It is difficult to discuss in detail each control factor in every step of the preparation process for Fiberglass SMC(Sheet Molding Compound)because of the different control parameters and the different complexity. Therefore, only a few main factors are briefly presented here.
1.Viscosity of Resin Solution:
In the preparation of resin solution, in addition to the correct calculation and weighing of ingredients, in order to make the resin penetrate and adhere evenly and quickly between fibers, it is generally necessary to add a suitable amount of solvent in the resin to adjust the viscosity. This is very important not only in the pre-mixing method, the pre-immersion method, but also in the felt method. The decrease of the viscosity of resin solution is beneficial to the permeation of resin into fibers and the loss of fibre strength. However, if the viscosity is too low, in the process of premixing, it will lead to the segregation of the fibers, affect the adhesion of the resin to the fibers, thus affecting the control of the quality index of the moulding material. Because there is a certain relationship between viscosity and relative density, the measurement of viscosity is not as simple as that of relative density. Therefore, in the production process, relative density is often used as the control index of viscosity. For example, in phenolic premix, the relative density of resin solution is controlled in the range of 1.00-1.025.
2.Short cut length of fibers:
The short cut length of fibers should not be cut too long in the production of moulding materials by pre-mixing method, otherwise it will lead to tangled materials, not easy to tear and dry. But it should not be too short, too short will cause the mechanical properties of fiberglass moulded products to decline. Usually, the cutting length of the fibers is (30 + 5) mm by mechanical premixing method and (40 + 5) mm by manual premixing method.
The impregnation time should be shortened as far as possible to ensure the uniform penetration of fibers. Especially in the process of preparation of premix, too long kneading time will lose the original strength of the fiber, and too much volatilization of solvent will increase the difficulty of impregnation process.
The main purpose of moulding material drying is to remove volatile components such as solvents, so that the resin part is converted from A to B. Therefore, drying conditions directly affect the quality of moulding materials, and the determination of drying temperature mainly depends on the type of moulding materials, more precisely on the curing properties of the resin used. When the premix is dried, the material layer should be laid evenly, and the material layer should not be too thick.