Optimization of temperatures across extruder is essential to reduce stress to polymer melt

Extruders should be set up with all temperatures across barrel, adaptor & die according to the recommendations of polymer producers. Generally the polymer producers recommend ideal melt temperature, which is the temperature of melt coming out of die. All other temperatures should be set up in order to attain melt temperature recommended for particular grade of polymer. When this type of operating condition is used, control of the extrudate temperature is totally dependent on screw geometry and the viscous heat developed from shear generated by channel depths, flight clearances, and screw speed. This is especially problematic when the extruder is using a barrier-type screw, which is commonly used in most extrusion processes to achieve optimum output and performance. Barrier screws are high-performance screws, which need finer tuning of barrel-temperature zone profiles.
The different zones are located in relation to the different sections of the screw. The general configuration of a single-screw extruder with 24:1 to 30:1 L/D and a barrier screw is to have a feed section, barrier section, and metering section, with a total of five temperature zones. Starting with the die, both the die and adapter zones should be set at the polymer manufacturer's recommended melt temperature. The cast-iron feed-throat section of the extruder should then be set to a temperature, which is warm to the touch (55-60 C. This is warm enough to help preheat the material as it enters the extruder, yet cool enough to prevent bridging in the feed throat.
A good way to monitor feed-throat temperature is to install an immersion thermometer in the return line of the feed-throat cooling water. The immersion thermometer can be fitted by installing a “T” fitting in the line. A globe valve after the thermometer to keep the water chamber of the feed throat full should be set up to eliminate cavitation of the system. The first barrel zone that should be set is zone 1, which has the most effect on solids conveying of the polymer. Three coefficients of friction (COF) take place in that zone: friction between barrel and pellet, friction between pellet and pellet, and friction between the root of the screw and pellet.
The main theory of solids conveying is that the polymer must stick to the barrel and slip on the screw. So cooling the root of the screw will reduce the COF between the steel of the screw and the plastic pellets. Screw cooling should always be installed in the core of the screw in the feed section to give the operator another zone of temperature control on the extruder.

Now zone 1 can be set to a temperature that will maximize solids conveying of the polymer. Zone 1 should be set approximately 5 – 15 C below the polymer supplier's suggested melt temperature—or, as mentioned earlier, 5- 15 C below the adapter and die zone settings. For most polyolefins, zone 1 should be set between 100-200 C. The higher the final recommended melt temperature for the polymer, the higher zone 1 could be set. It should be set as high as possible without causing bridging in the throat. With sufficient cooling in the feed-throat casing, higher temperatures can be set in zone 1. It takes energy to melt plastic. So, zone 2 should be set between 65 & 90 C higher than zone 1. This elevated zone temperature will not cause a higher melt temperature because the polymer is still in pellet form at this point. But a higher temperature in this zone will put more energy into the polymer and help melt the polymer. Energy can be imparted to the polymer via mechanical energy from the screw or energy from the heaters. Introducing large amounts of energy via the barrel heaters in the rear of the extruder normally will reduce the drive-motor load or amperage.
Temperatures in the remaining zones should be set in evenly declining steps between zone 2 and the final metering zone. For example, in an extruder with five zones, if there is a 30 C difference between zone 2 and zone 5, then zone 3 will be set 10 C below zone 2, and zone 4 will be 10 C below zone 3, leaving a final 10 C drop to zone 5.

The temperature settings suitable for the polyolefins for barrier screw are:

These recommended temperatures will provide the least stress to polymer.
Proper setting of barrel temperatures will also help reduce screw and barrel wear, which can be caused by forcing cold polymer into the barrier section of the screw. For example, a flat temperature profile for PP will not reduce polymer viscosity enough to pass through the barrier section. Too high viscosity then causes high barrel pressures, which accelerates screw and barrel wear.
(Source : Plastics Technology)