An email from "Mike" asks, "Color changes on a machine using a barrier screw are always very difficult. Why is this?"
Here's our theory of what's happening, Mike. In a barrier screw design (like the one shown here in a figure from the 1972 Dray-Lawrence patent) an auxiliary channel is introduced in the melt zone. The polymer melt flows over the barrier flite into the melt channel as the remaining solids continue along the primary channel. This permits more efficient melting and can yield an increase in extruder throughput.
But this is a low shear environment compared with a conventional screw, and it sacrifices mixing effectiveness. A great many barrier screws add a mixing section (like a Maddock mixer or a pin mixer) to ensure complete mixing of the melt.
Mixing effectiveness is essential to efficient purging. If you need to do a dark-to-light color change and your purging material does not readily mix into the resident production material, the purge will prove difficult. Thus, barrier screws are harder to purge for color changes than conventional screws of equivalent size.
What can you do about this? Increased agitation can compensate for reduced mixing. Consider trying a chemical purging compound or a hybrid purging compound in place of the resin or the mechanical purging compound you've been using. (See also Purging Tip of the Week #27.)