Sheet moulding compound (SMC) or sheet moulding composite is a ready to mould glass-fibre reinforced polyester material primarily used in

compression moulding Compression molding is a method of molding in which the molding material, generally preheated, is first placed in an open, heated mold cavity. The mold is closed with a top force or plug member, pressure is applied to force the material int ...

. The sheet is provided in rolls weighing up to 1000 kg. Alternatively the resin and related materials may be mixed on site when a producer wants greater control over the chemistry and filler. SMC is both a process and reinforced composite material. This is manufactured by dispersing long strands (usually >1”) of chopped fiber, commonly

glass fiber Glass fiber ( or glass fibre) is a material consisting of numerous extremely fine fibers of glass. Glassmakers throughout history have experimented with glass fibers, but mass manufacture of glass fiber was only made possible with the inventio ...

s or

carbon fiber Carbon fiber-reinforced polymers (American English), carbon-fibre-reinforced polymers (Commonwealth English), carbon-fiber-reinforced plastics, carbon-fiber reinforced-thermoplastic (CFRP, CRP, CFRTP), also known as carbon fiber, carbon compo ...

s on a bath of

thermoset In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening (" curing") a soft solid or viscous liquid prepolymer (resin). Curing is induced by heat or suitable radiation and ...

resin (typically

polyester resin Polyester resins are synthetic resins formed by the reaction of dibasic organic acids and polyhydric alcohols. Maleic anhydride is a commonly used raw material with diacid functionality in unsaturated polyester resins. Unsaturated polyester resins ...

vinyl ester resin Vinyl ester resin, or often just vinyl ester, is a resin produced by the esterification of an epoxy resin with acrylic or methacrylic acids. The "vinyl" groups refer to these ester substituents, which are prone to polymerize and thus an inhibitor is ...

epoxy resin Epoxy is the family of basic components or cured end products of epoxy resins. Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups. The epoxide functional group is also coll ...

). The longer fibers in SMC result in better strength properties than standard

bulk moulding compound Bulk moulding compound (BMC), bulk moulding composite, or dough moulding compound (DMC), is a ready-to-mold, glass-fiber reinforced thermoset polymer material primarily used in compression moulding, as well as in injection moulding and transfer mo ...

(BMC) products. Typical applications include demanding electrical applications, corrosion resistant needs, structural components at low cost, automotive, and transit.

Process

Paste reservoir dispenses a measured amount of specified resin paste onto a plastic carrier film. This carrier film passes underneath a chopper which cuts the fibers onto the surface. Once these have drifted through the depth of resin paste, another sheet is added on top which sandwiches the glass. The sheets are compacted and then enter onto a take-up roll, which is used to store the product whilst it matures. The carrier film is then later removed and the material is cut into charges. Depending on what shape is required determines the shape of the charge and steel die which it is then added to. Heat and pressure act on the charge and once fully cured, this is then removed from the mould as the finished product. Fillers both reduce weight and change the physical properties, typically adding strength. Production challenges include wetting the filler, which could consist of glass microspheres or aligned fibers rather than random chopped fibers; adjusting die temperature and pressure to provide the proper geometry; and adjusting chemistry to end use.

Advantages

Compared to similar methods, SMC benefits from a very high volume production ability, excellent part reproducibility, it is cost effective as low labor requirements per production level is very good and industry scrap is reduced substantially. Weight reduction, due to lower dimensional requirements and because of the ability to consolidate many parts into one, is also advantageous. The level of flexibility also exceeds many counterpart processes.

Physical properties

Properties vary depending upon filler and resin types, with compounds using aligned fibers (especially long fibers) being subject to greater

anisotropy Anisotropy () is the property of a material which allows it to change or assume different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's physic ...

. Typical ranges are listed below.

References

Plastics industry Composite materials Fibre-reinforced polymers {{Material-stub

Process

Advantages

Physical properties

See also

References