Peripheral Screw Speed vs. RPM
There are plenty of abundantly obvious differences between small and large screws, whether we were to compare injection molding or extrusion applications. However, beyond the physical differences in geometry and design approach, there is one important factor that is rarely understood or considered – peripheral screw speed.
Primary Melting Mechanisms in Single Screw Extrusion and Injection Molding
The primary melting mechanism in single screw extrusion (and injection molding screws) is viscous heat dissipation. There are some levels of conductive heat transfer present in the melting action within the barrel, primarily from the external heat provided by the heater bands, as well as the adjacent melt in the accumulated melt pool. However, almost all of the energy required for adequate melting is generated through viscous heat dissipation. As the solid bed melts, a thin melt film is formed at the solid bed-barrel interface. This melt film is continually scrapped off the barrel wall by the advancing flight, collecting the molten resin in the melt pool. As the screw rotates, the solid bed rotates. The viscous polymer in the melt film is continuously stretched and elongated by the forwarding action. The melt film between the solid bed and barrel wall is highly sheared by the scrapping action of the advancing flight during rotation. Because of the viscous nature of the resin, heat is readily generated within the melt film by dissipating the mechanical power from the screw drive. This mechanical energy (torque) is converted to thermal energy from the internal friction between the resin molecules.
Large Screws VS. Small Screws
Large diameter screws are generally more difficult to design than smaller screws when melt quality is carefully considered. With smaller screws, conductive heat transfer plays a far more significant role in melting and is readily controlled by the operator through barrel temperature settings; this is primarily due to the very thin polymer layers within the small screw channels and the higher watt density present. There are limitations with very small screws such as machining tolerances, cost of production, and pellet size. On the other hand, large diameter screws increasingly depend on optimal melting in the form of viscous heat dissipation. Solid bed width increases proportional to screw diameter. Melt film thickness also increases as screw diameter increases. Although it is apparent that larger screws can produce much higher theoretical outputs compared to smaller screws, rpm and peripheral screw speed plays a major factor along with optimal screw design.
Peripheral Screw Speed
Peripheral screw speed is the distance a given point on the screw’s circumference travels during rotation, expressed in a measure of distance per second. While rpm is simply the number of revolutions around an axis per minute. At an equivalent peripheral screw speed, the scraping frequency of the advancing flight decreases, and the melt film increases as the screw diameter increases. In other words, a designated rpm on a very small diameter machine does not perform in any way similar to a much larger diameter machine at the same rpm. Below is a simple calculation that solves for rpm at a set peripheral screw speed. In this example we will use a peripheral screw speed of 20 m/sec with two different screws: a small screw with a diameter of 25mm and a larger screw with a diameter of 150mm.
In this example we can see that at varying screw diameters there can be widely varied scraping frequency of the advancing flights. This scraping frequency is directly related to the primary melting mechanism in screw design – viscous heat dissipation. This highlights the importance of proper screw design not only in small diameter screws, but especially larger diameter screws. Large screws tend to be a fairly costly investment for a plastics processing business. Purchasing the wrong screw can be extremely detrimental, while investing in an optimized screw can lead to a very hefty return on investment.
Understanding peripheral screw speed vs rpm is important – not only in the case of comparing widely different screw diameters, but also for technicians to better approach processing amongst varied machines. One cannot rely on a familiar baseline rpm from machine to machine when the screw diameter and design vary. Different sized machines undoubtedly perform differently. Even if a small or large screw is designed properly, approaching both processes with the former or latter mindset will severely limit theoretical performance.
