Bulk molding compound

from Wikipedia, the free encyclopedia

Bulk Molding Compound or Bulk Molding Compound ( BMC ), sometimes also referred to as Dough Molding Compound ( DMC ), is a semi-finished fiber matrix product . It mostly consists of short glass fibers and a polyester or vinyl ester resin , other reinforcing fibers or resin systems are possible. Sheet-shaped fiber-matrix semi-finished products are called sheet molding compounds .

If necessary, the mixture is pressed into the casting mold, only slightly heated, and cured there with higher heat. In contrast to thermoplastics , it cannot be liquefied again afterwards by heating.

BMC is supplied as a shapeless mass in bags or other containers. Bulk Molding Compound became known under the trade name Premix in the early 1950s . Compared to pure resin, BMC has higher strengths, rigidity and temperature limits.

BMC achieved the breakthrough in 1980 with the introduction of the Farrel mixer , which enabled large-scale industrial production. The first BMC applications came from the electrical industry, later the automotive industry followed. In the latter, BMC was used particularly for headlight reflectors and cylinder head covers. In 2002, around 18,000 t of BMC were processed into headlight reflectors in Europe, until 1989 Citroën processed the same amount of BMC into tailgates .

processing

BMC is processed in a hot pressing process, which enables short cycle times. The BMC compound is placed centrally in a heated, split tool. When closing, the BMC is distributed in the mold cavity. Due to the short fiber lengths, even thin ribs and wall thicknesses can be filled during pressing. However, there is a risk that the BMC will unmix at bottlenecks. This happens when a constriction becomes blocked with fibers so that only the resin can continue to flow. The individual reinforcing fibers are usually oriented in the direction of flow so that locally strongly oriented fibers can occur. This effect is often desired and is specifically influenced by adapting the tool with the help of flow and filling simulations.

In special processes, BMC can also be processed by injection molding with correspondingly small fiber lengths .

composition

The exact composition of the bulk molding compound is only known to the respective manufacturer. Basically, polyester-based BMC consists of the following components (in each case percentage by mass):

The main task of the fillers is to make the BMC cheaper by replacing parts of fibers and resin with cheaper fillers. Additives are added depending on the desired properties, e.g. increased flame retardancy or low shrinkage. Magnesium oxide , for example, increases plasticity and kaolin increases resistance to acids.

Natural fibers are becoming increasingly popular as an inexpensive, renewable alternative to glass fibers.

literature