Abstract
A discharge screw element (DSE) A in a polyethylene resin extruder was found fractured after excessive temperature excursion and main motor power spike were experienced. An investigation was subsequently conducted to understand the root causes of the failure; determine the construction materials of the DSEs and the barrels; and provide recommendations to prevent similar failure.
The fracture occurred at a weak portion of the DSE A as a result of a frictional stress (shear stress) due to excessive adhesive wear between the hardfacings of DSE A and the corresponding barrel A. The weak portion, roughly 160 mm in length, was found to be absent of internal splines. The function of the internal splines is to couple with the external splines on the screw shaft to transmit the torque from the main motor to the die plate. Because of the large hardness difference between the hardfacings, the metal-metal contact induced adhesive wear between the barrel A hardfacing and the DSE A hardfacing, causing excessive thickness reduction on the DSE A hardfacing, excursions of melt temperature and motor power output. The adhesive wear induced such a high frictional stress that the first fracture crack propagated at a shear stress direction into the body of DSE A. With continuous application of the extrusion torque, the DSE A was fractured into four pieces starting at the end of the internal splines of DSE A, where stress was concentrated.
Recommendations were proposed for the new construction of discharge screw elements: 1) use relatively soft hardfacing material in the barrel to reduce the hardness difference and avoid adhesive wear; 2) strengthen the weak portion in DSE by building the internal splines on the full length of DSE. Based on the findings, the screw and barrel supplier has elected to incorporate the recommendations in the new designs, further validating this work.
