Surface fracture in injection molding of filled polymers

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Surface Fracture in Injection Molding of Filled Polymers R. L. BALLMAN,* R. L. KRUSE, and W. P. TAGGART Research Department, Monsanto Company, Indian Orchard, Massachusetts In injection molding certain polymers, fracture of the poly- mer stream sometimes occurs at the mold surface. This phc- nomenon has been found to be a tearing apart of the polymer surface layer accompanied hy downstream slip of the flowing melt at the polymer/mold interface. Fracture occurs early in mold filling and is initiated usually at the gate to the mold cavity. Analysis of the fracture mechanism indicates that fracture is caused by: (1) high shearing stress in the melt as it fills the mold; ( 2 ) poor polymer/mold adhesion; and (3) low polymer surface cohesive strength. INTRODUCTION he injection molding of filled or heterogeneous T polymers introduces problems that arc not en- countered in molding single-phase thermoplastics. One of these problems is surface fracture, which previously has been reported as smears and splays in the molding of polyvinyl acetal (1,2) and as scars or blushes in the molding of AHS (3-6). This phenomenon does not usually occur in thc molding of homogeneous polymers such as poly- methyl methacrylate, polystyrene, and styrene-acry- lonitrile copolymer. It is the purpose of this study to offer a mechanism for the formation of surface fracture and to discuss the relationship of fracture occurrence to polymer rheology. In order to simplify the problem, much of this work was done with a model system, e.g., poly- styrene filled with glass spheres. Nonetheless, it has been shown that the conclusions derived from this model system are generally valid for those filled materials of more commercial interest. EXPERIMENTAL One polymer used in this work was commercial polystyrene with an M , of 338,000 and an M , of 73,- 000 as determined by GPC. The filler used, obtained from the Microbeads Div. of the Cataphote Corp., was glass beads having a particle size of 1-30 p * Present address: Monsanto Co., Pensacola, Fla. 154 The glass beads were compounded with poly- styrene by mill rolling for 5 min at a temperature of 125C and roll speeds of 30 ft/min. The sample was pelletized in an Abbe cutter. Selected samples were analyzed for glass content by dissolving the polymer and filtering, washing and drying, and weighing the beads. Typical results were 32.97, 33.73, 34.23% found vs 33.00% charged, indicating negligible bead loss during compounding. Glass beads of 1-30 p diam were coupled with polystyrene by first reacting the beads with a silane ( Dow Corning, 2-6030). The silane was dissolved in methanol diluted with 0.1% acetic acid and coupled to the glass by refluxing for 30 min at 65C. Styrene monomer was charged, and the methanol and water were distilled off. Di-tert-butyl peroxide catalyst was added, and polymerization was con- ducted to completion. Also used in this study were several commercial ABS polymers. Rheology data were obtained using an Instron capillary rheometer according to standard pro- cedures ( 7 ) . A measure of recoverable strain y r was determined by extrudate swell, using y r = (d,J dCa,)-1 ( assuming extensional deformation to cyl- indrical volume element ) . No corrections were made at this time for velocity profile rearrangement, in- complete melt relaxation, or contraction in cooling. Moldings were prepared on a 1 oz Watson Still- man press set at 400F using a 1.75 in. diam x 0.170 in. thick disc die cooled to 77F. POLYMER ENGlNEZRlNG AND SCIENCE, MAY, 1970, Yo/. 10, No. 3 Surface Fracture in Injection Molding of Filled Polymers Fig. 3. Detail of individual fractures in gloss-filled polystyrene WOX). Fig. 1 . Surface fracture of a glass-filled polystyrene injection molding (27X). Fig. 2. Fracture initiation of a glass-filled polystyrene molding (27X). Fig. 4. Adhesive failure of glass/polystyrene interface (ZOOOX). Light micrographs were made using a Unitron microscope (Model BU-13) and electron micro- graphs were made using a scanning electron micro- scope Type JSM. RESULTS A molding of a filled system showing typical well developed fractures is shown in Fig. 1. Moldings made at lower injection pressure show the earliest stage of fracture in Figs. 2 and 3. A closeup of an individual glass bead at a fracture site is shown in Fig. 4 . These pictures indicate that surface fracture initiates at the filler/matrix interface and at numerous sites simultaneously. Other filled polymer systems, such as ABS show similar features as illustrated in Figs. 5, 6 and 7. As the individual fractures grow, they join together into one large tear as illustrated in Figs 1 and 7. The leading edge of the tear appears POLYMER ENGlNEERlNG AND SCIFNCE, MAY, 1970, Vol. 10, No. 3 Fig. 5. Surface fracture of an ABS injection molding (1X). 155 R. L. Ballman, R. L. Kruse, and W. P . Taggart Fig. 4. Fracture initiation of an ABS Molding (150X). Fig. '1. Uetuil of surface fracture of an ABS injection m o k h g (1 80X). Fig. 4. Stages in the formution of surface fracture during ABS injecqon molding. ~ 156 ~ I Fig. 9. Stages in the formation of surface fracture during ABS injection molding. Fig. 10. Stages in the formation of surface fracture during ABS injection molding. as an irregular line roughly bisecting the fracture area. Features characteristic of both brittle and duc- tile failures are evident. Brittle failures, usually OC- curring in a polymer below its T,, appear at the edges, while ductile failures characteristic of poly- mer above its T , are visible in the interior. Stringers in the region of ductile failure are oriented in the flow direction. The sequence of events in the formation of sur- face fracture during the molding operation is shown in Figs. 8, 9, 10, and 11. These moldings were made by interrupting the molding cycle and withdrawing the plunger at various stages in filling the mold. Figures 8 and 9 show the polymer jetting into the mold cavity. Figure 10 shows the point at which fracture occurs, and Fig. 11 shows the continued fill of the mold behind the jet and fractured area. THEORY A schematic diagram of the polymer melt flowing along the wall of the cold mold is shown in Fig. 12. At some point at distance L behind the wave front, exists a filler/matrix interface of length D. Acting on this interface are forces generated by a shearing stress T and a normal stress PI1 caused by the flow- POLYMER ENGINEERING AND SCIENCE, MAY, 1970, Vol. 10, No. 3 Surface Fracture in Injection Molding of Filled Polymers 15.0 14.0 13.0 12.0 - 11.0 E 10.0 6 8.0 - 9.0 t > 7.0 Fig. 11 . Stages in the formation of surface fracture ilitring ARS injection molding. L - - - - - - - DIRECTION t. 3 5 4.0 3.0 20 1.0 FILLER/MATRIX HOT POLYMER I - - - - I L PI I D I V @PI1 COLD DIE WALL I'ig. 13. Stirface fracture mechanism. ; 6.0 1 n 5.0 FILLER VOLUME FRACTION = 0.19 / I 100 200 300 400 5 0 0 600 700 800 9 0 0 loo0 INJECTION PRESSURE (pal l Fig. 13. Effect of shear stress on surface fracture sez;erity oj polystyrene filled with glass beads. ing polymer on the cooled surface layer. A force balance indicates that at the critical condition of interfacial fracture, the shearing stress causing frac- ture must be balanced by the adhesive strength of the polymer to the die wall and the cohesive strength of the interface or Fig. 14. Surface fracture of unfilled polystyrene due to pres- ence of silicone lubricant on mold surface. (1) rLW = p PllLW + uDW where W is the fracture width, p is the coefficient of polymer/wall friction and u is the interfacial co- hcsive strength. Fracture will occur when Substituting the product of viscosity and shear rate, .I+, for shear stress T and using Weissenberg's relationship between normal stress Pll and recov- erable strain ( 8 ) ) yr = P11/7, we rewrite Eq 2 as Tliis model indicates that the tendency of a ma- terial to surface fracture in molding will increase as the melt viscosity and shear rate are increased and as the cohesive strength, coefficient of friction and melt elasticity are decreased. These factors will now be considered. DISCUSSION The effect of shearing stress on surface fracture is shown in Fig. 13. Fracture severity is measured by its length on the molding surface. As predicted by E q 3, increasing shearing stress increases the degree of surface fracture. The effect of the interfacial cohesive strength is also shown on Fig. 13. It is assumed that glass coupled with polystyrene will have greater inter- facial cohesive strength with polystyrene matrix than does uncoupled glass. As shown here, increasing u decreases tendency to fracture. The effect of changing the coefficient of polymer/ die wall coefficient of friction is shown on Fig. 14. Reduced friction caused by spraying the mold sur- face with silicone lubricant results in fracture. The effect of melt elasticity on surface fracture is shown by Fig. 15. The data are scattered on this plot because a number of different filled polymer systems were used whose differing rheological prop- erties also affected other parameters in the fracture equation. Nevertheless, the trend with increased melt elasticity is to reduce fracture as predicted by the model. Hence, experimental testing of each of the indi- cated material and molding parameters gives at least qualitative agreement with theory. POLYMER ENGlNEERlNG AND SCIENCE, MAY, 1970, Vol. 10, No. 3 157 R. L. Ballman, R. L. Kruse, and W . P . Taggart 12.0 ? 511.0 Y * 10.0 k 9.0 2 8.0 w 7.0 2 6.0 2 5.0 4.0 3.0 2.0 1.0 v) a 0 LL 13.0 I - - - - - - - - - 0 0 0 0 0 0 0 0 0 0 0 ~ - v 0. I 0.2 0.3 0.4 0.5 0.6 Fig. IS. Correlation of fracture severity with recoverable strain (die swell) for filled polymer. SUMMARY Surface fracture in the injection molding of filled polymer has been studied using polystyrene filled with glass beads. An analysis was made using a force balance at the fracture site in the flowing polymer as it is filling the mold. This analysis indicates that fracture will occur more readily with increasing polymer viscosity and shear rate and with decreas- ing interfacial strength, coefficient of wall friction, and melt elasticity. Experiments give qualitative agreement with these predictions. ACKNOWLEDGMENT We wish to thank G. C. Claver and W. H. Farn- ham for contributing the micrographs, and Dr. N. E. Aubrey for his many stimulating ideas and helpful discussions concerning this work. The polystyrene coupled glass beads were prepared under the direc- tion of M. Baer. NOMENCLATURE shearing stress normal stress interfacial cohesive strength coefficient of friction fracture width length from wave front to fracture depth of fracture melt viscosity above cold layer shear rate of melt above cold layer recoverable strain of melt above cold layer REFERENCES 1. W. C. Filbert and T. M. Roder, Technical Papers, 9, SPE 21qt Annual Technical Conference, Los Angeles ( 1963). 2. A. G. Srrle, Plastics Technology, 14, No. 3, 67 (1968). 3. D. A. Videtto, Technical Papers, SPE Regional Technical 4 . R. E. Dunning, ibid., 98. 5. H. H. Fruinberger and W. B. Evans, ibid., 32. 6. W. B. Evans, Technical Papers, 13, 1004, SPE 25th Annual 7 . R. L. Ballman and J. J. Brown, Instron Application 8. A. Jobling and J. E. Roberts, Rheology, Ch. 13, ed. Conference, 124, Hartford, Conn. (1964). Technical Conference ( 1967). Series SA-2. F. R. Eirich, Academic Press ( 1958). I58 POLYMER ENGINEERING AND SCIENCE, MAY, 1970, Vol. 10, No. 3


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