There are more than 16,000 injection molding and plastic manufacturing facilities in the United States. It is a very broad industry, but it can be divided into three primary divisions: resin suppliers, suppliers of other materials, and compounders (companies that prepare plastic formulations). These three divisions send their materials to processors, which then sell products to other industries that use them. The major markets for plastic and plastic parts are the automotive, electronics, computers, telecommunications, and packaging industries. These industries use plastic pieces or parts in their products.

In general, plastics consist of a family of materials, not a single material. Each member of this family has its own distinct and special properties. From different combinations of elements, it is possible to create different plastics with almost any quality desired in an end product. Some plastic products are similar to existing conventional materials but can be produced more economically. Some plastic products represent significant property improvements over existing materials.

One generally accepted definition of plastics classifies it as any one of a large and varied group of materials consisting wholly or in part of combinations of carbon with oxygen, hydrogen, nitrogen, and other organic and inorganic elements. At some stage in their manufacture, these materials are liquefied and are thus capable of being formed into various shapes, usually through the application of heat and pressure.

Sometimes there is an overlapping of functions between industry and market. For example, customers such as automotive and packaging companies also process plastics into products and parts. There is also an overlapping of functions within the industry, as when materials manufacturers also do processing and finishing.

Materials and resins manufacturers use chemical reactions to transform basic feedstocks into plastic materials; about 500 different resins and compounds are produced. These materials are then sold in the form of granules, powder, pellets, flakes, or liquids for eventual processing into finished products.

Between these manufactured materials and the finished compound that goes to the processor, there is often an intermediate step that involves the addition of modifiers, chemicals, and additives that impart special properties to the plastic or upgrade existing ones. For example, plastics can be colored with pigments or dyes or made more flexible by being treated with a modifier. The companies that supply these modifiers are often the materials manufacturers themselves. In other instances, they are separate companies specializing in the production of one or more specific types of modifiers for plastics.

The compounding of the base resin into the finished plastic material that goes to the processor is usually performed by the materials manufacturer. However, an industrial segment of the plastics industry known as compounders, or custom compounders, buys the base polymer from the materials manufacturer and then specially compounds it by adding modifiers, additives, and so on for resale to processors. It is also possible for processors to buy base polymers, modifiers, and additives directly and do their own compounding.

Plastics processing, or plastics production, is the heart of the plastics industry. It is the function of the processor to turn plastics material into secondary products, component parts, or finished products. The miscellaneous products market is very fragmented. Some major categories include miscellaneous plastic packaging, fabricated plastics for automobiles, plastics for electrical devices, kitchenware, plastic furniture parts, and plastics for building construction. Other segments of the plastics products industry include laminated plastics, pipe, bottles, profile shapes, plumbing fixtures, foam products, and film and sheet.

The fabricator's role (in plastics production) is to turn secondary products such as film, sheet, rod, tube, and special shapes into end products. There are several common ways in which this is done: molding, casting, laminating, extrusion, and calendering.

Molding a secondary product is a simple procedure that involves taking the heated liquid plastic, injecting it into a mold, and using intense pressure to shape the piece. For all molded items, the plastic is poured into a mold of the desired shape, whether it be a hollow bottle or a round object. Casting is very similar to molding; however, the plastic sets without the use of pressure.

In the lamination process, a piece of paper is placed between two transparent sheets of plastic. The three items are then heated to form a bond. Products such as tabletops and electrical insulation are laminated. The most common example of laminated plastic is Formica.

Extrusion is the process of making plastic tubes such as pipes and hoses. Melted plastic is squeezed through an oblong object that expels the finished product. Inside the tube is a screw-like part that pulls the plastic through it and squeezes the substance into the desired shape. The most frequently produced piping (in descending order) are water piping; waste, vent, and drain pipe; sewer pipe; and mining, oil and gas, industrial, and other piping. Plastic piping is superior to many other types of piping because it is less expensive, lightweight, and relatively easy to install, among other factors.

Foam-forming of plastics involves the blowing of air into plastics during production. Products produced by foam-forming include furniture and furnishings (cushions, carpet underlay, and filler for pillows), consumer products (plates, cups, coolers, and trays), and packaging and transportation products.

Calendering, like lamination, coats materials. In a process similar to printing, the substance to be coated is fed through a series of rollers. The rollers contain the plastic that coats the substance.

Finishing, decorating, and assembling the plastic products can either be done in-house by the plastics processor or fabricator, or by companies that specialize in various finishing techniques.

The plastics industry employs workers of all education and skill levels, from Ph.D. chemists and engineers to unskilled workers. Scientists and engineers work in research and development, developing new types of plastics, finding new applications, and designing manufacturing processes.

Many workers operate machines that actually form plastic objects. For example, injection molders run machines that first liquefy plastic powders or pellets. This liquid pellet is then injected into a mold and then allowed to harden. The final result is a molded plastic object.

Some plastics are cut into shapes rather than molded. Shaping-machine operators cut spheres, cones, and other shapes from blocks of plastic.

Technicians play an important role in the manufacturing of plastics. Some technicians install molds, watch over the molding process, maintain production schedules, and test both raw materials and finished products. Other technicians work in laminating.

The plastics industry also employs technicians as sales and service workers. Some sell plastics materials to manufacturers. Others work for manufacturers of the machinery used in plastics production.