posted January 11, 2005 03:42 PM               

Welcome to the sheet extrusion forum, be sure to check out the threads in the single screw forum for ones relating to "purging".

The best way, when going from coloured/black back to white, is to tear the line down (split the die, separate die, adaptors, screen changer, pull and clean the screw). Generally we can't do "the best way", although some times it would be quicker and save raw materials. If you do hand clean, don't use copper scouring pads/wool to clean equipment used to process PP, copper traces can cause PP to degrade.

So we use purging agents such as Rapid Purge, Dynapurge(Tom's suggestion), I use a liquid purge (added to a resin/regrind mix) called Aquapurge (works preety well). You can also try bare PP resin of a diferent viscosity as a purge. Do a "google" for "disco purge method"

Re-engineering adaptors, die manifolds, breaker plates etc to eliminate "hang up" points/stagnant zones will help speed up colour changes on the fly. Don't leave the extruder parked at "run" temps for longer than 5 (or so) minutes, PP is fairly stable, pbut anything you can do to stop degradation occuring/ or additives plating flow surfaces will help colour changes. Shutting down on bare PP (without colourant) will help too, pigments love to stick to hot metal.