Application of Prepreg Winding Process and Automatic Fiber Placement Technology

 

Prepreg Winding Process Winding process Introduction

Under the conditions of controlled tension and predetermined linear shape, a special winding equipment is used to impregnate continuous fibers or cloth tapes with resin glue and then continuously, evenly and regularly wind them on a core mold or lining, and then solidify them under a certain temperature environment to form a composite material molding method of a certain shape product.

 

 

 

Characteristics of winding process High specific strength Compared with other molding methods, the fibers in the composite products formed by winding process are straightened and arranged in the specified direction with high neatness and precision, and the products can fully utilize the strength of the fibers, so the specific strength and specific stiffness are high.

 

Can achieve equal strength design of products Since the direction, layer and number of fiber arrangement can be determined according to the load-bearing requirements to achieve equal strength design, the product structure is reasonable.

 

Save raw materials and low manufacturing cost. The reinforcing materials used in winding products are mostly continuous fibers, untwisted rovings and weftless tapes. Since the reinforcing materials do not need to be woven, the weaving process is reduced, the cost is reduced, and the stress concentration at the interweaving points of the cloth and the ends of the chopped fibers is also avoided.

 

Highly repeatable product quality The fiber winding process is easy to mechanize and automate, the product quality is high and stable, the productivity is high, and it is easy to mass produce.

 

Limitations of Winding Process

 

 

Winding Technology Application

At present, fiber winding technology has been widely used in aerospace, military, various pressure pipelines, storage tanks, natural gas cylinders, bearings, energy storage flywheels, insulation products, sports equipment, transportation and other industrial and civil fields.

 

The wide application of fiber winding technology has driven the rapid development of the composite materials industry. At the same time, the fiber winding industry has made leaps and bounds with the continuous expansion of new material application fields and the introduction of new materials.

 

Pipeline

Pressure Vessels

 

Storage Tank

 

Solid Rocket Motor Casing

 

External Pressure Cylinder

 

 

 

Classification of Winding Process

 

According to the different chemical and physical states of the resin matrix during winding, the winding process can be divided into three types: dry, wet and semi-dry.

 

According to the different reinforcing materials during winding molding, the winding process can be divided into two types: fiber winding and cloth tape winding.

 

Wet winding is to bundle and dip the fibers, directly wind them on the core mold under tension control, and then solidify them into shape.

 

The equipment for wet winding is relatively simple, but since the yarn tape is immediately wound after being dipped in glue, it is difficult to control and inspect the glue content of the product during the winding process.

 

When the solvent in the glue solution solidifies, it is easy to form defects such as bubbles and pores in the product, and the tension is also difficult to control during winding.

 

Workers operate in an atmosphere of solvent evaporation and an environment with flying fiber hairs, and the working conditions are poor.

 

Dry winding uses pre-impregnated yarn tapes that are in the B stage after being pre-impregnated. Pre-impregnated yarn tapes are manufactured and provided in special factories or workshops.

 

During dry winding, the pre-impregnated yarn tapes must be heated and softened on the winding machine before being wound onto the core mold.

 

Because the glue content, tape size and quality of the pre-impregnated yarn tapes can be tested and screened before winding, the quality of the product can be controlled more accurately.

 

Dry winding has a high production efficiency, the winding speed can reach 100-200m/min, and the working environment is also cleaner.

 

 

 

Prepreg Tow Winding-Raw Material

Carbon fiber has a series of excellent properties such as high specific strength, high specific modulus, high temperature resistance, fatigue resistance, creep resistance, electrical conductivity, heat transfer and small thermal expansion coefficient. It can not only bear loads in the structure, but also play a role as a functional material.

 

Resin Matrix – Thermosetting Resin

 

Epoxy Resin

Epoxy resin is a type of polymer compound with a linear structure and belongs to thermosetting resin. This type of resin contains multiple epoxy groups and is a polymer prepolymer with aliphatic, alicyclic or aromatic chain segments as the main chain. Because the active groups it contains are easy to cross-link with various curing agents to form a network polymer with a certain cohesive force, it has good adhesion, stability, wear resistance, electrical insulation, processability, low curing shrinkage and high mechanical strength

 

Bismaleimide resin

Bismaleimide is a resin system derived from polyimide. It has similar fluidity and moldability to typical thermosetting resins and can be processed and molded by general methods similar to epoxy resins. BMI resin has a series of excellent properties such as high temperature resistance, radiation resistance, moisture and heat resistance, low moisture absorption rate and small thermal expansion coefficient.

 

Phenolic resin

The polycondensation product of phenols and aldehydes is generally called phenolic resin, which generally refers to a synthetic resin obtained by the polycondensation reaction of phenol and formaldehyde. Phenolic resin is a resin that is corrosion-resistant, heat-resistant, has good insulation, good flame retardancy and outstanding instantaneous high-temperature ablation resistance. Generally, small molecules are released during the curing process of phenolic resin reaction, so it must be cured under high pressure conditions.

 

Resin matrix-thermoplastic resin

 

Polyetheretherketone

Polyetheretherketone is a polymer composed of 1 ketone bond and 2 ether bond repeating units in the main chain structure. Because the macromolecular chain contains rigid benzene rings, flexible ether bonds and carbonyl groups that increase the intermolecular force, the structure is regular, so this type of resin has excellent comprehensive properties such as high heat resistance, high strength, high modulus, and high fracture toughness. Fiber-reinforced PEEK composites also have excellent flame retardant properties such as good self-extinguishing, low smoke and non-toxic combustion products, and can be used to manufacture interior decoration parts and cable components.

 

Polyphenylene sulfide

PPS is a polymer composed of phenylene sulfide as a repeating unit in the main chain structure. PPS has high strength, modulus and good product dimensional stability, small creep, extremely high fatigue resistance, good flame retardancy, low hygroscopicity, especially under high temperature and high humidity conditions, it does not deform, and can maintain excellent electrical insulation.

 

Polyetherimide

PEI is a polymer with ether and imide as the main chain structure characteristics, and the main monomers are aromatic diphenol and dinitrobisimide. In order to overcome the weaknesses of polyimide resin such as difficulty in processing and insufficient toughness, flexible ether bonds are introduced into the aromatic heterocyclic main chain to obtain polyetherimide.

 

Prepreg Tow Preparation Method

Currently, there are two main methods for preparing prepreg tow: direct prepreg method and slitting method.

 

The Two Methods Have Their Own Advantages and Disadvantages:

The direct prepreg method is to directly pre-impregnate the fiber bundle with resin and prepare the pre-impregnated tow by shaping. It can easily obtain a continuous length of pre-impregnated tow, and the requirements for equipment are not high, but the resin content and tow width are difficult to control, which has an adverse effect on the quality of the component; the slitting method is to accurately slit the traditional prepreg to obtain the pre-impregnated tow, and the resin content and tow width can be more accurately controlled, but the slitting may cause the tow to produce burrs or broken wires. At the same time, it is also difficult to control the resin pre-impregnation effect and prepare a continuous length of pre-impregnated yarn, which puts higher requirements on the slitting and rewinding equipment.

 

From the research results and application status at home and abroad, the direct prepreg method is mainly used for pre-impregnation/winding molding technology, and the slitting method is more suitable for automatic tow placement, which can achieve accurate placement of complex parts.

 

 

 

Prepreg Tow Winding Equipment

The winding machine is the main equipment of winding technology. The design intention and performance of the wound product must be realized through the winding machine. According to the control form, the winding machine can be divided into mechanical winding machine, digital control winding machine, microcomputer control winding machine and computer numerical control winding machine, which are actually the four stages of the development of winding machines. At present, the most commonly used ones are mainly mechanical and computer numerical control winding machines.

 

Mechanical winding machines can generally only achieve two degrees of freedom. Increasing the degrees of freedom will complicate the equipment. The emergence of microcomputer-controlled winding machines makes it easier to achieve multi-degree-of-freedom movement.

 

 

Differences Between Prepreg Tow Winding and Wet Winding Equipment

 

The prepreg tow winding system consists of a tension control system, a yarn guide device, and a special winding nozzle.

 

• Tension control system-the single filament bundle tension control range is larger than that of wet winding, and the yarn output speed is faster than that of wet winding.

 

• Yarn guide device-the prepreg tow is harder than wet winding under normal conditions, and the yarn guide device should avoid sharp bends to guide the yarn to reduce damage to the fiber.

 

• Winding nozzle-compared with wet winding, a prepreg tow heating and softening device is added.

 

Prepreg Winding Process-Winding Law

 

• The winding law is a law that describes the relative motion relationship between the winding nozzle and the core mold, so that the yarn tape can be evenly arranged on the surface of the core mold. The winding law is the technical key to ensure the quality of fiber winding products, and is the basis for the design of the winding machine movement mechanism and the strength of the product and the molding process.

 

• To achieve stable winding, the winding line must meet the following requirements: the fibers are neither overlapping nor separated, and are evenly and continuously distributed on the surface of the core mold; the fibers are stable on the surface of the core mold and do not slip.

 

Hoop Winding

Hoop winding is when the core mold rotates around itself at a constant speed, and the winding nozzle moves in a direction parallel to the axis of the core mold barrel. For each rotation of the core mold, the winding nozzle moves a yarn sheet width until the yarn sheet is evenly distributed on the surface of the core mold barrel section. Hoop winding can only be performed on the barrel section and only provides hoop strength. The hoop winding angle is usually between 85º and 90º

 

Spiral Winding

The core mold rotates at a constant speed around its own axis, and the winding nozzle reciprocates along the axis of the core mold at a specific speed. The basic line shape of spiral winding is composed of the space curve on the head and the spiral line of the cylinder section. The spirally wound fiber provides strength in both the longitude and latitude directions on the head, and provides strength in both the circumferential and longitudinal directions on the cylinder section.

 

 

 

• The angle between the winding fiber and the axis of rotation of the core mold is called the winding angle α•

 

Conditions for the fiber to be evenly distributed on the surface of the core mold

 

To make the fiber evenly wrapped around the surface of the core mold, several standard lines formed by continuously wound fibers are required.

 

The tangent points of a complete cycle divide the angle of rotation of the core mold equally, that is, the tangent points are evenly distributed on the circumference of the pole hole.

 

The yarn sheets corresponding to the previous complete cycle and the subsequent complete cycle are staggered in the barrel section by a distance equal to the width of a yarn sheet.

 

The main process parameters in the winding process are yarn sheet width, winding tension, winding speed, curing system, ambient temperature, humidity, glue impregnation and content distribution, etc.

 

Selecting reasonable winding process parameters is an important condition for giving full play to the characteristics of raw materials and manufacturing high-quality fiber winding products.

 

Prepreg Winding-Tape Winding

 

The tape winding molding process is one of the molding methods of the winding molding process.

 

The process of preheating the tape impregnated with resin glue on the winding machine with electric furnace and other heating equipment to soften and sticky, then winding it around the core mold according to the thread pattern, and then curing and demolding it into a composite material product is called tape winding process.

 

• The reinforcing materials of the tape winding molding process are mainly glass fiber cloth (tape) or polyester cloth, carbon cloth, etc., among which the glass fiber cloth includes various specifications of alkali-free glass fiber cloth, high silica glass fiber cloth, medium alkali glass fiber cloth, etc.;

 

• The resin matrix is ​​various synthetic resins, including barium phenolic resin, aminophenolic resin, epoxy resin, etc.

The tape winding molding process can be divided into cone winding and fixed-length tube winding according to the shape of the product; according to the direction of the cloth layer when the tape is wound, it can be roughly divided into parallel winding, overlapping winding and inclined winding.

 

 

Application

 

Cone Winding:

In aircraft, some products such as missile head shells and nozzle diffusers are designed into cone shapes due to aerodynamic performance requirements. For axisymmetric rotating cone products, the winding process undoubtedly has significant advantages. For cone products such as ablative nose cones that do not require structural functions but only require good ablation resistance, a single circumferential winding can be used.

 

 

Parallel Winding:

Wrapping the tape parallel to the core mold busbar is called parallel winding, that is, the core mold rotates one circle, and the distance the tape moves along the core mold axis is slightly less than the tape width. After wrapping the core mold surface in sequence, it is repeated to wrap another layer until the designed thickness is reached.

 

Overlapping Winding:

Overlapping winding is to wrap the tape continuously from the small end of the cone to the large end on the rotating core mold, and the direction of the tape is parallel to the direction of the cone axis. If used as a nozzle diffusion section, the airflow direction is in the direction of the overlapping seam of the inner layer of the cloth layer, so it has excellent erosion resistance.

 

Inclined Winding:

Inclined winding is also called oblique lap winding, which is to wrap the tape continuously from the large end of the cone to the small end on the rotating core mold, and the direction of the cloth layer is at a certain angle to the cone axis. When this kind of winding product is applied to the warhead cone heat protection layer, since the arrangement of the tape is in the direction of the airflow, it not only prevents the peeling layer phenomenon during airflow erosion, but also avoids the expansion of local defects. Thereby improving the ablation resistance of the product and improving the reliability of the product.

 

 

Automatic Wire Laying Technology

Automatic wire laying technology is developed on the basis of the integration of winding technology and automatic tape laying technology. It is based on the demand background of composite fuselage manufacturing. It makes corresponding improvements to the limitations of winding technology (the yarn trajectory must meet the winding rules of “periodicity, stability, and no overhead”, and the line shape and thickness changes are limited) and the shortcomings of automatic tape laying technology (the laying trajectory must comply with the “natural line” and “equal bandwidth laying” rules, so it is only suitable for laying small curvature wall panels, wings, etc.), and integrates the advantages of winding technology and automatic tape laying technology to innovate and form a new composite molding technology.

 

Automatic wire laying technology, like CNC machining, has extremely strong processing (laying) adaptability, and can achieve precise laying of complex parts including openings, reinforcements and other detailed structures.

 

In fact, if the laying time is not counted, automatic wire laying technology can manufacture complex components similar to mechanical processing.

 

Compared with automatic tape laying technology, the advantages of automatic fiber laying technology are mainly: it has the function of increasing or decreasing the number of yarn bundles according to the component profile, can automatically adjust the adaptation boundary according to the component shape, and the waste rate (3% to 8%) is very low.

 

It can complete local thickening, reinforcement, ply reduction, opening reinforcement and other operations, with greater freedom of laying trajectory and variable angle laying, which can adapt to the forming of complex components with large curvature.

 

Automatic tape laying technology uses CNC laying equipment to achieve continuous automatic cutting and laying of composite prepreg tapes through digital and automated means.

 

The composite prepreg tape roll is installed in the laying head, and the prepreg tape is exported by a set of rollers and pressed on the tool or the laid prepreg sheet by a pressing roller or a conformable mechanism. The cutting knife cuts the prepreg tape in the set direction.

 

While laying the prepreg tape, the return roller recycles the prepreg tape backing material.

 

Automatic tape laying technology is one of the composite material automation manufacturing technologies widely used in Europe and the United States. It mainly involves automatic laying equipment technology, prepreg tape cutting technology, laying CAD/CAM (computer-aided design/manufacturing) technology, automatic laying process technology, laying quality monitoring, mold technology, cost analysis, etc.

 

Automatic tape laying and automatic fiber laying use prepreg as raw materials, but the width of the prepreg used is different.

 

Generally, the prepreg used in automatic tape laying is called prepreg tape (Tape), and the prepreg used in automatic fiber laying is called prepreg tow (Tow or Fiber). The laying property of prepreg has a great influence on the laying quality, so automatic laying requires higher viscosity of prepreg than manual laying.

 

Automatic tape laying is mainly aimed at small curvature surface structures. In order to improve efficiency, prepreg tapes with a width of 75mm, 150mm, and 300mm are generally used for laying. Automatic fiber placement is mainly aimed at the forming of complex structures, so narrower prepreg tows are required. Currently, the widths generally used are 3.2mm (1/8) and 6.35mm (1/4), and 6.35mm (1/4) is mainly used in engineering applications.

 

The common features of automatic tape laying and automatic wire laying are automatic high-speed forming, reliable quality, and are mainly suitable for the forming of large composite components. Among them, automatic tape laying is mainly used for the automatic laying of small curvature curved surface components (such as wing surfaces and wall panels. Since the prepreg tape is wider, it is known for its high efficiency; while automatic wire laying focuses on the realization of complex double-curved surfaces (such as fuselages and wing-body fusions). It has a wide range of applications, but its efficiency is inferior to the former.

 

 

Automatic Wire Laying Equipment

 

Automatic wire laying equipment uses industrial robots as the main actuators for laying actions.

 

Typical automatic tow laying machines generally have 7 or more motion axes, including automatic tow laying machines, component support mechanisms, laying actuators, raw material supply mechanisms, control mechanisms, auxiliary mechanisms, and laying head mechanisms.

 

The laying actuators include industrial robots and robot mobile workbenches, and the laying instructions are executed by industrial robots.

 

The raw material supply mechanism mainly refers to the storage and conveying device of the prepreg tows, which stores the prepreg tows in a set environment and is transported to the laying head by a computer-controlled conveying shaft of 12 to 32 tows (referring to prepreg tows with a width of 3mm or 6mm), and the prepreg tows are laid on the surface of the core mold through the laying head.

The control and auxiliary mechanisms mainly refer to the robot control cabinet, compressed air source and other auxiliary devices.

 

 

 

 

The placement head is the core component of the automatic placement machine. Its function is to place the prepreg tow on the surface of the core mold. It is the most important actuator of the entire equipment. In the typical placement head structure, the placement head independently transports the prepreg tow, compacts it, and cuts it. After each prepreg tow is drawn out from the tow barrel, it reaches the placement head through the prepreg tow delivery system and is bundled by the placement head and placed on the mold surface.

 

The placement head includes: heating mechanism, placement mechanism, clamping mechanism, and connecting mechanism. The placement mechanism can complete the functions of re-feeding, cutting, and clamping prepreg. The clamping mechanism presses the prepreg tow delivered from the placement mechanism on the surface of the core mold. The connecting mechanism connects the heating mechanism, placement mechanism, and clamping mechanism with the placement robot.

 

 

Wire Laying Path Planning

 

The core issue of automatic wire laying technology is the wire laying path design. The wire laying path refers to the arrangement and distribution of composite fiber on the core mold wire laying surface, which is designed for automatic wire laying processing.

 

 

Automatic wire laying trajectory planning includes overall layer planning and single layer planning. Overall layer planning mainly determines the orientation, laying order, and the proportion and thickness of each layer relative to the total number of layers based on the design requirements of composite components. Through overall layer planning, the design requirements are refined to each layer, preparing for the design of the wire laying path on a single layer.

 

The components formed by wire laying are generally free-form curved components with smooth surfaces. The surface shape and structural differences of the components will directly affect the differences in the generation methods of the wire laying path.

 

 

Therefore, before planning the path, it is necessary to perform a geometric analysis of the surface, including the change trend and overall characteristics of the surface. In addition, continuous multi-layer laying of fiber bundles with a certain thickness on the mandrel will thicken the surface and cause geometric changes, especially some detailed features on the surface, such as local curvature of concave and convex parts, will change significantly, and even interference and collision of the laying head will occur.

 

 

• The automatic laying software system shows the entire process of composite materials from design to manufacturing. First, the designer completes the structural design of the composite parts in the 3D design software. Then, the process personnel convert the design input into manufacturing data according to the laying process requirements, and import it into the programming software to generate the laying trajectory.

 

 

Then, the trajectory data is post-processed and converted into a processing program executable by the CNC system, and the laying process is simulated in the virtual environment of the machine tool to detect the reliability of the processing program. Finally, the processing program is passed to the laying equipment to complete the manufacturing of the part.