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

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<ul><li><p>Residual Stresses in Polymers 111: The Influence of Injection-Molding Process Conditions </p><p>A. SIEGMANN*, A. BUCHMAN and S. KENIG </p><p>Department of Materials Engineering Technion, lsrael lnstitute of Technology </p><p>Haifa 32000, lsrael </p><p>Following the evaluation of Residual Stresses (K.S.) i n quenched specimens (Part I ) and the resulting mechanical- physical properties (Part II), the,,ixesent study deals with the effect of iiijectioii-nroldiiig process conditions on K.S. and the respective properties of amorphous polymers. Melt tempera- ture, mold temperature, injection rate, and injection pressure were the parameters studied. Experimental results indicated that the melt temperature caused two maxima in K.S. The sec- ond one rmerses from compressive to tensile. In general, most changes occur in the surface regions, while K.S. decreases with increasing melt temperature, a s is the case i n zones far away from the gate. Furthermore, tensile modulus increased, i n gen- eral, with rising melt temperature, In the case where the effect of mold temperatiire was studied, it was found that K.S. are conipressive i n the surface layers ancl tend to decrease upon iir- crease in Inold temperature ancl distance froin the entrance re- gion. Significant changes in K.S. were also detected i n the in- terior layers. As the niold temperature approached Tg, low values of K.S. were measured, a s was the case i n quenched specinlens. Injection rate affects surface R.S. to a large extent. With low flow rates, tensile stresses were developed i n the ex- terior, 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, a s high as 30 percent, were measured at high injec- tion rates. As far as injection and holding pressures are con- cerned, experimental results showed that a maximum in K.S. was obtained, with increasing pressure, at the siirface. Close to the gate entrance, a reverse from compressive to tensile K.S. was detected at high injection pressnres. As i n 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 K.S. nreasured in thc core re&amp; 1 0 1 1 s . </p><p>INTRODUCTION transition, on the R.S. of Norvl were investigated and </p><p>esidual stresses (K. S.) in polymeric materials have Rb ecome the subject of an increasing number of stud- ies. Although most ofthe studies are of a preliminary na- ture, the phenomenon itself has been defined and dis- tinguished from other phenomena that frequently accompany residual stresses, such as molecular orienta- tion. In recent studies, the level and distribution of R. S. in thermally treated amorphous polymers (1-3) as well as in melt-processed (4-7) and cold-rolled (8-9) polymers, have been investigated. The effect of thermal history, including quenching through a variety of temperature gradients and levels with respect to the polymer glass </p><p>discussed in Part I of the presint study (10).The R.S. were generated by quenching from temperature levels above to below the glass transition. From static-eon- trolled experiments, basic information on the buildup of R. S. induced by thermal gradient only was obtained. Si- multaneously, the effect of these stresses on the me- chanical tensile behavior was studied and reported in Part I1 of the present study (11). Pursuant to the con- trolled experiments, the effect of process conditions on R. S. in injection-molded specimens was investigated. </p><p>During the injection-molding process, the polymer melt, at elevated temperature, flows into a cold mold, the temperature of which is much below Tg. Under these dynamic conditions, both temperature gradient and high shear rates affect the buildup of residual stress. </p><p>560 POLYMER ENGINEERING AND SCIENCE, JUNE, 7982, Vol. 22, No. 9 </p></li><li><p>Residual Stresses in Polymers Il l: The InfIuence of Injection-Molding Process Conditions </p><p>Since the final properties of injection-molded products are of major interest and are influenced by R.S. , this issue has been the subject of research in several labora- tories. For example, the distribution of R.S. in injec- tion-molded Nylon-6 were investigated by Russel and Beaumont (5). They measured a parabolic stress distri- bution, compressive at the surface and tensile at the center. Results indicated that the magnitude of the re- sidual stresses was inversely proportional to the mold temperature. Generally, the experimental results were much lower than the calculated values based on the model proposed by Agganvala and Saibel(12). This dif- ference could have been smaller provided that suitable values for the temperature difference (AT) would have been used in the calculations. The value used for AT was the difference between melting point and mold temper- ature. No account was taken for the actual melt temper- ature and the crystallization that takes place at the crys- tallization temperature. Moreover, since Nylon-6 is a semicrystalline polymer, part of the crystallization takes place at elevated temperature and the remaining amor- phous phase, is cooled first to the mold temperature, (which is either above or below Tg) , and then to ambient temperature. Such thermal history does not result in the buildup of stresses, according to Aggarwala, et al (12); furthermore, it allows for partial stress relaxation. Re- laxation of residual stresses was shown to occur in injection-molded polypropylene, even at low tempera- tures such as -40C (below T g ) (4). The relaxation rate was found to be temperature-dependent and related to the relaxation spectrum of the polymer. Stress relaxa- tions are common in light of the significant changes in the behavior of amorphous polymers, which have been shown by Struick (13) to occur upon aging. Hogberg (14) and Peiter (15) measured R.S. in injection-molded polystyrene and found them to be compressive at the surface. </p><p>One of the first studies on the influence of injection- molding process parameters on the R. S. in polystyrene was reported by Fett (16). He estimated the stresses on the surface only, using the surface hardness method (17), and found residual tensile stresses. By removing thin layers with the aid of a microtom, he found that the stresses become compressive at 50 p below the surface. The stresses remained compressive through a 0.7-mm- thick layer and reversed to tensile stresses at deeper lay- ers. Thus, Fett suggested that molded specimens con- sist of three layers: a thin surface layer (in contact with the mold), in which tensile stresses are developed upon contraction; an intermediate layer, where most of the tensile stresses are allowed to relax; and a core layer, in which tensile stresses are built up, and simultaneously, from equilibrium considerations, compressive stresses are developed in the already-solidified outer layers. The surface residual stresses were found to decrease with increasing melt temperature as well as mold tem- perature. It was suggested that the high melt tempera- ture resulted in high core temperature, which had an annealing effect on the outer layers. Increasing melt temperatures prolonged the effective annealing and thus allowed for further stress relaxation. It was further </p><p>suggested that high mold temperature directly affects the relaxation of the surface residual stresses. </p><p>A comprehensive study was conducted by Menges and coworkers (7, 18, 19). In this study, residual stresses were calculated, using the finite elements method, tak- ing into consideration the dependence of the linear ex- pansion coefficient, a, elastic modulus, E, and poisson ratio, p, on temperature, T , and time, t, as well as the stress relaxation, u,l, and its dependence on tempera- ture and time. The following expression was derived for the residual stresses profile: </p><p>A v ~ =- { [ - a i A T i + Ae]Ei - Aai,,l} - AP 1 - P </p><p>where i denotes the section number (the specimen was divided into i sections), A a is the residual stress, and AE is the shrinkage. The first term in brackets describes the stresses due to thermal gradient, and the second one takes stress relaxation into account. Finally, AF is the hydrostatic pressure that the material is exposed to, prior to the mold opening. Menges, et al, measured the R.S. in polystyrene as affected by melt temperature, mold temperature, injection pressure, holding pres- sure, and time. The study concludes that R.S. are com- pressive at the surface layers and tensile in the interior; high injection pressures especially high holding pres- sures and long holding time, cause reduction of stresses at the surface and an increase of stresses in the center. Under extreme conditions, tensile stresses were devel- oped at the surface and compressive ones in the center. Moreover, they found that the R.S. were not signif- icantly affected by melt temperature; however, they were reduced with increasing mold temperature. It was emphasized that the holding time had a decisive effect on the level and distribution of the residual stresses. </p><p>The present investigation is aimed at further broad- ening the understanding of the R.S. phenomenon in injection-molded products and elucidating the effects of process conditions on the final R. S. and on the prod- uct properties distribution at various thicknesses and locations. </p><p>EXPERIMENTAL Modified poly(ethy1ene oxide) (PPO blended with </p><p>polystyrene) was used throughout the present study. PPO is an engineering thermoplastic material, commer- cially known as Noryl, produced by General Electric. Noryl SE-1, the grade presently used, has a glass transi- tion temperature of 130C. </p><p>To study the effect of the injection-molding process conditions on the R.S. buildup, a mold was designed to produce flat plates, 110 x 110 x 5 mm in dimension. Injection molding was carried out with an ES 99/50 AS Engel machine (max. shot size of 140 g) having a noz- zle diameter of 0.4 cm. The process conditions were changed in a wide range. A certain set of conditions, so- called standard conditions, was chosen and kept con- stant, while only one of the parameters was changed at a time. The injection conditions are summarized in TabZe 1. The injection temperature, so-called melt tempera- ture, is the polymer temperature at the die and is always </p><p>POLYMER ENGINEERING AND SCIENCE, JUNE, 1982, Vol. 22, No. 9 561 </p></li><li><p>A. Siegmann, A. Buchman and S. Kenig </p><p>I I I </p><p>I </p><p>- C I l l C </p><p>I 1 </p><p>Table 1. Injection-Molding Process Conditions </p><p>Melt Temperature, ("C) </p><p>Mold Temperature, ("C) In iect io n Pressu re/H o Id i ng </p><p>20, 215, 230, 245, 260, 275, 280*. 20, 35, 50, 65, 80'. 90. 14011 20, 1 251 1 05, 7 2017 OO* </p><p>Pressure (atrniatm.) 110190, 95/75. 80160, 65/45, 50130. 35/20. </p><p>Injection Rate, (g/s) 75,60,40,34', 20,15,9.7,5,3. </p><p>* Standard Condltlonr </p><p>preceded by a 30C temperature gradient along the machine cylinder. The mold temperature was kept constant by circulating oil in both parts of the mold to achieve symmetrical cooling. Combinations of injection and holding pressures were chosen in a wide range ac- cording to the machine capability. The injection rate (flow rate or fill rate) at the standard injection condi- tions was calibrated for a wide range of machine injec- tion speeds. </p><p>To measure residual stresses, seven rectangular spec- imens (50 x 10 x 5 mm) were cut out from the molded plate, as described in Fig. 1 . These specimens were cho- sen from symmetry considerations, to enable the mea- surement of stress distribution through the plate thick- ness in four different points, at various distances from the injection gate. </p><p>R.S. were measured by the "layer removal" method described by Treuting, et al (20). The details of this method as applied in the present work are described elsewhere (10). The dependence of R.S. on the time elapsed from injection to actual measurement was checked by measuring the stresses in several identical plates, which were allowed to age at ambient tempera- ture for one, five, and 12 days. For this purpose, plates molded at three different rates (six of each) were cho- sen. Only minor differences between the stresses pro- files in the aged plates could have been observed. Thus, no time-dependent behavior was assumed. Menges, et aZ(18), showed that stress relaxation occurred only dur- ing the first minutes after the plate ejection, and no significant further relaxation was observed. </p><p>g : o x </p><p>i' F i g . 1 . Locutions at which H . S . measurements were perforined. </p><p>To evaluate the degree of anisotropy in the injection- molded plates at various operating conditions, tensile bars were cut out of the plate center, perpendicular and parallel to the flow direction. The tensile mechanical properties were measured using an Instron testing ma- chine. It should be emphasized that the results so ob- tained represent average values through the specimen thickness and not actual values for the different layers. </p><p>RESULTS AND DISCUSSION The present study deals with the effect of the injec- </p><p>tion-molding conditions on the buildup of R.S. The injection-molding parameters studied include: melt temperature, mold temperature, injection rate, and in- jection holding pressures. </p><p>The need to investigate and characterize the R. S. dis- tribution in various regions of the molded plates stem from the fact that a respective distribution of physical properties exists in such plates (21). Consequently, stresses were measured as a function of thickness and distance from the injection gate. </p><p>A general remark should be made regarding the R. S. method of calculation. The latter was based on the Treuting and Read (20) derivation, which assumes, among other things, a uniform tensile modulus through- out the specimen. It has recently been shown, however, that this assumption is not accurate (22); in fact, the modulus values of quenched material change markedly through the specimen thickness, especially in the sur- face layers. Moreover, the moduli are expected to change even more in injection-molded products, as a re- sult of molecular orientation distribution, in addition to the different thermal histories in various regions. Con- sequently, the data obtained by the above-mentioned, simplified model should. be regarded as a first approxi- mation and cautiously analyzed. Nevertheless, the data obtained using the Treuting and Read (20) method gives a good practical description of the R. S. and of their level and distribution. Another result of the above-men- tioned method (20) could be the experimentally deter- mined, unbalanced tensile and compressive stresses in the molded specimens, which practically could not exist in a body free of external stresses. Following the re- marks, the effects of the various process parameters on R.S. will be discussed. </p><p>Melt Temperature </p><p>The effect of the polymer (so-called melt) tempera...</p></li></ul>


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