Residual stresses in polymers III: The influence of injection‐molding process conditions

Abstract

AbstractFollowing the evaluation of Residual Stresses (R.S.) in quenched specimens (Part I) and the resulting mechanical‐physical properties (Part II), the, present study deals with the effect of injection‐molding process conditions on R.S. and the respective properties of amorphous polymers. Melt temperature, mold temperature, injection rate, and injection pressure were the parameters studied. Experimental results indicated that the melt temperature caused two maxima in R.S. The second one reverses from compressive to tensile. In general, most changes occur in the surface regions, while R.S. decreases with increasing melt temperature, as is the case in zones far away from the gate. Furthermore, tensile modulus increased, in general, with rising melt temperature. In the case where the effect of mold temperature was studied, it was found that R.S. are compressive in the surface layers and tend to decrease upon increase in mold temperature and distance from the entrance region. Significant changes in R.S. were also detected in the interior layers. As the mold temperature approached Tg, low values of R.S. were measured, as was the case in quenched specimens. Injection rate affects surface R.S. to a large extent. With low flow rates, tensile stresses were developed in the exterior, reversing to compressive stresses at higher speeds. The reversal in sign depends on the location relative to the gate. Once compressive stresses were formed, further increase in rate caused a reduction in R.S. In addition, variations in tensile modulus, as high as 30 percent, were measured at high injection rates. As far as injection and holding pressures are concerned, experimental results showed that a maximum in R.S. was obtained, with increasing pressure, at the surface. Close to the gate entrance, a reverse from compressive to tensile R.S. was detected at high injection pressures. As in the other cases, injection pressure influenced mostly the exterior layers. Only in zones close to the entrance and at high pressures were high levels of R.S. measured in the core regions.