SMC are versatile materials: their formulation can be adjusted and tailored to meet the requirements of adiverse range of applications. Compression molding of SMC allows processing complex and large shapes on a rapid cycle time.Features such as inserts, ribs, bosses, and attachments can be molded in. This process needs little mold prepara-tion and generates few scraps, thus reducing the cost of trimming operations.
Good surface finishes are obtainable,contributing to lower part-finishing cost. This process can also be automated.
Compared, for instance, to steels, SMC provide design freedom and flexibility by accommodating shape complex-ity and geometric details; reduced manufacturing com-plexity through part integration in a single assembly; and combining structural, assembly, and integration functions (e.g., antennae embedment); substantial weight reduction (∼20–35% lighter than equivalent steel parts); superior corrosion resistance; and reduced tooling costs (40% less than tools for steel stamping). SMC also offer enhanced damage resistance from dents and dings, especially in exterior cladding (body panels), as compared to aluminum alloys and steels; good harshness properties; and good noise and vibration properties. The benefits of using SMC are also: in-mold coloring and powder priming for painted parts with the requirement of high temperature resistance from 150 to 200 ◦ C for e-coat application.
Compared with injection molded parts, particularly those produced using the BMC process, SMC parts have better mechanical properties. For instance, equivalent parts exhibit typically an average thickness of 2.5mm in the SMC case, whereas this thickness is about 3.5mm in the case of injection molded BMC to meet the same mechanical requirements. This is due to the highest length of the fibers used in SMC. Even if fibers having the same length can be used in both processes, it is known that injection processes have a clear tendency to damage the fibrous reinforcement resulting in a drastic length reduc-tion. This shows a limitation and an inefficiency in the injection processes. On the contrary, compression molding allows a lower freedom of design and complex geometry to be molded. This can be seen as an advantage when com-paring SMC with prepreg fabrics: the latter indeed offer higher mechanical properties, but the parts have simpler geometry.
For many applications, SMC compete with fiber-reinforced thermoplastic materials. Those materials can be injection molded or compression molded as GMT. SMC parts have better mechanical properties, heat resistance, better than usual dimensional stability and environmental resistance, and cost-effective thermoplastic composite parts. In contrast, fiber-reinforced thermoplas-tics generate fewer scraps than SMC, thus lowering labor costs. Furthermore, SMC have some disadvantages: the continuous thickening of the paste, the variations of the rheological properties, as well as the emission of styrene,which impacts the curing reaction, the variations of the basis weight, and the cooling of molding parts, which may induce some geometrical distortions if not controlled properly, still remain tricky to control at the industrial scale. But there is no clear trend yet that SMC will be replaced by composite thermoplastics. Indeed, assembly can be manufactured from a subset of parts. SMC can be used for the structural parts of the assembly, whereas visible skins can be composed of thermoplastic composites.This type of solution is currently implemented in the automotive industry. Nevertheless, painted SMC body panels are still being produced without major insoluble industrial problems.
ALICE CHEN
EMAIL:alice@ningguangmould.com
MOBILE PHONE:+8613989617789
FAX:86-576-84616787
Wechat:13989617789
Website:www.smctooling.com
