The function of the pultrusion mold is to ensure the compaction, form and solidification of materials. Molding shrinkage of resin should be considered when designing the sectional dimension of the die. The length of the die is related to solidification speed, temperature, size of the product, pultrusion speed, properties of reinforced materials, etc. The normal length is 600 centimeters to 1200 centimeters.
A high degree of finish of the die cavity can reduce the friction, extend the lifespan and make it easier to demold. It is generally heated with electric while high-performance composite materials are heated with a microwave. The cooling device should be set at the entry of the die in case the glue solution solidifies prematurely. Relative density (viscosity) and impregnation time are important in the impregnation process and the demand and influence factors are the same as the prepreg materials.
Mastering the molding temperature, the distribution of the temperature of the die and time materials going through the die (pultrusion speed) is the key point of the solidification process while solidification is the most important part of pultrusion process. Prepreg generates a series of physical, chemistry and physicochemical complicated changes when it goes through the die and they are not very clear so far. The die is divided into three areas according to the state of prepreg. Reinforced materials go through the die uniformly while resin does it in a different way. At the entrance of the die, resin acts like Newtonian liquid, viscous resistance between the resin and inner of the die makes resin slow down, as the distance to the inner surface of the die decreases, it gradually comes to the equal lever to fiberglass.
When the prepreg goes through the die, cross-linking reaction happens to the heated resin, the viscosity of resin becomes lower, viscosity resistance increases and it starts to solidify and comes to the solidification area. Resin becomes harder and shrinks, then leaves the die. Resin and fiberglass moved forward together at the same speed. They continue to solidify by heated in the solidification area to ensure the specified solidification degree. The solidification temperature is normally higher than the peak value of the exothermic peak of the glue solution and temperature, time and pulling speed should match. The temperature of the preheating area should be relatively low and make the solidification exothermic peak a little behind the middle and point of departure in the middle of the die. The temperature gradient should not be too large, the normal difference in temperature is 20 degrees to 30 degrees. influence of the solidified reaction should be considered, too. Three pairs of heating systems are used to control these three areas respectively.
Pulling force is the key to ensure successful demold. It is determined by the interface shearing stress between the product and the die. Shearing stress decreases as the pulling speed increases and three peak values appear at the entrance, middle and exit of the die. The first peak occurs because of the viscosity resistance of resin and it is determined by the property of resin liquid, the temperature at the entrance and the content of padding. Viscosity of resin decreases as the temperature becomes higher, the shearing stress decreases in the meantime. Viscosity and shearing stress increase as the solidified reaction goes on. The second peak value corresponds to the point of departure and decreases sharply with the increase of pulling speed. The third peak value is at the exit, it appears when a solidified product rubs the inner of the die and it’s small. It is very important to have good control of pulling force. To get a bright and clean product, the second peak value should be small and the product should get out of the die as early as possible. The reactive state of profiles is showed by the change of the pulling force. Fiberglass content, shape of profile, size, release agent, temperature and pulling speed are all relative to the change of pulling force.
