Introduction to the Main Parameters of Pultrusion Process

 

Overview of FRP Pultrusion Molding Process

 

As the application field of FRP pultrusion products continues to expand, the specifications and varieties of foreign pultrusion products are also increasing. At present, in addition to the standard pultrusion product shapes such as L-type, O-type, U-type, flat type, hollow or solid core, various special-shaped structures can also be produced according to customer requirements. The core materials of some porous cavity products have now been standardized.

 

The size of pultruded composite products ranges from a few square millimeters to tens of square meters, such as pultruded products for bridge decks.

 

 

There are also many types of reinforcing materials used in the FRP pultrusion process, such as glass fiber roving, felt, thin cloth or glass fiber fabric, carbon fiber, aramid fiber and their fabrics.

 

The matrix resin materials used in pultrusion are divided into two categories: thermoplastic resin and thermosetting resin. Thermosetting resins such as polyester resin, epoxy resin, vinyl ester resin and phenolic resin are often used in the production of large batches of pultruded products; while thermoplastic resin matrix is ​​in the stage of development and production.

 

At present, the standard equipment for horizontal pultrusion is generally 20 to 30 meters long and the maximum width is about 1.5 meters. The entry end of this standard equipment production line is a glass fiber yarn supply warehouse, followed by dried or preheated glass fiber yarn, which passes through the thermosetting resin dipping tank, is formed in the mold, and is cured after heating.

 

Usually, there is a relatively long distance between the molding mold and the puller, and the FRP product can complete the curing process and gradually cool down within this distance. On the production line, a clamp is used to clamp the product from the pultrusion die and pull the FRP product out. Finally, the pultruded product is cut into fixed-length products by a cutting machine.

 

The Process and Control Parameters of FRP Pultrusion Molding

 

The FRP pultrusion molding process has a total of 8 processes: spinning, pre-impregnation, heating, product curing and calibration measurement of dimensions, cooling, pulling and cutting.

 

Usually, each process has a process parameter that can be adjusted within a certain range. Some of these process parameters can be directly adjusted through the pultrusion equipment, such as the temperature of the mold, the speed of pulling, etc. However, some other process parameters, such as the temperature of the pultruded product, the stress condition, the viscosity of the resin, etc., cannot be directly adjusted through the equipment.

 

 

Obviously, all process parameters will have a certain impact on the quality of pultruded products, including mechanical properties and optical properties. The most important processes are pre-impregnation, molding and curing.

 

It must be pointed out that the process parameters of a certain process will have a certain impact on other processes. For example, the speed of pulling will have a certain impact on the above three main processes.

 

Due to the mutual influence of pultrusion process parameters, it is still impossible to establish a practical process model to achieve the predetermined goal of pultrusion product quality.

 

FRP Pultrusion Process Parameter Control Element

 

As mentioned above, due to the thermosetting resin pultrusion process parameter conditions, it is restricted by some complex factors occurring in the molding die, and is also subject to the mutual influence between other process parameters. Therefore, during pultrusion, the polymerization reaction in the raw materials is also difficult to accurately predict.

 

At present, the commonly used monitoring and control sensor elements in FRP pultrusion dies are: temperature sensors, pressure sensors and dielectric sensors. The above sensors must first solve the problem of pultrusion wear resistance.

 

In addition, the temperature of the pultrusion die, the volume content of the glass fiber, and the speed of the pulling will also have a certain impact on the pultrusion process parameter sensor.

 

The temperature detection sensor element used in the currently commonly used FRP pultrusion equipment is often placed along the direction of the fiber and on the surface of the molding die; while the pressure detection sensor element is often placed at the entrance of the pultrusion die and in the middle of the die (usually the length of the pultrusion die is about 1000mm). The surface of this tension pressure sensor element is often coated with a chromium layer to improve its resistance to glass fiber pultrusion wear.

 

The dielectric sensor elements used in pultrusion equipment are of two types, thin film type and fixed ceramic type. The basic principle of this dielectric sensor element is that the polymer is used as the medium between the two plates. When it is in an alternating electric field, the polymer molecules will move.

 

Due to the change of the alternating frequency, the performance parameters such as the molecular weight of the polymer (which can also be expressed as the degree of polymerization), viscosity, and conductivity will also change. That is, the lower the viscosity of the polymer, the higher the conductivity and the lower its resistance.

 

The thin-film dielectric sensor is pulled out of the mold cavity along with the glass fiber. The sensor will remain in the mold after the product is cured, so it can only be used once and is not suitable for industrial mass production. Fixed dielectric sensors are a type of sensor that belongs to the double-plate capacitor type.

 

The sensor will serve as one of the plates, and the other plate is the mold itself. But the Index thin-film sensor itself is equipped with two plates. The arrangement between their plates is similar to the structure of a printed circuit board.

 

Due to their different structures, the conductivity performance of the above two dielectric sensors cannot be directly compared.