Composition and performance of BMC and SMC

sheet moulding compound

SMC and BMC, as well as the following derivatives: TMC (thick molding compound), HPC (high performance compound), CIC (continuous impregnant) and AMC (molding compound), are fiber reinforced composites, which are mainly composed of thermosetting resin (usually up unsaturated polyester), reinforcing material (usually glass fiber) and inorganic filler.

Common SMC formula

Other components (such as low shrinkage agent, curing initiator, thickener, processing aid and release agent) are used to improve the machinability of materials and the performance of parts. In SMC / BMC formulations, less than 30% of crude oil derivatives are unsaturated polyester resin or thermoplastic resin additives. The rest is inorganic filler and reinforced glass fiber, the cutting length is as follows: smc25-50mm, bmc6-12mm.

sheet moulding compound

Classic BMC formula.

Thermosetting resin is the matrix of SMC / BMC system and unsaturated polyester (up) or vinyl ester (VE) resin with chemical main chain. When cured under heating and pressure, cross-linking reaction occurs. Ve resin is suitable for occasions with high technical requirements, such as those requiring durable heat resistance and chemical corrosion resistance. High heat resistance is the characteristic of all thermosetting plastics. The difference between them and thermoplastics is that once the compound is solidified into a hard solid, it will not soften at high temperature and will not become brittle at low temperature. Therefore, SMC and BMC components can maintain their original characteristics and dimensional accuracy over a wide temperature range.

Production of SMC raw materials

SMC is the abbreviation of sheet forming material, which is called sheet forming material in China. Production process: firstly, the prefabricated resin paste is transferred to the scraper box, and the scraper box flows through the mobile carrier film below, and the valve on the scraper box controls the amount of resin paste. At the same time, the glass fiber roving is quickly cut through the rotary cutting machine above the resin carrier film. The fibers are cut into lengths (usually 25 mm or 50 mm) and deposited randomly in a resin slurry.

The content of FRP is controlled by the movement speed of FRP cutting blade and current carrying film. Under the cutting operation, the two-layer film is coated with resin paste, and the glass fiber will fall on the resin paste. At this stage, the resin paste and the glass fiber “sandwich” are formed, and then the glass fiber and the resin are pressed together through a series of compaction rollers.

SMC boards are usually 4mm thick and 1200mm wide. The length and weight of SMC plates depend on the operation requirements of the mold workers, and they are usually stored in 350kg (standard) or 1500kg (standard) drums or folded in half into the large material box. SMC production is a highly automated, computer-controlled production process.

SMC production line

SMC must be matured before it is used in molding. In order to reduce the relative viscosity of resin, the mature period is necessary. The adhesive will be maintained at a controlled temperature (usually at 30 ℃ for 48 hours) and it usually takes 2-5 days to achieve the required molding viscosity. SMC from the date of production, its shelf life is generally a few weeks a few months. Storage time can be extended or shortened according to SMC formula and storage conditions. It is recommended that the storage temperature should not exceed 20 ℃.