Barrier screws have a number of advantages, but the most important is the ability of the designer to pinpoint the location where melting is to be completed. In conventional screws, the typical melting pattern occurs in the compression section. The channel depth is gradually reduced in this region, forcing unmelted polymer outward, where it is subjected to high shear forces that cause it to melt.
However, in conventional screws the compression section must stop at some point to allow polymer to move forward to the metering section to create the desired output. As a consequence of this transition, some of the unmelted material is not subjected to high shear and is instead conveyed into the metering section unmelted. At that point any unmelt acts like an ice cube in water, drifting along and melting very slowly.
Polymers are poor conductors of heat. If the screw is long enough, polymer will eventually be completely melted by the heat conducted from the surrounding fully melted material. But if the screw is not long enough, some material will emerge from it as either unmelted or in a state where its temperature and viscosity characteristics are inconsistent with the surrounding polymer. This will result in erratic die flow.
Calculating the amount of unmelt escaping into the metering section is complex and difficult to ascertain with complete accuracy, since it varies with screw speed. The typical solution is to equip the screw with a high-shear mixer, such as the widely used Maddock-style mixer, located somewhere in the metering section. This device forces the polymer to pass through very tight clearances, imparting high shear to complete melting.
Although it effectively serves its purpose of completely melting the material, these types of mixing devices have several disadvantages. First, forcing unmelted material through a tight clearance requires pressure, and that reduces the screw output and increases melt temperature.
Secondly, such a mixer does not correct the non-homogeneous melt-temperature problem, since the already melted polymer also gets subjected to additional shear, raising its temperature along with that of the unmelted polymer. This is particularly true if the mixer is located at or near the end of the screw so that no further homogenization and residence time is available.
A properly designed barrier screw, on the other hand, allows for complete elimination of unmelt while not restricting output. The unmelted polymer can be contained in the solids channel until it is melted, with the melted polymer contained in a second melt channel. This allows for the completion of melting to be “designed in.”
Additionally, the thermal history of the entire melt is more uniform since it was all melted the same way. And since the melting was completed well before the screw discharge, the melt is thermally homogeneous to encourage uniform die flow. This design also eliminates the need for a mixer in most cases.
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