Surging

Surging is generally caused by one or any combination of four things; poor hear control, trash in the extruder, fines in the regrind and/or uneven mixing of regrind and virgin resin.

During the occurance of extruder surging, the following things may happen concurrently;

Condition A
. . . Pressure decreases
. . . Amperage decreases
. . . Extrusion rate decreases
. . . Extruder speed (RPM) increases

Condition B
. . . Pressure rises
. . . Amperage rises
. . . Extrusion RATE rises
. . . Extruder SPEED (RPM) decreases

If conditions A and B alternately occur with a slow, somewhat cyclic rhythm, the trouble can usually be attributed to heat control problems.

If conditions A and B occur in a random fashion with no cyclic tendencies, the trouble more likely results from improper blending of regrind and virgin.

If only condition A occurs, fines in the regrind may well be the cause . This usually happens when the fines stick to the screw in the feed zone. When this happens it is often possible to clear the problem by placing small chunks of resin such as tops and tails in the feed throat. This has a tendency to clear the offending stuck resin from the feed zone and re-establishing proper feed.

Occasionally, when a machine has sat idle for an hour or three, it may be difficult to establish a proper feed of resin in the extruder resulting from the screw and the barrel both being the same temperature. This results in the resin sticking evenly to both the screw and the barrel. Often it may be possible to start the offending extruder by raising the temperature of the feed zone by 20 or 30 degrees (f.) for a few minutes to raise the temperature of the barrel. This should cause the resin to stick to the barrel a bit quicker than to the screw, establishing the proper feed. Once this has occurred the feed zone temperature should be returned to its normal setting. The normal action of the resin moving through the extruder will slightly draw down the temperature of the feed zone on the screw, reducing the friction against the screw and causing the resin to once again properly move through the extruder.