In the extrusion process plastic is fed into a heated extruder cavity or cylinder. Typically, the raw plastic is in bead form and may be mixed with colorants before the extrusion process begins. In some plastic extrusion processes, ultraviolet (UV) inhibitors may be added to the raw plastic beads as well. Once inside the plastic extrusion machine, the plastic beads, and any accompanying material, move through an opening in the extruder cavity towards a screw mechanism. The screw rotates, forcing the plastic material to advance through the extruder cavity.

Inside the extruder cavity, the temperature is very high, often reaching about 400°F (200 °C). The cavity needs to be so high in order to match the preferable melt temperature of the plastic material. Many plastic extrusion processes involve the use of three different heaters set to gradually increase heat inside the cavity. This reduces the potential for overheating.

Friction and pressure within the cavity of the extruder serve to produce extra heat that is independent of the heaters. Sometimes, the pressure and friction inside the cavity produces so much heat that the heaters may be shut off. When this happens, the desired temperature is maintained by the friction and pressure. Cooling fans are also employed frequently, helping to keep the plastic extrusion cavity at the desired temperature.

When the molten plastic reaches the front of the barrel, it moves away from the screw and journeys through a special screen designed to filter contaminants from the plastic. The molten plastic then moves into a die. The die is responsible for giving the molten plastic its profile. The plastic must then be cooled, often by a sealed-water bath; care must be taken to prevent the collapse of the newly formed product in its still molten state. Plastic sheeting and certain other products are cooled by special cooling rolls, instead of water baths.

After cooling, the product is spooled, coiled, or cut to length. In addition to plastics, a variety of other materials may be extruded. Aluminum and rubber are examples of materials that can be extruded. Even clay and certain types of foods may be extruded. However, the process used may differ from that employed for the purpose of plastic extrusion.