Modern extruders usually have three or more temperature control zones along the barrel length. These can be individually set to precise temperatures to optimize the extrusion process. However, there are no firm guidelines on how to select the best processing temperature settings.
Usually, the final one or two downstream barrel zones are set reasonably close to the desired exit melt temperature. However, the proper settings for other zones depend a great deal on the particular polymer being extruded and the screw design being used. Generally speaking, we can describe different barrel profiles as being flat, reverse, or normal.
A normal profile reflects a situation where the rear zone temperature is set significantly below the exit melt temperature. The temperatures of the intermediate zones gradually taper up to match the temperature of the final downstream zone. A reverse temperature profile describes the opposite situation: the rear zone is hotter than the final downstream temperature zone (which is sometimes set below the exit melt temperature). A flat temperature profile reflects a situation where all barrel zones are set at approximately the same temperature.
Resin supplier technical bulletins often suggest barrel temperature profiles. However, these recommendations may not apply to all screw designs. To establish a good barrel temperature profile, start with these recommendations but be prepared to compare the results after making adjustments. Pay particular attention to the effect of changing the rear zone temperatures. A Du Pont nylon extrusion study noted that exit-melt-pressure fluctuations were seven times more extreme after the temperature of the two rear zones was dropped 50 deg F. A Mobay processing handbook indicates that a 10-deg-F change in a 390F rear barrel zone can affect screw power by as much as 20 percent in extrusion of Texin 355D polyurethane.
Some polymer/screw combinations are relatively insensitive to small changes in rear zone barrel temperature. However, with some commercial screw designs I have seen major output increases for polystyrene and low-density polyethylene occur after the rear zone temperature was lowered. I have also seen situations where a reverse temperature profile gives better output with a high rear zone temperature, especially with polypropylene. Results of such experiments vary widely depending on the polymer, screw design, and downstream barrel temperature settings, as well as the rear zone temperature setting.
If rear zone temperature changes have little effect on output or pressure stability, either a flat or normal barrel temperature profile will be acceptable in most cases. Avoid barrel temperatures so low that they affect stability and melt quality. Also avoid high barrel temperatures that needlessly lead to degradation or a downstream cooling problem.
For different extruder models with different zone configurations and thermocouple locations, the barrel temperature settings may need to be modified even if the same screw design is used. Assuming no difference in screw and barrel wear, we want to obtain the same temperature at the inside barrel wall of each extruder—not merely the same temperature reading at the thermocouple locations.
Sophisticated instruments may do an excellent job of keeping the metal in the immediate vicinity of these instruments at a constant temperature. However, unless we also consider the process and surrounding hardware, we may end up with fine-tuned control over only the metal that immediately surrounds the thermocouple.
Location of the thermocouples may be extremely important in interpreting what temperature environment actually exists at the inside barrel wall. If the thermocouple for each zone is imbedded halfway into the thick metal barrel wall (a common practice), the inside barrel wall will have a similar temperature if the zone operates largely without heating or cooling. But the inside wall will be hotter than indicated if the zone calls for cooling, and colder than indicated if the zone calls for heating.
Some extruders have thermocouples located deep inside the barrel wall, and others have very shallow thermocouples. Some use a combination of deep and shallow thermocouples. With modem sophisticated control instruments, well-engineered extruders can be run properly under steady-state operation with any of these thermocouple locations. However, it is important to recognize that the same inside barrel wall temperature may require widely different temperature settings for two different extruders. Recognize also that any of these systems can malfunction. Suppose, for example, that water-flow passages become fouled in one aluminum block of a barrel zone, but not in the other blocks. The metal temperature surrounding the thermocouple may represent a compromise between a heavily cooled block and an uncooled block. Temperature differences of more than 100 degrees F between blocks in the same zone have been recorded after fouling occurred.
After a tentative barrel temperature profile has been established, it is often useful to estimate how much heating or cooling is required in each zone. It may help to shut off both heating and cooling for one zone at a time for brief periods after the extruder reaches steady-state operation. Then note how long it takes the zone to change temperature up or down about 20 degrees F. This can indicate whether some zones are being subjected to unusual heating or cooling rates. If any zone requires extreme cooling, the extrusion process may be generating excessive localized polymer heating in that particular zone. If verified, this condition could justify a change in barrel temperature profile or modification in the screw design.
Use of a high-compression screw or application of a high crammer force to the feed may lead to intensive heat development in the barrel wall, even in the rear barrel zones. By contrast, if cold uncompressed powder feed brushes the barrel wall through the rear barrel zone, this can cool the barrel wall effectively. You should consider all of these factors when selecting process conditions, including barrel zone temperatures.