In the SMC/BMC molding process, attention must be paid to controlling "3 points": molding temperature, molding pressure and molding time.
First, the mold temperature
The temperature change of SMC/BMC molding compound in the molding process is more complicated. Because the plastic is a poor conductor of heat, the temperature difference between the center of the material and the edge at the beginning of the molding is large, which will result in the curing cross-linking reaction in the inner and outer layers of the material.
The molding temperature of the SMC/BMC molding compound depends on the exothermic peak temperature and the curing rate of the curing system. Usually, the temperature range where the curing peak temperature is slightly lower is the curing temperature range, which is generally about 135 to 170° C. and determined by experiments. The system with a fast curing rate takes the low temperature, and the system with a slow curing rate takes a higher temperature. The upper limit of the temperature range for forming thin-walled products and the lower limit of the temperature range for thick-walled products may be taken.
Second, the pressure of molding
Molded materials with large compression ratios consume more energy when they are densified. Therefore, higher molding pressures are required for molding. Therefore, the pressure of molded bulk materials is higher than that of molded materials, and SMC/BMC molding materials are used. It is also higher than the pressure of molded powder material. Molding materials with high melt viscosity and fast cross-linking rate, and when processing products with complex shapes, thin walls, deep or large areas, mold cavity needs to be filled due to the need to overcome large flow resistance, and thus higher mold pressing is required. pressure. The high molding temperature will accelerate the cross-linking reaction, resulting in a rapid increase in viscosity of the melt, so it needs to be matched with a high molding pressure. Although the high molding pressure increases the product density, mold shrinkage decreases, prompts rapid flow filling, overcomes swelling and prevents pores and other advantages. However, the molding pressure is too low to reduce the service life of the mold, increase the hydraulic power consumption, and increase the residual stress in the product. Therefore, in the processing of thermoset plastic molded products, pre-pressing, preheating, and proper increase of the mold temperature are used in order to avoid high molding pressures. If the preheating temperature is not properly increased or the preheating time is prolonged, the partial curing fluidity during the preheating process is reduced. Not only the mold pressure cannot be reduced, but a higher molding pressure is used to ensure that the material fills the mold cavity.
Third, the molding time
Molding time is also called compression molding and holding pressure time. Refers to the time after the mold is completely closed or after the last deflation and mold closing, and the material is heated and solidified in the mold. The role of the molding time in the molding process is mainly to allow sufficient time for the molding to obtain the cavity shape to complete the curing. Curing refers to the formation process of the body-shaped structure during thermosetting plastic molding. From the nature of the chemical reaction, the curing process is the process of cross-linking reaction. However, the process of "curing completely" does not mean that the cross-linking reaction has been carried out to the end, that is, all the reactive groups that can participate in cross-linking have all participated in the reaction. This term refers in process to the fact that the cross-linking reaction has proceeded to an appropriate degree, and the product's comprehensive physical and mechanical properties or other specifically specified properties have reached the desired target. Obviously, the degree of cross-linking of the product is unlikely to reach 100%, but the degree of curing can exceed 100%, and the phenomenon that cross-linking exceeds the degree required for complete curing is generally referred to as "over-ripening," and vice versa as "under-cooked." Molding time and the curing rate of SMC/BMC molding compound, the shape and wall thickness of the product, the structure of the mold, the molding temperature and the molding pressure, as well as factors such as the pre-compression, preheating, and whether it is exhausted during molding, etc. Relevant, among all these factors, the effect of molding temperature, product wall thickness and preheating conditions on molding time is the most significant.