Application of Honeycomb Sandwich Composite Materials in Automotive Exterior Parts

 

1. Introduction:

In recent years, my country’s automobile industry has gradually become a pillar industry of the national economy. How to reduce energy consumption and reduce environmental pollution has become a key issue that needs to be solved in the automobile industry. In order to meet the needs of energy saving and consumption reduction, the most direct and effective way is to achieve automobile lightweighting. The main way to achieve automobile lightweighting is to optimize the structure of parts, use lighter materials, and innovate manufacturing processes.

 

PHC structural materials can be used in spare tire covers, shelves, sunroof visors and support plates of automobiles due to their light weight and high strength. PHC has been widely used in European models. After nearly two years of development, the application of this material in the Chinese automotive industry is also growing rapidly.

 

PHC is a sandwich structure, which uses core material as the substrate, upper and lower composite surface layers, and surface composite decorative layers. According to product requirements, its surface decorative layer can be composite fabrics, non-woven fabrics and PVC leather. The surface material is usually made of glass fiber or carbon fiber felt material sprayed with PU composite material. The core material can be honeycomb paperboard, corrugated paperboard, aluminum plate and plastic plate.

 

Considering the cost and quality requirements of the product, the core material of paperboard is currently widely used; honeycomb paperboard has high strength, compressibility and low transportation cost. Some flat products (such as spare tire covers and shelves) mostly use honeycomb paperboard; corrugated paperboard is easy to bend, so some products that need to bend often use corrugated paperboard as the core material. Since the bonding between PC and PU is the best, PC material is often used when selecting plastic core materials.

 

PHC products are environmentally friendly and pollution-free, and VOC meets environmental standards; the “one-step molding method” of lightweight core sandwich components is adopted, and this sandwich structure component can achieve economical production in a shorter period of time; the process adaptability is strong, its expansion width and depth are large, and it can produce new products in combination with other modules; PHC product sandwich components are light in weight, good in seismic resistance and high in rigidity, and its weight can be reduced by more than 30~50% compared with similar products made of traditional materials.

 

2. Discussion On Weight Reduction Of Automotive Exterior Products Under The Premise Of Unchanged Strength And Rigidity Performance

As automotive exterior parts, especially large exterior parts of commercial vehicles, they should have good strength and rigidity, so that the parts can support the bending deformation caused by their own weight and resist external force loads under the premise of designing as few fixing points as possible (fewer fixing points can simplify the assembly relationship of parts and reduce product costs). Therefore, the realization of lightweight automobiles must not be at the expense of sacrificing the strength and rigidity performance of products.

 

First, discuss the factors related to product rigidity. Under the action of three-point bending load, the overall deflection deformation of the component is mainly composed of bending deformation and shear deformation. The deflection deformation formula of the entire sandwich structure is:

 

 

Where: D1 and D2 are the equivalent bending stiffness and shear stiffness of the honeycomb structure respectively; P is the load; L is the span; E is the elastic modulus of the panel; J is the moment of inertia of the sandwich structure; b is the width of the sandwich structure; tf is the thickness of the panel of the sandwich structure; h is the thickness of the sandwich structure.

 

It can be seen from the formula that the bending stiffness of the sandwich structure is proportional to the elastic modulus of the panel and the moment of inertia of the sandwich structure. If the cross section of the product is regarded as a rectangular cross section, it is proportional to the cube of the product thickness.

 

To achieve the lightweight requirement, intuitively speaking, it is necessary to reduce the weight per unit area. There are three solutions to achieve this goal:

 

(1) Under the premise of ensuring that the material remains unchanged, the product thickness is reduced (the elastic modulus remains unchanged);

 

(2) Under the premise of ensuring that the product thickness remains unchanged, a low-density material is selected (the conventional elastic modulus generally changes with density);

 

(3) Select a low-density material and increase the product thickness at the same time (ensuring that the cubic deviation of the thickness is greater than the change in the average elastic modulus).

 

It can be seen from the above sandwich structure deflection deformation formula that: in order to ensure the stiffness of the lightweight product, the first solution is obviously not feasible; for the second solution, for conventional non-metallic materials, low-density materials often have a relatively low elastic modulus, which is also not feasible; for the third solution, if you want to achieve a lower average density, the ideal approach is to select materials with corresponding characteristics based on the distribution of bending positive stress in different areas of the product under bending load.

 

Figure 1 is a cross-sectional bending positive stress distribution diagram of a rectangular beam when it is bent externally. It can be seen from the figure that the positive stress at any point on the cross section is proportional to the distance from the point to the neutral axis. If the product is equivalent to the bending condition of a rectangular beam, it can be known that the positive stress on the two surfaces of the product is the largest. Therefore, the epidermis can be made of high-strength materials. Since the middle area mainly bears shear stress and compressive stress, a hollow or loose material with slightly better compressive strength can be selected. In this way, the stress characteristics of the material can be maximized while increasing the section thickness, while reducing the weight as much as possible, thereby achieving the lightweight goal of the product under the premise of ensuring product strength and stiffness.

 

 

3. Lightweight Solution Of Honeycomb Sandwich Structure

Based on the above lightweight design scheme, it is easy to think of composite materials with honeycomb sandwich structure. Honeycomb sandwich structure is a special type of composite material.

 

Because this lightweight structural material has the characteristics of optimal specific strength, specific stiffness, maximum fatigue resistance, smooth surface, etc., it has been widely used in China’s aerospace field.

 

As a typical honeycomb sandwich structure composite material process, PHC process is widely used in the automotive field. As automotive interior parts, its core material is mostly paper honeycomb (plastic honeycomb or aluminum honeycomb, etc. can also be used), and the skin (strength layer) is made of glass fiber reinforced polyurethane foam material. It is molded by high-temperature foaming and compression molding of polyurethane, and has the characteristics of light weight, good strength and stiffness, and good dimensional stability.

 

However, due to the insufficient surface finish of the polyurethane foam skin of the PHC process, it presents a porous structure in the microscopic state, and it is impossible to achieve surface coating or provide a smooth and bright surface effect. Therefore, if you want to apply the PHC process to automotive exterior parts, you must compound a layer of skin material on the outer surface.

 

4. The material selection of the skin layer is based on the technical performance requirements of the exterior parts and the above-mentioned lightweight solution. The skin layer of the exterior parts must have the following performance characteristics:

(1) Appearance: The surface must have good A-level surface quality and a certain impact resistance and scratch resistance;

(2) Physical properties: low molding shrinkage, good thermal stability, high strength and rigidity;

(3) Chemical properties: The material must be corrosion-resistant and have stable molecular structure performance.

 

In addition, from the perspective of PHC process production, the skin material must meet the following characteristics:

(1) The material has good high temperature resistance and can withstand the high temperature of up to 150°C of the lower mold during molding, and transfer the heat of the lower mold to the polyurethane material in contact with it;

(2) The material has good surface polarity and can achieve good bonding performance with the polyurethane material;

(3) The material has good thermal stability and can achieve good matching with the mold when placed in the mold for molding, thereby ensuring good production stability and consistency; in addition, good thermal stability can make the skin material and the substrate have very close linear thermal expansion coefficients, thereby ensuring that the product does not twist or deform under different ambient temperatures.

 

From the above requirements, the most suitable material process is SMC (sheet molding compound), that is, glass fiber reinforced unsaturated polyurethane resin, which fully possesses all the above characteristics, but due to its high density of 1.8g/cm3~2.0g/cm3, its thickness must be controlled to control its proportion in the weight of all materials.

 

5. Research On The Process of SMC and Honeycomb Sandwich Structure Composite

 

Through the above research and discussion, if SMC is used as the skin of the product and PHC is used as the base material of the product, the good surface quality characteristics and dimensional stability of SMC, as well as the good anti-deformation ability and lightweight effect of PHC can be fully utilized.

 

Combining the two, it is possible to develop lightweight, high-strength and good surface quality exterior products. This process is named SPHC. Under the premise of ensuring strength and stiffness, its weight is reduced by about 50% compared with the SMC process. Figure 2 shows the cross-sectional view of the material.

 

 

To Design and Develop Products Using the SPHC Process, the Following Design Points Should be Noted:

(1) Due to the high density of SMC materials, in order to control the surface density of the product assembly, the average thickness of the SMC skin should be reduced as much as possible and controlled within 1.3mm~1.8mm;

(2) The strength and stiffness of SMC materials are good. In areas where local stress is too large, the SMC skin can be reinforced accordingly to increase the strength of the entire product assembly;

(3) Since the PHC process is a hollow structure with poor local pressure bearing capacity, in areas with large local stress, such as the location of the fixed point, the PHC hollow structure needs to be compressed as thin as possible to improve the local strength;

(4) In view of the design requirements of products with unequal thickness, the thickness variation area should be increased as much as possible to reduce the imprint of the honeycomb core on the surface due to different pressures. Figure 3 shows the main production process of this process.

 

 

The Specific Preparation Process Is As Follows:

(1) Preparation of SMC products: Use SMC molding process to make thin plate parts with a diameter of 1.3mm~1.8mm; (2) Stretching molding: Use a honeycomb paper stretching machine to stretch the folded honeycomb core raw materials into a regular shape;

(3) Wrapping of auxiliary materials: Wrap the two layers of cut glass fiber felt on both sides of the honeycomb core material and fix them together with staples;

(4) Placement of SMC parts: Place the pre-formed SMC thin plate parts on the lower mold of the pressing mold to ensure that the SMC parts fit well with the molding mold;

(5) Placement of embedded parts: Place metal embedded reinforcement parts or plastic connectors at the fixed point of the upper mold;

(6) Loading before spraying: Place the wrapped semi-finished auxiliary materials on the spraying equipment =Prepared loading platform;

(7) Polyurethane spraying: The robot clamps the coating auxiliary material on the loading platform to the spraying area, and the spray gun head sprays a fixed amount of polyurethane material on the glass fiber layer surface on both sides of the auxiliary material in sequence according to the specified path and speed. After spraying, the robot clamps it and puts it into the mold cavity;

(8) Molding: The robot arm withdraws from the press platform, and the press drives the mold to close the mold. The upper mold temperature is set to 100℃ and the lower mold temperature is set to 120℃;

(9) Curing: After closing the mold, the press pressure reaches 15MPa, and the polyurethane in the mold begins to cure at high temperature and completes curing after 150s;

(10) Demolding: After the pressure is maintained, the upper mold is opened by the press and returned to its original position, the automatic latch is locked, the product is taken out, the waste is removed, and the edge is sanded, etc.

 

The performance parameters of the SPHC product obtained by the above preparation process are shown in Table 1.

 

 

6. Practical Application 6.1 Example 1 As the most important exterior trim for heavy-duty commercial vehicles, the lower door trim panel is particularly required to reduce weight. In addition to energy saving and emission reduction, it can also reduce the load on the door hinge and increase the reliability of door closing. At the same time, because it is fixed on the door, it often bears the external force when closing the door. This part has only one row of fixed points under the door inner panel, which is a cantilever fixed structure, so the rigidity of the product itself is very high.

 

Figure 4 is a description of the scheme change. The part originally used the double-layer SMC bonding process. Due to its excessive weight, it had the disadvantages of difficult assembly and easy damage to the door hinge. Finally, the SPHC process was used to completely solve the above problems. Table 2 is a parameter comparison of the door trim panel products made by the SMC process and the SPHC process.

 

 

6.2 Example 2 The conventional process material for the engine hood product of a passenger car is a double-layer sheet metal stamping structure, which is then welded together. However, this process has high tooling and mold costs, a long development cycle, many processes, and a heavy product weight. It is not suitable for new energy vehicles with multiple varieties, small batches, and lightweight requirements. The process mentioned in this article can avoid the above problems. The modification plan is shown in Figure 5.

 

Figure 5 is an explanation of the scheme change.

 

The part originally used a double-layer sheet metal structure, which was stamped and formed separately and then welded together. After switching to the SPHC process, the product assembly weight was reduced by 3.5kg, accounting for about 48%. The specific parameter comparison is shown in Table 3.

 

6.3 Example The front face mask product of a triple truck is shown in Figure 6.

 

At present, most manufacturers mainly use SMC technology. However, due to the large size of the product, the dimensions in the body coordinate direction are generally: 2000mm-800mm-100mm (Y-Z-X), the thickness is relatively thin, generally 4, and the number of fixed points is limited, generally 4, located at the flip hinge and lock mechanism, so the stiffness performance of the product is very poor, especially when the product is subjected to unilateral force after opening, the product often shows severe distortion and deformation, and the force on the gas spring support rods on the left and right sides is uneven.

 

In order to make up for the lack of stiffness of the product, a large number of reinforcing ribs are usually designed on the back of the product, but if the reinforcing ribs are too high, the root will be too thick, and the surface of the product will easily shrink, thereby affecting the surface quality. In addition, if the reinforcing ribs are too high, the material fluidity will be poor during compression molding. The top of the reinforcing ribs lack glass fiber, making it insufficient in strength, and cracking often occurs when the product is opened and closed. After adopting the SPHC process, it not only ensures the excellent appearance of the product, but also gives full play to the physical performance characteristics of the sandwich structure, ensuring the good stiffness of the product.

 

 

7. Results and Discussion

 

The SHPC process can maximize the lightweight of vehicles while ensuring good surface quality and sufficient strength and stiffness. The process material can be designed into different shapes according to needs. It not only plays a decorative role, but also can be used as the outer cover of the car body. Its honeycomb core and polyurethane foam material are good sound insulation and heat insulation materials, which play a good role in sound insulation, heat insulation and noise reduction for the driving environment in the car, thereby bringing good NVH comfort to the cab.

 

For commercial trucks, this process material can be used to develop front face masks, bumpers, top air deflectors, side air deflectors, fenders, side skirts and other exterior decorative parts, and for commercial buses, it can also be used in products such as trunk door panels, rear door panels, and engine hoods. It is believed that in the near future, this process and material will be widely promoted, used and recognized.