Plastics are available in two main types: thermoplastics, which are meltable, and thermosets, which are not. The difference is in how their polymers are formed. Polymers, or chains of atoms, in thermoplastics are like one-dimensional strings, and if they are melted, they can be reshaped. Thermosets are three-dimensional chains that always keep their shape. A variety of processes are used to shape or mold plastic, some that can make only thermoplastics or thermosets and others that can make both.
The extrusion molding process begins with raw plastic such as pellets, powder or beads. A hopper feeds the plastic into a revolving chamber. The chamber, called an extruder, turns and melts the plastic. The melted plastic is forced out of a die and becomes the shape of the finished product. The item is dropped onto a conveyer belt, where it is cooled with water, cut and finished. Products made by extrusion include sheets, film and pipes.
Injection molding uses the same principle as extrusion. The raw plastic is fed from a hopper to a melting chamber. However, instead of being forced out of a die, the melted plastic is forced into a cold mold under high pressure. Once the mold cools and solidifies, the product is cleaned and finished. Products made by injection molding include butter containers, bottle caps, toys and lawn furniture.
Blow molding is a process that uses a blowing method after extrusion or injection molding. Extrusion blowing uses a die that creates a heated plastic tube with a chilled mold around it. Compressed air is blown through the tube to force the plastic to conform to the shape of the inside of the tube. This allows manufacturers to create a continuous, uniformly melted, hollow shape without having to attach separate injection-molded parts. Injection blowing still uses an injection mold, but instead of a finished product, the mold is an intermediate form that is heated to be blown into a final shape in a different cold mold.
Compression molding is the process of taking a pre-specified volume of plastic material, putting it into a mold, and then using another mold to flatten or compress the plastic into the previous mold. The process can be automated or manual, and it can use either thermoplastics or thermosets.
Thermoforming is the process of taking heated film and softening it to conform to a mold shape. The film is not melted, but heated so that it can be soft enough to be pressed into a mold. The manufacturer forces the plastic into the desired shape through the use of high pressure, a vacuum or a plug. After the finished product has cooled, it is sheared from the mold and scraps are recycled to be put in new film.