Twin Screw Compounding Extruders - Is Faster Better??
“A powerful extruder capable of giving high output” is a catchy statement which is bound to attract your attention. The important thing to note here is under what specific condition an extruder can be termed as “powerful” and whether all the available power is actually usable or not. The following equation shows how powerful an extruder may be:
Sp. T. = Md ÷ a3
Sp.T. = Specific Torque
Md = Max. torque per shaft
a = center distance between the two shafts.
In simple terms, Sp.T. is a true measure of what we like to call “extruder power”. Notice that “a”, the center distance, is in the denominator and is raised to the cube. This means that it has a profound influence on the Specific Torque and a small reduction in the center distance will result into a relatively large increase in the Sp.T. However, center distance may be reduced only if shafts are bought closer. This requires the shaft diameter to be reduced and a whole lot of other redesigning.
So, the easy way is to simply increase the screw RPM, which means that for the same shaft diameter, there is a rise in the torque carrying capacity, thereby increasing Md and Sp.T., making the extruder “powerful”. Increasing RPM, of course, needs higher power input and hence a larger motor and gearbox. An extruder with a maximum screw speed of 1000RPM is twice as “powerful” as an extruder of the same diameter with a maximum speed of 500RPM. Such high RPM extruders are capable of giving much higher output for the same screw diameter. Thus, the trend of increasing Screw RPM for making a “powerful” extruder capable of giving high output was evolved.
The paradox here is that even if the extruder is powerful and capable of high RPM and Sp.T., it is not necessary that this high Sp.T. is always usable. This is because while processing shear or heat sensitive materials, it may not be possible to run the extruder at high RPM which it is capable of. This situation results into a significant reduction in output and overall performance of the extruder. For example, consider the case of compounding glass-reinforced (or flame-retardant materials or Engineering Plastics like PC or PBT) materials. High RPM generate high peak shear rates which may destroy the fibers and/or degrade the ingredients resulting into an inferior product. Therefore, in spite of a powerful high RPM extruder, the operator may be forced to run the machine at a much lower RPM and lower output resulting into an inefficient process.
An over-specified RPM is a disadvantage. Such extruders are costly due to the expensive power train components and its high power consumption requirement. This extra investment cannot be utilized, resulting into inefficiency. Materials to be processed decide extruder specifications, not the other way around. And “one universal extruder for all” does not exist. It is always a compromise!!
(Note: “Constant Torque” motors have partly resolved this issue).
If you have any other questions or would like to suggest topics for us to write about, please feel free to contact us at info@polymerupdateacademy.com
Author
Dr. Pradeep Bakshi
Trainer, Polymerupdate Academy
