The raw materials for the fiberglass are gathered together. These raw materials include borax, silica sand, limestone, soda ash and other minerals such as feldspar. This often requires that fiberglass manufacturers be in touch with quarries and chemical companies from many areas, since some of the components are only mined or produced in certain places where the resources are plentiful. Some of the components are foundational, providing the actual material for the fiberglass. Others are included simply because they lower the melting temperature of the other materials. This is important for heating process that follows.
The gathered raw materials, called a batch, is heated in a furnace. The temperature of the furnace is high enough to melt the components of the batch. Usually furnaces used in fiberglass manufacturing are kept at around 2500 degrees F, and this temperature is controlled carefully. If the temperature fluctuates, then it is more difficult to maintain a good, even flow of the molten components in the next step, and the quality of the fiberglass is not as good.
The molten components of the batch are transferred to forming equipment through a channel that is attached to the end of the melting furnace. This channel, known as the forehearth, is built to withstand the extremely high temperature of the molten batches.
The molten glass, either directly from the furnace or after been put into a marbling machine, is fed into heated spinnerets that have anywhere from several hundred to several thousand tiny openings. The molten glass is pressed through the openings and exits as glass fibers. If the marbling process is used during this step, then inspection of the marbled glass is done visually rather than mechanically to check for impurities. This can take more time than feeding the spinnerets with non-marbled glass, but the quality can be higher because of the visual inspection.
The glass fibers are put onto a winding machine that spins at high speed. This machine revolves at a rate of 10,000 feet per second or more, creating tension on the glass fibers. This stretches the fibers out into long filaments, which then are treated chemically to keep them from breaking. After this treatment, the fibers are placed on large tubes and are ready to be wound into fiberglass yarn.
Depending on the intended use for the fiberglass, a variety of processes are used to make the final fiberglass products. Fiberglass for insulation, for instance, is interlaced on a conveyor, pressed together and cured into semi-rigid or rigid insulation boards. These are then readied for shipment and sale.