finally fine cleaning treatment of the etched area.

3. The design method for a police shield based on composite metal layup and bonding technology according to claim 1, characterized in that: In step two, the laying temperature and pressure are obtained by using an integrated pressure-temperature sensor. The sensor is disassembled and installed between the rubber roller used for laying and the laying substrate to monitor the contact pressure and temperature of the laying area in real time during the laying process. The interlayer adhesion force data under different laying pressures and temperatures are collected by a universal testing machine in conjunction with an extensometer. Then, an outlier removal algorithm is used to remove invalid data to ensure data reliability.

4. The design method for a police shield based on composite metal layup and bonding technology according to claim 1, characterized in that: In step three, the heterogeneous interface bonding construction process standard is as follows: the metal surface is pretreated by laser etching, 5% short-cut carbon fiber is added to the adhesive, the laying temperature is 305℃, the laying pressure is 0.55MPa, and the laying speed is 0.9m / min. At this time, the interface shear strength reaches more than 35MPa, which meets the impact resistance and puncture resistance performance requirements of the police shield.

5. The design method for a police shield based on composite metal layup and bonding technology according to claim 1, characterized in that: In step four, the coating thickness of the carbon fiber modified epoxy bonding transition layer is 0.25 mm.

6. The design method for a police shield based on composite metal layup and bonding technology according to claim 1, characterized in that: In step five, 10 layers of thermoplastic CFRP prepreg are laid sequentially on the surface of the adhesive layer, with the fibers laid in a 0° / 90° cross-laying pattern.

7. A police shield based on composite metal layup and bonding technology, implemented by the police shield design method based on composite metal layup and bonding technology described in claims 1-6, characterized in that: include: First layer: front wear-resistant and scratch-resistant protective surface layer (1), the material is modified high-density polyethylene wear-resistant protective layer, which is bonded to the surface of thermoplastic CFRP multilayer main protective layer (2) by hot pressing composite molding method; Second layer: Thermoplastic CFRP multilayer laying main protective layer (2), the main material is PPS matrix thermoplastic carbon fiber prepreg, the fiber laying arrangement of PPS matrix thermoplastic carbon fiber prepreg is laid in a cross-alternating manner. The PPS matrix thermoplastic carbon fiber prepreg layer was laid layer by layer on the heterogeneous interface modified bonding transition layer (3) at a laying temperature of 305℃, a laying pressure of 0.55MPa, and a laying speed of 0.9m / min. The third layer: heterogeneous interface modified bonding transition layer (3), the main material used is: epoxy high temperature modified adhesive filled with short carbon fiber, which is used to improve the bonding strength. It is coated on the inner aluminum alloy substrate surface in a uniform thin coating manner, without accumulation and overflow of adhesive. The outer CFRP protective layer and the inner aluminum alloy substrate are firmly bonded by the epoxy modified adhesive filled with short carbon fiber. Fourth layer: High-strength aluminum alloy load-bearing base layer (4), which is made of high-strength, lightweight and rust-proof aluminum alloy and is formed by one-piece stamping. As the overall frame of the shield, the surface of the high-strength aluminum alloy load-bearing base layer (4) needs to be pre-treated during installation. Fifth layer: Back high elastic cushioning and shock absorption layer (5), this layer uses high density flame retardant EVA foam, which is fixedly connected to the back surface of the aluminum alloy substrate; The sixth layer: back functional assembly layer. This layer consists of an integrated reinforced grip base (701), a protective strap mounting groove (702), and a protective strap (703). The integrated reinforced grip base (701) passes through the back high-elasticity buffer and shock-absorbing layer (5) and is fixed in the middle of the aluminum alloy high-strength load-bearing base layer (4). The aluminum alloy high-strength load-bearing base layer (4) has symmetrical protective strap mounting grooves (702) on both sides. The protective strap mounting grooves (702) pass through the back high-elasticity buffer and shock-absorbing layer (5), and the protective strap (703) can be detachably installed through the protective strap mounting grooves (702).

8. A police shield based on composite metal layup and bonding technology according to claim 7, characterized in that: The high-strength load-bearing aluminum alloy substrate layer (4) is pretreated by laser etching during installation. The laser power is 80W, the etching speed is 15mm / s, and the etching depth is 8μm. The surface roughness Ra=1.0μm, and the mechanical bonding effect is optimal.

9. A police shield based on composite metal layup and bonding technology according to claim 7, characterized in that: The thickness of the high-density flame-retardant EVA foam is 1.2mm.

10. A police shield based on composite metal layup and bonding technology according to claim 7, characterized in that: The shield edges are sealed with hard PU (6) with a sealing width of 15mm to prevent the edges from cracking, peeling, or getting wet, while also preventing the sharp shield edges from accidentally injuring others.

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