Why is pultrusion the preferred process for high-precision fiberglass profiles?

Pultrusion is a continuous process for producing composite profiles. It involves impregnating untwisted fiberglass rovings and other continuous reinforcing materials, such as polyester surface mat, onto a scaffold with resin. The resin is then passed through a mold maintaining a specific cross-sectional shape, cured within the mold, and continuously extruded to form pultruded products. This is an automated production process.

 

Products produced using the pultrusion process have higher tensile strength than ordinary steel. The resin-rich surface layer also provides excellent corrosion resistance, making it the best alternative to steel in corrosive environments. It is widely used in transportation, electrical engineering, electrical insulation, chemical industry, mining, marine engineering, shipbuilding, corrosive environments, and various domestic and civilian applications.

 

Pultrusion Molding Process Flow: Pultrusion processes come in many forms and can be classified in many ways. These include intermittent and continuous processes, vertical and horizontal processes, wet and dry processes, tracked and clamped traction, in-mold curing and in-mold gelation with out-of-mold curing, and heating methods such as electric heating, infrared heating, high-frequency heating, microwave heating, or a combination of these methods.

 

The typical pultrusion process flow is as follows: Glass fiber roving arrangement – ​​Impregnation – Preforming – Extrusion molding and curing – Traction – Cutting – Finished product

 

Pultrusion equipment components:

 

1. Reinforcing material conveying system: such as roving frame, mat spreading device, roving holes, etc.

 

2. Resin impregnation: The straight groove impregnation method is most commonly used. Throughout the impregnation process, the fibers and mats should be arranged very neatly.

 

3. Preforming: The impregnated reinforcing material passes through the preforming device, being carefully and continuously conveyed to ensure their relative positions, gradually approaching the final shape of the product, and extruding excess resin before entering the mold for molding and curing.

 

4. Mold: The mold is designed under defined system conditions. Based on the resin curing exothermic curve and the frictional properties between the material and the mold, the mold is divided into three different heating zones, the temperature of which is determined by the properties of the resin system. The mold is the most critical part of the pultrusion process; the typical mold length ranges from 0.6 to 1.2 meters.

 

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

 

6. Cutting Device: The profile is cut to the required length by an automatically moving synchronous cutting saw.

 

The function of the molding die is to achieve compaction, molding, and curing of the preform. The die cross-sectional dimensions should take into account the molding shrinkage rate of the resin. The die length is related to the curing speed, die temperature, product size, pultrusion speed, and the properties of the reinforcing material, and is generally 600–1200 mm. A high surface finish in the die cavity is necessary to reduce friction, extend service life, and facilitate demolding. Electric heating is usually used; microwave heating is used for high-performance composite materials. A cooling device is required at the die inlet to prevent premature curing of the adhesive. The impregnation process mainly controls the relative density (viscosity) of the adhesive and the impregnation time. Its requirements and influencing factors are the same as for prepreg.

 

The curing and molding process mainly controls the molding temperature, die temperature distribution, and the time the material passes through the die (pultrusion speed). This is the key process in pultrusion molding. During pultrusion, the prepreg undergoes a series of complex physical, chemical, and physicochemical changes as it passes through the mold, which remain largely unclear. Broadly speaking, the mold can be divided into three zones based on the state of the prepreg as it passes through. The reinforcing material passes through the mold at a constant speed, while the resin behaves differently. At the mold entrance, the resin behaves approximately like a Newtonian fluid; the viscous resistance between the resin and the mold's inner wall slows its forward velocity, gradually returning to a level comparable to that of the fiber as the distance from the inner mold surface increases.

 

As the prepreg moves forward, it undergoes a cross-linking reaction upon heating, its viscosity decreases, its viscous resistance increases, and it begins to gel, entering the gel zone. It gradually hardens, shrinks, and detaches from the mold. The resin moves forward uniformly with the fiber at the same speed. In the curing zone, it continues to cure under heat, ensuring the specified degree of curing is achieved upon demolding. The curing temperature is typically higher than the peak value of the exothermic peak of the resin, and the temperature, gel time, and traction speed are matched.

 

The preheating zone temperature should be relatively low, and the temperature distribution should be controlled so that the curing exothermic peak appears slightly later in the middle of the mold, with the detachment point controlled in the middle of the mold. The temperature difference between the three sections should be controlled between 20 and 30°C, and the temperature gradient should not be too large. The effect of exothermic curing reaction should also be considered. Typically, three pairs of heating systems are used to control the temperature of each of the three zones.

 

Traction force is crucial for ensuring smooth demolding. The magnitude of the traction force depends on the interfacial shear stress between the product and the mold. Shear stress decreases with increasing traction speed, exhibiting three peaks at the mold inlet, middle, and outlet. The peak at the inlet is caused by the viscous resistance of the resin at that point. Its magnitude depends on the properties of the resin viscous fluid, the inlet temperature, and the filler content. Inside the mold, resin viscosity decreases with increasing temperature, and shear stress decreases. As the curing reaction proceeds, viscosity and shear stress increase. The second peak corresponds to the release point and decreases significantly with increasing traction speed. The third peak is at the outlet, generated by the friction between the cured product and the inner wall of the mold, and its value is relatively small. Traction force is very important in process control. To achieve a smooth product surface, the shear stress at the release point (the second peak) must be small, and the product must be released from the mold as early as possible. Changes in traction force reflect the reaction state of the product in the mold and are related to fiber content, product shape and size, release agent, temperature, traction speed, etc.