Experimental Determination of Residual Molded-in Stress Level in Plastic Rotating Bands

Abstract : Polyethersulfone (PES) has been investigated for use as an injection molded rotating band material on 20 mm projectiles. Molded bands have been observed to develop crazing when exposed to solvent laden atmospheres. The presence of residual stresses resulting from thermal contraction during cool- down and melt stresses in the polymer on filling the cavity exacerbate this stress cracking problem. The purpose of this investigation was to experimentally establish the magnitude of this residual molded-in stress level. Tensile specimens of PES were subjected to solvent induced stress-rupture tests in a laboratory test set-up. A relationship between both the time-to-craze and the time-to-failure and the applied stress was developed over the stress range of 500 to 5000 psi. Three different solvents were used to encompass this wide stress range within reasonable testing times. After this relationship between tensile stress and time-to-craze was established, actual projectiles were immersed in the solvents and the time-to-craze recorded. Correlation of the projectile test results with the tensile test results indicated that the residual molded-in stress in the rotating bands was 1750 + or - 800 psi (12.06 + or - 5.51 MPa).