Carbon fiber vacuum bag compression molding process and carbon fiber vacuum infusion molding process

The research on the properties and manufacturing process of honeycomb sandwich structure, as a special porous composite material, has a history of more than half a century. Due to its high specific strength and stiffness, good electrical insulation, low thermal conductivity, high wave permeability, vibration isolation, and impact resistance, it has become an indispensable material in the fields of aviation, aerospace, shipbuilding, railways, automobiles, and construction. The drones we manufacture are lightweight, have good maneuverability, and are equipped with advanced equipment. They have a wide range of applications in both military and civilian fields. The fuselage, wings, and tail fins are all made of honeycomb sandwich structures. This honeycomb sandwich structure consists of carbon fiber composite inner and outer panels and Nomex honeycomb core. The inner and outer panel forming materials still use self-made carbon fiber/epoxy prepreg. Due to the very thin design of the inner and outer panels, with only two layers of non-woven fabric thickness, and the epoxy resin being a medium temperature curing system, it is suitable to use vacuum bag compression molding technology and can adopt co curing and co bonding technology.

1) Characteristics of honeycomb sandwich structure
The schematic diagram of the honeycomb sandwich structure is shown in the following figure. The inner and outer panels are 0.25mm thick carbon fiber composite thin plates, and the sandwich is a uniformly thick Nomex core cut into slices. The mechanical properties of this sandwich structure depend not only on the inherent properties of the panel and core material, but also on the bonding performance between the panel and core material. At the same time, the molding process also contributes greatly to the comprehensive performance of the component. Therefore, adopting the vacuum bag compression molding process requires solving the following four technical problems simultaneously:
(1) Surface quality control measures for pre impregnated low-pressure formed parts.
(2) Selection of sandwich structure materials and overall mechanical properties.
(3) The bonding performance between carbon fiber composite panel and Nomex sandwich interface.
(4) Weight control of sandwich structure components.

In terms of material selection, it is important to consider the performance requirements of drone components and the four vacuum bag forming processes mentioned above
Technical issues have determined the selection of the following materials:
(1) Flame retardant surface adhesive coating (color adjustable).
(2) Carbon fiber prepreg non-woven fabric 8=0.125mm, with a glue content of (40 ± 2)%.
(3) Nomex core material: aramid paper honeycomb, pre impregnated phenolic adhesive, pore size 1.83mm, 8=5mm, p=
48g/cm'.
(4) Board core adhesive film SY-24C.

2) Forming process flow
(1) Mold preparation.
In the research and development stage, in order to shorten the mold manufacturing cycle and reduce prices, fiberglass molds are used for molding, and the surface of the mold is subjected to water polishing treatment. In order to ensure that the mold does not deform when used at a medium temperature of around 130 ℃, the thickness of the fiberglass mold is over 20mm, and a steel frame is used to reinforce the back of the mold. After cleaning the surface of the mold, first apply a sealing putty strip around the perimeter, and then manually apply liquid release agent on the mold surface. If the release agent is applied first, it will cause the sealing putty strip to not stick firmly to the mold surface.

(2) Flame retardant adhesive coating spraying.
In order to control the weight of the components, the honeycomb sandwich structure panel is very thin. If it is treated by spraying paint after molding, the outer panel must be polished to remove residual release agent, which can easily damage the quality of the thin-walled panel. The pressure curing molding is carried out by vacuum pumping. During the vacuum pumping process, the outer panel is tightly pressed against the surface. After the vacuum is released and demolded, due to the different pressures inside and outside the honeycomb cell, honeycomb cell indentations will be formed on the thin-walled outer panel. Using ordinary spray painting method, in order to control weight, ordinary putty scraping method cannot be used for treatment. Even if the paint layer is thickened, the honeycomb pattern on the surface of the paint layer is difficult to completely eliminate. Therefore, in terms of process, the method of directly spraying surface flame-retardant adhesive coating on the mold is adopted instead of surface painting.
First, color the flame-retardant adhesive coating according to the requirements of the product. The amount of adhesive coating used should be sufficient at once, otherwise it may cause color difference. Mix the resin, curing agent, and diluent of the adhesive coating accurately according to the formula, and stir evenly. At this point, the adhesive coating has the viscosity suitable for spraying, and pour it into a handheld spray gun for spraying. The thickness of the adhesive coating sprayed should be controlled between 0.15~0.2mm, so that the weight of the component can be controlled while ensuring that the adhesive coating completely covers the honeycomb indentation.
(3) External panel molding.
The gel coat is sprayed on the mold surface and placed at room temperature. After its gel is sprayed, two layers of pre cut carbon fiber weft free cloth are laid on the mold surface, and then the vacuum system is directly laid. As shown in Figure, the laying sequence of auxiliary materials is as follows: release cloth, adhesive material, isolation film, breathable felt, and vacuum bag film. After the bagging is completed, vacuum leak detection is carried out. After confirming that there are no errors, the mold is pushed into a large oven and heated up for the first solidification molding.

Curing parameters: temperature of 125 ℃, holding time of 2h, pressure of vacuum negative pressure above 0.09MPa. In order to control the adhesive content of carbon fiber composite materials, the amount of adhesive absorbing material should be accurately calculated according to the following formula:

In the formula, X, The amount of adhesive absorbed per unit weight of the adhesive material; W, The weight of the adhesive material; W. The weight of the pre impregnated material for the outer panel. The outer panel must be separately cured and formed first, so that all its areas can withstand vacuum pressure. If honeycomb is laid and then pressure is transmitted to the outer panel through the honeycomb core material, it will cause a phenomenon of high pressure at the honeycomb wall and low pressure at the honeycomb cell and honeycomb side walls, which seriously affects the forming quality of the sandwich structure load-bearing outer panel. The surface coating and the outer panel prepreg are co cured and formed in an oven, improving the bonding strength between the two. Rubber clothing can cover the honeycomb indentations on the outer panel. The rich resin surface layer also fills the gap between fibers in the contact surface of the prepreg during low-pressure molding in non hot press tanks, making its outer surface smooth and flat, and improving the surface quality of the body components.
The use of adhesive coating can not only replace the secondary coating process on the surface of components, reduce the weight of body parts, but also improve the flame retardant effect of components by selecting flame-retardant adhesive coating.

(4) Nomex honeycomb core laying.
According to the thickness of the components, select honeycomb chip materials of equal thickness. The density of the Nomex honeycomb core directly provided by the honeycomb manufacturing factory after cutting has a significant impact on the mechanical properties. Therefore, it is necessary to choose a sheet with uniform density and color. The smaller the honeycomb core grid, the better the bending flexibility, and the more aramid material it contains, the higher the corresponding price.
After the outer panel is cured, it should be lowered to below 50 ℃. After removing the auxiliary materials on the mold, use a steel needle to mark the edge line of the honeycomb core on the inner surface. At this time, it is necessary to prevent the outer panel from moving on the mold. Lay a layer of SY-24C adhesive film along the edge line, and lay Nomex honeycomb core on top of the adhesive film. Honeycomb cores should be laid in whole pieces as much as possible. If two pieces need to be spliced, sufficient foam rubber strips should be placed on the splicing end face. The foam rubber strips will expand due to heat and solidify synchronously with the pre impregnated material and adhesive film, bonding the two honeycomb cores into one. This type of foam adhesive can also be used for bonding the sides of honeycomb cores to the frame skeleton.
The Nomex honeycomb core is divided into L and W directions, as shown in the figure below. The flexibility of the honeycomb along the W direction is much stronger than that along the L direction. Therefore, when laying the honeycomb, the L direction should be along the longitudinal direction of the drone to better fit the mold surface. Use a scraper to chamfer the edges of the honeycomb core at a 45 ° angle, as shown in the right Figure, to create a Z-shaped edge seal on the edges of the sandwich structure, in order to improve the processability of the honeycomb edge seal and enhance the bonding strength between the honeycomb core and the inner panel.

(5) Co curing molding.
After laying the honeycomb core, clean up any remaining debris from edge processing, and lay two layers of carbon fiber on top. Connect the edges of the honeycomb core to the outside frequently, and prevent displacement of the honeycomb core during placement. Lay the vacuum system again as shown in the picture of the vacuum bag combination system. This time, no adhesive material is placed, allowing a certain amount of matrix resin in the prepreg to be discharged into the honeycomb and climb up to the honeycomb wall to form adhesive lumps, increasing the bonding strength between the honeycomb and the inner panel. After vacuum leak detection, push the mold into the oven and heat it up for the second solidification molding. After curing, in order to prevent thermal deformation, the components are vacuum sealed and cooled to room temperature in the furnace, and then the vacuum is stopped to remove the vacuum auxiliary materials. After demolding the parts, the edge residue cutting process is carried out.
Adopting a two-step method to manufacture honeycomb sandwich structures has reduced costs. During the co curing process, the outer panel is pre formed in the first step to ensure the quality of the outer panel and the surface quality of the body components. The outer panel is bonded to the honeycomb core through adhesive film, which has high mechanical properties and ensures the load-bearing performance of the outer panel.
The inner panel prepreg forms a depression under vacuum pressure, and its overall shape is undulating, resulting in a significant decrease in its mechanical properties. There is no adhesive film between the honeycomb and the inner panel, and the peel strength is not very high. However, the bonding strength between the inner panel and the honeycomb can ensure the stability of the sandwich structure and meet the stiffness requirements of the components.
By designing the thin-walled structure of the panel and strictly controlling the adhesive content of the composite material, the weight of the entire sandwich structure is relatively low, which has excellent weight control effect on unmanned aerial vehicles.
The vacuum bag compression molding process has the characteristics of simple molding, low cost, and short manufacturing cycle. It is easy to form complex unmanned aerial vehicle structural components as a whole. The composite sandwich structural components formed by it have high specific strength and stiffness, which better meet the needs of unmanned aerial vehicles for structural lightweighting.

Vacuum infusion molding process
In recent years, vacuum assisted RTM molding technology (VARTM) has been developed abroad. Compared with the traditional RTM process, its mold cost can be reduced by 50%~70%. Using this process, there are very few organic volatiles in the molding process, fully meeting people's requirements for environmental protection. Moreover, the molding adaptability is good because vacuum assistance can fully eliminate bubbles.
The vacuum infusion molding process is to directly lay dry fiber-reinforced materials on the mold, and lay a peeling layer on top of the fiber-reinforced materials. The peeling layer is usually a thin layer of low porosity, low permeability fiber fabric peeling layer with high permeability medium laid on it, and then wrapped and sealed with a vacuum bag.
The resin infusion system is shown in the following figure. The mold is wrapped and sealed with a vacuum bag film, and the vacuum pump is pumped to a negative pressure state. Various layers are shown in the figure. The release cloth is a layer of easily peelable low porosity fiber fabric, the flow guide net is a high permeability medium, and the flow guide pipes are distributed on top of the flow guide cloth. The resin enters the entire system through the inlet tube and is guided in the main direction of resin flow through the guide tube. The guide net distributes the resin to every corner of the layer, and after curing, the release cloth is peeled off to obtain a structure layer with high density and low adhesive content.
1) Characteristics of vacuum infusion molding process
(1) The use of vacuum infusion molding technology can greatly reduce the labor intensity of workers and improve work hygiene
Conditions.
(2) Due to the closed nature of the entire tooling system, working conditions have been improved, operator exposure to harmful substances has been reduced, the working environment has been improved, and the process operation is simple.
(3) In terms of product performance, vacuum infusion can reduce the porosity of the product, effectively control the gel content of the product, and produce products that are less affected by human factors. The product has high quality stability and good reproducibility.
(4) The apparent quality of the product is good, with the same layer thickness being thin and high strength, and the tensile strength is increased by more than 20% compared to hand lay up molding.
The vacuum infusion molding process is a closed mold, controllable, and continuous process. Fibers can be pre formed with stable material usage, and high-quality layup and sandwich structures can be manufactured without the need for high equipment investment.
The prerequisite for successful infusion is a 100% airtight mold, a 100% airtight system, low viscosity resin, and appropriate infusion strategy and method.
The requirements for the matrix resin are as follows.
(1) Low viscosity: generally 150-300Pa · s is optimal. If the viscosity is high, the molded product is not easy to evenly fill the mold cavity; If the viscosity is too low, it is easy to carry air and cause pinholes in the product.
(2) Low curing heat release peak: When vacuum forming larger and thicker products, if the heat release peak is too high, local heat is not easily dissipated, which can lead to coking.
(3) Curing time: The length of curing time should be determined according to the product being manufactured, and an appropriate curing time is beneficial for shortening the working cycle.
(4) Physical properties: The selected resin should have good mechanical properties, high tensile and bending strength, good corrosion resistance, and minimal curing shrinkage.
(5) Affordable, non-toxic, and widely sourced.
 

2) The use of vacuum infusion molding technology on unmanned aerial vehicle components includes reinforced ribs in the drone body, support walls in the tail wing, and other components made of carbon fiber composite material on both sides of the panel, with a flat sandwich structure consisting of aviation layer panels in the middle. The forming process adopts vacuum infusion, and the scheme is to first form large-sized boards, and then cut them according to the shape of use. This manufacturing method has good product component quality and high production efficiency. During the research and development of UAV, the sandwich material can be replaced by foam contour plate, and the molding method remains unchanged.
3) Vacuum infusion molding process flow
The molding mold can be made of aluminum plate, steel plate, tempered glass, etc. We used flat aluminum molds and polished the surface during the research and development process. The mold is equipped with an electric heating system inside, which can control the heating process. After cleaning the surface of the mold with acetone, apply a sealing putty strip around the perimeter, and then manually coat the mold surface with liquid release agent. The release agent adopts a semi permanent type, which can be applied once to form parts more than seven times, avoiding the accumulation of release agent caused by repeated coating and affecting the surface quality of the product.
(2) Material selection.
Reinforcement material: Carbon fiber grid cloth 150g/m.
Matrix material: The viscosity of the system mixed with epoxy resin is 300cP.
Sandwich material: aviation laminates, or foam wheel plates such as PVC, PET, etc,
(3) Material laying.
When cutting carbon fiber woven fabric, use scissors to cut it directly along the fiber direction. On the aviation layer board, many small holes with a diameter of 3 are regularly opened for adhesive penetration, arranged in sizes of 50mm x 50mm. This can not only improve the resin infusion efficiency, but also enhance the bonding strength between the panel and the sandwich panel.
Lay the carbon fiber grid flat on the flat mold, then place the aviation layer board, and lay the carbon fiber square on the layer board
Cloth.
Cover the entire area inside the sealing strip with polyester release cloth, and fix it with a small amount of 3M super multi-purpose spray glue produced by 3M Company in the United States.
(4) Installation of infusion system.
Lay perforated isolation film, diversion net, diversion pipe, exhaust pipeline, and vacuum bag film on the release cloth in sequence. The laying method of the infusion system is shown in the following figure.

Use a vacuum pump to evacuate the system to 0.03MPa, stop pumping, adjust the vacuum bag film, and ensure that the vacuum film is not locally tight, especially in areas with high and low steps such as the feeding port, pumping port, and product edge. Manually adjust the position of each auxiliary material inside the vacuum bag.
After adjusting the straight air system, evacuate the system to 0.1 MPa and check its airtightness; Then turn off the vacuum pump and maintain the pressure for 10 minutes until the vacuum degree does not decrease before proceeding with the infusion process.
(5) Injection molding.
Prepare a sufficient amount of resin and curing agent, weigh them in proportion, stir evenly, and place them in a defoamer box for vacuum defoaming for 10 minutes to remove any air that may be trapped during resin mixing.
Place the feeding port into the resin and open the pipeline for injection. During the injection process, the port must always be immersed in the resin, otherwise it will inhale a large amount of air and cause product defects.
The timing for closing the feed inlet should be based on the resin seeping into the edge line of the product parts.
After the product is poured, it is cured and the mold heating system is turned on, with a temperature of 80 ℃ and a holding time of 60 minutes. During the heating process, constant attention should be paid to the curing condition and no air leakage should occur; Always test the surface temperature of the product to prevent explosive aggregation.
After the product is cured, the hardness is tested. When the hardness value reaches 80 or above, all auxiliary materials on the carbon fiber product can be removed. Use a plastic triangular wedge to insert the material from the edge of the product, pry it loose, and remove the product from the mold. Be careful not to damage the mold surface and product surface.
Draw the shape of the component using AutoCAD software and place it on a waterjet cutting platform for automatic cutting. The dimensions of the component are precise and the edges are smooth.