The raw materials used for preparing SMC include resins, curing agents, fillers, thickeners, release agents, colorants, low shrinkage agents, and fiber-reinforced materials.

（1） Resin

1. Unsaturated polyester resin

There are many varieties of unsaturated polyester suitable for SMC, and the resin can be selected according to the requirements of the product. The representative resin formula and properties are as follows:

1. The product is required to have a Class A precision of meta phenylene polyester SMC, and the formula and properties of this resin are shown in Tables (7-14) and (7-15).

2. Resilient SMC

3. Low density, high-strength SMC

4. SMC with low shrinkage rate

5. Universal SMC

6. Structural SMC

7. High strength SMC (HMC)

2. Vinyl resin

Ethylene based resin, as the resin matrix of SMC, is used in the same way as polyester resin, but requires further efforts in thickening. The following measures are usually taken: thickening with interpenetrating polymer network IPN; Introduce reactant thickening; Thicken with metal salts.

3. Phenolic resin

Phenolic resin has the advantages of temperature and corrosion resistance, high mechanical degree, and easy control.

（2） Initiator

The initiator used for SMC of unsaturated polyester has always been organic peroxide. When selecting, the following points should be noted:

① Reactivity, considering the molding cycle;

② Liquidity, considering the situation where complex products fill the mold cavity;

③ Storage period, pay attention to the stability of quality during the storage period;

④ Physical mechanical properties, which must meet the usage requirements of the product;

⑤ Performance quality: luster, color, etc;

⑥ The residual styrene content is related to the issue of curing efficiency.

（3） Thickener

When selecting resin, its molding adaptability must be considered. In the formula, it is necessary to meet both strength and corrosion resistance, as well as attention to impregnation and flow issues. In order to avoid excessive fluidity, resin thickening is crucial.

（4） Low shrinkage agent

In thermosetting molding materials, unsaturated polyester resin has a large curing shrinkage, with a volume shrinkage rate of about 7% to 8%. Curing shrinkage reduces the dimensional accuracy of molded products, causing problems such as unevenness and warping. Therefore, in order to manufacture unsaturated polyester SMC, low shrinkage agents must be selected.

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