Effect of molding conditions on crystallization kinetics and mechanical properties of poly(lactic acid)

Although Poly(lactic acid) (PLA) possesses many desirable properties, above all biodegradability, its heat deflection temperature is too low for many desirable applications. Similarly, to any other polymers, also for PLA the physical and mechanical properties in the solid state depend on the morphology and crystallinity degree, which in their turn are determined by the thermomechanical history experienced during solidification. A large crystallinity degree is highly desirable to increase the heat resistance of PLA but is rather difficult to reach during injection molding due to the very slow crystallization kinetics of this material. In this work, the crystallization kinetics of an injection molded PLA grade was assessed in function of the thermal history by using calorimetric analysis. The cold crystallization kinetics (starting from the amorphous glassy sample) turned out to be faster than melt crystallization kinetics. Following the indications gained from crystallization kinetics, some samples were injection molded imposing different thermal histories. The effect of molding conditions on crystallinity was determined. This finding was adopted to develop a post‐molding stage which allows obtaining crystalline samples in times much shorter (of a factor about two) with respect to samples injection molded in a hot mold kept at temperatures close to the maximum crystallization rate. POLYM. ENG. SCI., 57:306–311, 2017. © 2016 Society of Plastics Engineers     

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     

Optical monitoring of polypropylene injection molding

We have constructed an optical fiber sensor for monitoring injection molding and we have developed a model to describe sensor behavior. The sensor consists of a sapphire window at the end of a sleeved ejector pin into which an optical fiber is inserted. The optical view with this sensor is through the thickness of the molded product. The measured optical signal was light that transmitted through the resin, reflected off the back wall of the mold, and retraced its path through the resin to the optical sensor, i.e., light transmitted through twice the thickness of the resin. While monitoring polypropylene during the packing and cooling phase of the molding cycle, we observed a decrease in light intensity due to scattering of light by the growing microcrystals. A characteristic minimum in the transmitted light intensity versus time curve is attributed to scattering by growing crystalling spherulites at the core of the molded product. Cavity pressure was also measured and was found to be an essential parameter in the process model. The model illustrates how temperature, pressure, and crystallinity affect the detected light intensity and clarifies the roles that temperature and pressure play in the crystallization process.     

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.     

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.     

Fluorescence monitoring of polymer injection molding: Model development

We have developed models to describe the behavior of an optical fiber sensor which was used to detect fluorescence from a polymer resin during the cooling phase of injection molding. The optical fiber sensor was positioned at the wall of the mold cavity by using the ejector pin channel as access to the cavity. The sources of fluorescence were dyes, which were chosen because of their sensitivity to temperature and which were mixed with the resin at dopant concentrations (parts per million by weight). The behaviors of a molecular rotor dye, dimethylamino diphenyl hexatriene, doped into polyethylene, and an excimer producing dye, bis-(pyrene) propane, doped into polystyrene were the subjects of the modeling calculations. The models consist of two modules: (a) a solution to the thermal diffusion equation for the resin cooling in the mold and (b) using temperature/time profiles and, in the case of polyethylene, crystallinity/time profiles obtained from the thermal diffusion equation, fluorescence intensity as a function of time was computed. Factors incorporated in the models are: adiabatic heating and cooling, light scattering due to microcrystals of polyethylene, crystallization kinetics, temperature and pressure shift factors for viscoelastic relaxation near the glass transition temperature of polystyrene, and the thermal resistance at the resin/mold interface.     

Effect of Clay Dispersion on the Nonisothermal and Isothermal Crystallization Behaviors of Polyethylene Composites

The presence of nanosized clay fillers can greatly influence the crystallization behaviors and crystalline morphologies of polymer composites. In this study, the modified montmorillonite polyethylene nanocomposites have been prepared by melt mixing, followed by injection molding. The morphology and microstructure of the composites were characterized by scanning electron microscopy and Fourier transform infrared spectrometry. The nanosized clay particles act as a nucleating agent in the solidification process of the matrix. The relative crystallinity ratio decreases with the decrement of polymer loading. Nonisothermal and isothermal crystallizations were conducted to characterize the crystallization kinetics of nanocomposites. The effects of clay dispersion on the crystallization of polymer were quantified by Avrami and Hoffman models. The addition of 1?wt% clay into polymer matrix has significantly increased the activation energy. The crystallization rate constants (G) have been determined which are higher for the composites than that for the polymer matrix.     

注射成型中聚合物剪切诱导结晶行为的三维模拟

在考虑剪切导致分子链取向并升高其平衡熔点的基础上,建立了基于Nakamura方程的剪切诱导结晶动力学模型。在WLF-Cross黏度模型中引入结晶对黏度系数的影响,构建了考虑结晶的注射成型过程模型。采用改进的有限体积法对聚合物剪切诱导结晶行为进行了三维数值模拟,模拟中耦合了流动场、熔体压力、温度、诱导时间与结晶度。结果表明,本方法可清晰模拟出注射成型过程中聚合物的三维“喷泉”流动行为以及3层“皮-芯”结晶结构,同时,诱导结晶时间指数与相对结晶度的模拟结果与理论及实验结果吻合。     

Correlation between morphological properties and thermal conductivity of injection‐molded polyamide 46

The morphology and thermal conductivity of injection‐molded polyamide 46 (PA46) samples were investigated in this study. It was found that injection molding parameters had no influence on the thermal conductivity. This was attributed to the high crystallization speed and therefore imperfect crystal structure of PA46. By annealing of some samples at 260°C for 24 h the thermal conductivity was increased by 30%. Polarization light microscopy revealed only minor changes of the visible morphological structure for the as molded and annealed samples. For the investigation of the sample crystallinity via Raman spectroscopy an analysis method was established and the term “Raman crystallinity” is introduced as the intensity ratio of characteristic Raman bands. Via Raman crystallinity it was possible to distinguish between different mold temperatures and the annealed PA46 samples showed a significantly increased Raman crystallinity. Our results show that the thermal conductivity of PA46 primarily depends on the crystal structure on a length scale of crystallites. The size of the visible spherulite‐like structures did not correlate with the change in thermal conductivity. A correlation of the Raman crystallinity with the thermal conductivity of PA46 was shown. POLYM. ENG. SCI., 55:2231–2236, 2015. © 2015 Society of Plastics Engineers     

In-line optical monitoring of polymer injection molding

An optical sensor, consisting of optical fibers to transmit light to and from the mold cavity, was constructed for the purpose of measuring the onset of polymer solidification during injection molding. The sensor was used to detect characteristic fluorescence radiation from a dye which had been doped into the resin at very low concentration. By measuring changes in fluorescence intensity it was possible to detect whether the state of the resin was liquid or solid. We observed that, as the resin cooled in the mold, the onset of solidification was indicated by highly characteristic and distinct changes in the fluorescence intensity/time profile. Application of the method involved the use of a calibration relationship between the fluorescence intensity and temperature of the doped polymer in order to determine the distict features which characterize the onset of solidification. Injection molding of a glass forming polymer (polystyrene) and a crystallizable polymer (polyethylene) was monitored by this technique.     

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.     

The Numerical Simulation of the Crystallization Morphology Evolution of Semi-Crystalline Polymers in Injection Molding

In polymer processing, crystallization plays a major role in the structure development of polymer, and structure development has important influence on the final properties of the products. Crystallization simulation should be highly concerned with the accurate simulation of polymer processing. In this work, a general formulation system based on the nucleation and growth process of polymer crystallization was adopted to describe the crystallization morphology evolution, and the crystallization process under isothermal and non-isothermal conditions was simulated. The model was introduced in injection molding, and the simulation of morphology evolution of polymer in injection molding was implemented.     

Optical monitoring of the injection molding of intercalated polypropylene nanocomposites

Intercalated polypropylene (PP)/clay nanocomposites were produced by twin screw extrusion; afterwards, the optical monitoring of their injection molding was done using a laser sensor. The transmitted light intensity as a function of molding time was measured. The mold and melt temperatures, packing pressure and flow rate were changed. The nanocomposite had higher induction times than the PP, that is, scattering structures were detected later in the nanocomposite than in the PP, which was attributed to a retardation effect promoted by the clay on the PP crystallization growth rate. The morphologies of the injection molded samples were analyzed by polarized light optical microscopy, differential scanning calorimetry and transmission electron microscopy. The nanocomposite samples showed a second core, a thicker skin layer, highly oriented nanoclay's tactoids in the skin region and average spherulites' sizes smaller than the PP. The final light intensity I_f was correlated with the spherulites' sizes: high values of I_f represented samples with large spherulites. The PP sample had average spherulites' sizes larger than the Nano samples. However, the surging of a second core with large spherulites in the Nano samples changed the expected pattern: the PP samples showed I_f lower than the Nano samples. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Effect of polymer properties on the structure of injection-molded parts

In this study, the distributions of both molecular orientation and crystallinity along the flow direction as well as across the thickness direction of injection-molded specimens of poly(ethylene terephthalate) (PET) molded at different mold temperatures were investigated. The degree of molecular orientation at the surface of the specimens was compared with that of other injected materials (polystyrene, high density polyethylene, liquid crystal polymer) showing different thermal, rheological, and crystallization characteristics. It was found that the molecular orientation at the skin layer of the molding increases with the polymer relaxation time, the rigidity of the polymer molecules, and the crystallization rate of the polymer. Moreover, in the case of PET, it was found that the crystallinity at the skin layer and in the core of the molding depends on the mold temperature. For low mold temperatures, near the gate, the maximum of crystallinity was observed at the subskin layer because of the “shear-induced crystallization” generated during the filling stage. On increasing the mold temperature, the maximum of crystallinity was found to shift to the skin layer as a result of the decrease of the thickness of this layer. For low mold temperatures, the variation of the molecular orientation in the thickness direction was found to be much the same as for the crystallinity of the polymer. This result indicates that the shear-induced crystallization process improves the degree of molecular orientation in the flow direction since it inhibits the relaxation process of the polymer molecules.     

Injection molding of semicrystalline polymers. II. Modeling and experiments

The injection molding of an isotactic polypropylene was computer-simulated with both quiescent and shear-induced crystallization taken into account. A one-dimensional finite difference model was used to simulate the filling, packing, and cooling stages of the injection-molding cycle. The Spencer-Gilmore equation was used to relate the density variations to the pressure and temperature traces in the packing simulation. The quiescent crystallization kinetics was modeled by the differential form of the Nakamura equation. The theory developed by Janeschitz-Kriegl and co-workers was used to model the shear-induced crystallization kinetics. The pressure traces during the filling and packing stages of the molding cycle, the thickness of the shear-induced crystallization layer, and the crystallinity profile throughout the thickness of the part were measured and compared with predicted values. © 1995 John Wiley & Sons, Inc.     

Ultrasonic characterization of the crystallization behavior of poly(ethylene terephthalate)

On the basis of the previous observations that the ultrasonic signals are sensitive to the crystallization of polymers (Tatibouet and Piché, Polymer 1991, 32, 3147), we have expanded our efforts to study the detail relationship between the ultrasonic signals and crystallization process in this work. The nonisothermal and isothermal crystallization of virgin poly(ethylene terephthalate) (PET) and PET samples after degradation were studied by using a specially designed pressure‐volume‐temperature (PVT) device, with which an ultrasonic detector was combined. The results showed that the evolution of the ultrasonic signals not only can be used to probe the crystallization process but also can qualitatively characterize the crystallization rate, crystallinity, crystallite size, and amorphous. DSC measurement was used to verify such results. Ultrasonic signals could be as a complementary tool to polymer chain movement and well be applied to characterize the crystallization behavior. Furthermore, the ultrasonic measurement has the potential use to characterize crystallization of products in‐line during processing (i.e., injection molding, micromoulding). © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

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     

Development of a pressure–volume–temperature measurement method for thermoplastic materials based on compression injection molding

The pressure–volume–temperature (pvT) relationship of polymers is vitally important information in designing and manufacturing polymers. Because of the special behavior of polymers, however, it is extremely difficult to accurately measure the data in a way that matches the thermal conditions of injection molding, which is one of the most widely used processing methods. As neither widely used, commercially available measuring devices, nor special equipment mentioned in the literature can fully satisfy this need, it was decided to build a new device able to determine the pvT relationship during injection molding of the polymer. The new device consists of a special mold that can be used on an injection molding machine and a data collection system connected to it. With the help of this device, pvT data can be measured during processing according to the thermal conditions of injection molding, and also considerably faster, and even in an industrial setting. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41140.