SMC/BMC material compression molding control three elements

In the process of SMC/BMC compression molding, we should focus on controlling the "three points", that is, the three main process parameters: molding temperature, molding pressure and molding time.

 

The molding temperature is the mold temperature specified during the mold molding, and this process parameter determines the heat transfer conditions of the mold to the material in the mold cavity, which has a decisive impact on the melting, flow and curing process of the material. The temperature change of SMC/BMC molding compound during the molding process is more complicated, because the plastic is a poor conductor of heat, and the temperature difference between the center and edge of the material at the beginning of molding is large, which will cause the curing cross-linking reaction to start at the same time in the inner and outer layers of the material. The surface material is cured earlier by heating and forms a hard shell, and the curing and shrinkage of the inner material later is limited by the outer hard shell, resulting in residual compressive stress in the surface layer of the molded product, and the inner layer has residual tensile stress, and the existence of residual stress will cause warping, cracking and strength reduction of the product. Therefore, taking measures to minimize the temperature difference between the inside and outside of the material in the mold cavity and eliminating uneven curing is one of the important conditions for obtaining high-quality products. The molding temperature of SMC/BMC molding compound depends on the exothermic peak temperature and curing rate of the curing system, and the temperature range of the curing peak temperature is usually slightly lower as its curing temperature range, which is generally about 135~170°C and determined by test; The system with a fast curing rate takes a lower temperature, and the system with a slow curing rate takes a higher temperature. When molding thin-walled products, the upper limit of the temperature range can be taken, and the lower limit of the temperature range can be taken for forming thick-walled products, but when forming thin-walled products with a large forming depth, the lower limit of the temperature range should also be taken due to the long process to prevent the material from solidifying during the flow process. Under the premise of not damaging the strength and other performance indicators of the product, appropriately increasing the molding temperature is beneficial to shortening the molding cycle and improving the quality of the product. The molding temperature is too low, not only the viscosity and poor fluidity of the melted material, but also because the cross-linking reaction is difficult to carry out fully, the strength of the product is not high, the appearance is dull, and the mold sticking and ejection deformation occur during demoulding.

 

 

The molding pressure is usually expressed by the molding pressure (MPa), that is, the ratio of the total force exerted by the FRP hydraulic press on the mold to the projected area of the mold cavity in the direction of pressure. The role of molding pressure in the molding process is to close the mold tightly and densify the material, as well as to promote the flow of melt and balance the pressure generated by the volatilization of low molecular matter in the mold cavity. The molded material with large compression ratio consumes more energy when making it denser, so it needs to use higher molding pressure when molding, so the pressure of molded bulk material is higher than that of molded blank, and the pressure of SMC/BMC molded material is higher than that of molded powder material. When molding materials with high melt viscosity and fast cross-linking rate, as well as processing products with complex shapes, thin walls, depths or large areas, it is necessary to use high molding pressure because it is necessary to overcome large flow resistance to fill the mold cavity. The high molding temperature will accelerate the crosslinking reaction, resulting in a rapid increase in the viscosity of the melt, so it is necessary to cooperate with it with high molding pressure. Although the high molding pressure has a series of advantages such as increasing the density of the product, reducing the molding shrinkage, promoting rapid flow mold filling, overcoming swelling and preventing the appearance of pores. However, excessive molding pressure will reduce the service life of the mold, increase the power consumption of the hydraulic press, and increase the residual stress in the product. Therefore, when processing thermosetting plastic molded products, pre-pressing, preheating, and appropriately increasing the molding temperature are used to avoid high molding pressure. If the preheating temperature is not appropriately increased or the preheating time is prolonged, the fluidity of the partial curing has been reduced in the preheating process, not only can not reduce the molding pressure, but a higher molding pressure should be used to ensure that the material fills the mold cavity.

 

The molding time is also known as the compression molding insulation and holding time. It refers to the time when the material is heated and solidified in the mold after the mold is completely closed or after the last gas release and the mold is opened. The role of molding time in the molding process is mainly to give the molded object that has obtained the shape of the mold cavity enough time to complete the curing. Curing refers to the formation process of body structure during the molding of thermosetting plastics, and the curing process is the process of cross-linking reaction from the essence of chemical reaction. However, the "complete curing" in the process does not mean that the crosslinking reaction has been carried out to the end, that is, all the active groups that can participate in the crosslinking have all participated in the reaction. This term in process refers to the fact that the cross-linking reaction has been carried out to an appropriate extent, and the comprehensive physical and mechanical properties of the product or other specially specified properties have reached the expected indicators. Obviously, the degree of cross-linking of the product cannot reach 100%, but the degree of curing can exceed 100%, and the phenomenon of cross-linking exceeding the required degree of complete curing is usually called "overripe", and vice versa is called "underripe". The determination of molding time is related to 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 level of molding pressure, as well as whether the preloading, preheating and whether it is exhausting during molding. Suitable preheating conditions are conducive to shortening the molding time because the heating process of the material in the mold cavity and the process of filling the mold cavity can be accelerated, and the molding time is shortened when the molding temperature is increased, and the molding time should be extended accordingly when the thickness of the product wall is increased. When the molding temperature and molding pressure are constant, the molding time becomes the key factor that determines the performance of the product, the molding time is too short, the resin can not be cured completely, the product is not ripe, so the mechanical properties are poor, the appearance lacks luster, and it is easy to warp and deform after demoulding. Appropriately extending the molding time can not only overcome the above shortcomings, but also reduce the molding shrinkage of the product and improve its heat resistance, strength performance and electrical insulation performance. However, prolonging the molding time too much will make the product overripe, which will not only reduce the production efficiency and increase the energy consumption, but also increase the shrinkage rate due to excessive cross-linking, resulting in a large internal stress between the resin and the filler; It also often makes the surface of the product dark and blistered, and the product will crack in serious cases.