The main raw materials of SMC are composed of SMC special yarn, unsaturated resin, low shrinkage additives, fillers and various additives. SMC has the advantages of superior electrical performance, corrosion resistance, lightweight and easy and flexible engineering design. Its mechanical properties are comparable to some metal materials, so it is widely used in transportation vehicles, construction, electronics/electricity and other industries.
1. Preparation before molding
(1) The quality of SMC sheet has a great influence on the molding process and SMC part quality. Therefore, it is necessary to understand the formulation of resin paste, glass fiber content, etc. before molding.
(2) Determine the shape and size of the sheet material cutting according to the structural shape of the SMC part, the feeding position, and the process. In order to prevent contamination by external impurities, the upper and lower films are only removed before loading.
(3) Familiar with the various operating parameters of the press, especially to adjust the working pressure and the running speed of the press and the parallelism of the table. The mold installation must be level, and ensure that the installation position is in the center of the press table. Before molding, the mold must be thoroughly cleaned and coated with mold release agent. Before feeding, wipe the release agent with clean gauze to avoid affecting the appearance of the SMC part.
(1) Determine the feeding amount and feeding area. The size of the feeding area directly affects the compactness of the SMC part and the surface quality of the SMC part. It is related to the flow and curing characteristics of SMC, SMC part performance requirements, mold structure, etc.
(2) Feeding position and method. The feeding position and method directly affect the appearance, strength and directionality of the SMC part. Normally, the feeding position of the material should be in the middle of the cavity. For asymmetric and complex SMC parts, the feeding position must ensure that the material flow reaches the ends of the molding cavity at the same time during molding. The feeding method must be conducive to exhaust. When stacking multiple layers of sheets, it is best to stack the materials up and down.
3. After the material enters the mold cavity, the press goes down quickly. When the upper and lower molds coincide, slowly apply the required molding pressure. After a certain amount of curing, the molding of the SMC part ends. During the molding process, various molding process parameters and press operating conditions should be selected reasonably.
(1) The molding temperature depends on the type of resin, SMC part thickness, SMC partion efficiency and SMC part structure. Generally speaking, the forming temperature of thick SMC parts should be lower than that of thin-walled SMC parts. For example, if the thickness of the SMC part is 25-32mm, the forming temperature is 135-145 ℃. And thinner SMC parts can be formed at 171 ℃. It is generally believed that the SMC molding temperature is between 120-155 ° C.
(2) Molding pressure varies with SMC part structure, shape and size. SMC parts with simple shapes only need a forming pressure of 5-7MPa; SMC parts with a complicated shape can have a forming pressure of 7-15MPa. The size of the molding pressure is also related to the mold structure. The molding pressure required by the vertical parting structure mold is lower than that of the horizontal parting structure mold. In short, the determination of molding pressure should consider many factors. Generally speaking, SMC molding pressure is between 3-7MPa.
(3) The curing time is related to factors such as material type, molding temperature, SMC part thickness and color. The curing time is generally calculated at 40s / mm. For thick SMC parts above 3mm, each additional 4mm increases the curing time by 1min.