Warpage–crystallinity relations in rotational molding of polypropylene

The authors present a study of the crystalline properties of selected series of polypropylene (PP) materials in relation to their warpage at conditions relevant for rotational molding. The PP materials have different crystallization temperatures and kinetics. The authors study the crystalline features of the materials using hot‐press experiments and differential scanning calorimetry (DSC). In the DSC, the authors do constant cooling rate runs as well as isothermal crystallization and subsequent heating. A multimode crystallization kinetics model was fitted to the DSC cooling runs. Regression analysis was done to relate the results from the crystallization experiments to warpage. The derived empirical model shows that the crystallization temperature, crystallization half‐time, and heat of fusion are the most significant parameters influencing warpage. Materials with warpage deviating from the model average are discussed taking aspects of the multimode kinetics into account. The present work could represent a basic platform for understanding and predicting the warpage during rotational molding process. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Dimensional stability of single‐site ethylene copolymers in rotational molding

The dimensional stability of ethylene copolymers in rotational molding was studied by comparing the warpage observed for a series of conventional and single‐site catalyzed ethylene copolymers. Bench‐scale molding trials were carried out under controlled molding conditions. The rapid cooling of the mold using a water spray resulted in greater warpage. Under such conditions, molded parts made using the single‐site resins showed less warpage compared to the Ziegler‐catalyzed copolymers with otherwise comparable densities. The Ziegler‐catalyzed copolymers were characterized by a faster crystallization rate, and were shown to generate larger crystallinity gradients through the part thickness during the cooling process. Second to temperature gradients, crystallinity gradients are a leading cause for the development of residual stresses and causing warpage. Differences in the crystallization rates between single‐site and Ziegler‐catalyzed copolymers are discussed based on their intermolecular comonomer distributions. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Thermal residual stress development for semi‐crystalline polymers in rotational molding

In rotational molding, polymer powders are subjected to heating, melting, cooling, and subsequent crystallization processes. Because of the asymmetrical cooling condition, thermally induced residual stresses are created inside molded parts leading to part warpage. A detailed theoretical heat transfer model is presented for the entire rotational molding process including the consideration of endothermic and exothermic transitions. At the same time, the development of residual stress inside the molded parts is simulated with thermoelastic model. The warpage values are calculated under different processing cases, and the generated numerical results are in good agreement with data reported in the literature. Our results show that both crystallinity and temperature gradients developing within the polymer during the cooling process greatly affect the dimensional stability of ethylene copolymers typically processed in rotational molding. The latter is found to be the determining factor in evaluating the effect of cooling conditions on the warpage generated in a molded product. Our results also demonstrate the importance of the crystallization kinetics, the material stiffness, and its evolution during the solidification process on the dimensional stability of the molded products. POLYM. ENG. SCI., 2008. © 2007 Society of Plastics Engineers     

Computer simulations of crystallinity gradients developed in injection molding of slowly crystallizing polymers

A model is developed to simulate the crystallinity gradients developed in injection molding of slowly crystallizing polymers. In this model, effects of nonisothermal and stress-induced crystallization kinetics are taken into account through phenomenological relationships. Computer simulations included calculations of the temperature, velocity, and pressure distributions as well as two dimensional crystallinity distributions in the final products. In addition, effects of various processing conditions: mold temperature, injection flow rate, and holding time are also included in the calculations. The crystallinity gradients obtained through computer simulations agree with the experimental results obtained with poly (p-phenylene sulfide) under a variety of processing conditions.     

Effect of flow rate and temperature on crystallization kinetics,crystallinity index,and elastic modulus of PEEK

Dynamic, in situ wide angle X-ray scattering (WAXS) studies of the melt crystallization of injection-molded poly(ether ether ketone) (PEEK) have been carried out using an X-ray diffractometer and a position-sensitive detector. A test cell has been fabricated to fit inside the diffractometer and yet work as a complete injection molding apparatus. The rate of crystallization has been shown to increase with decreasing crystallization temperature and/or increasing flow rate in the mold. The crystallization rate decreases dramatically with increase in melt soak time at 400°C. The crystallinity index, which affects the stiffness, toughness, and fracture behavior of PEEK, has been measured under various processing conditions, by wide angle X-ray scattering, so as to optimize the process parameters: molding time, mold temperature, melt temperature, soak time at melt temperature, and flow rate. It has been shown that the crystallinity and hence the elastic modulus increase with increase in crystallization temperature and/or flow rate. Chain orientation has been shown to be absent in the bulk of the injection-molded specimens under normal molding conditions.     

Comparative study of warpage,global shrinkage,residual stresses,and mechanical behavior of rotationally molded parts produced from different polymers

A comparative study of warpage, global shrinkage, and residual quench stresses developed in rotational molding is made for a series of thermoplastics including various polythylenes, polypropylene, polyamide-6, polycarbonate, and polystyrene. The influence of rate of quench on uniaxial stress strain and impact behavior of rotomolded parts was also studied. Generally, warpage, global shrinkage, and residual stresses increase with increasing quench rate for all the polymers. Further, the levels of warpage and global shrinkage increases with extent of crystallization, i.e., products from glassy polymers exhibit little warpage and those from highly crystalline polymers are highly warped. Increasing rate of quench tends to increase elongation to break and impact strength.     

Optical monitoring of polyesters injection molding

An optical fiber sensor similar to the one developed by Thomas and Bur 1 was constructed for the monitoring of the crystallization of three polyesters during the injection molding process. The polyesters studied were: polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), and polyethylene terephthalate (PET). With this optical system it was possible to obtain, in real time, some essential parameters of the polyester crystallization kinetics at different processing conditions. Thus, a study of the influence of injection molding variables on the nonisothermal crystallization kinetics of these polyesters was done. The processing variables were: mold wall and injection temperatures, T_w and T_i, respectively; flow rate, Q; and holding pressure, P_h. The experiments were done following a first order central composite design statistical analysis. The morphology of the samples was analyzed by polarized light optical microscopy, PLOM. The signal of the laser beam during the filling and the crystallization stages of the injection molding of these materials was found to be reproducible. The measurements showed that this system was sensitive to variations of the crystallization of different types of polymers under different processing conditions. The system was not able, however, to monitor the crystallization process when the crystallinity degree developed by the sample was very low, as in the PET resin. It was also observed that T_w and T_i were the most influential variables on the crystallization kinetics of PBT and PTT. Due to its slower crystallization kinetics, PTT was found to be more sensitive to changes in these parameters than the PBT. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 563–579, 2006     

Cure shrinkage analysis of epoxy molding compound

EGA (ball grid array), one of the structures Used for semiconductor packages, involves a laminated structure. BGA inevitably involves significant warpage, owing to differences in shrinkage among constituent materials. The extent of warpage is governed by total shrinkage (= cure shrinkage + thermal shrinkage) of the epoxy molding compound that encapsulates the IC chip. In particular, the cure shrinkage exerts great influence on warpage. Cure shrinkage has been understood as the decrease in free volume at the time of curing. However, the cure shrinkage rate cannot be sufficiently explained by the free volume of the cured epoxy resin. We have developed an evaluation method based on the epoxy group reaction ratio, and have eventually confirmed that cure shrinkage depends on the reaction ratio of the epoxy group after curing, and on epoxy group density.     

Computer simulation of stress‐induced crystallization in injection molded thermoplastics

Injection molding of semicrystalline plastics was simulated with the proposed stress‐induced crystallization model. A pseudo‐concentration method was used to track the melt front advancement. Stress relaxation was considered using the WFL model. Simulations were carried out under different processing conditions to investigate the effect of processing parameters on the crystallinity of the final part. The simulation results reproduced most of the experimental results in the literature. Comparison is made between the slow‐crystallizing polymer (PET) and fast‐crystallizing polymer (PP) to demonstrate the effect of stress on the crystallization kinetics during the injection molding process for materials with different crystallization properties. The results show that for fast‐crystallizing plastics, stress has little effect on the final crystallinity in the injection molded parts.     

Bubble removal in rotational molding

Closed form solutions have been obtained for bubble dissolution in typical polymer melts encountered in rotational molding. The solutions are in excellent agreement with experimental data available in the literature. Using these solutions, it is shown that under typical rotational molding conditions the polymer melts may be almost saturated. As a result, bubble shrinkage occurs over long periods. Depending on the degree of saturation, surface tension may contribute substantially to the concentration gradient that drives bubble shrinkage. It is also shown that a pressure increase imposed on a nearly saturated polymer melt leads to a steep concentration gradient at the bubble/melt interface that can cause extremely fast bubble shrinkage. Applied to the rotational molding process, such a pressure increase can result in substantial cycle‐time shortening through elimination (or reduction) of the currently used excessive heating. A further benefit may be that additional resins, which at present cannot be used because of oxidation at sustained high‐temperatures, can become available to the rotational molding industry. Under the under‐saturated conditions created by a pressure increase, the effect of surface tension on the rate of bubble shrinkage is negligible.     

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.     

Flow‐induced crystallization in the injection molding of polymers: A thermodynamic approach

The prediction of the crystallinity and microstructure that develop in injection molding is very important for satisfying the required specifications of molded products. A novel approach to the numerical simulation of the skin‐layer thickness and crystallinity in moldings of semicrystalline polymers is proposed. The approach is based on the calculation of the entropy reduction in the oriented melt and the elevated equilibrium melting temperature by means of a nonlinear viscoelastic constitutive equation. The elevation of the equilibrium melting temperature that results from the entropy reduction between the oriented and unoriented melts is used to determine the occurrence of flow‐induced crystallization. The crystallization rate enhanced by the flow effect is obtained by the inclusion of the elevated equilibrium melting temperature in the modified Hoffman–Lauritzen equation. Injection‐molding experiments at various processing conditions were carried out on polypropylenes of various molecular weights. The thickness of the highly oriented skin layer and the crystallinity in the moldings were measured. The measured data for the microstructures in the moldings agree well with the simulated results. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 502–523, 2005     

Effects of molding pressure on the warpage and the viscoelasticities of HVQFN packages

Molding temperature, molding pressure, and time are three major parameters affecting the curing quality of molding materials. In this article, the effect of molding pressure on warpage of HVQFN packages is studied. The typical map‐chips with 65% silica particle‐filled epoxy resin are manufactured under five molding pressure levels from 1.8 to 12.3 MPa. The curvature measurements are performed for the packages after demolding and postcuring, respectively. The viscoelastic tensile relaxation modulus of molding materials is obtained by using dynamic mechanical analysis. The characterization of evolution of equilibrium moduli is analyzed for future modification of the model. The experimental results show that the molding pressure has a significant effect on the warpage after molding as well as after postcuring of HVQFN packages. The curvatures of HVQFN packages at both lower (1.8 MPa) and higher molding pressures (12.27 MPa) are about 45% less than the average max curvature at 3.6–8.66 MPa. The molding pressure has obvious influences on the glassy Young's modulus but has little effects on the rubbery modulus and relaxation time of the package materials. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Modeling the morphology development of ethylene copolymers in rotational molding

A model is proposed to describe the solidification and crystallization phenomena in rotational molding. To capture the morphology development with crystallization behavior, a two‐dimensional theoretical simulation was carried out, consisting of a phase‐field model emphasizing the metastability of polymer crystallization and a heat‐transfer model describing the molding cycle. The model parameters were experimentally evaluated with differential scanning calorimetry and isothermal crystallization tests. Molding trials were also conducted with bench‐scale rotational molding equipment, and the cross sections of the molded products were examined under polarized light optical microscopy. The model predictions capture the formation of transcrystalline structures near the mold surface, which is more apparent under moderate cooling conditions. Our results show that the model predictions are in general agreement with the experimental results obtained in our laboratory as well as those presented in the literature. Because morphological features are important contributing factors to product performance, the model will be useful for the formulation of new materials and process optimization. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5903–5917, 2006     

Metallocene polypropylene crystallization kinetic during cooling in rotational molding thermal condition

This article is part of an ambitious project. The aim is to simulate mechanical properties of rotomolded part from microstructure consideration. Main objective here is to consider metallocene polypropylene crystallization kinetic (PP) during cooling stage in rotational molding. Crystallization kinetic of metallocene PP is so rapid that microscopy cannot help to observe nucleation and growth. Crystallization rate can be estimated by a global kinetic. Given that cooling in rotational molding is dynamic with a constant rate, Ozawa law appears more appropriate. Ozawa parameters have been estimated by differential scanning calorimetry. In rotational molding thermal condition, Avrami index identifies a complex nucleation intermediate between spontaneous and sporadic. Ozawa rate constant is 68 times higher than this obtained for Ziegler–Natta PP. By coupling transformation rate from Ozawa model and a thermal model developed earlier, the difference between theory and experimental is less than 1%. To optimize rotational molding, study has been completed by sensitivity to adjustable parameters. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Analysis of the effects of internal heating and cooling during the rotational molding of plastics

In rotational molding, shortening the cycle time is one of the most important requirements for increasing production rates and reducing product cost. A characteristic feature of the process is that the mold and plastic powder are heated from room temperature to the melting temperature of the plastic and then back to room temperature. In addition, in the vast majority of cases the heat input and subsequent heat extraction occur at the outside surface. In order to improve the heat transfer, this paper considers the effects of internal heating and cooling. A mathematical model has been developed in which an internal heating term can be incorporated. The experiments with rotomolding powders show that the predictions made by the model are accurate. In particular, it is found that the introduction of internal heating is very effective in shortening the cycle time and that the introduction of internal cooling in rotational molding provides a more uniform structure and less likelihood of warpage.     

Modeling and Optimization of Injection Molding Process Parameters for Thin-Shell Plastic Parts

This paper deals with the application response surface methodology (RSM) integrating with statistical technique to discuss variation of warpage and tensile stress properties depended on injection molding parameters during production of thin-shell plastic components. By applying RSM analysis, a mathematical predictive model of the warpage and tensile stress properties was developed in terms of the injection molding parameters. The trim operation has been optimized for a given injection molding condition by desirability function approach and the response surface contours were constructed for determining the optimum conditions. Additionally, the analysis of variance is also applied to identify the most significant factor.     

Improving mechanical performance of injection molded PLA by controlling crystallinity

Currently, use of poly(lactic acid) (PLA) for injection molded articles is limited for commercial applications because PLA has a slow crystallization rate when compared with many other thermoplastics as well as standard injection molding cycle times. The overall crystallization rate and final crystallinity of PLA were controlled by the addition of physical nucleating agents as well as optimization of injection molding processing conditions. Talc and ethylene bis‐stearamide (EBS) nucleating agents both showed dramatic increases in crystallization rate and final crystalline content as indicated by isothermal and nonisothermal crystallization measurements. Isothermal crystallization half‐times were found to decrease nearly 65‐fold by the addition of only 2% talc. Process changes also had a significant effect on the final crystallinity of molded neat PLA, which was shown to increase from 5 to 42%. The combination of nucleating agents and process optimization not only resulted in an increase in final injection molded crystallinity level, but also allowed for a decreased processing time. An increase of over 30°C in the heat distortion temperature and improved strength and modulus by upwards of 25% were achieved through these material and process changes. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Relationship between the crystallization behavior and the warpage of film‐insert‐molded parts

The dimensional variation of an injection‐molded, semicrystalline polymer part is larger than the variation of an amorphous polymer part because the shrinkage of a crystalline polymer is generally greater than the shrinkage of an amorphous one. We investigated the warpage of film‐insert‐molded (FIM) specimens to determine the effect of the crystallization behavior on the deformation of FIM parts. More perfect crystalline structures and higher crystallinity developed in the core region of the FIM specimens versus other regions. Relatively imperfect crystalline structures and low crystallinity developed in the adjacent regions of the inserted films, whereas a thin, amorphous skin layer developed in the adjacent regions of the metallic mold wall. The crystallizable substrate in the FIM specimens caused very large warpage because nonuniform shrinkage occurred in the thickness direction of the specimens. Therefore, the warpage of an experimentally prepared FIM poly(butylene terephthalate) specimen was greater than that predicted numerically because of its complex crystallization behavior. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Study of polyamide 12 crystallization behavior within rotational molding process

Our study is focused on the investigation of polyamide 12 (PA 12) grade behavior in rotational molding process. Hence, some rotational molding tests of polyamide 12 were conducted on a STP LAB 40 machine. To simulate the cooling stage within the rotational molding, the crystallization behavior of polyamide 12 was studied using differential scanning calorimetry technique and the obtained results for non-isothermal crystallization were fitted with Ozawa model. Furthermore, morphology survey has been carried out by a hot stage method using a microscope to investigate the spherulites evolution which depends on the temperature. The micro-tensile properties have been studied using micro-tensile bench (MVTV2) to explain the mechanical behavior of polyamide 12 during crystallization. As a result, the rotational molding of PA 12 was successfully carried out. The simulation of the melting and crystallization stages, by application of Ozawa model coupled with enthalpy method gave a good representation of experimental data on one hand. On the other hand, all characterization revealed useful information to understand the different phenomena that govern the rotational molding process.