Abstract: The effect of pressure on viscosity is an important but often overlooked aspect of the flow properties of polymeric materials. In this work, two polymers (an atactic and a syndiotactic Polystyrene) were characterized to determine the effect of pressure on viscosity. In particular, a device was adopted to increase the exit pressure of a standard capillary rheometer, thus obtaining data of viscosity under high pressure and high shear rates. The Simha-Somcynsky equation of state was applied to the pressure-volume-temperature experimental data of both materials to obtain the dependence of free volume on temperature and pressure. The Doolittle equation was eventually employed to verify the dependence of viscosity on free volume. It was found that, for both materials, a linear relationship holds between the logarithm of zero-shear-rate viscosity (at several temperatures and pressures) and the inverse of free volume.
Abstract: In polymer crystallization, the correct determination of the melting temperature is of crucial importance since crystallization kinetics mainly depends on the amount of undercooling. In this work, the thermodynamic melting temperature and the reference temperature for crystallization kinetics (also called "zero growth rate temperature") are calculated by means of alternative procedures with the objective of providing a general discussion on problems occurring in their determination and of identifying the relationship between them. Syndiotactic polystyrene is used as test material due to its very interesting polymorphic behavior and because many data have been recently reported in the literature concerning its melting temperature. It was found that the thermodynamic melting temperature resulting by differential scanning calorimetry heating analysis provides a poor description when applied to crystallization kinetic data. The reference temperature to be adopted in crystallization kinetic equations resulted to be significantly lower. This seems to agree with recent findings and somewhat to contradict conventional understanding.
Abstract: The study of polymer crystallization enhanced by flow has attracted much interest because it implies the possibility of controlling the final morphology and the resulting mechanical and functional properties of semicrystalline polymers. An improved understanding of the fundamentals of flow-enhanced crystallization effects can help to tailor advanced processing strategies. Indeed, a complete understanding of the fundamentals of structure development during processing remains a challenge. In this work, the effect of a steady shear flow applied during crystallization on the morphology evolution and on the kinetics of isothermal crystallization of an iPP has been studied experimentally. In particular, measurements of nucleation and growth rates of spherulites during continuous and constant steady shear flow were performed by means of a Linkam shearing cell coupled with an optical microscope. During all the tests carried out in this work, the dominant crystalline structure was fully spherulitic. It was found that nucleation density in quiescent conditions remained constant with time (i.e., no nucleation rate was observed during the test). On the contrary, under shear flow, an increase of nucleation density with time was observed. This increase was found to be essentially linear with time, The linear dependence allowed to calculate a constant nucleation rate, which was found to be dependent on shear rate according to a power law expression whose exponent was found to be about 3. The evolution of crystallinity under shear conditions, calculated combining the results obtained on nucleation and growth rate, was also successfully compared with data obtained in the same conditions by means of in situ wide-angle X-ray diffraction. Finally, an attempt was made to determine scaling rules which can describe the effect of flow on crystallization kinetics.
Abstract: Infrared (IR) spectroscopy was adopted to study the hydrolytic degradation of films of aliphatic polyesters in alkaline environment. A measurable increase with the time of immersion of the absorbance of the peak centered at about 1570 cm- I was observed. Analysis of the IR spectra showed that the integrated peak area in that region can be used to quantify changes in the concentration of degradation products and thus to provide indications regarding the kinetic constant of the hydrolysis reaction. It was found that the hydrolysis of ester bonds proceeds linearly with time, and this result suggests that the controlling mechanism is the chemical reaction rather than water diffusion. The results also show that degradation rate increases with increasing polydispersity. (c) 2007 Elsevier Ltd. All rights reserved.
Abstract: 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. (c) 2007 Elsevier Ltd. All rights reserved.
Abstract: A study of the crystallization process of syndiotactic polystyrene is carried out, with particular emphasis on the effect of previous melt annealing on the subsequent crystallization kinetics and polymorphism, with a combination of thermal analysis, infrared spectroscopy, and X-ray diffraction. An effort is made to obtain quantitative results in terms of the relative crystallinity content of the alpha and beta phases after a given thermal treatment. A kinetics of melting for the crystalline memory is proposed, which enables the determination of the time at a given temperature that ensures the complete fusion of the a crystals. This leads to a generalization of the effects of the time-temperature couple on the memory effect. A particular protocol is identified, which upon solidification from the melt induces the development of just one crystalline phase, either alpha or beta. This allows the determination of the enthalpy of crystallization, the maximum attainable crystal volume fraction, the crystallization half-time as a function of temperature, and the Avrami index for each of the two crystalline phases. (c) 2006 Wiley Periodicals, Inc.
Abstract: A study of as-molded shrinkage of Poly Vinylidene Fluoride (PVDF) injection molded samples was carried out in this work. The polymer was injected into a simple rectangular cavity under different holding pressures and the dimensions of the resulting samples were accurately measured at room temperature 10 min after demolding. As expected, the relative difference between the sample dimensions and the corresponding cavity dimensions (namely the shrinkage) decreased on increasing holding pressure. Furthermore, the shrinkage increased on increasing distance from the injection point. A somewhat unexpected feature of experimental data was the fact that the shrinkage was much higher along the flow direction than along the transverse on-plane direction. A thorough characterization of the elastic properties of the solid samples also showed a marked anisotropy of the elastic modulus and of the coefficient of linear thermal expansion. The injection molding tests were simulated by a software developed at University of Salerno to obtain the evolution of temperature, pressure, and crystallinity inside the samples. The predicted pressure profiles were satisfactorily compared with the measured values. The simulated histories of temperature, pressure and crystallinity were used as input for a thermomechanical model for shrinkage evolution. It was found that the reason for anisotropy in shrinkage had to be ascribed mainly to the anisotropy in material properties. It was in fact shown that, adopting the correct anisotropic values for the elastic modulus and thermal expansion coefficient, a satisfactorily description of shrinkage data could be reached.
Abstract: In this work, FTIR spectroscopy was applied to the determination of crystalline content in an aliphatic polyester. To this goal, a range of wavelengths (from 560 cm(-1) to 680 cm(-1)), non conventional for this kind of polymers, was selected by analyzing spectra collected during isothermal crystallization tests. A deconvolution of the IR spectrum in that range showed the presence of three peaks sensitive to the amorphous content and one sensitive to the crystal content. An analysis of the time evolution of the absorbances allowed to determine the parameters needed to determine the absolute crystallinity degree. The time evolution of crystallinity during isothermal tests at different temperatures was successfully compared with results obtained by DSC. The procedure was also favorably compared with the result obtained by WAXD on a solid sample at room temperature.
Abstract: The crystallization of the isotactic poly(propylene) (I-PP) has been studied carrying out measurements by means of a special calorimeter connected to a microscope and a digital acquisition system of images. To authors’ knowledge, this is the first time that simultaneous calorimetric and optical measurements are carried out on polymers. The analysis of Polarized Optical Microscopy images has allowed the appraisal of nucleation density and growth rate in isothermal and non isothermal conditions. The results obtained in isothermal conditions have been analyzed through the Kolmogoroff model and the crystallinity calculated from the model has been compared with that obtained from the calorimetric measurements.
Abstract: This study aims at exploring the effect of a commercial organoclay montmorillonite (MMT) on the final properties of syndiotactic polystyrene (sPS) injection-molded samples. To this goal, injection-molded specimens made from neat sPS and commercial MMT modified with various organic compounds were prepared in different molding conditions. Dispersion of clay was attained via melt blending, directly in the injection chamber of the injection-molding machine. The obtained specimens were analyzed by IR spectroscopy, X-ray diffraction, thermogravimetry, and differential thermal analysis, with the aim of elucidating the effect of clay on the microstructures of the samples. Results clearly show that, depending on the organic modification, the presence of clay can induce strong effects on final crystallinity. This behavior can be attributed mainly to the role played by clay on the kinetics of the crystallization process. Eventually, it was found that the addition of a small percentage of clay (1%) in sPS can substantially widen the processing window of the material.
Abstract: The integrated knowledge of the injection molding process and the material changes induced by processing is essential to guarantee the quality of technical parts. In the case of parts with deep cavities, quite often the ejection phase of the molding cycle is critical. Thus, in the mold design stage, the aspects associated with the ejection system will require special consideration. In particular, the prediction of the ejection force will contribute to optimizing the mold design and to guarantee the integrity of the moldings. In this work, a simulation algorithm based on a thermomechanical model is described and their predictions are compared with experimental data obtained from a fully-instrumented mold (pressure, temperature, and force). Three common thermoplastics polymers were used for the tubular moldings: a semicrystalline polypropylene and two amorphous thermoplastics: polystyrene and polycarbonate. The thermomechanical model is based on the assumption of the polymer behavior changing from purely viscous to purely elastic below a transition point. This point corresponds to solidification determined by temperature in the case of amorphous materials and by critical crystallinity for semicrystalline polymers. The model results for the ejection force closely agree with the experimental data for the three materials used. (c) 2005 Society of Plastics Engineers.
Abstract: The effect of pressure on viscosity is an important but often overlooked aspect of the flow properties of polymeric materials. Generally, an exponential dependence ( the so-called Barus equation: eta=eta(0)exp(beta P)) can be adopted to describe this effect. In this work two polymers (an atactic and a syndiotactic Polystyrene) were characterized as far as the effect of pressure on viscosity is concerned by analyzing the non-linearities in the so-called Bagley plots. The results obtained show that for both materials the average value of beta is in the range 1-3 10(-8)Pa(-1). No relevant effects of temperature and shear rate were detected in the range analyzed. The data obtained were also described by means of a Cross-Vogel model, which reproduces the main features of experimental data.
Abstract: A thorough analysis of the effect of operative conditions of injection molding process oil the morphology distribution inside the obtained moldings is performed, with particular reference to semi-crystal line polymers. The paper is divided into two parts: in the first part, the state of the art on the subject is outlined and discussed; in the second part, an example of the characterization required for a satisfactorily understanding and description of the phenomena is presented, starting from material characterization, passing through the monitoring of the process cycle and arriving to a deep analysis of morphology distribution inside the moldings. In particular, fully characterized injection molding tests are presented using an isotactic polypropylene, previously carefully characterized as far as most of properties of interest. The effects of both injection flow rate and mold temperature are analyzed. The resulting moldings morphology (in terms of distribution of crystallinity degree, molecular orientation and crystals structure and dimensions) are analyzed by adopting different experimental techniques (optical, electronic and atomic force microscopy, IR and WAXS analysis).Final morphological characteristics of the samples are compared with the predictions of a simulation code developed at University of Salerno for the simulation of the injection molding process. (c) 2005 Elsevier Ltd. All rights reserved.
Abstract: The crystallization of polymers under pressure has recently attracted particular interest for being a powerful method of obtaining different crystal structures and morphologies. A specific molecular conformation may indeed induce remarkable changes in polymer solvent resistance and mechanical performance. In this work an experimental apparatus was designed and developed to investigate the effects of pressure and cooling rate on polymer samples. The apparatus, based on the confining fluid technique, is able to impose constant external pressures and different cooling rates during the solidification of polymer samples. In its current configuration, the device can reach 1250 bars and a maximum cooling rate of 40 degrees C/s (measured at 200 degrees C) with water at 5 degrees C as a cooling medium. Preliminary results obtained with Syndiotactic Polystyrene confirm that, in addition to thermal history, external pressure is indeed a significant factor for inducing changes in crystalline polymeric structures. The stable orthorhombic beta form is favored for specimens solidified under pressure, whereas the overall final crystallinity degree (alpha and beta forms) gradually decreases with both cooling rate and pressure. (c) 2005 American Institute of Physics.
Abstract: The current goal in the simulation of injection molding is the description of material morphology. The path to reach this goal passes through the prediction of molecular orientation and strain, namely the molecular conformation. To obtain this information, the viscoelastic nature of the polymer must be taken into account. The aim of this paper is to check if a simple, recently proposed, non-linear dumbbell model is sufficiently accurate to quantitatively describe birefringence distribution in injection molded PS samples. To this goal, a series of theological measurements were performed in a parallel plate rheometer, measuring in the meantime the birefringence. By choosing an appropriate stress-optical coefficient, the model could describe the whole set of data. The results obtained allowed to reinterpret some results of molecular orientation in injection molding and to reach a quantitative description of data of birefringence distribution in molded PS samples. (c) 2005 Elsevier Ltd. All rights reserved.
Abstract: A model for crystallization kinetics that accounts for the formation of different crystalline phases and is able to describe the morphological characteristics of samples solidified under quiescent conditions, has been enriched to account for the effect of solidification pressure. The effect of pressure was considered by assuming a linear increase of melting and glass transition temperatures (which are involved in the description of the growth rate and nucleation density of the alpha phase). Moreover pressure was incorporated in the kinetic constant adopted to describe the evolution of the mesomorphic phase. The parameters of the model were identified on the basis of literature data on the distribution of crystalline phases in samples solidified under different pressures. The modified model also satisfactorily described PVT curves up to 100 MPa, and is now able to describe the evolution of morphology during solidification at cooling rates as fast as several hundreds of Kelvin degrees per second and under pressures of as high as 100 MPa.
Abstract: Gate solidification time is an important topic in injection molding technology, as it determines cycle time, which itself is an important issue in the economics of the production process. In this work, a study of the effect of both gate and cavity geometries on gate solidification time was conducted, using a commercial polymer, injection molded with constant holding pressure into a rectangular cavity. Three cavity lengths were used, and for each, two cavity thicknesses were adopted. Special dies containing different gates were assembled in the mold. Gate thickness was found to be the most important factor deter-mining gate sealing time. However, the cavity geometry is also quite important. A clear indication on gate solidification could be drawn by analyzing time evolution of pressure distribution inside the mold. The solidification phenomenon leading to gate sealing was analyzed by a simple model. which also takes into account the effect of cavity geometry, by comparing the heat flow through the gate walls and the energy required to solidify the packing flow rate. Model results satisfactorily describe the main features of the experimental data.
Abstract: The so-called "fluoropolymers" gained in recent years a considerable industrial success, and the increasing industrial interest to this class of materials causes a need of a better characterization of the properties of interest for processability. In this work, the crystallisation kinetics of a Fluorinated Copolymer of Tetrafluoroethylene (MFA, produced by Solvay), was studied by both standard calorimetric tests and fast cooling tests performed by an apparatus which allows on line determination of crystallisation phenomena. Material crystallisation kinetics resulted to be so fast that the polymer reached the maximum degree of crystallisation for all solidification conditions, also those obtained at cooling rates of the order of hundreds of degrees per second. Calorimetric tests also gave indications about the dependence of maximum crystallinity degree upon temperature. The crystallisation kinetics was described by the non-isothermal formulation due to Nakamura of the well-known Avrami equation. Results were compared with experimental data. (C) 2004 Elsevier Ltd. All rights reserved.
Abstract: Modeling and simulation of the injection molding process of thermoplastic polymers has remarkably improved over the last decade. One of the most challenging scientific objectives is currently the reliable prediction of molecular orientation simulations of the molding process. Indeed, although pressure and velocity distribution can be satisfactorily described by viscous models, the viscoelastic nature of the polymer needs to be accounted for in the description of molecular orientation evolution. In this work, an amorphous PS was injection molded into a line gate rectangular cavity. Molding tests are carefully characterized and all information needed for further analysis is provided. The molds contained special dies that could accept various sized gates. In particular two gates were used whose thicknesses were 1.5 mm and 0.5 mm, respectively. Birefringence distribution (which for PS is essentially the orientation distribution) along the thickness direction was measured by using the wedge method at different positions in the moldings, and inside the gates. Data regarding the amount of frozen-in molecular strain were gathered by measuring the thermal shrinkage at different positions along the flowpath. Molding tests were simulated by means of a software developed at the University of Salerno, and a simple viscoelastic model was used to describe the evolution of molecular orientation due to the effect of kinematics obtained using a viscous approach. Simulation results describe the main features of experimental data collected from the molded samples; in particular, the effect of the packing flow is clear in both the data and simulations. In addition, the importance of the effect of pressure on relaxation time is discussed.
Abstract: The exact knowledge of postprocessing polymer-specific volume is often a factor of enormous strategic importance from an industrial point of view. The subject is complicated by the fact that the specific volume of solid polymers at a constant temperature and pressure is not only a function of the current temperature and pressure, but is also a consequence of the whole formation history from the melt. In this work, specific volumes of samples solidified in different conditions are analyzed and related to their formation history. A wide range of cooling rates (from 5 X 10(-3) to 300 K/s) and solidification pressures (from 0.1 to 80 MPa) are examined. The results show a synergic effect of the cooling rate and solidification pressure: Lower cooling rates result in a much higher pressure-induced densification with respect to higher cooling rates. A simple phenomenological model which essentially links the densification effect to the dependence of the glass transition temperature upon the cooling rate and solidification pressure is adopted to describe the experimental data. Starting from the densification effect, the effect of the pressure and cooling rate on the glass transition temperature is evaluated. Furthermore, some conclusions about the dependence of the volume relaxation time on the temperature and pressure in the glass transition range are achieved. (C) 2003 Wiley Periodicals, Inc.
Abstract: A wide set of isothermal and non-isothermal crystallization experiments were carried out in this work on an iPP resin. Several experimental techniques were adopted in order to characterize crystallization kinetics and final morphology of the material, also under cooling rates comparable to those encountered during material processing (up to several hundred K/s). The whole set of data was taken as a reference to identify a kinetic model which describes the evolution of the structural organization of iPP (a crystalline phase and mesomorphic phase) as a parallel of two non-interacting kinetic processes competing for the available amorphous volume. Kolmogoroff equation was adopted to describe the crystallization of the a form. Avrami-Evans-Nakamura isokinetic approach was adopted to describe the evolution of the mesomorphic phase. Resulting kinetic model satisfactorily describes the whole set of experimental data including those obtained on samples solidified under high cooling rates, and reveals that a correct description of the evolution of the a phase during solidification can be attained only if the evolution of the competing mesomorphic phase is kept into account. The effect of cooling rate during solidification from the melt on diameters of spherulites, observed on solidified samples, is also satisfactorily described by model predictions. (C) 2002 Elsevier Science Ltd. All rights reserved.
Abstract: The so-called fluoropolymers have gained, in recent years, considerable industrial success, and the increasing industrial interest in this class of materials has caused a need for better characterization of the properties of interest for processability, for instance, for injection molding or extrusion. In this work, the pressure-volume-temperatre (PVT) relationship of a poly(vinylidene fluoride) is described by combining specific volumes of amorphous and crystalline phases present in the material. The volumes of the two phases are described simply by thermal expansion and compressibility coefficients drawn from standard PVT data below and above the crystallization range. Within the crystallization range, the material volume is assumed to change from amorphous to crystalline according to the evolution of an overall crystallinity degree, which is described by the Nakamura nonisothermal formulation of an Avrami crystallization kinetic model. Model parameters are identified by comparison with standard calorimetric results, PVT data, and final densities of thin samples solidified during quenches conducted with cooling rates of several hundreds of Kelvins/second. The resulting model allows the description of the PVT behavior of PVDF in the pressure -and cooling-rate ranges of interest for processing. (C) 2003 Wiley Periodicals, Inc.
Abstract: Control of volume changes with time has a critical industrial relevance for the production of objects made of thermoplastic materials (obtained, e.g., by injection molding), but this phenomenon is completely disregarded by commercial codes for simulation of processes. In this work, attention is focused on the relevance of thermo-mechanical history on volume relaxation at room conditions of an amorphous polystyrene. A set of data of volume relaxation of samples obtained in an extremely wide range of thermomechanical treatments was collected. Data were analyzed with the aim of applying a simplified model on the basis of the well-known KAHR model, which describes the postprocessing volume relaxation of amorphous polymers by adopting a minimum number of material parameters. Despite the fact that only two relaxation times are considered, the model satisfactorily describes volume evolution (either contraction or expansion) at room conditions after a given thermomechanical treatment if an appropriate partition of free volume into two fractions is provided. Furthermore, in its present form that neglects the effect of pressure on volume relaxation, the model satisfactorily describes the effect of a given thermal treatment (at room pressure), starting from the melt, on both specific volume and its relaxation rate after treatment. (C) 2003 Wiley Periodicals, Inc.
Abstract: The prediction of ejection forces in tubular moldings (pipe fittings, cups, ..) is relevant for the optimization of the ejection systems in molds. An instrumented mold (pressure, temperature and force) for a tubular part was developed for measuring the actual ejection forces for two amorphous materials, polystyrene and polycarbonate. The ejection forces depend mostly on the holding pressure and the mould core temperature. The experimental data is used to validate a thermo-mechanical model that predicts the shrinkage and internal stresses.
Abstract: Guaranteeing the quality of technical parts made by injection molding implies a precise characterization of the processing phase. In the case of parts with deep cores, the ejection step of the molding cycle is often critical. The prediction of the ejection force may contribute to optimizing the mold design and guaranteeing the integrity of the moldings. Data obtained from a fully instrumented mold (pressure, temperature and force) producing a tubular molding are compared with predictions from a simulation algorithm based on a thermo-mechanical model. Semi-crystalline (iPP) and amorphous (PS) materials were used to expand the amplitude of the research.
Abstract: Modelling of the injection moulding process is carried out in this work on the basis of Williams and Lord model and its recent extensions to postfilling stages. The emphasis is devoted to identifying the role of crystallisation kinetics in the process simulation. Data of pressure histories during injection moulding of an iPP are taken as reference to the analysis. Crystallisation kinetics of the material was described by means of a nonisothermal formulation of Avrami model whose parameters where determined either by accounting for only of calorimetric results or by describing also final density data of thin samples subjected to characterised quenching histories. Predictions of pressure histories are analysed in relation to the crystallisation kinetics adopted. The effect of pressure on crystallisation is also discussed.
Abstract: In this work an amorphous PS was injection molded into a simple line-gated rectangular cavity. The effect on post-filling pressure curves and temperatures (at the polymer-mold interface) of different holding pressures and times, and of two gate thickness configurations, was analyzed. Molding tests were simulated leading to a good description of pressure evolution in different positions along the flow path for all molding conditions analyzed. Comparison between simulation results and experimental data shows that: considering a rigid mold can lead to predicted values of post-filling pressure profiles much different from experimental ones; heat transfer coefficient description plays a minor role in the prediction of pressure evolution, but its effect is relevant on temperature evolution in layers close to the mold surface.
Abstract: The structural organization of blends of isotactic polypropylene (iPP) and linear low-density polyethylene (LLDPE), with different compositions, was studied and correlated with the thermal history followed by the samples during solidification from the molten state. The materials were cooled at two extreme controlled rates: 0.1 and 500 degreesC/s. The resulting structure was investigated both in the crystalline and the amorphous phases. In particular, attention was focused toward the analysis of the diffusion parameters of dichloromethane vapors, and the morphological organization of the amorphous phases was interpreted using models that consider them (in terms of resistance to diffusion) combined in series and in parallel. (C) 2001 John Wiley & Sons, Inc.
Abstract: The phenomenon of shrinkage in injection moulding is particularly, evident in semi-crystalline polymers. During cooling these materials experience a transition from a completely amorphous to a partially crystalline status, which results in a severe change of all material characteristics, including volumetric parameters. In this study an iPP was injection moulded in a rectangular impression (120 mm x 30 mm x 2 mm) with a 1.5 mm thick line gate. The influence of holding pressure and time, and geometry constraints on linear shrinkage was explored. In-mould shrinkage was measured by means of a recent method based on strain gauges. Experimental results are compared with predictions for shrinkage obtained by C-Mold and a code developed ar the University of Salerno, which takes into account crystallisation kinetics. The solidification criterion resulted to be extremely relevant for shrinkage predictions.
Abstract: Modern approaches to the phenomenon of dimensional accuracy in injection moulding link the evolution of shrinkage from the moment of first solidification to a force balance between restraining and constraining forces which sets in inside the mould before ejection. Such an approach needs a complete understanding of what happens inside the cavity during the moulding cycle. In a recent paper [9], a new technique has been presented, by which it is possible to follow the shrinkage development of an injection moulded sample from the moment it starts solidifying to soon after ejection.In this work, with reference to a similar rectangular cavity, shrinkage curves measured by strain gages in different moulding conditions are analysed by means of a simple thermomechanical model recently proposed. Results show that the model satisfactorily predicts the moment shrinkage starts inside the mould and reveal the existence of a restraining force not due to pressure, which builds in gradually in a few second from the instant of first solidification. Knowing the value of this force - which can be drawn by Hooke’s low from the strain jump at mould opening - the model gives a good description of both shrinkage development inside the mould and ejection shrinkage.
Abstract: An experimental study of shrinkage in injection molded products is presented in this paper and documented with all information needed for any further analysis. In particular, the effect of holding pressure, holding time and mold geometry on product shrinkage was investigated for amorphous polystyrene. It turned out with increasing holding pressure the in-plane shrinkages varied from 0.6% to 0.1%, while the product thickness increased from about 1% to 10%. The holding time only affected product shrinkage for settings less than gate freeze-off time. Shrinkage usually increases along the flow path unless back flow sets in at the end of the holding stage. It was also found that if a constraint prevents in-mold shrinkage to take place, final shrinkage may decrease if holding pressure and time are small. The effects of all the variables analyzed were described by a thermomechanical model recently proposed.
Abstract: The problem of shrinkage in injection moulding has been studied by means of a new technique based on strain gages placed on mould surface before injection. The local shrinkage development from the moment it starts inside the mould to soon after ejection can be followed by means of this method.With reference to a simple rectangular cavity, the effects of different holding pressures, of geometrical constraints placed inside the mould, of position in the cavity and of total cavity length on local shrinkage of injected PS samples are analysed. Shrinkage inside the mould is, indeed, registered: it starts later if higher holding pressures are applied; furthermore, any factor which produces and enhancement of shrinkage evolution before complete solidification gives rise to a corresponding increase of final shrinkage.
