Cross contamination, what about material that plates out on the wall of the main resin line?=

This can be caused by a number of things. Of course, softer materials like PE or PP are most likely to plate out, especially on the insides of elbows but lots of materials may do the same thing if the pickup velocity is too high. See the attached formula to determine what the proper pickup velocity should be for your material. The minimum pickup velocity for plastic pellets is 3000 fpm.
You may also overcome the problem by installing elbows with rifling on the interior surfaces as they tend to make the pellets tumble instead of sliding around the curves. This reduces frictional heat.

500 x the square root of Material Bulk Density = Pickup Velocity
500 x the square root of 36 lb/cu ft = 500 x 6 = 3000 fpm

500 x sq root of 50 lb/cu ft = 500 x 7.07 = 3535 fpm

Other Factors:

  • Particle size
  • Flowability – coefficient of friction

When you purge the pellets during material change, where do they go?

There are two reasons for purging. First, purging lines after each loading sequence will ensure that all the pellets conveyed will go to the intended destination. Secondly, purging clears the lines to avoid contamination of the material being conveyed next.