Generation of large residual stresses in injection moldings

Large residual stresses have been generated in injection molded bars by ejecting them prematurely and completing the cooling process by quenching into ice water or liquid nitrogen. The stress distribution formed under these conditions was found to be much closer to parabolic than is the case when the moldings cool conventionally. Limited testing on moldings made in this way indicated significant property enhancement and improved resistance to ultraviolet degradation.     

Dependence of crack trajectories on stress distributions in the surfaces of injection moldings produced under high packing pressure

The curved trajectories of solvent-induced cracks in the surfaces of polycarbonate injection moldings produced under high packing pressures have been rationalized in terms of the residual body stresses that exist largely in a thin surface layer. The analysis indicates that the residual tensile stress in the skin of the molded plaque can reach values as large as 5 MPa and the tangential tensile stress values as large as 12 MPa, depending on location in the plaque and on molding conditions. The inward penetration of the crack is stopped eventually by the interior compressive stresses that counterbalance the tensile stresses in the “skin.” The crack tends to turn sideways and grow further in Mode II as a result of the intense interlayer shear stress set up at the crack tip by the difference between the skin tension and core compression. The most important practical conclusion from this analysis is that in the absence of externally applied stress, these so-called edge cracks are unlikely to penetrate the molding's interior since the tensile stress in the surface layer is necessarily counterbalanced by the subsurface compression.     

Residual thermal stresses in injection moldings of thermoplastics: A theoretical and experimental study

Internal stresses in injection molded components, a principal cause of shrinkage and warpage, are predicted using a three‐dimensional numerical simulation of the residual stress development in moldings of polystyrene and high‐density polyethylene. These residual stresses are mainly frozen‐in thermal stresses due to inhomogeneous cooling, when surface layers stiffen sooner than the core region as in free quenching. Additional factors in injection molding are the effects of melt pressure history and mechanical restraints of the mold. Transient temperature and pressure fields from simulation of the injection molding cycle are used for calculating the developing normal stress distributions. Theoretical predictions are compared with measurements performed on injection molded flat plates using the layer removal method on rectangular specimens. The thermal stress development in the thinwalled moldings is analyzed using models that assume linear thermo‐elastic and linear thermo‐viscoelastic compressible behavior of the polymeric materials. Polymer crystallization effects on stresses are examined. Stresses are obtained implicitly using displacement formulations, and the governing equations are solved numerically using a finite element method. Results show that residual stress behavior can be represented reasonably well for both the amorphous and the semicrystalline polymer. Similarities in behavior between theory and experiment indicate that both material models provide satisfactory results, but the best predictions of large stresses developed at the wall surface are obtained with the thermo‐viscoelastic analysis.     

Effect of machine compliance on mold deflection during injection and packing of thermoplastic parts

Minimizing mold deflection is essential when manufacturing plastic parts to tight tolerances. Both the mold and the machine are compliant and deform upon loading, which can affect the part quality. Therefore, understanding mold deflection during injection molding is critical for determining the final geometry of the part. It is also critical for secondary processes such as the in‐mold coating process. This article presents work in quantifying both mold deflection during an injection‐molding cycle and the effect of machine compliance on mold behavior. The mold cavity pressure obtained using MoldFlow? was used as input for the subsequent finite element mold deflection analysis. Two different structural models were used: the first model included only the mold, the mold base units and the ejector platen; the second model included the effect of the injection‐molding machine compliance. To validate the model, strain gage rosettes were placed on the mold and the machine. Validating experiments were conducted using process parameters identical to those used in the simulations. A comparison of the experimental and simulation results for both models is presented. POLYM. ENG. SCI., 46:844–852, 2006. © 2006 Society of Plastics Engineers     

The effect of a temperature gradient on residual stresses and distortion in injection moldings

Distortion of bars injection-molded from polystyrene, polypropylene, and glass-fiber-filled polypropylene and subsequently placed in a temperature gradient has been examined. Residual stress distributions have been measured both for the as-molded state and after annealing in a temperature gradient. In the as-molded state all moldings showed the usual residual stress distribution with compressive stresses near the surface and tensile stresses in the interior. In all three materials it was found that tensile stresses could be developed near to the warmer surface on gradient annealing and that tensile stresses still remained at this surface when the bar was cooled and permitted to bend to restore internal equilibrium. It is shown therefore that in addition to the dimensional changes which occur and which may render the molding unserviceable after temperature gradient annealing, another undesirable change takes place, leaving the molding much more susceptible to fracture from a surface flaw. Uniform annealing is found to be much less likely to cause stress reversal and the stresses remain balanced so that distortion is minimal.     

Morphology evolution during injection molding: Effect of packing pressure

Injection molding is one of the most widely employed methods for manufacturing polymeric products. The final properties and then the quality of an injection molded part are to a great extent affected by morphology. Thus, the prediction of microstructure formation is of technological importance, also for optimizing processing variables. In this work, some injection molding tests were performed with the aim of studying the effects of packing pressure on morphology distribution. The resulting morphology of the moldings was characterized and it was compared with previous results gathered on samples obtained by applying a lower holding pressure. Furthermore, the molding tests were simulated by means of a code developed at University of Salerno. The results obtained show that on increasing holding pressure the molecular orientation inside the samples increases, and simulations show that this is due mainly to the increase of relaxation time caused by the higher pressures. On discussing the simulation results, some considerations are made on the effects of pressure on crystallization kinetics and on rheology.     

Internal stress,molecular orientation,and distortion in injection moldings: Polypropylene and glass-fiber filled polypropylene

Residual stress measurements and distortion analyses have been conducted on injection molded plaques made from polypropylene (PP) and a short glass-fiber filled polypropylene (GFPP). The residual stress analyses include measurements both parallel and perpendicular to the direction of flow during mold filling. Residual stresses are very anisotropic in GFPP, but not in PP. The residual stress levels in PP fall on aging at room temperature, whereas in GFPP the proportion of stress relaxation is smaller, and significant stresses remain even after heating to elevated temperatures. A significant contribution to distortion has been linked to the ejection process, and the long- and short-term distortion of moldings is discussed within the framework of the properties of the materials measured here.     

保压时间和保压压力对注塑制品厚度分布的影响

通过实验研究了保压压力和保压时间对制品厚度分布的影响,所得的结论可以指导塑件和模具的设计。     

The thermomechanical environment and the mechanical properties of injection moldings

Axisymmetric specimens were injection molded in a propylene copolymer with systematic variations of the melt and mold temperatures and the injection flow rate, in a total of 15 different processing conditions. From computer simulations of the mold filling stage using commercially available software packages, two thermomechanical indices were calculated. They aim at evaluating the level of orientation of the skin and the degree of crystallinity of the core layers. Assuming that these morphological features determine the mechanical response of the moldings, the thermomechanical indices were weighted by the relative thickness of the skin and core layers. The tensile behavior of the moldings was assessed at two velocities of 3.3 × 10^?5 (2 mm/min) and 3 m/s. The mechanical properties studied were the initial modulus, the yield stress and the strain at break. The relationships between the weighted thermomechanical indices and these mechanical properties were analyzed from 3D response surfaces obtained by polynomial fittings. Globally, a marked effect of the strain rate on the mechanical response along with a distinct sensitivity on the weighted thermomechanical indices was found. At high strain rates the microstructural differences were enhanced. The dependence of the yield stress on the thermomechanical indices was not significantly affected by the strain‐rate. However, the strain‐rate dependence of the other mechanical properties was strongly influenced by the initial microstructural state. Furthermore, the maximization of different mechanical properties could not be made simultaneously due to their distinct microstructural dependences. The concept of the thermomechanical indices is evidenced as a simple, valid and valuable tool to establish straightforward relationships between the processing and the mechanical behavior. Polym. Eng. Sci. 44:1522–1533, 2004. © 2004 Society of Plastics Engineers.     

Tensile properties in the oriented blends of high-density polyethylene and isotactic polypropylene obtained by dynamic packing injection molding

In this article, tensile properties have been discussed in terms of phase morphology, crystallinity and molecular orientation in the HDPE/iPP blends, prepared via dynamic packing injection molding, with aid of scanning electron microscopy (SEM), differential scanning calorimetry (DSC) as well as two dimensional X-ray scattering (2D WAXS). For the un-oriented blends, the tensile properties (tensile strength and modulus) are mainly dominated by the phase morphology and interfacial adhesion related to the influenced crystallization between HDPE and iPP component. A maximum in tensile strength and modulus is found at iPP content in the range of 70-80 v/v%. As for the oriented blends, however, the presence of dispersed phase in the blends, independent of phase morphology and crystallinity, always makes tensile properties to be deteriorated through reducing molecular orientation of matrix. It is molecular orientation of matrix that determines the tensile properties of oriented blends. In the blends with HDPE as matrix, steep decreasing of tensile properties is related to the rapid reducing of molecular orientation of HDPE, whereas in the blends with iPP as a major component, slight decreasing of molecular orientation of iPP results in slight reducing of tensile properties. Other factors, such as interfacial properties and phase morphology, seem to be little contribution to the modulus and tensile strength.     

Birefringence in gas‐assisted tubular injection moldings: Simulation and experiment

The influence of the processing variables on the birefringence and polymer/gas interface distribution is analyzed for polystyrene moldings obtained by gas‐assisted injection molding (GAIM) under various processing conditions. The processing variables studied were: melt and mold temperatures, shot size, gas pressure, injection speed, and gas‐delay time. Measurements and viscoelastic simulations of the radial distribution of birefringence components, Δn and n_rr ? n_θθ, the variation of the average birefringence, 〈n_zz ? n_θθ〉, along the molding and polymer/gas interface along the length of spiral‐shaped tubular moldings are presented. The polymer/gas interface distribution and flow stresses were simulated using a numerical scheme based on a hybrid finite element/finite difference/control volume method. The birefringence was calculated from the flow‐induced stresses using the stress‐optical rule. Simulations qualitatively agreed with measurements and correctly described theeffect of the processing variables on the birefringence andthe polymer/gas interface distribution in GAIM moldings. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Effect of injection pressure and crazing on internal stresses in injection-moulded polystyrene

The influence of injection pressure on the internal stress distribution in injection-moulded polystyrene bars has been found to be rather small using the layer removal method. On the other hand, substantial differences in the stress relaxation behaviour have been discovered in specimens moulded at pressures in the range 37–143 MPa. The Kubát and Rigdahl internal stress parameter for all sets of specimens was numerically fairly small, but changed from negative to positive on increasing the injection pressure. The parameter most sensitive to variations in injection pressure appears to be the index in the power law expression used to describe the stress relaxation behaviour. Specimens were surface-crazed by bending around a cylindrical former and tested by the same techniques. A further increase in the magnitude of stress at all positions was indicated by the layer removal procedure while the gradients of the Kubát and Rigdahl plots from stress relaxation data were unchanged.     

Fiber orientation in simple injection moldings. Part I: Theory and numerical methods

This paper sets out the theory and numerical methods used to simulate filling and fiber orientation is simple injection moldings (a film-gated strip and a center-gated disk). Our simulation applies to these simple geometry problems for the flow of a generalized Newtonian fluid where the velocities can be solved independently of fiber orientation. This simplification is valid when the orientation is so flat that the fibers do not contribute to the gapwise shear stresses. A finite difference solution calculates the temperature and velocity fields along the flow direction and through the thickness of the part, and fiber orientation is then integrated numerically along pathlines. Fiber orientation is three-dimensional, using a second-rank tensor representation of the orientation distribution function. The assumptions used to develop the simulation are not valid near the flow front, where the recirculating fountain flow complicates the problem. We present a numerrical scheme that includes the effect of the fountain flow on temperature and fiber orientation near the flow front. The simulation predicts that the orientation will vary through the thickness of the part, causing the molding to appear layered. The outer “skin” layer is predicted only if the effects of the fountain flow and heat transfer are included in the simulation.     

Yield and internal stresses in aluminum filled epoxy resin. A compression test and positron annihilation analysis

The influence of the filler content on the mechanical properties of an epoxy resin composite filled with aluminum powder was investigated. Compressive tests were performed at room temperature and at different strain rates. The response of the composites was also studied by positron annihilation lifetime spectroscopy. The dependence of the yield stress on the filler content is shown. The results are discussed in terms of a proposed model that takes into account the contribution of the filler powder. To this purpose information from positron spectroscopy is important since it allows to correctly evaluate the internal stresses introduced in the composite epoxy lattice by the metal filler.     

Ejection force in tubular injection moldings. Part I: Effect of processing conditions

The development and manufacture of injection molds for high quality technical parts are complex tasks involving the knowledge of the injection molding process and the material changes induced by processing. In the case of some specific shapes (boxes, tubular fittings), the shrinkage is partially restricted by the mold. The molding shrinks against the core, inserts or pins. Thus, upon ejection, it will be necessary to overcome the frictional forces resulting from the shrinkage. The knowledge of the ejection force is a useful contribution to optimizing the design of molds with these features, and to guaranteeing the structural integrity of the moldings. A study on the effect of conditions on the ejection force required for deep tubular moldings is described for the cases of three common thermoplastic polymers. The studies were based on tubular moldings (60 mm diameter, 146 mm length, and 2 mm thickness). The injection unit cell consisted of a 1 MN clamp force injection molding machine, thermal regulator, and material dryer. During processing, pressure, temperature and ejection force evolutions were recorded. The results show that the processing conditions noticeably influence the ejection force. Polym. Eng. Sci. 44:891–897, 2004. © 2004 Society of Plastics Engineers.     

Crystallinity and microstructure in injection moldings of isotactic polypropylenes. Part II: Simulation and experiment

The injection moldings of isotactic polypropylenes with various molecular weights were simulated using finite difference method. In the simulations, the unified crystallization model proposed in our previous paper was applied. The prediction of crystallinity and microstructure development in the moldings was based upon the crystallization kinetics and the “competing mechanisms” for introducing various microstructure layers in the moldings. Extensive injection molding experiments were carried out. The pressure traces during the molding experiments were recorded. The crystallinity distribution in the moldings was determined using differential scanning calorimetry. The measurements on the microstructure embedded in the moldings were performed, including the thickness of the highly oriented skin layer and the gapwise distribution of the spherulite sizes. The measured data for the crystallinity and microstructure in the moldings were compared with the simulated results. The effects of molecular weight and processing conditions on the development of crystallinity and microstructure in the moldings were elucidated. Theoretical predictions were found to be in a good agreement with experimental measurements.     

Theoretical and experimental studies of anisotropic shrinkage in injection moldings of semicrystalline polymers

A novel approach to predict anisotropic shrinkage of semicrystalline polymers in injection moldings was proposed using flow‐induced crystallization, frozen‐in molecular orientation, elastic recovery, and PVT equation of state. The anisotropic thermal expansion and compressibility affected by the frozen‐in orientation function and the elastic recovery that was not frozen during moldings were introduced to obtain the in‐plane anisotropic shrinkages. The frozen‐in orientation function was calculated from amorphous and crystalline contributions. The amorphous contribution was based on the frozen‐in and intrinsic amorphous birefringence, whereas the crystalline contribution was based on the crystalline orientation function, which was determined from the elastic recovery and intrinsic crystalline birefringence. To model the elastic recovery and frozen‐in stresses related to birefringence during molding process, a nonlinear viscoelastic constitutive equation was used with temperature‐ and crystallinity‐dependent viscosity and relaxation time. Occurrence of the flow‐induced crystallization was introduced through the elevation of melting temperature affected by entropy production during flow of the viscoelastic melt. Kinetics of the crystallization was modeled using Nakamura and Hoffman‐Lauritzen equations with the rate constant affected by the elevated melting temperature. Numerous injection molding runs on polypropylene of various molecular weights were carried out by varying the packing time, flow rate, melt temperature, and mold temperature. The anisotropic shrinkage of the moldings was measured. Comparison of the experimental and simulated results indicated a good predictive capability of the proposed approach. POLYM. ENG. SCI., 46:712–728, 2006. © 2006 Society of Plastics Engineers     

Theoretical and experimental studies of anisotropic shrinkage in injection moldings of various polyesters

A novel approach to predict anisotropic shrinkage of slow crystallizing polymers in injection moldings was proposed, using the flow‐induced crystallization, frozen‐in molecular orientation, elastic recovery, and PVT equation of state. In the present study, three different polyesters, polyethylene terephthalate, polybutylene terephthalate, and polyethylene‐2,6‐naphthalate (PEN), are used. The anisotropic thermal expansion and compressibility affected by the frozen‐in orientation function and the elastic recovery that was not frozen during moldings were introduced to obtain the in‐plane anisotropic shrinkages. The frozen‐in orientation function was calculated from the amorphous contribution based on the frozen‐in and intrinsic amorphous birefringence and crystalline contribution based on the crystalline orientation function determined from the elastic recovery and intrinsic crystalline birefringence. To model the elastic recovery and frozen‐in stresses related to birefringence during molding process, a nonlinear viscoelastic constitutive equation was used with the temperature‐dependent viscosity and relaxation time. Occurrence of the flow‐induced crystallization was introduced through the elevation of melting temperature affected by entropy production during flow of the viscoelastic melt. Kinetics of the crystallization was modeled using Nakamura and Hoffman‐Lauritzen equations with the rate constant affected by the elevated melting temperature. Numerous injection molding runs were carried out by varying the packing time, packing pressure, flow rate, melt and mold temperature, and anisotropic shrinkage of moldings were measured. The experimental results were compared with the simulated data and found in a fair agreement. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 3526–3544, 2006     

Simulations of a full three‐dimensional packing process and flow‐induced stresses in injection molding

Numerical investigations of a full three‐dimensional (3D) packing process and flow‐induced stresses are presented. The model was constructed on the basis of a 3D nonisothermal weakly compressible viscoelastic flow model combined with extended pom‐pom (XPP) constitutive and Tait state equations. A hybrid finite element method (FEM)–finite volume method (FVM) is proposed for solving this model. The momentum equations were solved by the FEM, in which a discrete elastic viscous stress split scheme was used to overcome the elastic stress instability, and an implicit scheme of iterative weakly compressible Crank–Nicolson‐based split scheme was used to avoid the Ladyshenskaya–Babu?ka–Brezzi condition. The energy and XPP equations were solved by the FVM, in which an upwind scheme was used for the strongly convection‐dominated problem of the energy equation. Subsequently, the validity of the proposed method was verified by the benchmark problem, and a full 3D packing process and flow‐induced stresses were simulated. The pressure and stresses distributions were studied in the packing process and were in agreement with the results of the literature and experiments in tendency. We particularly focused on the effects of the elasticity and pressure on the flow‐induced stresses. The numerical results show that normal stress differences decreased with incremental Weissenberg number and increased with incremental holding pressure. The research results had a certain reference value for improving the properties of products in actual production processes. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012