Troubleshooting and Mitigating Gels in Polyolefin Products

The incumbent resin effect for single-screw extrusion of polyethylene resins: The effect of resin changeover on gels in the product

“…These flaws are regions where resin stagnates, degrades, and then separates from the screw, causing defects in the film product. 3 The gel level is manageable at steady-state conditions for the incumbent resin. However, slight upsets that occur by introducing a challenger resin can cause the degraded material to separate from the screw at a faster rate.…”

“…The most common design defect is the flight radii size. 2,3 Most screw manufacturers design the screws with flight radii that are about half the local channel depth. In many cases, the flight radii are even smaller.…”

“…The degradation at small flight radii are caused by long residence times due to secondary re-circulation flows known as Moffat eddies. 3,5 Large flight radii will not allow Moffat eddies to form, eliminating degradation at this location. As a general guideline, the flight radius should be 1.5 times the local channel depth for channels filled with molten PE resins.…”

“…There are many types of gels. [1][2][3] The gels relevant to this discussion are those generated in the extruder screw channels due to resin degradation caused by long residence times and often oxygen exposure. They are:…”

“…Improperly designed extrusion equipment and processes are common and can lead to the oxidative degradation of PE resins and the formation of crosslinked gels. 2,3 For most cases, the incumbent resin has been running on the film line for an extended time, producing a product with an acceptable quality. Here, the gel level would be relatively low with only minor black specks and brown soft gels.…”