There are several methods that are used to convert plastic materials into formed shapes and parts such as pellets and granules. You may head to https://www.universalplastic.com/, if you want to know more about plastic tubing.
Injection molding is the process of heating and homogenization plastic granules in a cylinder until they are fluid enough to allow for pressure injection into a mold. In the mold the material will take the shape of the mold cavity. The benefits of this technique are the speed of production and the minimal requirements for the post molding operations. It is very effective for high-speed molding of such materials such as nylons, polyesters and polyurethanes.
Rotational molding is a process in which finely ground powders are heated in a rotating mold until they reach melting point or fusion happens. The melted or fused material coats the inner surface of the mold. When cooled, a hollow finished shape can be removed.
Blow molding is the process of forming hollow plastic tubes. The process begins with melting down the plastic and forming it into a parison. A parison is a tube like piece of plastic with a hole in one end so that compressed air can pass through. The air pressure then pushes the plastic out to match the pattern of the mold.
Thermoforming is where plastic sheets are formed into part through heat and pressure. Plastic sheets are heated in a large oven to the correct temperature that they can stretched into or onto a mold and cooled to the finished shape. This method is quite cost effective and is often used for sample and prototype parts.