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.     

Computer modeling of isothermal crystallization in short fiber reinforced composites

Computer modeling and simulation for the isothermal crystallization of short fiber reinforced composites with athermal nucleation is presented. The “pixel coloring” technique which is capable of capturing the morphology evolution and calculating the crystallization kinetics is implemented. A parametric study is used to explore the influences of fibers on the crystallization in the reinforced system. The results indicate that the fibers depress the crystallization rate when compared to that of neat polymers or accelerate the crystallization rate by providing nucleation sites. The constraining effect is mainly dependent on fiber content while the enhancing effect is mainly determined by fiber surface and fiber nucleation density. Fiber orientation has almost no effect on the crystallization kinetics but changes the ultimate morphology. Fiber length and fiber diameter have different influences on crystallization kinetics and mean size of spherulites. Results show that fiber diameter affects the crystallization much more than that of fiber length.     

Simulation and analysis of industrial crystallization processes through multidimensional population balance equations. Part 2: a study of semi-batch crystallization

A bi-dimensional population balance model was presented in the previous part of this series of papers to simulate the time variations of two characteristic sizes of hydroquinone particles during crystallization. The multidimensional population balance equations combined with kinetic models and mass balance equations were shown to allow the simulation of the solution crystallization of hydroquinone characterized by a rod-like habit. Semi-continuous isothermal operations were performed at the lab-scale in the presence of various additive concentrations. Both the experimental solute concentration trajectory and the final bi-dimensional crystals size distribution were correctly predicted by the model. The simulated elongation shape factor characterizing the crystal shape was therefore in agreement with the experimental one. Due to the use of tailor-made additive, inhibition effects were observed to affect both primary nucleation and growth kinetics in the length direction. For secondary nucleation, indirect effects were assumed to occur which allowed satisfactory predictions of the final number of fine particles. The representation of the kinetics involved required the evaluation of a set of nine parameters. As a result it was observed that the elongation ratio characterizing the shape of the rod-like particles increases with the length in a nonlinear way. A major interest of the two-dimensional model lies in its ability to relate the time variations of the crystal habit: the particles lengthen in the first moments of their growth and then progressively get thicker until the end of the process.     

Modeling Growth Rate Dispersion in Industrial Crystallizers

The phenomenon of healing appears to be a plausible explanation for the growth rate dispersion observed in many industrial crystallizers. In this paper a growth model is postulated, which describes the healing of plastically deformed attrition fragments. The rate of healing is assumed to be inversely proportional to the initial strain and to the rate of change of either the length, the area, or the volume of the crystal. The validity of the proposed model is verified by the simulation of growth of the smallest crystals (L₀) in time in a growth experiment for specific combinations of the model parameters. In addition, the applicability of the proposed model is evaluated through simulations of steady state experimental data obtained in a 75‐liter Draft Tube (DT) crystallizer. It is concluded that the proposed model is able to fit reasonably well the experimental crystal size distribution. The model predicts the existence of a ‘dead time’ during which attrition fragments with large initial strain do not grow and which may last several residence times.     

注射螺杆研究进展

综述了国内外注射螺杆的实验研究和理论研究进展,展望了注射螺杆的未来发展方向。     

Micro-flowers of poly(p-phenylene pyromelliteimide) crystals

Morphology control of poly(p-phenylene pyromelliteimide) (PPPI) crystals was examined using reaction-induced crystallization of oligomers during solution polymerization of self-polymerizable N-(4′-aminophenyl)-3-carboxyl-4-alkoxycarbonylphthalimide. Micro-flowers of the PPPI needle-like crystals were formed in which the needle-like crystals grew radially from the center part as petals. The molecules aligned regularly along the long axis of the needle-like crystal. The structure of alkoxy group in the monomer and the monomer concentration influenced the size of the needle-like crystals, and their average length and width were changeable from 640 nm to 1.69 μm and from 110 nm to 210 nm, respectively. The average thickness was 20 nm. The obtained micro-flowers possessed high crystallinity and exhibited excellent thermal stability.     

Morphology development in immiscible polymer blends during crystallization of the dispersed phase under shear flow

Crystallization under shear of dispersed polybutylene terephthalate (PBT) fibers in copolymer polyethylene-methyl acrylate matrix (EMA) was investigated using a hot optical shear device. Crystallization during isotherm and cooling process was studied. Static crystallization experiments were carried out for comprehension purpose. Differential scanning calorimetry (DSC) analysis was performed in order to predict the crystallization behavior of PBT. Shear enhancement of its crystallization was thus demonstrated from rheological experiments. Interfacial tension of EMA/PBT blend was experimentally measured using the hot optical shear device. Theoretical break-up times of PBT fibers were also calculated. Control of the morphology through shear rate and crystallization time balance was demonstrated. Static crystallization experiments show that decreasing crystallization time favor fibrillar morphology. Breaking up of fibers was brought to the fore during dynamic crystallization experiments due to heterogeneous development of the crystallization along the fiber. During the dynamic crystallization, rapid quenching enables fibrillar morphology. Long crystallization times associated with low shear rates allow nodular morphology.     

基于正交试验和MPI二次开发技术多目标优化注射工艺参数

工艺条件是影响注射成型制品质量的重要因素,采用CAF模拟软件进行注射成型的工艺优化不但能降低成本,提高效率,而且能提高注射制品的质量。本文通过MPI二次开发技术开发出注射成型工艺方案优选分析器,采用正交试验方法为用户提供多目标值下的最优工艺参数组合,进一步提高了工艺优化的效率,也为用户提供了实用方便的工艺优化工具。     

基于Pro/E塑料顾问的手机前盖注塑成型工艺分析

从实用角度出发,以主流三维模具设计软件Pro/E中的塑料顾问为平台,以手机前盖注塑零件为实例,对塑料制品进行注塑成型工艺分析和注塑成型模拟。结果表明:Pro/E塑料顾问可提高塑料制品的成型质量,减少缺陷的产生降,低生产成本缩,短生产周期。     

熔体振动成型技术的应用

简要介绍了熔体振动成型技术在注射、挤出、吹塑等方面的应用。通过大量的实验表明 ,熔体振动成型可提高生产率、质量及力学等方面的性能     

Effect of processing on the crystalline orientation, morphology, and mechanical properties of polypropylene cast films and microporous membrane formation

Cast films of a high molecular weight linear polypropylene (L-PP) were prepared by extrusion followed by stretching using a chill roll. An air knife was employed to supply air to the film surface right at the exit of the die. The effects of air cooling conditions, chill roll temperature, and draw ratio on the crystalline orientation, morphology, mechanical and tear properties of the PP cast films were investigated. The crystallinity and crystal size distribution of the films were studied using differential scanning calorimetry (DSC). It was found that air blowing on the films contributed significantly to the uniformity of the lamellar structure. The orientation of crystalline and amorphous phases was measured using wide angle X-ray diffraction (WAXD) and Fourier transform infrared (FTIR). The amount of lamellae formation and long period spacing were obtained via small angle X-ray scattering (SAXS). The results showed that air cooling and the cast roll temperature have a crucial role on the orientation and amount of lamellae formation of the cast films, which was also confirmed from scanning electron microscopy (SEM) images of the films. Tensile properties and tear resistance of the cast films in machine and transverse directions (MD and TD, respectively) were evaluated. Significant increases of the Young modulus, yield stress, tensile strength, and tensile toughness along MD and drastic decreases of elongation at break along TD were observed for films subjected to air blowing. Morphological pictograms are proposed to represent the molecular structure of the films obtained without and upon applying air cooling for different chill roll temperatures. Finally, microporous membranes were prepared from annealed and stretched films to illustrate the effect of the PP cast film microstructure on the morphology and permeability of membranes. The observations of SEM surface images and water vapor transmission rate of the membranes showed higher pore density, uniform pore size, and superior permeability for the ones obtained from the precursor films prepared under controlled air cooling.     

Crystallization, melting, and morphology of a poly(ethylene oxide) diblock copolymer containing a tablet-like block of poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene}

A poly(ethylene oxide) diblock copolymer containing a short block of poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} (PEO-b-PMPCS) has been successfully synthesized via atom transfer radical polymerization (ATRP) method. The number average molecular weights (M_n) of the PEO and PMPCS blocks are 5300 and 2100 g/mol, respectively. Combining the techniques of differential scanning calorimetry (DSC), optical microscopy (OM), wide angle X-ray diffraction (WAXD), and small angle X-ray scattering (SAXS), we have found that the PMPCS blocks, which are tablet-like, can significantly affect the crystallization and melting of the diblock copolymer. The sample studied can form the crystals with a monoclinic crystal structure identical to that of the homo-PEO. The melting temperature (T_m) of the diblock copolymer increases monotonically with crystallization temperature (T_c), which is remarkably similar to the behavior of long period. On the basis of Gibbs-Thomson relationship, the equilibrium T_m of the diblock copolymer is estimated to be 65.4 °C. In a wide undercooling (ΔT) range (14 °C<ΔT<30 °C), the isothermal crystallization leads to square-shaped crystals. The PEO-b-PMPCS crystallization exhibits a regime I→II transition at ΔT of 19 °C. The PEO blocks are non-integral folded (NIF) in the crystals, and the PMPCS blocks rejected to lamellar fold surfaces prevent the NIF PEO crystals from transforming to integral folded (IF) ones. Furthermore, the PMPCS tablets may adjust their neighboring positions up or down with respect to the lamellar surface normal, forming more than one PMPCS layer to accompany the increase in the PEO fold length with increasing T_c.     

Crystallization of hydroxybutyrate oligomers: 1. morphology and folding

Exact length hydroxybutyrate oligomers containing 24 and 32 repeat units have been prepared and their crystallization behaviour and crystal morphology observed.The oligomers form crystals from dilute solution and from the melt that have similar overall morphologies to crystals of the polymer, poly(hydroxybutyrate). Electron diffraction indicates that the chains are closely perpendicular to the basal planes of the crystals.Wide angle X-ray diffraction suggests that all crystals, no matter what their thickness, have a high degree of crystallinity and the same crystal structure.The crystals have a range of thicknesses, as measured by small angle X-ray diffraction. The most common thicknesses correspond to the extended stem length and to one half or, surprisingly, two thirds of that value. A model is proposed for the non-integer fraction crystals in which half of the chain ends are incorporated in the crystals themselves.     

Crystalline morphology of poly(dimethylsiloxane)

The crystalline morphology of poly(dimethylsiloxane) was studied using a scanning electron microscope equipped with a cold stage. Samples of two different molecular weights were used. In both cases, spherulitic morphology is seen, from −70 °C, with spherulites of about 100 μ in size. Small single crystals of about a micron in size are also seen, and these are attributed to the presence of cyclics.     

塑料注射模模腔尺寸的计算

本文从实际生产中出现的两种极端状态出发推导了模腔尺寸计算的新公式，给出了塑件尺寸公差与塑件名义尺寸，收缩率波动范围，模具制造误差定量关系式，并以此为依据提出了判断塑件及成型尺寸精度的准则、提出了将塑件产量与模具尺寸计算相结合的模具设计思想。     

Injection Molding Nanoscale Features with the Aid of Induction Heating

During injection molding of micron or submicron scale features, incomplete filling frequently occurs, resulting from premature freezing of the polymer melt in contact with a cold mold. In order to overcome the filling difficulty without increasing the total cycle time, the mold surface temperature was raised rapidly by induction heating. A prototype mold insert with cooling channels was fabricated and integrated with a nickel stamp having nanoscale-grating structures. The nickel stamp surface was successfully heated from 25 to 258°C in 2.7 sec. Four different mold surface temperatures, 100, 150, 200 and 250°C, were tested to determine if the nanograting structures can be replicated with an optical quality cyclic olefin copolymer. Experimental results indicate that the nanocavities were successfully filled when the surface temperature reached 250°C, but mold release caused drag damages on the nanogratings. Further, coupled thermoelectromagnetic analyses were carried out to simulate the induction heating process of the nanostructured mold insert. The predicted surface temperature responses in general agree with the experimental ones and the simulation model can be used in the further development of process control and mold design in micro/nano molding.