Key elements in BMC molding process

During the molding process of BMC molded products, there are three important factors that are indispensable: molding pressure, molding temperature, and molding time. These three factors have a significant impact on the quality of the finished product and need to be controlled within a reasonable range.

The molding pressure is usually expressed as the molding pressure (MPa), which is the ratio of the total force exerted by the fiberglass hydraulic press on the mold to the projected area of the mold cavity in the direction of pressure application. The role of molding pressure in the molding process is to tightly close the mold and increase material density, as well as to promote the flow of molten material and balance the pressure generated by the volatilization of low molecular substances in the mold cavity. Molded materials with high compression rates require a higher compression pressure during molding due to the consumption of more energy during densification. Therefore, molded loose materials have higher pressure than molded blanks, while SMC/BMC molded materials have higher pressure than molded powder materials. When molding materials with high viscosity and fast crosslinking rate, as well as processing products with complex shapes, thin walls, deep or large areas, high molding pressure is required to overcome large flow resistance in order to fill the mold cavity.

The molding temperature is the specified mold temperature during molding, which determines the heat transfer conditions of the mold to the material inside the mold cavity and has a decisive impact on the melting, flow, and solidification process of the material. The temperature changes of SMC/BMC molded plastics during the molding process are complex. Due to the fact that plastics are poor conductors of heat, there is a large temperature difference between the center and edge of the material at the beginning of the molding stage. This will result in curing and cross-linking reactions not starting at the same time on the inner and outer layers of the material.

The molding time, also known as the compression molding insulation and pressure holding time, refers to the time when the mold is completely closed and the material is heated and solidified inside the mold. The main function of molding time in the molding process is to provide sufficient time for the molded material to achieve the shape of the mold cavity to complete solidification. If the molding time is too short, it will cause the resin to fail to cure completely, and BMC molded products will be immature, resulting in poor mechanical properties, lack of gloss in appearance, and easy warping and deformation after demolding. Extending the molding time appropriately can not only overcome the above drawbacks, but also reduce the molding shrinkage of the product and improve its heat resistance, strength performance, and electrical insulation performance.