Sheet Moulding Compound and Bulk Moulding Compound (or Dough Moulding Compound) are part of the so-called thermosetting materials, for which there is no softening point: for SMC, BMC and DMC the cross-linking between resin and solvent leads to an irreversible hardening. SMC comes in the form of a mat welded in boxes or wrapped on rolls, while BMC and DMC are in raw form: a pasty material packed in bags.
In general, the composition of Bulk Moulding Compound and DMC are very similar, but in some countries connected to the Commonwealth (UK, India, South Africa, Australia, New Zealand) the term DMC (Dough Moulding Compound) is preferred. The distinction between BMC and DMC, as well as geographically, can related to the presence of a thickener (BMC) or its absence (DMC).
Sheet Moulding Compound is a sheet material composed primarily of polyester resin, mineral fillers and glass fibers. Resin, fillers, and additives are mixed together resulting in a paste of honey-like consistency, which is then pumped into an “impregnating” machine equipped with a cutter that adds the reinforcing fiber in a predetermined and constant amount.
Bulk Moulding Compound is a material that appears as a homogeneous mass, composed mainly of polyester resin, mineral fillers and glass fibers. The polyester resin and additives are first mixed, then the resulting paste is transferred to a kneading machine where the mineral fillers and chopped (pre-cut) fiberglass, typically 6 or 12 mm in length, are then added.
The components of our materials
The two materials mainly consist of the following components:
These are mainly unsaturated polyester resins, usually composed of maleic anhydride, orthophthalic anhydride and glycols. The presence of maleic anhydride is necessary to obtain the cross-linking and is present in greater percentage where it is essential to the flatness of the surface of the printed product.
These are styrene thermoplastic solutions that make it possible to counteract the natural shrinkage of polyester resins following cross-linking, thus avoiding excessive deformation of the products. Depending on the type and quantity of additive, it is possible to obtain LS (Low shrinkage) or LP (Low Profile, expanding piece) materials.
Typically calcium carbonate or tri-hydrated alumina. Where flame resistance is required, tri-hydrated alumina is preferred. If, on the other hand, products for the transport industry have to be molded, calcium carbonate is typically used.
Fiberglass or carbon fiber. The glass fiber, in relative cheapness, guarantees the characteristics for the most common applications. The carbon fiber, on the other hand, is used for special applications, where it is essential to obtain high flexural and tensile modulus.
Internal thickeners and release agents. The thickener is used to improve the handling of the material and the transport capacity of the glass fibers in the molding phase. The internal release agent, typically stearate, allows the part to be removed from the mold.
These are substances that enable the chemical reaction between resin and solvent (styrene) at typical molding temperatures. Initiators are temperature-sensitive molecules, so to avoid pre-gelification of the material, both SMC and BMC should be stored away from heat sources and preferably at temperatures below 25°C.