Common screw materials

The details of screw design will be discussed Chapter 8. In the U. S., a very common screw material is 4140 steel, which is a medium carbon, relatively low-cost material. A table of common screw materials used in polymer extrusion with their chemical composition is shown in Table 3.4. Table 3.6 shows some physical properties and cost comparison data. The selection of the proper screw base material and hard-facing material will be discussed in detail in Section 11.2.1.4. 

The screw configuration is normally either a ribbon-type helical screw or a series of paddles mounted on a common shaft. Materials of construction are selected based on the specific application and materials to be mixed. Typically, the screws are either steel or stainless steel, but other materials are available. 

From the early 1900s, when the first power-driven assembly line for vehicle production was introduced, manufacturers have sought ways to decrease labor costs, increase safety, improve turnaround times and implement efficient processes in an effort to positively impact their bottom line. The first assembly lines commonly included materials such as screws, bolts, nails and wet adhesives. 

Screw Feeders. Screws are primarily used when feed over a slotted outlet is requited. Screws are a good choice when an enclosed feeder is requited, when space is restricted, when handling dusty or toxic materials, or when attrition (particle breakage) is not a problem. A screw is composed of a series of flights that are wound around a common shaft. The flights have a particular diameter and pitch (the distance between flights). Some screws have constant pitch flights others vary. The screw shaft has to be sized to prevent deflection (12). 

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