posted July 05, 2004 07:21 AM               

One method is to set your process up as you nrmally do, run long enough to establish equilibrium, then empty the hopper completely while maintaining all extrusion parameters - including screw(s) RPM, temps, etc.

Just after your hopper is "dry" and you can "see" the feed flights of the screw, drop in your material that has a marked color change (i.e. using a master-batch). Do not over-dose it though. A Red would be fine. With your stop watch, in hand, you start it (the watch) as you allow the new feed to enter the feed zone, and allow it to come through the tooling. Hit "stop" on your watch. Mark that time down.

Now, allow virgin (un-colored feed stock) to enter the hopper after you emptied it out and let the screw run dry. Repeat this agiain, and "mark the time of the start and only "stop" the watch when all the red appears to be completely out.

The best "guess" then is taken by adding the two together, and dividing it by two. This will give you an approximate "residence" time.

There are many mathamatical calculations available from your extruder supplier and resin vendors, but one thing that is rarely taken into account is the wear of your screw, barrel, and tooling.