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.
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.