The machine which forces the mix through the die is a plastic extruder, and the mix is known as the extrudate. A typical extruder usually consists of seven major components, such as:
An important part of the extruder is the motor, which provides the energy to rotate the extruder screws. But the first step in raw material processing with Bausano extrusion process is when the raw thermoplastic material is gravity fed from a hopper (top mounted) into the barrel, which is the first part after the feeder of the extruder. The raw material is in the form of small pellets and is generally called resin. Before the material is put in the feeder, it can be mixed with different additives like colorants & UV inhibitors to enhance properties and quality of the raw material.
In the extrusion process, the raw material is subjected to compaction, shearing, particle size reduction, phase transition, and molecular breakdown, because of shear which is generated by the action of the screws and particles of the material rubbing against each other. So, the material once it goes into the barrel and screw it typically changed into a melt form thanks to high pressure, high temperature and shear.
This melt is finally pushed through the die at the end of the screw and barrel, to form the desired shape. The internal temperature varies according to the type of raw material processed. When plastic is extruded from the die, it is fed onto a conveyor belt for cooling, which can be air or water. This phase is very important for the success of the process. At the end of the cooling process some extruders have also end of line equipment like haul-off units or cutters to complete production.
Extrusion systems have evolved over the decades and one of the critical, and most important elements, is the extruders screw.
The screw along with the barrel accomplishes the roles of conveying, mixing, metering, and kneading the raw material in the barrel. The different extruders are distinguished by the number of screws they have. This means that principally two are the types of Extruders, Single Screw Extruders and Twin-Screw Extruders. Single-screw extruders can be classified based on shear capabilities, heat source, screw design (segmented vs solid) while twin-screw extruders are classified into counter-rotating or corotating with intermeshed or non-intermeshed screw systems. The major difference between single- and twin-screw systems, apart from the fact that they have single versus double screws, is that twin-screw extruders have a wide range of operational capabilities in terms of the type of raw materials they can handle and the operating conditions. So Twin screw extruders are more versatile in terms of material processing.
For example, for processing PVC, a Twin Screw is needed due to the characteristics of the material that is harder to process and a Twin screw extruder is more efficient and withstands more difficult materials, because Twins have a better mixing capability. In addition to the two types of extruders available, there isalso the possibility of combining several extruders to produce more complex products and shapes from different materials. This process is called Co-extrusion and involves combining two streams of products in the final die, resulting in the products that have multiple textures. Generally, the end products have an inner core and an outer shell.
