Revealed! FRP Pultrusion Process Parameters

 

My country’s pultrusion industry has gone through more than ten years of history from scratch, and has made gratifying progress in both product variety and output. However, there is still a big gap compared with the advanced level of foreign countries.

 

In addition to the limitations in raw material selection, the accuracy, stability and mutual matching of pultrusion process parameters are the key to the success or failure of pultrusion process.

 

Pultrusion molding process parameters are a mutually restrained and large and precise system, including molding temperature, traction speed, formula design, filling amount, etc. Fully understanding the resin reaction kinetics in the pultrusion process, the mutual influence of process parameters and their interaction with product performance are the key to determining whether the product can achieve design requirements and achieve smooth production.

 

1. Molding Temperature

 

In the pultrusion molding process, the changes that occur when the material passes through the die are the most critical and the focus of pultrusion process research. So far, although there are many research methods, such as the application of mathematical models, computer simulation, and available tools such as pressure sensors, etc., researchers still do not understand what is happening in the mold, but only put forward a series of speculations and hypotheses based on experimental and theoretical research.

 

Generally speaking, it is believed that after the glass fiber is impregnated with glue, it passes through a heated metal mold, and the mold is divided into three parts according to its different states in the mold, as shown in the figure.

 

Schematic diagram of the velocity curve of the resin in the pultrusion die and the viscosity and friction forces in different areas

 

The figure above shows the main characteristics of the material as it passes through the die. Although the reinforcement material must pass through the die at the same speed, there are areas where the resin and fiber have relative flow. The figure plots the resin velocity distribution in the area near the die inlet and outlet. In the die inlet area, the resin behaves like a Newtonian fluid, and the boundary condition of the wall velocity means zero. At a short distance from the die wall, the flow velocity of the resin increases to a level comparable to that of the reinforcement material. On the inner wall surface of the die, the resin produces viscous resistance.

 

In the three-stage die, this continuous pultrusion process is artificially divided into a preheating zone, a gelling zone, and a curing zone. Three pairs of heating plates are used on the die to heat it, and the temperature is controlled by a computer. The breakaway point is the point where the resin breaks away from the die. During the heating process of the resin, the temperature gradually increases and the viscosity decreases. After passing through the preheating zone, the resin system begins to gel and solidify. At this time, the viscous resistance at the interface between the product and the mold increases, the boundary condition of zero speed on the wall is broken, and the resin has a sudden change in speed at the detachment point. The resin and the reinforcement material move evenly at the same speed. The product continues to solidify under heating in the curing zone to ensure sufficient curing degree when it is ejected from the mold.

 

1. Determination of Temperature

The heating conditions of the mold are determined according to the resin system. Taking the polyester resin formula as an example, the resin system is first dynamically scanned by a differential scanning calorimeter (DSC) to obtain the exothermic peak curve.

 

Generally speaking, the mold temperature should be greater than the exothermic peak of the resin, and the upper temperature limit is the degradation temperature of the resin. At the same time, the gelling experiment of the resin is carried out, and the temperature, gelling time, and pulling speed should match. The temperature of the preheating zone can be lower, and the temperature of the gelling zone is similar to that of the curing zone. The temperature distribution should make the product curing exothermic peak appear in the middle of the mold, and the gelling and curing separation point should be controlled in the middle of the mold. Generally, the temperature difference between the three sections is controlled at about 20-30℃, and the temperature gradient should not be too large.

 

2. Optimal Mold Temperature Distribution and Analysis

Previously, when analyzing heat transfer in pultruded profiles and profile curing, it was assumed that the mold temperature was known. In fact, a complete and scientific pultrusion process model must include heat transfer in the profile and in the mold. Once the fiber impregnated with resin enters the mold, its heat is transferred from the mold wall to the profile. The resin close to the mold is heated before the resin in the center of the profile, resulting in gelation; after curing, the exothermic reaction will cause the center temperature to be higher than the temperature of the mold wall.

 

After curing, due to volume shrinkage, the resin will detach from the mold wall due to shrinkage. Under several assumed conditions, a model for heat transfer in the profile was established, and relevant scholars have conducted in-depth research on this. Because the pultrusion mold is a metal mold and a good thermal conductor, the heat energy of the mold will be lost in the longitudinal and transverse directions of the mold. Establishing a mold temperature model helps us understand the mold temperature distribution law.

 

The configuration of the heater has a great influence on the temperature in the core and the mold temperature. Generally, under certain agreed conditions, the position of the curing peak moves with the movement of the heater, while the distance between the heating belt and the core temperature peak remains basically unchanged. This movement of the exothermic position is normal, the heat flux from the heater is limited, and under these conditions, the curing is controlled by the heater. When the heat transfer is controlled by “dynamics”, the core temperature peak is not sensitive to the position of the heater due to the constraints of line speed and preheating temperature.

 

The effects of heat preservation around the mold and reducing the heat transfer coefficient of the air are the same. When the heat transfer coefficient is reduced, the temperature of the back half of the mold increases, and the heat distribution of the entire mold is more uniform. Because most resin curing occurs near the heater, the effect of heat preservation on the core temperature is small. When the exothermic peak is far away from the heating belt, the mold is best to choose heat preservation.

 

Using the mold temperature model to analyze the pultrusion process and computer-aided design of the pultrusion process parameters is currently a reasonable, simple and efficient design tool.

 

2. Determination of Pultrusion Speed

The length of the pultrusion die is generally 0.6 to 1.2m. The mold temperature is determined by the curing exothermic curve of the resin system. The temperature must also fully consider the product gelling and solidifying in the middle of the mold, that is, the detachment point is in the middle and as far forward as possible.

 

If the pultrusion speed is too fast, the product is not well cured or cannot be cured, which directly affects the product quality, and the product surface will have a thick, resin-rich layer; if the pultrusion speed is too slow, the profile stays in the mold for too long, the product is over-cured, and the production efficiency is reduced.

 

The general experimental pultrusion speed is about 300mm/min. At the beginning of the pultrusion process, the speed should be slowed down and then gradually increased to the normal pultrusion speed. The general pultrusion speed is 300-500mm/min. One of the development directions of modern pultrusion technology is high speed. At present, the fastest pultrusion speed can reach 15m/min.

 

3. Traction

The traction is the key to ensure the smooth demolding of the product. The magnitude of the traction is determined by the shear stress on the interface between the product and the mold. By measuring the traction of the resin-impregnated reinforcing fiber being pulled through a short distance of the mold, the shear stress on the above interface can be measured and its characteristic curve can be drawn.

 

Figure 9-48 shows the change of average shear stress when three different traction speeds pass through the mold.

 

Although these results are not very accurate, they are sufficient as a qualitative analysis.

 

 

Pulling speed and shear stress

 

From the figure, we can see that the shear force curve in the mold changes with the change of pulling speed. Temporarily ignoring the influence of pulling speed, it can be found that the shear force is different at different positions of the mold. There are three peaks in the curve of the whole mold, which are discussed below.

 

The shear stress peak at the mold inlet is consistent with the viscous resistance of the resin near the mold wall. By heating, in the mold preheating zone, the resin viscosity decreases with the increase of temperature, and the shear stress also begins to decrease. The change of the initial peak value is determined by the properties of the resin viscous fluid. In addition, the filler content and the mold inlet temperature also have a great influence on the initial shear force.

 

Due to the curing reaction of the resin, its viscosity increases and produces the second shear stress peak. This value corresponds to the separation point between the resin and the mold wall, and is closely related to the pulling speed. When the pulling speed increases, the shear stress at this point is greatly reduced.

 

Finally, in the third area, that is, at the mold outlet, continuous shear stress appears, which is caused by the friction between the product and the mold wall in the curing zone. This friction is small.

 

Traction force is very important in process control. If you want to make the surface of the product smooth during molding, the shear stress of the product at the detachment point is small and it is required to detach from the mold as soon as possible. The change of traction reflects the reaction state of the product in the mold, which is related to many factors, such as fiber content, product geometry and size, release agent, temperature, pulling speed, etc.

 

1. Correlation of Various Pultrusion Process Variables

 

1. Relationship Between Thermal Parameters, Pultrusion Speed and Traction

Among the three process parameters of thermal parameters, pulling speed and traction, the thermal parameter is determined by the characteristics of the resin system and is the primary factor that should be solved in the pultrusion process. The temperature values ​​of each section of mold heating are determined by the peak value and related conditions of the DSC curve of the resin curing system.

The principle of determining the pultrusion speed is to ensure that the product is gelled and cured in the middle of the mold at a given in-mold temperature.

 

There are many restrictive factors for traction, such as: it is closely related to the mold temperature and is controlled by the pultrusion speed. From the previous analysis, it can be seen that the increase in pulling speed directly affects the second peak of shear stress, that is, the shear stress at the detachment point; the influence of the release agent is also a factor that cannot be ignored.

 

2. Optimization of T-V-F Process Parameters

 

The mold temperature distribution determined by the exothermic peak curve of the resin system curing is the premise for us to determine other process parameters. The pulling speed selected must match the temperature. When the mold temperature is high, the pulling speed should be increased. The gel point of the resin can be determined by adjusting the mold temperature and the pulling speed. When the mold temperature is too high or the reaction is too fast, it will cause thermal cracking of the product. Therefore, using the zone heating mold to divide the heating zone into the preheating zone, the gelling zone and the curing reaction zone can optimize the pultrusion process and reduce the thermal cracking of the product.

 

In order to improve production efficiency, the pulling speed is generally increased as much as possible. This can increase the shear stress of the mold and the surface quality of the product. For thicker products, a lower pulling speed or a longer mold should be selected to increase the mold temperature. The purpose is to make the product better solidified, thereby improving the performance of the product.

 

In order to reduce the pulling force and make the product demold smoothly, it is necessary to use a good demolding agent, which sometimes plays a decisive role in the molding process.

 

1. Resin Preheating and Product Post-Curing

 

Preheating the resin before entering the mold is very beneficial to the process. This may reduce the curing reaction temperature of the resin and make the product surface excellent. Radio frequency (RF) preheating has a good effect. Preheating increases the resin temperature, reduces the viscosity, increases the fiber impregnation effect, and creates conditions for increasing the pulling speed. Many resin systems, such as epoxy resin, require preheating.

 

The effect of preheating is also reflected in reducing the temperature gradient inside and outside the impregnated fiber bundle. Because after entering the mold, the heat transferred from the mold to the product is distributed in a ladder shape from the surface of the product to the center of the product, and the temperature of the center line of the product is lower than the temperature of the surface of the product. Similarly, the curing of the center of the product lags behind the curing of the surface of the product. If the pultrusion speed is increased, the lag in temperature and curing degree between the center line and the surface of the product will increase.

 

Then, the lag will decrease inversely with the increase in curing heat release, and finally even the center temperature of the product will be higher than the surface temperature. In order to achieve uniform curing inside and outside the product and reduce thermal stress, the resin should be preheated.

 

If the curing degree of the product after demolding does not meet the requirements, post-curing treatment is required. Generally speaking, the product is naturally cooled in the air after demolding. During this process, the curing reaction continues. The general post-curing treatment is: put the cut products in a constant temperature box for a period of time to allow the products to reach the required degree of curing.