Our country's pultrusion industry has experienced more than ten years of history from scratch, and has made gratifying progress in terms of product variety and output. But compared with the advanced level abroad, there is still a big gap. In addition to the limitations in the selection of raw materials, the accuracy, stability, and mutual compatibility of the pultrusion process parameters are the key to the success or failure of the pultrusion process. Pultrusion process parameters are a huge and precise system that is mutually constrained, including forming temperature, traction speed, formula design, filling amount, etc. Fully understanding the resin reaction kinetics in the pultrusion process, the interaction between process parameters and their interaction with the performance of the product is the key to determining whether the product can meet the design requirements and achieve smooth production.
1. Molding temperature
In the pultrusion process, the changes that occur when the material passes through the mold are the most critical, and it is also the focus of research on the pultrusion process. Up to now, although there are many research methods, such as the application of mathematical models, computer simulation, and available tools such as pressure sensors, etc., so far researchers still do not understand what is happening in the mold, just based on experiments And theoretical research put forward a series of speculations and assumptions.
Generally speaking, the metal mold that is heated by dipping glass fiber is considered to be divided into three parts according to its different states in the mold, as shown in the figure.
Schematic diagram of the speed curve of the resin in the pultrusion mold and the viscous and frictional forces in different areas
The figure above shows the main features of the material as it passes through the mold. Although the reinforcing material must pass through the mold at the same speed, in certain areas, the resin and fibers have a relative flow. The figure shows the distribution of resin velocity in the vicinity of the mold inlet and outlet. In the mold inlet area, the resin behaves like Newtonian fluid, and the boundary condition of the wall velocity means zero. At a short distance from the mold wall, the resin flow rate increases to a level comparable to that of the reinforcing material. On the inner wall surface of the mold, the resin generates viscous resistance.
In the three-stage die, this continuous pultrusion process is artificially divided into preheating zone, gelling zone and curing zone. Use three pairs of heating plates to heat the mold, and use a computer to control the temperature. The detachment point refers to the point at which the resin detaches from the mold. During the heating 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 velocity on the wall is broken, and the speed of the resin changes suddenly at the point of separation. The resin and the reinforcing material They move uniformly at the same speed together, and the product continues to solidify in the curing zone by heating to ensure sufficient curing degree when the mold is released.
1. Determination of temperature
The heating conditions of the mold are determined according to the resin system. Taking the polyester resin formulation as an example, first of all, the resin system is dynamically scanned by a differential scanning calorimeter (DSC) to obtain an exothermic peak curve. In general, the mold temperature should be greater than the peak exotherm of the resin, and the upper temperature limit is the degradation temperature of the resin. Simultaneously for resin gelation experiments, the temperature, gelation time, and pulling speed should match. The temperature in the preheating zone can be lower, and the temperature in the gelling zone and the curing zone are similar. The temperature distribution should make the product curing exothermic peak appear in the middle of the mold, and the gel solidification separation point should be controlled in the middle of the mold. Generally, the three-stage temperature difference is controlled at about 20-30℃, and the temperature gradient should not be too large.
2. The best mold temperature distribution and analysis
Previous analysis of heat transfer in pultruded profiles and curing of profiles assumes that the die temperature is known. In fact, a complete and scientific pultrusion process model must include the heat transfer in the profile and the mold. Once the resin-impregnated fiber 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 reaction exotherm will cause the center temperature to be higher The temperature of the mold wall. Due to volume shrinkage after curing, the resin will detach from the mold wall due to shrinkage. Under several assumptions, a model has been established for the heat energy transfer in the profile, and relevant scholars have made in-depth research on it. Because the pultrusion die is a metal die and a good heat conductor, the heat energy of the die is lost in the longitudinal and lateral directions of the die. Establishing the 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, and the distance between the heating belt and the core temperature peak is basically unchanged. This movement of the heat release position is normal, the heat flux from the heater is limited, and under these conditions, the curing is controlled by the heater, and when the heat transfer is controlled by "kinetics", the linear velocity And preheating temperature constraints, the temperature peak in the core is not sensitive to the position of the heater.
The effects of keeping the heat around the mold and reducing the heat transfer coefficient of the air are the same. When the heat transfer coefficient decreases, the temperature of the second half of the mold increases, and the heat distribution of the entire mold is more uniform. Because most resin curing takes place close to the heater, the effect of heat preservation on the temperature of the core is small. When the heat release peak is far from the heating belt, the mold is best to choose heat preservation.
Using the die 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 curing temperature of the resin system determines the die temperature. This temperature must also be fully considered to make the product gel in the middle of the die, that is, the point of separation is in the middle and as far as possible. If the pultrusion speed is too fast, the product is poorly cured or cannot be cured, which directly affects the quality of the product, the product surface layer will have a thick, resin-rich layer; if the pultrusion speed is too slow, the profile stays in the mold for too long, and the product is over cured , And affect the reduction of production efficiency.
The general experimental pultrusion speed is about 300mm/min. When the pultrusion process starts, the speed should be slowed down and then gradually increased to the normal pultrusion speed. Generally, the pultrusion speed is 300～500mm/min. One of the development directions of modern pultrusion technology is high speed. The fastest pultrusion speed is up to 15m/min.
The traction force is the key to ensure the smooth exit of the product. The traction force is determined by the shear stress at the interface between the product and the mold. The shear stress on the above interface can be measured by measuring the traction force of the resin-impregnated reinforcing fiber drawn through the mold for a short distance, and its characteristic curve is drawn.
Traction speed and shear stress
It can be seen from the figure that the shear force curve in the mold changes with the change of the pulling speed. Ignoring the influence of the pulling speed for a while, 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 entire mold, which are discussed separately below.
The peak of the shear stress at the entrance of the mold is consistent with the viscous resistance of the resin near the mold wall. By increasing the temperature, in the preheating zone of the mold, the resin viscosity decreases with increasing temperature, and the shear stress also begins to decrease. The change of the initial peak value is determined by the nature of the resin viscous fluid. In addition, the filler content and mold inlet temperature also greatly affect the initial shear force.
Due to the curing reaction of the resin, its viscosity increases and a second shear stress peak occurs. This value corresponds to the detachment point of the resin from the mold wall and has a great relationship with the pulling speed. When the traction speed increases, the shear stress at this point is greatly reduced.
Finally, in the third area, that is, at the exit of the mold, there is continuous shear stress, which is caused by the friction of the product with the mold wall in the curing zone. This friction force is small.
Traction is very important in process control. If you want to make the surface of the product smooth during molding, it is required that the shear stress of the product at the detachment point is small, and it should be detached from the mold as soon as possible. The change of traction force reflects the reaction state of the product in the mold. It is related to many factors, such as: fiber content, geometric shape and size of the product, release agent, temperature, drawing speed, etc.
4. Correlation of various pultrusion process variables
1. The relationship between thermal parameters, pultrusion speed and traction
Among the three process parameters of thermal parameter, drawing speed and traction force, 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. Through the peak value of the DSC curve of the resin curing system and related conditions, determine the temperature value of each segment of the mold heating. The principle of determining the pultrusion speed is the gel time at a given temperature in the mold to ensure that the product gels and solidifies in the middle of the mold. There are many factors restricting the traction force, such as: it has a great relationship with the mold temperature, and is controlled by the pultrusion speed. From the previous analysis, it can be seen that the increase in the tensile speed directly affects the second peak of the shear stress, that is, the shear stress at the breakaway 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 cured by the resin system is the premise for us to determine other process parameters. The drawing speed thus selected must match the temperature, the mold temperature is high, and the traction speed should be increased. The gel point of the resin can be determined by adjusting the mold temperature and traction speed. When the mold temperature is too high or the reaction is too fast, it will cause thermal cracking of the product. Therefore, the use of zone heating molds to distinguish heating into preheating zone, gelling zone and curing reaction zone can optimize the pultrusion process and reduce thermal cracking of the product.
In order to improve production efficiency, the pulling speed is generally increased as much as possible. This can improve the mold shear stress and the surface quality of the product. For thicker products, you should choose a lower pulling speed or use a longer mold to increase the temperature of the mold. The purpose is to make the product cure better, thereby improving the performance of the product.
In order to reduce the traction and make the product demold smoothly, it is necessary to use a good mold release agent, which sometimes plays a decisive role in the molding process.
Five, 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 works well. Preheating increases the resin temperature and decreases the viscosity, which increases the fiber wetting effect and creates conditions for increasing the drawing speed. Many resin systems, such as epoxy resin, require preheating.
The effect of preheating is also manifested in the reduction of the temperature gradient inside and outside of the dipped fiber bundle. Because after entering the mold, the heat transferred from the mold to the product is distributed stepwise from the product surface to the center of the product, and the temperature of the product center line is lower than the temperature of the product surface. Similarly, the curing of the product center lags behind the curing of the product surface. If the pultrusion speed is increased, the temperature and curing degree lag between the centerline and the surface of the product will increase. Then, the amount of hysteresis decreases conversely as the heat of curing increases, and finally the center temperature of the product is higher than the surface temperature. To achieve uniform curing inside and outside the product and reduce thermal stress, the resin should be preheated.
Post-curing treatment is required when the degree of curing of the product after mold release does not meet the requirements. Generally speaking, the products are naturally cooled in the air after being released from the mold. During this period, the curing reaction continues. The general post-curing treatment is to put the cut products in a constant temperature box for a period of time to make the products reach the required curing degree.