The raw materials of SMC are composed of three major categories

　　The raw materials of SMC are composed of three major categories: synthetic resins, reinforcing materials, and auxiliary materials

　　(1) Synthetic resin Synthetic resin is unsaturated polyester resin. Different unsaturated resins have a direct impact on the thickening effect of resin paste, process characteristics, product performance, shrinkage and Surface states. SMC has the following requirements for unsaturated polyester resin:

　　① Low viscosity, good wetting performance for glass fibers;

　　② The same thickener has sufficient reactivity to meet the thickening requirements;

　　③ Rapid solidification, short production cycle, and high efficiency;

　　④ The solidified material has sufficient thermal strength to facilitate the thermal demolding of the product;

　　⑤ The solidified material has sufficient toughness to prevent cracking when the product undergoes certain deformations;

　　⑥ Lower shrinkage rate.

　　(2) The reinforcing material is short cut glass fiber roving or raw silk. In unsaturated polyester resin molded plastics, only short cut glass fiber felt is currently used as the reinforcing material for SMC, while there are more reinforcing materials used for premixes, including short cut glass fiber, asbestos fiber, hemp, and various other organic fibers. In SMC, the glass fiber content can be adjusted between 5% and 50%.

　　(3) Auxiliary materials include curing agents (initiators), surface treatment agents, thickeners, low shrinkage additives, release agents, colorants, fillers, and crosslinking agents.