Extruder Screw Cooling
There are basically three Coefficients of Friction (Figure 1) that take place in the feed section of the screw, (1) between the barrel and the pellet, (2) between pellet to pellet, and (3) between the root of the screw and the plastic pellet.
Screw cooling on the feed section core of the screw should always be installed. In most cases, screw cooling will be a benefit to the process. It will give the operator another “zone” of control of the extruder. The main theory of “solids conveying” is that the resin must “stick to the barrel” and “slip on the screw”. By cooling the root of the screw it will reduce the Coefficient of Friction between the steel of the screw and the plastic pellet.
By installing a rotatory union to an extruder screw, as shown in Figure 2, you can have many positive affects to your processing issues.
One advantage to using screw cooling is when regrind is part of the feedstock recipe; there are always traces of “fines” in the regrind. These fine particles will always melt quicker that pellets and pellet-size regrind particles and therefore stick to the root of the screw in feed section. This resin that has stuck to the root of the screw in the feed section will eventually degrade and break loose and ultimately exit through the die and into the final product being made. By circulating 100?F to 120?F water through the feed section of the screw, this will control the temperature of the steel in the feed section of the screw and reduce the possibility of the fines building up on the root in this area.
Another advantage to using screw cooling is to increase the throughput rate of the extruder. For resins that have poor coefficients of friction in the solid phase, such as polypropylene and polyamides, by increasing the first barrel temperature zone and keeping the root of the screw cooled to 100?F to 120?F, the differential of the COF between the pellet/barrel and pellet/screw can be maximized and therefore solids conveying will be maximized.
The easiest way to make sure that 100?F to 120?F can be maintained on the screw, is to simply use tower water and control the flow so that the return water out of the screw is maintained. To do this, all that needs to be done is to install a shut-off valve and immersion thermometer on the return side of the rotary union as shown in Figure 3.
So, by spending about $500 for a rotary union, an immersion thermometer and the necessary plumbing components, many of your feeding problems may go away.