When using mechanical purging compounds to purge co-extrusion dies, it is important to not purge all the extruders through the die at once to prevent excessive die pressure buildup.
Question: I have been told "For co-extrusion it is important to purge from first to last and not all at once". Could you explain this?
My Response: Every co-extrusion die is constructed differently, but the main point is to avoid purging all the extruders with a mechanical compound at once if the die is somewhat restrictive. Since non-abrasive mechanical purging compounds tend to increase the die pressure, filling the die with compound from each extruder all at once may result in damage to the die. Some companies have had the die blown right off of the extruders when they have purged concurrently.
Safe procedures for using mechanical purging compounds in co-extrusion dies would involve purging one extruder at a time. This can either be purging to either the final material, a virgin resin, or a more neutral material such as polypropylene before moving on to the next extruder. Many molders will also reduce the RPM of all the extruders to help reduce pressure buildup. This reduces the pressure spikes in the die making it safer and less stressful to the die when purging.