Bulk moulding compound

Bulk Molding Compound or Bulk Moulding Compound (BMC ) is a fiber-matrix semis. It usually consists of short fibers and a polyester or vinyl ester resin, reinforcing fibers or other resin systems are possible. Natural fibers such as inexpensive alternative to glass fibers, are becoming increasingly common. BMC is supplied as a shapeless mass in bags or other containers. Bulk Molding Compound became known in the early 1950s under the trade name premix. Compared with the pure resin BMC has higher strength, stiffness and temperature limits.

The breakthrough came in 1980 with the introduction of the BMC Farrel mixer which enabled a large-scale industrial production. The first BMC applications stemmed initially from the electrical industry, later followed by the automotive industry. In the latter BMC was used for on headlight reflectors and cylinder head cover. 2002 in Europe about 18,000 t BMC processed into headlight reflectors, to 1989 Citroën processed the same amount of BMC to tailgates.

Processing

BMC is processed in the hot press method, which permits short cycle times. The BMC mass is to centrally inserted into a heated, shared tool. During closure, the BMC distributed in the mold cavity. The short fiber lengths and thin ribs and wall thicknesses can be filled during pressing. However, there is a risk that the BMC segregate at constrictions. This occurs when a narrow clogged with fibers, so that only the resin can continue to flow. The individual reinforcing fibers are oriented generally in the direction of flow, so that can occur locally highly oriented fibers. This effect is often desired and is influenced specifically by adjusting the tool with the help of flow and filling simulations.

BMC can in special processes, with correspondingly small lengths of fiber, is processed in the injection molding process.

Composition

The exact composition of Bulk Molding Compounds is only known to the respective manufacturers. Basically, BMC polyester based of the following components (each mass percent):

• Reaction resin 10 to 35%
• Fibers 10 to 50%
• Fillers 0 to 70% calcium carbonate
• Aluminum trihydrate
• ...

The fillers have here mainly the task to cheapen the BMC by fractions of fibers and resin are replaced by cheaper fillers. Depending on the desired properties, for example improved flame-resistant, or low- shrink additives are added. Such as magnesium oxide increases the plasticity and kaolin, the acid resistance.