Extruder Screw Design and Purpose
At its core, the extruder is a very simple machine. The various configurations of shaft speed, number of chambers and types of screws/steamlocks enables the operator to create a multitude of products. Different products usually require different configurations and run parameters. An important parameter to consider is retention time in the barrel. Depending on what your product requires, you may have to increase or decrease the amount of time product stays in the barrel. A common way to do this is vary the screws inside each chamber.
The screws inside each chamber serve two purposes:
1) Convey product through the barrel
2) Create shear and heat to perform necessary treatment to the product
There are two main factors that affect how the screw performs: pitch and flighting (see diagram).
Pitch: The distance between adjacent flights
Flights: Raised, helical sections of the screw (similar to threads on a machine bolt)
Flights can vary in design and are commonly either single or double (triple and quadruple are possible, but not very common). In the picture of the two screws, the one on the left is a single flight while the one on the right is double. Notice how the double appears to have two lobes on the end while the single only has one.
- You can think of this like wrapping a string around a pencil: Single flighting is the same as wrapping one string around while double is like wrapping two strings around (anchored on opposite ends of each other).
Double flights increase retention time and work since the product has to travel further in the same amount of space. This is good when the product must be heat treated to create proper chemical reactions.
Altering these geometries will affect how your product cooks and conveys. Very tall flighting at a steep angle and large pitch will convey a lot of product very quickly, but will not apply very much work to it. On the other hand, short flighting with narrow pitch will convey less product but potentially do more work. Single flights are generally used at the beginning of an extruder to move product away from the intake quickly and double flights are used in the middle section to cook the product. A combination of these in your extruder will help you make the product you want.