An Article to Understand the Composite Pultrusion Process

 

Pultrusion Process Overview

Composite material pultrusion process technology is an advanced production method, mainly used to produce products with glass fiber, carbon fiber and other reinforced materials. This technology originated in the United States in 1948 and is used to manufacture FRP (fiber reinforced plastic) profiles with constant cross-sections. It has been developed and promoted throughout the world. Its advantages are high production efficiency, stable product quality, and low cost. With the continuous advancement of science and technology, this technology has developed rapidly in the past few decades and has gradually achieved industrial production.

 

 

Pultruded profiles are widely used in electrical equipment, corrosion-resistant components, construction engineering, transportation industry and military, and are currently in a stage of rapid development. Pultrusion can theoretically produce products of any length. The typical pultrusion line speed is 0.2~1.5m/min, the rapid molding rate can reach more than 4m/min, and multiple products can be produced at the same time, which greatly improves the molding efficiency and is suitable for mass production; in addition, the production process can be fully automated, and the cross-sectional shape of the product can be serialized and standardized, which significantly reduces the discreteness of the quality of composite products and has stable performance; the fiber content is high, up to 80%, because the fiber is fully straightened under tension during molding, the fiber performance can be fully utilized, the longitudinal mechanical properties are outstanding, and the raw material utilization rate can reach more than 95%.

 

02 Development of Pultrusion Molding Process

 

The research on pultruded FRP molding technology in China is not too late. In 1968, Beijing No. 251 Factory produced FRP tubes by pultrusion. In 1974, it produced grooved FRP profiles. In 1982, it produced the horizontal bars of parallel bars and uneven bars for gymnastic equipment, and successfully trial-produced electromechanical slot wedges based on phenolic resin.

 

In the 1970s, Wuhan University of Technology produced small-diameter round-section tie rods and receiving antennas by pultrusion. The above products are all made of domestic resin and glass fiber raw materials, and the pultrusion technology developed by self-explored process technology and equipment research and development.

 

Since 1985, more than 30 pultrusion FRP production lines have been introduced from abroad. The relevant units have also combined production reality, digested and absorbed foreign technology to independently design and process 70 production lines. The total production capacity of pultruded FRP molding in the country is nearly 30,000 tons.

 

In the early 1990s, the Hubei Shashi Steel Pipe Factory of the Petroleum and Natural Gas Corporation and the Qinhuangdao Yaohua FRP Factory combined the introduction of technology with self-development to develop and produce oil extraction sucker rods, which were recognized by the petroleum department and have been used in actual production. my country’s pultruded FRP industry ushered in its first spring. Large and small pultrusion plants have been established and began to develop the production of FRP door and window profiles by pultrusion.

 

03 Pultrusion Molding Process

 

3.1 Typical Pultrusion Molding Process

 

Glass fiber roving arrangement-impregnation-preforming-extrusion molding and curing-traction-cutting-products

 

After the untwisted roving is drawn out from the creel, it passes through the yarn dispensing device and enters the impregnation tank to be soaked in the resin glue, then enters the preforming mold to discharge the excess resin and bubbles, and then enters the molding mold to gel and solidify. The solidified product is continuously pulled out of the mold by the traction machine, and finally cut to a fixed length by the cutting machine. In the molding process, each process can have different methods: for example, in the yarn feeding process, continuous fiber mats can be added, hoop-wound yarn or three-way fabric can be used to improve the lateral strength of the product; the traction process can be a crawler traction machine or a manipulator; the curing method can be in-mold curing or curing with a heating furnace; the heating method can be high-frequency electric heating or molten metal (low melting point metal), etc.

 

Raw Materials for Pultrusion Process

① Resin matrix: In the pultrusion process, the most used is unsaturated polyester resin, which accounts for more than 90% of the resin used in this process. In addition, there are epoxy resin, vinyl resin, thermosetting methacrylate resin, modified phenolic resin, flame retardant resin, etc.

② Reinforcement materials: The reinforcement materials used in the pultrusion process are mainly glass fibers and their products, such as untwisted roving, continuous fiber mats, etc. In order to meet the special performance requirements of the products, aramid fibers, carbon fibers and metal fibers can be selected. Regardless of the type of fiber, when used in the pultrusion process, its surface must be treated so that it can be well bonded to the resin matrix.

③ Auxiliary materials: The auxiliary materials of the pultrusion process mainly include release agents and fillers.

 

3.2 Composition of Pultrusion Molding Equipment

 

① Reinforcement material delivery system: such as yarn racks, felt spreading devices, yarn holes, etc.

② Resin impregnation: The straight slot impregnation method is most commonly used. During the entire impregnation process, the fibers and felts should be arranged very neatly.

③ Preforming: The impregnated reinforcement materials pass through the preforming device and are carefully transferred in a continuous manner to ensure their relative positions, gradually approach the final shape of the product, and squeeze out excess resin, and then enter the mold for molding and curing.

④ Mold: The mold is designed under the conditions determined by the system. According to the resin curing exothermic curve and the friction performance between the material and the mold, the mold is divided into three different heating zones, and its temperature is determined by the performance of the resin system. The mold is the most critical part of the pultrusion process, and the length of the typical mold ranges from 0.6 to 1.2m.

⑤ Traction device: The traction device itself can be a crawler-type puller or two reciprocating clamping devices to ensure continuous movement.

⑥ Cutting device: The profile is cut to the required length by an automatic synchronously moving cutting saw.

 

3.3 Precautions for Pultrusion Process Operation

 

The function of the molding mold is to achieve the compaction, molding and curing of the blank. The cross-sectional size of the mold should take into account the molding shrinkage of the resin. The mold length is related to the curing speed, mold temperature, product size, pultrusion speed, and the properties of the reinforcing material, and is generally 600 to 1200mm.

 

The mold cavity should have a high finish to reduce friction, extend service life, and be easy to demold. It is usually heated by electricity, and microwave heating is used for high-performance composite materials. A cooling device is required at the mold entrance to prevent the glue from curing prematurely. The dipping process mainly controls the relative density (viscosity) of the glue and the dipping time. Its requirements and influencing factors are the same as those of prepreg.

 

The curing molding process mainly controls the molding temperature, mold temperature distribution, and the time (pultrusion speed) of the material passing through the mold. This is the key process of the pultrusion molding process. During the pultrusion molding process, the prepreg produces a series of complex physical, chemical and physicochemical changes when passing through the mold, which is still not very clear.

 

Generally speaking, according to the state of the prepreg when passing through the mold, the mold can be divided into three areas. The reinforcement material passes through the mold at a constant speed, while the resin is different. At the entrance of the mold, the behavior of the resin is similar to that of a Newtonian fluid. The viscous resistance between the resin and the inner wall surface of the mold slows down the forward speed of the resin, and gradually returns to a level equivalent to that of the fiber as the distance from the inner surface of the mold increases.

 

During the forward process of the prepreg, the resin undergoes a cross-linking reaction when heated, the viscosity decreases, the viscous resistance increases, and it begins to gel, enters the gel zone, gradually hardens, shrinks and separates from the mold. The resin and the fiber move forward evenly at the same speed. The curing is continued by heating in the curing zone, and the specified degree of curing is ensured when the mold is removed. The curing temperature is usually greater than the peak value of the exothermic peak of the glue, and the temperature, gel time and pulling speed are matched. The temperature of the preheating zone should be low, and the temperature distribution should be controlled so that the curing exothermic peak appears later in the middle of the mold, and the release point is controlled in the middle of the mold.

 

The temperature difference of the three sections is controlled at 20-30℃, and the temperature gradient should not be too large. The effect of the exothermic reaction of the curing reaction should also be considered. Usually, three pairs of heating systems are used to control the temperature in the three areas.

 

The pulling force is the key to ensure the smooth demolding of the product. The magnitude of the pulling force depends on the interface shear stress between the product and the mold. The shear stress decreases with the increase of the pulling speed, and three peaks appear at the entrance, middle and exit of the mold.

 

The peak at the entrance is caused by the viscous resistance of the resin at that location. Its size depends on the properties of the resin viscous fluid, the temperature at the entrance and the filler content. In the mold, the viscosity of the resin decreases with the increase of temperature, and the shear stress decreases. As the curing reaction proceeds, the viscosity and shear stress increase. The second peak corresponds to the detachment point and decreases significantly with the increase of traction speed. The third peak is at the exit, which is caused by the friction between the product and the inner wall of the mold after solidification, and its value is smaller.

 

Traction force is very important in process control. To make the surface of the product smooth, the shear stress (the second peak) at the detachment point is required to be small and to detach from the mold as soon as possible. The change of traction force reflects the reaction state of the product in the mold and is related to the fiber content, product shape and size, release agent, temperature, traction speed, etc.

 

Pultrusion is a Special Process in the Composite Material Molding Process. Its Advantages are:

① The production process is fully automated and the production efficiency is high;

② The fiber content in the pultruded product can be as high as 80%, and the dipping is carried out under tension, which can give full play to the role of the reinforcing material and the product strength is high;

③ The longitudinal and transverse strength of the product can be adjusted arbitrarily to meet the use requirements of products with different mechanical properties;

④ There is no scrap in the production process, and the product does not need post-processing, so it saves labor, raw materials and energy consumption compared with other processes;

⑤ The product quality is stable, the repeatability is good, and the length can be cut arbitrarily.

 

04 Application of Composite Pultrusion Technology

 

Composite pultrusion technology has been widely used in various fields. The following are several major application areas:

 

1. Aerospace field: Composite materials have the characteristics of light weight and high strength, so they are widely used in the aerospace field. The composite pultrusion technology can produce high-quality aerospace parts, such as wings and fuselages.

 

2. Automobile field: The automobile field is one of the important application fields of composite pultrusion technology. This technology can be used to produce lightweight and high-strength automotive parts, such as frames and bodies, thereby improving the performance and fuel economy of automobiles.

 

3. Construction field: In the construction field, composite pultrusion technology is mainly used to prepare building materials such as glass fiber reinforced concrete. These materials have the characteristics of light weight, high strength, and corrosion resistance, which can greatly improve the safety and service life of buildings.

 

4. Other fields: In addition to the above fields, composite pultrusion technology is also widely used in petrochemical, electronic and electrical fields. For example, this technology can be used to produce high-quality pipes, containers and other products for use in the petrochemical industry; it can also be used to produce lightweight, high-strength circuit boards and other products for use in the electrical and electronic fields.